Bug Summary

File:lib/CodeGen/BranchFolding.cpp
Warning:line 1339, column 39
Called C++ object pointer is null

Annotated Source Code

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clang -cc1 -triple x86_64-pc-linux-gnu -analyze -disable-free -disable-llvm-verifier -discard-value-names -main-file-name BranchFolding.cpp -analyzer-store=region -analyzer-opt-analyze-nested-blocks -analyzer-eagerly-assume -analyzer-checker=core -analyzer-checker=apiModeling -analyzer-checker=unix -analyzer-checker=deadcode -analyzer-checker=cplusplus -analyzer-checker=security.insecureAPI.UncheckedReturn -analyzer-checker=security.insecureAPI.getpw -analyzer-checker=security.insecureAPI.gets -analyzer-checker=security.insecureAPI.mktemp -analyzer-checker=security.insecureAPI.mkstemp -analyzer-checker=security.insecureAPI.vfork -analyzer-checker=nullability.NullPassedToNonnull -analyzer-checker=nullability.NullReturnedFromNonnull -analyzer-output plist -w -mrelocation-model pic -pic-level 2 -mthread-model posix -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb -momit-leaf-frame-pointer -ffunction-sections -fdata-sections -resource-dir /usr/lib/llvm-7/lib/clang/7.0.0 -D _DEBUG -D _GNU_SOURCE -D __STDC_CONSTANT_MACROS -D __STDC_FORMAT_MACROS -D __STDC_LIMIT_MACROS -I /build/llvm-toolchain-snapshot-7~svn337490/build-llvm/lib/CodeGen -I /build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen -I /build/llvm-toolchain-snapshot-7~svn337490/build-llvm/include -I /build/llvm-toolchain-snapshot-7~svn337490/include -U NDEBUG -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/x86_64-linux-gnu/c++/8 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/x86_64-linux-gnu/c++/8 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/backward -internal-isystem /usr/include/clang/7.0.0/include/ -internal-isystem /usr/local/include -internal-isystem /usr/lib/llvm-7/lib/clang/7.0.0/include -internal-externc-isystem /usr/lib/gcc/x86_64-linux-gnu/8/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -O2 -Wno-unused-parameter -Wwrite-strings -Wno-missing-field-initializers -Wno-long-long -Wno-maybe-uninitialized -Wno-comment -std=c++11 -fdeprecated-macro -fdebug-compilation-dir /build/llvm-toolchain-snapshot-7~svn337490/build-llvm/lib/CodeGen -ferror-limit 19 -fmessage-length 0 -fvisibility-inlines-hidden -fobjc-runtime=gcc -fdiagnostics-show-option -vectorize-loops -vectorize-slp -analyzer-output=html -analyzer-config stable-report-filename=true -o /tmp/scan-build-2018-07-20-043646-20380-1 -x c++ /build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp -faddrsig
1//===- BranchFolding.cpp - Fold machine code branch instructions ----------===//
2//
3// The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass forwards branches to unconditional branches to make them branch
11// directly to the target block. This pass often results in dead MBB's, which
12// it then removes.
13//
14// Note that this pass must be run after register allocation, it cannot handle
15// SSA form. It also must handle virtual registers for targets that emit virtual
16// ISA (e.g. NVPTX).
17//
18//===----------------------------------------------------------------------===//
19
20#include "BranchFolding.h"
21#include "llvm/ADT/BitVector.h"
22#include "llvm/ADT/DenseMap.h"
23#include "llvm/ADT/STLExtras.h"
24#include "llvm/ADT/SmallPtrSet.h"
25#include "llvm/ADT/SmallSet.h"
26#include "llvm/ADT/SmallVector.h"
27#include "llvm/ADT/Statistic.h"
28#include "llvm/CodeGen/Analysis.h"
29#include "llvm/CodeGen/LivePhysRegs.h"
30#include "llvm/CodeGen/MachineBasicBlock.h"
31#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
32#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
33#include "llvm/CodeGen/MachineFunction.h"
34#include "llvm/CodeGen/MachineFunctionPass.h"
35#include "llvm/CodeGen/MachineInstr.h"
36#include "llvm/CodeGen/MachineInstrBuilder.h"
37#include "llvm/CodeGen/MachineJumpTableInfo.h"
38#include "llvm/CodeGen/MachineLoopInfo.h"
39#include "llvm/CodeGen/MachineModuleInfo.h"
40#include "llvm/CodeGen/MachineOperand.h"
41#include "llvm/CodeGen/MachineRegisterInfo.h"
42#include "llvm/CodeGen/TargetInstrInfo.h"
43#include "llvm/CodeGen/TargetOpcodes.h"
44#include "llvm/CodeGen/TargetPassConfig.h"
45#include "llvm/CodeGen/TargetRegisterInfo.h"
46#include "llvm/CodeGen/TargetSubtargetInfo.h"
47#include "llvm/IR/DebugInfoMetadata.h"
48#include "llvm/IR/DebugLoc.h"
49#include "llvm/IR/Function.h"
50#include "llvm/MC/LaneBitmask.h"
51#include "llvm/MC/MCRegisterInfo.h"
52#include "llvm/Pass.h"
53#include "llvm/Support/BlockFrequency.h"
54#include "llvm/Support/BranchProbability.h"
55#include "llvm/Support/CommandLine.h"
56#include "llvm/Support/Debug.h"
57#include "llvm/Support/ErrorHandling.h"
58#include "llvm/Support/raw_ostream.h"
59#include "llvm/Target/TargetMachine.h"
60#include <cassert>
61#include <cstddef>
62#include <iterator>
63#include <numeric>
64#include <vector>
65
66using namespace llvm;
67
68#define DEBUG_TYPE"branch-folder" "branch-folder"
69
70STATISTIC(NumDeadBlocks, "Number of dead blocks removed")static llvm::Statistic NumDeadBlocks = {"branch-folder", "NumDeadBlocks"
, "Number of dead blocks removed", {0}, {false}}
;
71STATISTIC(NumBranchOpts, "Number of branches optimized")static llvm::Statistic NumBranchOpts = {"branch-folder", "NumBranchOpts"
, "Number of branches optimized", {0}, {false}}
;
72STATISTIC(NumTailMerge , "Number of block tails merged")static llvm::Statistic NumTailMerge = {"branch-folder", "NumTailMerge"
, "Number of block tails merged", {0}, {false}}
;
73STATISTIC(NumHoist , "Number of times common instructions are hoisted")static llvm::Statistic NumHoist = {"branch-folder", "NumHoist"
, "Number of times common instructions are hoisted", {0}, {false
}}
;
74STATISTIC(NumTailCalls, "Number of tail calls optimized")static llvm::Statistic NumTailCalls = {"branch-folder", "NumTailCalls"
, "Number of tail calls optimized", {0}, {false}}
;
75
76static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
77 cl::init(cl::BOU_UNSET), cl::Hidden);
78
79// Throttle for huge numbers of predecessors (compile speed problems)
80static cl::opt<unsigned>
81TailMergeThreshold("tail-merge-threshold",
82 cl::desc("Max number of predecessors to consider tail merging"),
83 cl::init(150), cl::Hidden);
84
85// Heuristic for tail merging (and, inversely, tail duplication).
86// TODO: This should be replaced with a target query.
87static cl::opt<unsigned>
88TailMergeSize("tail-merge-size",
89 cl::desc("Min number of instructions to consider tail merging"),
90 cl::init(3), cl::Hidden);
91
92namespace {
93
94 /// BranchFolderPass - Wrap branch folder in a machine function pass.
95 class BranchFolderPass : public MachineFunctionPass {
96 public:
97 static char ID;
98
99 explicit BranchFolderPass(): MachineFunctionPass(ID) {}
100
101 bool runOnMachineFunction(MachineFunction &MF) override;
102
103 void getAnalysisUsage(AnalysisUsage &AU) const override {
104 AU.addRequired<MachineBlockFrequencyInfo>();
105 AU.addRequired<MachineBranchProbabilityInfo>();
106 AU.addRequired<TargetPassConfig>();
107 MachineFunctionPass::getAnalysisUsage(AU);
108 }
109 };
110
111} // end anonymous namespace
112
113char BranchFolderPass::ID = 0;
114
115char &llvm::BranchFolderPassID = BranchFolderPass::ID;
116
117INITIALIZE_PASS(BranchFolderPass, DEBUG_TYPE,static void *initializeBranchFolderPassPassOnce(PassRegistry &
Registry) { PassInfo *PI = new PassInfo( "Control Flow Optimizer"
, "branch-folder", &BranchFolderPass::ID, PassInfo::NormalCtor_t
(callDefaultCtor<BranchFolderPass>), false, false); Registry
.registerPass(*PI, true); return PI; } static llvm::once_flag
InitializeBranchFolderPassPassFlag; void llvm::initializeBranchFolderPassPass
(PassRegistry &Registry) { llvm::call_once(InitializeBranchFolderPassPassFlag
, initializeBranchFolderPassPassOnce, std::ref(Registry)); }
118 "Control Flow Optimizer", false, false)static void *initializeBranchFolderPassPassOnce(PassRegistry &
Registry) { PassInfo *PI = new PassInfo( "Control Flow Optimizer"
, "branch-folder", &BranchFolderPass::ID, PassInfo::NormalCtor_t
(callDefaultCtor<BranchFolderPass>), false, false); Registry
.registerPass(*PI, true); return PI; } static llvm::once_flag
InitializeBranchFolderPassPassFlag; void llvm::initializeBranchFolderPassPass
(PassRegistry &Registry) { llvm::call_once(InitializeBranchFolderPassPassFlag
, initializeBranchFolderPassPassOnce, std::ref(Registry)); }
119
120bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
121 if (skipFunction(MF.getFunction()))
122 return false;
123
124 TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
125 // TailMerge can create jump into if branches that make CFG irreducible for
126 // HW that requires structurized CFG.
127 bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
128 PassConfig->getEnableTailMerge();
129 BranchFolder::MBFIWrapper MBBFreqInfo(
130 getAnalysis<MachineBlockFrequencyInfo>());
131 BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true, MBBFreqInfo,
132 getAnalysis<MachineBranchProbabilityInfo>());
133 return Folder.OptimizeFunction(MF, MF.getSubtarget().getInstrInfo(),
134 MF.getSubtarget().getRegisterInfo(),
135 getAnalysisIfAvailable<MachineModuleInfo>());
136}
137
138BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist,
139 MBFIWrapper &FreqInfo,
140 const MachineBranchProbabilityInfo &ProbInfo,
141 unsigned MinTailLength)
142 : EnableHoistCommonCode(CommonHoist), MinCommonTailLength(MinTailLength),
143 MBBFreqInfo(FreqInfo), MBPI(ProbInfo) {
144 if (MinCommonTailLength == 0)
145 MinCommonTailLength = TailMergeSize;
146 switch (FlagEnableTailMerge) {
147 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
148 case cl::BOU_TRUE: EnableTailMerge = true; break;
149 case cl::BOU_FALSE: EnableTailMerge = false; break;
150 }
151}
152
153void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
154 assert(MBB->pred_empty() && "MBB must be dead!")(static_cast <bool> (MBB->pred_empty() && "MBB must be dead!"
) ? void (0) : __assert_fail ("MBB->pred_empty() && \"MBB must be dead!\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 154, __extension__ __PRETTY_FUNCTION__))
;
155 LLVM_DEBUG(dbgs() << "\nRemoving MBB: " << *MBB)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nRemoving MBB: " <<
*MBB; } } while (false)
;
156
157 MachineFunction *MF = MBB->getParent();
158 // drop all successors.
159 while (!MBB->succ_empty())
160 MBB->removeSuccessor(MBB->succ_end()-1);
161
162 // Avoid matching if this pointer gets reused.
163 TriedMerging.erase(MBB);
164
165 // Remove the block.
166 MF->erase(MBB);
167 EHScopeMembership.erase(MBB);
168 if (MLI)
169 MLI->removeBlock(MBB);
170}
171
172bool BranchFolder::OptimizeFunction(MachineFunction &MF,
173 const TargetInstrInfo *tii,
174 const TargetRegisterInfo *tri,
175 MachineModuleInfo *mmi,
176 MachineLoopInfo *mli, bool AfterPlacement) {
177 if (!tii) return false;
178
179 TriedMerging.clear();
180
181 MachineRegisterInfo &MRI = MF.getRegInfo();
182 AfterBlockPlacement = AfterPlacement;
183 TII = tii;
184 TRI = tri;
185 MMI = mmi;
186 MLI = mli;
187 this->MRI = &MRI;
188
189 UpdateLiveIns = MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF);
190 if (!UpdateLiveIns)
191 MRI.invalidateLiveness();
192
193 // Fix CFG. The later algorithms expect it to be right.
194 bool MadeChange = false;
195 for (MachineBasicBlock &MBB : MF) {
196 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
197 SmallVector<MachineOperand, 4> Cond;
198 if (!TII->analyzeBranch(MBB, TBB, FBB, Cond, true))
199 MadeChange |= MBB.CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
200 }
201
202 // Recalculate EH scope membership.
203 EHScopeMembership = getEHScopeMembership(MF);
204
205 bool MadeChangeThisIteration = true;
206 while (MadeChangeThisIteration) {
207 MadeChangeThisIteration = TailMergeBlocks(MF);
208 // No need to clean up if tail merging does not change anything after the
209 // block placement.
210 if (!AfterBlockPlacement || MadeChangeThisIteration)
211 MadeChangeThisIteration |= OptimizeBranches(MF);
212 if (EnableHoistCommonCode)
213 MadeChangeThisIteration |= HoistCommonCode(MF);
214 MadeChange |= MadeChangeThisIteration;
215 }
216
217 // See if any jump tables have become dead as the code generator
218 // did its thing.
219 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
220 if (!JTI)
221 return MadeChange;
222
223 // Walk the function to find jump tables that are live.
224 BitVector JTIsLive(JTI->getJumpTables().size());
225 for (const MachineBasicBlock &BB : MF) {
226 for (const MachineInstr &I : BB)
227 for (const MachineOperand &Op : I.operands()) {
228 if (!Op.isJTI()) continue;
229
230 // Remember that this JT is live.
231 JTIsLive.set(Op.getIndex());
232 }
233 }
234
235 // Finally, remove dead jump tables. This happens when the
236 // indirect jump was unreachable (and thus deleted).
237 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
238 if (!JTIsLive.test(i)) {
239 JTI->RemoveJumpTable(i);
240 MadeChange = true;
241 }
242
243 return MadeChange;
244}
245
246//===----------------------------------------------------------------------===//
247// Tail Merging of Blocks
248//===----------------------------------------------------------------------===//
249
250/// HashMachineInstr - Compute a hash value for MI and its operands.
251static unsigned HashMachineInstr(const MachineInstr &MI) {
252 unsigned Hash = MI.getOpcode();
253 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
254 const MachineOperand &Op = MI.getOperand(i);
255
256 // Merge in bits from the operand if easy. We can't use MachineOperand's
257 // hash_code here because it's not deterministic and we sort by hash value
258 // later.
259 unsigned OperandHash = 0;
260 switch (Op.getType()) {
261 case MachineOperand::MO_Register:
262 OperandHash = Op.getReg();
263 break;
264 case MachineOperand::MO_Immediate:
265 OperandHash = Op.getImm();
266 break;
267 case MachineOperand::MO_MachineBasicBlock:
268 OperandHash = Op.getMBB()->getNumber();
269 break;
270 case MachineOperand::MO_FrameIndex:
271 case MachineOperand::MO_ConstantPoolIndex:
272 case MachineOperand::MO_JumpTableIndex:
273 OperandHash = Op.getIndex();
274 break;
275 case MachineOperand::MO_GlobalAddress:
276 case MachineOperand::MO_ExternalSymbol:
277 // Global address / external symbol are too hard, don't bother, but do
278 // pull in the offset.
279 OperandHash = Op.getOffset();
280 break;
281 default:
282 break;
283 }
284
285 Hash += ((OperandHash << 3) | Op.getType()) << (i & 31);
286 }
287 return Hash;
288}
289
290/// HashEndOfMBB - Hash the last instruction in the MBB.
291static unsigned HashEndOfMBB(const MachineBasicBlock &MBB) {
292 MachineBasicBlock::const_iterator I = MBB.getLastNonDebugInstr();
293 if (I == MBB.end())
294 return 0;
295
296 return HashMachineInstr(*I);
297}
298
299/// Whether MI should be counted as an instruction when calculating common tail.
300static bool countsAsInstruction(const MachineInstr &MI) {
301 return !(MI.isDebugValue() || MI.isCFIInstruction());
302}
303
304/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
305/// of instructions they actually have in common together at their end. Return
306/// iterators for the first shared instruction in each block.
307static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
308 MachineBasicBlock *MBB2,
309 MachineBasicBlock::iterator &I1,
310 MachineBasicBlock::iterator &I2) {
311 I1 = MBB1->end();
312 I2 = MBB2->end();
313
314 unsigned TailLen = 0;
315 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
316 --I1; --I2;
317 // Skip debugging pseudos; necessary to avoid changing the code.
318 while (!countsAsInstruction(*I1)) {
319 if (I1==MBB1->begin()) {
320 while (!countsAsInstruction(*I2)) {
321 if (I2==MBB2->begin()) {
322 // I1==DBG at begin; I2==DBG at begin
323 goto SkipTopCFIAndReturn;
324 }
325 --I2;
326 }
327 ++I2;
328 // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
329 goto SkipTopCFIAndReturn;
330 }
331 --I1;
332 }
333 // I1==first (untested) non-DBG preceding known match
334 while (!countsAsInstruction(*I2)) {
335 if (I2==MBB2->begin()) {
336 ++I1;
337 // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
338 goto SkipTopCFIAndReturn;
339 }
340 --I2;
341 }
342 // I1, I2==first (untested) non-DBGs preceding known match
343 if (!I1->isIdenticalTo(*I2) ||
344 // FIXME: This check is dubious. It's used to get around a problem where
345 // people incorrectly expect inline asm directives to remain in the same
346 // relative order. This is untenable because normal compiler
347 // optimizations (like this one) may reorder and/or merge these
348 // directives.
349 I1->isInlineAsm()) {
350 ++I1; ++I2;
351 break;
352 }
353 ++TailLen;
354 }
355 // Back past possible debugging pseudos at beginning of block. This matters
356 // when one block differs from the other only by whether debugging pseudos
357 // are present at the beginning. (This way, the various checks later for
358 // I1==MBB1->begin() work as expected.)
359 if (I1 == MBB1->begin() && I2 != MBB2->begin()) {
360 --I2;
361 while (I2->isDebugInstr()) {
362 if (I2 == MBB2->begin())
363 return TailLen;
364 --I2;
365 }
366 ++I2;
367 }
368 if (I2 == MBB2->begin() && I1 != MBB1->begin()) {
369 --I1;
370 while (I1->isDebugInstr()) {
371 if (I1 == MBB1->begin())
372 return TailLen;
373 --I1;
374 }
375 ++I1;
376 }
377
378SkipTopCFIAndReturn:
379 // Ensure that I1 and I2 do not point to a CFI_INSTRUCTION. This can happen if
380 // I1 and I2 are non-identical when compared and then one or both of them ends
381 // up pointing to a CFI instruction after being incremented. For example:
382 /*
383 BB1:
384 ...
385 INSTRUCTION_A
386 ADD32ri8 <- last common instruction
387 ...
388 BB2:
389 ...
390 INSTRUCTION_B
391 CFI_INSTRUCTION
392 ADD32ri8 <- last common instruction
393 ...
394 */
395 // When INSTRUCTION_A and INSTRUCTION_B are compared as not equal, after
396 // incrementing the iterators, I1 will point to ADD, however I2 will point to
397 // the CFI instruction. Later on, this leads to BB2 being 'hacked off' at the
398 // wrong place (in ReplaceTailWithBranchTo()) which results in losing this CFI
399 // instruction.
400 while (I1 != MBB1->end() && I1->isCFIInstruction()) {
401 ++I1;
402 }
403
404 while (I2 != MBB2->end() && I2->isCFIInstruction()) {
405 ++I2;
406 }
407
408 return TailLen;
409}
410
411void BranchFolder::replaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
412 MachineBasicBlock &NewDest) {
413 if (UpdateLiveIns) {
414 // OldInst should always point to an instruction.
415 MachineBasicBlock &OldMBB = *OldInst->getParent();
416 LiveRegs.clear();
417 LiveRegs.addLiveOuts(OldMBB);
418 // Move backward to the place where will insert the jump.
419 MachineBasicBlock::iterator I = OldMBB.end();
420 do {
421 --I;
422 LiveRegs.stepBackward(*I);
423 } while (I != OldInst);
424
425 // Merging the tails may have switched some undef operand to non-undef ones.
426 // Add IMPLICIT_DEFS into OldMBB as necessary to have a definition of the
427 // register.
428 for (MachineBasicBlock::RegisterMaskPair P : NewDest.liveins()) {
429 // We computed the liveins with computeLiveIn earlier and should only see
430 // full registers:
431 assert(P.LaneMask == LaneBitmask::getAll() &&(static_cast <bool> (P.LaneMask == LaneBitmask::getAll(
) && "Can only handle full register.") ? void (0) : __assert_fail
("P.LaneMask == LaneBitmask::getAll() && \"Can only handle full register.\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 432, __extension__ __PRETTY_FUNCTION__))
432 "Can only handle full register.")(static_cast <bool> (P.LaneMask == LaneBitmask::getAll(
) && "Can only handle full register.") ? void (0) : __assert_fail
("P.LaneMask == LaneBitmask::getAll() && \"Can only handle full register.\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 432, __extension__ __PRETTY_FUNCTION__))
;
433 MCPhysReg Reg = P.PhysReg;
434 if (!LiveRegs.available(*MRI, Reg))
435 continue;
436 DebugLoc DL;
437 BuildMI(OldMBB, OldInst, DL, TII->get(TargetOpcode::IMPLICIT_DEF), Reg);
438 }
439 }
440
441 TII->ReplaceTailWithBranchTo(OldInst, &NewDest);
442 ++NumTailMerge;
443}
444
445MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
446 MachineBasicBlock::iterator BBI1,
447 const BasicBlock *BB) {
448 if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
449 return nullptr;
450
451 MachineFunction &MF = *CurMBB.getParent();
452
453 // Create the fall-through block.
454 MachineFunction::iterator MBBI = CurMBB.getIterator();
455 MachineBasicBlock *NewMBB = MF.CreateMachineBasicBlock(BB);
456 CurMBB.getParent()->insert(++MBBI, NewMBB);
457
458 // Move all the successors of this block to the specified block.
459 NewMBB->transferSuccessors(&CurMBB);
460
461 // Add an edge from CurMBB to NewMBB for the fall-through.
462 CurMBB.addSuccessor(NewMBB);
463
464 // Splice the code over.
465 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
466
467 // NewMBB belongs to the same loop as CurMBB.
468 if (MLI)
469 if (MachineLoop *ML = MLI->getLoopFor(&CurMBB))
470 ML->addBasicBlockToLoop(NewMBB, MLI->getBase());
471
472 // NewMBB inherits CurMBB's block frequency.
473 MBBFreqInfo.setBlockFreq(NewMBB, MBBFreqInfo.getBlockFreq(&CurMBB));
474
475 if (UpdateLiveIns)
476 computeAndAddLiveIns(LiveRegs, *NewMBB);
477
478 // Add the new block to the EH scope.
479 const auto &EHScopeI = EHScopeMembership.find(&CurMBB);
480 if (EHScopeI != EHScopeMembership.end()) {
481 auto n = EHScopeI->second;
482 EHScopeMembership[NewMBB] = n;
483 }
484
485 return NewMBB;
486}
487
488/// EstimateRuntime - Make a rough estimate for how long it will take to run
489/// the specified code.
490static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
491 MachineBasicBlock::iterator E) {
492 unsigned Time = 0;
493 for (; I != E; ++I) {
494 if (!countsAsInstruction(*I))
495 continue;
496 if (I->isCall())
497 Time += 10;
498 else if (I->mayLoad() || I->mayStore())
499 Time += 2;
500 else
501 ++Time;
502 }
503 return Time;
504}
505
506// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
507// branches temporarily for tail merging). In the case where CurMBB ends
508// with a conditional branch to the next block, optimize by reversing the
509// test and conditionally branching to SuccMBB instead.
510static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
511 const TargetInstrInfo *TII) {
512 MachineFunction *MF = CurMBB->getParent();
513 MachineFunction::iterator I = std::next(MachineFunction::iterator(CurMBB));
514 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
515 SmallVector<MachineOperand, 4> Cond;
516 DebugLoc dl = CurMBB->findBranchDebugLoc();
517 if (I != MF->end() && !TII->analyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
518 MachineBasicBlock *NextBB = &*I;
519 if (TBB == NextBB && !Cond.empty() && !FBB) {
520 if (!TII->reverseBranchCondition(Cond)) {
521 TII->removeBranch(*CurMBB);
522 TII->insertBranch(*CurMBB, SuccBB, nullptr, Cond, dl);
523 return;
524 }
525 }
526 }
527 TII->insertBranch(*CurMBB, SuccBB, nullptr,
528 SmallVector<MachineOperand, 0>(), dl);
529}
530
531bool
532BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
533 if (getHash() < o.getHash())
534 return true;
535 if (getHash() > o.getHash())
536 return false;
537 if (getBlock()->getNumber() < o.getBlock()->getNumber())
538 return true;
539 if (getBlock()->getNumber() > o.getBlock()->getNumber())
540 return false;
541 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
542 // an object with itself.
543#ifndef _GLIBCXX_DEBUG
544 llvm_unreachable("Predecessor appears twice")::llvm::llvm_unreachable_internal("Predecessor appears twice"
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 544)
;
545#else
546 return false;
547#endif
548}
549
550BlockFrequency
551BranchFolder::MBFIWrapper::getBlockFreq(const MachineBasicBlock *MBB) const {
552 auto I = MergedBBFreq.find(MBB);
553
554 if (I != MergedBBFreq.end())
555 return I->second;
556
557 return MBFI.getBlockFreq(MBB);
558}
559
560void BranchFolder::MBFIWrapper::setBlockFreq(const MachineBasicBlock *MBB,
561 BlockFrequency F) {
562 MergedBBFreq[MBB] = F;
563}
564
565raw_ostream &
566BranchFolder::MBFIWrapper::printBlockFreq(raw_ostream &OS,
567 const MachineBasicBlock *MBB) const {
568 return MBFI.printBlockFreq(OS, getBlockFreq(MBB));
569}
570
571raw_ostream &
572BranchFolder::MBFIWrapper::printBlockFreq(raw_ostream &OS,
573 const BlockFrequency Freq) const {
574 return MBFI.printBlockFreq(OS, Freq);
575}
576
577void BranchFolder::MBFIWrapper::view(const Twine &Name, bool isSimple) {
578 MBFI.view(Name, isSimple);
579}
580
581uint64_t
582BranchFolder::MBFIWrapper::getEntryFreq() const {
583 return MBFI.getEntryFreq();
584}
585
586/// CountTerminators - Count the number of terminators in the given
587/// block and set I to the position of the first non-terminator, if there
588/// is one, or MBB->end() otherwise.
589static unsigned CountTerminators(MachineBasicBlock *MBB,
590 MachineBasicBlock::iterator &I) {
591 I = MBB->end();
592 unsigned NumTerms = 0;
593 while (true) {
594 if (I == MBB->begin()) {
595 I = MBB->end();
596 break;
597 }
598 --I;
599 if (!I->isTerminator()) break;
600 ++NumTerms;
601 }
602 return NumTerms;
603}
604
605/// A no successor, non-return block probably ends in unreachable and is cold.
606/// Also consider a block that ends in an indirect branch to be a return block,
607/// since many targets use plain indirect branches to return.
608static bool blockEndsInUnreachable(const MachineBasicBlock *MBB) {
609 if (!MBB->succ_empty())
610 return false;
611 if (MBB->empty())
612 return true;
613 return !(MBB->back().isReturn() || MBB->back().isIndirectBranch());
614}
615
616/// ProfitableToMerge - Check if two machine basic blocks have a common tail
617/// and decide if it would be profitable to merge those tails. Return the
618/// length of the common tail and iterators to the first common instruction
619/// in each block.
620/// MBB1, MBB2 The blocks to check
621/// MinCommonTailLength Minimum size of tail block to be merged.
622/// CommonTailLen Out parameter to record the size of the shared tail between
623/// MBB1 and MBB2
624/// I1, I2 Iterator references that will be changed to point to the first
625/// instruction in the common tail shared by MBB1,MBB2
626/// SuccBB A common successor of MBB1, MBB2 which are in a canonical form
627/// relative to SuccBB
628/// PredBB The layout predecessor of SuccBB, if any.
629/// EHScopeMembership map from block to EH scope #.
630/// AfterPlacement True if we are merging blocks after layout. Stricter
631/// thresholds apply to prevent undoing tail-duplication.
632static bool
633ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
634 unsigned MinCommonTailLength, unsigned &CommonTailLen,
635 MachineBasicBlock::iterator &I1,
636 MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB,
637 MachineBasicBlock *PredBB,
638 DenseMap<const MachineBasicBlock *, int> &EHScopeMembership,
639 bool AfterPlacement) {
640 // It is never profitable to tail-merge blocks from two different EH scopes.
641 if (!EHScopeMembership.empty()) {
642 auto EHScope1 = EHScopeMembership.find(MBB1);
643 assert(EHScope1 != EHScopeMembership.end())(static_cast <bool> (EHScope1 != EHScopeMembership.end(
)) ? void (0) : __assert_fail ("EHScope1 != EHScopeMembership.end()"
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 643, __extension__ __PRETTY_FUNCTION__))
;
644 auto EHScope2 = EHScopeMembership.find(MBB2);
645 assert(EHScope2 != EHScopeMembership.end())(static_cast <bool> (EHScope2 != EHScopeMembership.end(
)) ? void (0) : __assert_fail ("EHScope2 != EHScopeMembership.end()"
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 645, __extension__ __PRETTY_FUNCTION__))
;
646 if (EHScope1->second != EHScope2->second)
647 return false;
648 }
649
650 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
651 if (CommonTailLen == 0)
652 return false;
653 LLVM_DEBUG(dbgs() << "Common tail length of " << printMBBReference(*MBB1)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Common tail length of "
<< printMBBReference(*MBB1) << " and " << printMBBReference
(*MBB2) << " is " << CommonTailLen << '\n';
} } while (false)
654 << " and " << printMBBReference(*MBB2) << " is "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Common tail length of "
<< printMBBReference(*MBB1) << " and " << printMBBReference
(*MBB2) << " is " << CommonTailLen << '\n';
} } while (false)
655 << CommonTailLen << '\n')do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Common tail length of "
<< printMBBReference(*MBB1) << " and " << printMBBReference
(*MBB2) << " is " << CommonTailLen << '\n';
} } while (false)
;
656
657 // It's almost always profitable to merge any number of non-terminator
658 // instructions with the block that falls through into the common successor.
659 // This is true only for a single successor. For multiple successors, we are
660 // trading a conditional branch for an unconditional one.
661 // TODO: Re-visit successor size for non-layout tail merging.
662 if ((MBB1 == PredBB || MBB2 == PredBB) &&
663 (!AfterPlacement || MBB1->succ_size() == 1)) {
664 MachineBasicBlock::iterator I;
665 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
666 if (CommonTailLen > NumTerms)
667 return true;
668 }
669
670 // If these are identical non-return blocks with no successors, merge them.
671 // Such blocks are typically cold calls to noreturn functions like abort, and
672 // are unlikely to become a fallthrough target after machine block placement.
673 // Tail merging these blocks is unlikely to create additional unconditional
674 // branches, and will reduce the size of this cold code.
675 if (I1 == MBB1->begin() && I2 == MBB2->begin() &&
676 blockEndsInUnreachable(MBB1) && blockEndsInUnreachable(MBB2))
677 return true;
678
679 // If one of the blocks can be completely merged and happens to be in
680 // a position where the other could fall through into it, merge any number
681 // of instructions, because it can be done without a branch.
682 // TODO: If the blocks are not adjacent, move one of them so that they are?
683 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
684 return true;
685 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
686 return true;
687
688 // If both blocks are identical and end in a branch, merge them unless they
689 // both have a fallthrough predecessor and successor.
690 // We can only do this after block placement because it depends on whether
691 // there are fallthroughs, and we don't know until after layout.
692 if (AfterPlacement && I1 == MBB1->begin() && I2 == MBB2->begin()) {
693 auto BothFallThrough = [](MachineBasicBlock *MBB) {
694 if (MBB->succ_size() != 0 && !MBB->canFallThrough())
695 return false;
696 MachineFunction::iterator I(MBB);
697 MachineFunction *MF = MBB->getParent();
698 return (MBB != &*MF->begin()) && std::prev(I)->canFallThrough();
699 };
700 if (!BothFallThrough(MBB1) || !BothFallThrough(MBB2))
701 return true;
702 }
703
704 // If both blocks have an unconditional branch temporarily stripped out,
705 // count that as an additional common instruction for the following
706 // heuristics. This heuristic is only accurate for single-succ blocks, so to
707 // make sure that during layout merging and duplicating don't crash, we check
708 // for that when merging during layout.
709 unsigned EffectiveTailLen = CommonTailLen;
710 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
711 (MBB1->succ_size() == 1 || !AfterPlacement) &&
712 !MBB1->back().isBarrier() &&
713 !MBB2->back().isBarrier())
714 ++EffectiveTailLen;
715
716 // Check if the common tail is long enough to be worthwhile.
717 if (EffectiveTailLen >= MinCommonTailLength)
718 return true;
719
720 // If we are optimizing for code size, 2 instructions in common is enough if
721 // we don't have to split a block. At worst we will be introducing 1 new
722 // branch instruction, which is likely to be smaller than the 2
723 // instructions that would be deleted in the merge.
724 MachineFunction *MF = MBB1->getParent();
725 return EffectiveTailLen >= 2 && MF->getFunction().optForSize() &&
726 (I1 == MBB1->begin() || I2 == MBB2->begin());
727}
728
729unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
730 unsigned MinCommonTailLength,
731 MachineBasicBlock *SuccBB,
732 MachineBasicBlock *PredBB) {
733 unsigned maxCommonTailLength = 0U;
734 SameTails.clear();
735 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
736 MPIterator HighestMPIter = std::prev(MergePotentials.end());
737 for (MPIterator CurMPIter = std::prev(MergePotentials.end()),
738 B = MergePotentials.begin();
739 CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) {
740 for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) {
741 unsigned CommonTailLen;
742 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
743 MinCommonTailLength,
744 CommonTailLen, TrialBBI1, TrialBBI2,
745 SuccBB, PredBB,
746 EHScopeMembership,
747 AfterBlockPlacement)) {
748 if (CommonTailLen > maxCommonTailLength) {
749 SameTails.clear();
750 maxCommonTailLength = CommonTailLen;
751 HighestMPIter = CurMPIter;
752 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
753 }
754 if (HighestMPIter == CurMPIter &&
755 CommonTailLen == maxCommonTailLength)
756 SameTails.push_back(SameTailElt(I, TrialBBI2));
757 }
758 if (I == B)
759 break;
760 }
761 }
762 return maxCommonTailLength;
763}
764
765void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
766 MachineBasicBlock *SuccBB,
767 MachineBasicBlock *PredBB) {
768 MPIterator CurMPIter, B;
769 for (CurMPIter = std::prev(MergePotentials.end()),
770 B = MergePotentials.begin();
771 CurMPIter->getHash() == CurHash; --CurMPIter) {
772 // Put the unconditional branch back, if we need one.
773 MachineBasicBlock *CurMBB = CurMPIter->getBlock();
774 if (SuccBB && CurMBB != PredBB)
775 FixTail(CurMBB, SuccBB, TII);
776 if (CurMPIter == B)
777 break;
778 }
779 if (CurMPIter->getHash() != CurHash)
780 CurMPIter++;
781 MergePotentials.erase(CurMPIter, MergePotentials.end());
782}
783
784bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
785 MachineBasicBlock *SuccBB,
786 unsigned maxCommonTailLength,
787 unsigned &commonTailIndex) {
788 commonTailIndex = 0;
789 unsigned TimeEstimate = ~0U;
790 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
791 // Use PredBB if possible; that doesn't require a new branch.
792 if (SameTails[i].getBlock() == PredBB) {
793 commonTailIndex = i;
794 break;
795 }
796 // Otherwise, make a (fairly bogus) choice based on estimate of
797 // how long it will take the various blocks to execute.
798 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
799 SameTails[i].getTailStartPos());
800 if (t <= TimeEstimate) {
801 TimeEstimate = t;
802 commonTailIndex = i;
803 }
804 }
805
806 MachineBasicBlock::iterator BBI =
807 SameTails[commonTailIndex].getTailStartPos();
808 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
809
810 LLVM_DEBUG(dbgs() << "\nSplitting " << printMBBReference(*MBB) << ", size "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nSplitting " << printMBBReference
(*MBB) << ", size " << maxCommonTailLength; } } while
(false)
811 << maxCommonTailLength)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nSplitting " << printMBBReference
(*MBB) << ", size " << maxCommonTailLength; } } while
(false)
;
812
813 // If the split block unconditionally falls-thru to SuccBB, it will be
814 // merged. In control flow terms it should then take SuccBB's name. e.g. If
815 // SuccBB is an inner loop, the common tail is still part of the inner loop.
816 const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ?
817 SuccBB->getBasicBlock() : MBB->getBasicBlock();
818 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB);
819 if (!newMBB) {
820 LLVM_DEBUG(dbgs() << "... failed!")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "... failed!"; } } while
(false)
;
821 return false;
822 }
823
824 SameTails[commonTailIndex].setBlock(newMBB);
825 SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
826
827 // If we split PredBB, newMBB is the new predecessor.
828 if (PredBB == MBB)
829 PredBB = newMBB;
830
831 return true;
832}
833
834static void
835mergeOperations(MachineBasicBlock::iterator MBBIStartPos,
836 MachineBasicBlock &MBBCommon) {
837 MachineBasicBlock *MBB = MBBIStartPos->getParent();
838 // Note CommonTailLen does not necessarily matches the size of
839 // the common BB nor all its instructions because of debug
840 // instructions differences.
841 unsigned CommonTailLen = 0;
842 for (auto E = MBB->end(); MBBIStartPos != E; ++MBBIStartPos)
843 ++CommonTailLen;
844
845 MachineBasicBlock::reverse_iterator MBBI = MBB->rbegin();
846 MachineBasicBlock::reverse_iterator MBBIE = MBB->rend();
847 MachineBasicBlock::reverse_iterator MBBICommon = MBBCommon.rbegin();
848 MachineBasicBlock::reverse_iterator MBBIECommon = MBBCommon.rend();
849
850 while (CommonTailLen--) {
851 assert(MBBI != MBBIE && "Reached BB end within common tail length!")(static_cast <bool> (MBBI != MBBIE && "Reached BB end within common tail length!"
) ? void (0) : __assert_fail ("MBBI != MBBIE && \"Reached BB end within common tail length!\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 851, __extension__ __PRETTY_FUNCTION__))
;
852 (void)MBBIE;
853
854 if (!countsAsInstruction(*MBBI)) {
855 ++MBBI;
856 continue;
857 }
858
859 while ((MBBICommon != MBBIECommon) && !countsAsInstruction(*MBBICommon))
860 ++MBBICommon;
861
862 assert(MBBICommon != MBBIECommon &&(static_cast <bool> (MBBICommon != MBBIECommon &&
"Reached BB end within common tail length!") ? void (0) : __assert_fail
("MBBICommon != MBBIECommon && \"Reached BB end within common tail length!\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 863, __extension__ __PRETTY_FUNCTION__))
863 "Reached BB end within common tail length!")(static_cast <bool> (MBBICommon != MBBIECommon &&
"Reached BB end within common tail length!") ? void (0) : __assert_fail
("MBBICommon != MBBIECommon && \"Reached BB end within common tail length!\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 863, __extension__ __PRETTY_FUNCTION__))
;
864 assert(MBBICommon->isIdenticalTo(*MBBI) && "Expected matching MIIs!")(static_cast <bool> (MBBICommon->isIdenticalTo(*MBBI
) && "Expected matching MIIs!") ? void (0) : __assert_fail
("MBBICommon->isIdenticalTo(*MBBI) && \"Expected matching MIIs!\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 864, __extension__ __PRETTY_FUNCTION__))
;
865
866 // Merge MMOs from memory operations in the common block.
867 if (MBBICommon->mayLoad() || MBBICommon->mayStore())
868 MBBICommon->setMemRefs(MBBICommon->mergeMemRefsWith(*MBBI));
869 // Drop undef flags if they aren't present in all merged instructions.
870 for (unsigned I = 0, E = MBBICommon->getNumOperands(); I != E; ++I) {
871 MachineOperand &MO = MBBICommon->getOperand(I);
872 if (MO.isReg() && MO.isUndef()) {
873 const MachineOperand &OtherMO = MBBI->getOperand(I);
874 if (!OtherMO.isUndef())
875 MO.setIsUndef(false);
876 }
877 }
878
879 ++MBBI;
880 ++MBBICommon;
881 }
882}
883
884void BranchFolder::mergeCommonTails(unsigned commonTailIndex) {
885 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
886
887 std::vector<MachineBasicBlock::iterator> NextCommonInsts(SameTails.size());
888 for (unsigned int i = 0 ; i != SameTails.size() ; ++i) {
889 if (i != commonTailIndex) {
890 NextCommonInsts[i] = SameTails[i].getTailStartPos();
891 mergeOperations(SameTails[i].getTailStartPos(), *MBB);
892 } else {
893 assert(SameTails[i].getTailStartPos() == MBB->begin() &&(static_cast <bool> (SameTails[i].getTailStartPos() == MBB
->begin() && "MBB is not a common tail only block"
) ? void (0) : __assert_fail ("SameTails[i].getTailStartPos() == MBB->begin() && \"MBB is not a common tail only block\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 894, __extension__ __PRETTY_FUNCTION__))
894 "MBB is not a common tail only block")(static_cast <bool> (SameTails[i].getTailStartPos() == MBB
->begin() && "MBB is not a common tail only block"
) ? void (0) : __assert_fail ("SameTails[i].getTailStartPos() == MBB->begin() && \"MBB is not a common tail only block\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 894, __extension__ __PRETTY_FUNCTION__))
;
895 }
896 }
897
898 for (auto &MI : *MBB) {
899 if (!countsAsInstruction(MI))
900 continue;
901 DebugLoc DL = MI.getDebugLoc();
902 for (unsigned int i = 0 ; i < NextCommonInsts.size() ; i++) {
903 if (i == commonTailIndex)
904 continue;
905
906 auto &Pos = NextCommonInsts[i];
907 assert(Pos != SameTails[i].getBlock()->end() &&(static_cast <bool> (Pos != SameTails[i].getBlock()->
end() && "Reached BB end within common tail") ? void (
0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 908, __extension__ __PRETTY_FUNCTION__))
908 "Reached BB end within common tail")(static_cast <bool> (Pos != SameTails[i].getBlock()->
end() && "Reached BB end within common tail") ? void (
0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 908, __extension__ __PRETTY_FUNCTION__))
;
909 while (!countsAsInstruction(*Pos)) {
910 ++Pos;
911 assert(Pos != SameTails[i].getBlock()->end() &&(static_cast <bool> (Pos != SameTails[i].getBlock()->
end() && "Reached BB end within common tail") ? void (
0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 912, __extension__ __PRETTY_FUNCTION__))
912 "Reached BB end within common tail")(static_cast <bool> (Pos != SameTails[i].getBlock()->
end() && "Reached BB end within common tail") ? void (
0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 912, __extension__ __PRETTY_FUNCTION__))
;
913 }
914 assert(MI.isIdenticalTo(*Pos) && "Expected matching MIIs!")(static_cast <bool> (MI.isIdenticalTo(*Pos) && "Expected matching MIIs!"
) ? void (0) : __assert_fail ("MI.isIdenticalTo(*Pos) && \"Expected matching MIIs!\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 914, __extension__ __PRETTY_FUNCTION__))
;
915 DL = DILocation::getMergedLocation(DL, Pos->getDebugLoc());
916 NextCommonInsts[i] = ++Pos;
917 }
918 MI.setDebugLoc(DL);
919 }
920
921 if (UpdateLiveIns) {
922 LivePhysRegs NewLiveIns(*TRI);
923 computeLiveIns(NewLiveIns, *MBB);
924 LiveRegs.init(*TRI);
925
926 // The flag merging may lead to some register uses no longer using the
927 // <undef> flag, add IMPLICIT_DEFs in the predecessors as necessary.
928 for (MachineBasicBlock *Pred : MBB->predecessors()) {
929 LiveRegs.clear();
930 LiveRegs.addLiveOuts(*Pred);
931 MachineBasicBlock::iterator InsertBefore = Pred->getFirstTerminator();
932 for (unsigned Reg : NewLiveIns) {
933 if (!LiveRegs.available(*MRI, Reg))
934 continue;
935 DebugLoc DL;
936 BuildMI(*Pred, InsertBefore, DL, TII->get(TargetOpcode::IMPLICIT_DEF),
937 Reg);
938 }
939 }
940
941 MBB->clearLiveIns();
942 addLiveIns(*MBB, NewLiveIns);
943 }
944}
945
946// See if any of the blocks in MergePotentials (which all have SuccBB as a
947// successor, or all have no successor if it is null) can be tail-merged.
948// If there is a successor, any blocks in MergePotentials that are not
949// tail-merged and are not immediately before Succ must have an unconditional
950// branch to Succ added (but the predecessor/successor lists need no
951// adjustment). The lone predecessor of Succ that falls through into Succ,
952// if any, is given in PredBB.
953// MinCommonTailLength - Except for the special cases below, tail-merge if
954// there are at least this many instructions in common.
955bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
956 MachineBasicBlock *PredBB,
957 unsigned MinCommonTailLength) {
958 bool MadeChange = false;
959
960 LLVM_DEBUG(do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
961 dbgs() << "\nTryTailMergeBlocks: ";do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
962 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i) dbgs()do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
963 << printMBBReference(*MergePotentials[i].getBlock())do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
964 << (i == e - 1 ? "" : ", ");do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
965 dbgs() << "\n"; if (SuccBB) {do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
966 dbgs() << " with successor " << printMBBReference(*SuccBB) << '\n';do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
967 if (PredBB)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
968 dbgs() << " which has fall-through from "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
969 << printMBBReference(*PredBB) << "\n";do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
970 } dbgs() << "Looking for common tails of at least "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
971 << MinCommonTailLength << " instruction"do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
972 << (MinCommonTailLength == 1 ? "" : "s") << '\n';)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nTryTailMergeBlocks: "
; for (unsigned i = 0, e = MergePotentials.size(); i != e; ++
i) dbgs() << printMBBReference(*MergePotentials[i].getBlock
()) << (i == e - 1 ? "" : ", "); dbgs() << "\n"; if
(SuccBB) { dbgs() << " with successor " << printMBBReference
(*SuccBB) << '\n'; if (PredBB) dbgs() << " which has fall-through from "
<< printMBBReference(*PredBB) << "\n"; } dbgs() <<
"Looking for common tails of at least " << MinCommonTailLength
<< " instruction" << (MinCommonTailLength == 1 ?
"" : "s") << '\n';; } } while (false)
;
973
974 // Sort by hash value so that blocks with identical end sequences sort
975 // together.
976 array_pod_sort(MergePotentials.begin(), MergePotentials.end());
977
978 // Walk through equivalence sets looking for actual exact matches.
979 while (MergePotentials.size() > 1) {
980 unsigned CurHash = MergePotentials.back().getHash();
981
982 // Build SameTails, identifying the set of blocks with this hash code
983 // and with the maximum number of instructions in common.
984 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
985 MinCommonTailLength,
986 SuccBB, PredBB);
987
988 // If we didn't find any pair that has at least MinCommonTailLength
989 // instructions in common, remove all blocks with this hash code and retry.
990 if (SameTails.empty()) {
991 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
992 continue;
993 }
994
995 // If one of the blocks is the entire common tail (and not the entry
996 // block, which we can't jump to), we can treat all blocks with this same
997 // tail at once. Use PredBB if that is one of the possibilities, as that
998 // will not introduce any extra branches.
999 MachineBasicBlock *EntryBB =
1000 &MergePotentials.front().getBlock()->getParent()->front();
1001 unsigned commonTailIndex = SameTails.size();
1002 // If there are two blocks, check to see if one can be made to fall through
1003 // into the other.
1004 if (SameTails.size() == 2 &&
1005 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
1006 SameTails[1].tailIsWholeBlock())
1007 commonTailIndex = 1;
1008 else if (SameTails.size() == 2 &&
1009 SameTails[1].getBlock()->isLayoutSuccessor(
1010 SameTails[0].getBlock()) &&
1011 SameTails[0].tailIsWholeBlock())
1012 commonTailIndex = 0;
1013 else {
1014 // Otherwise just pick one, favoring the fall-through predecessor if
1015 // there is one.
1016 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
1017 MachineBasicBlock *MBB = SameTails[i].getBlock();
1018 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
1019 continue;
1020 if (MBB == PredBB) {
1021 commonTailIndex = i;
1022 break;
1023 }
1024 if (SameTails[i].tailIsWholeBlock())
1025 commonTailIndex = i;
1026 }
1027 }
1028
1029 if (commonTailIndex == SameTails.size() ||
1030 (SameTails[commonTailIndex].getBlock() == PredBB &&
1031 !SameTails[commonTailIndex].tailIsWholeBlock())) {
1032 // None of the blocks consist entirely of the common tail.
1033 // Split a block so that one does.
1034 if (!CreateCommonTailOnlyBlock(PredBB, SuccBB,
1035 maxCommonTailLength, commonTailIndex)) {
1036 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
1037 continue;
1038 }
1039 }
1040
1041 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
1042
1043 // Recompute common tail MBB's edge weights and block frequency.
1044 setCommonTailEdgeWeights(*MBB);
1045
1046 // Merge debug locations, MMOs and undef flags across identical instructions
1047 // for common tail.
1048 mergeCommonTails(commonTailIndex);
1049
1050 // MBB is common tail. Adjust all other BB's to jump to this one.
1051 // Traversal must be forwards so erases work.
1052 LLVM_DEBUG(dbgs() << "\nUsing common tail in " << printMBBReference(*MBB)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nUsing common tail in "
<< printMBBReference(*MBB) << " for "; } } while
(false)
1053 << " for ")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nUsing common tail in "
<< printMBBReference(*MBB) << " for "; } } while
(false)
;
1054 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
1055 if (commonTailIndex == i)
1056 continue;
1057 LLVM_DEBUG(dbgs() << printMBBReference(*SameTails[i].getBlock())do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << printMBBReference(*SameTails
[i].getBlock()) << (i == e - 1 ? "" : ", "); } } while (
false)
1058 << (i == e - 1 ? "" : ", "))do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << printMBBReference(*SameTails
[i].getBlock()) << (i == e - 1 ? "" : ", "); } } while (
false)
;
1059 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
1060 replaceTailWithBranchTo(SameTails[i].getTailStartPos(), *MBB);
1061 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
1062 MergePotentials.erase(SameTails[i].getMPIter());
1063 }
1064 LLVM_DEBUG(dbgs() << "\n")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\n"; } } while (false)
;
1065 // We leave commonTailIndex in the worklist in case there are other blocks
1066 // that match it with a smaller number of instructions.
1067 MadeChange = true;
1068 }
1069 return MadeChange;
1070}
1071
1072bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
1073 bool MadeChange = false;
1074 if (!EnableTailMerge) return MadeChange;
1075
1076 // First find blocks with no successors.
1077 // Block placement does not create new tail merging opportunities for these
1078 // blocks.
1079 if (!AfterBlockPlacement) {
1080 MergePotentials.clear();
1081 for (MachineBasicBlock &MBB : MF) {
1082 if (MergePotentials.size() == TailMergeThreshold)
1083 break;
1084 if (!TriedMerging.count(&MBB) && MBB.succ_empty())
1085 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(MBB), &MBB));
1086 }
1087
1088 // If this is a large problem, avoid visiting the same basic blocks
1089 // multiple times.
1090 if (MergePotentials.size() == TailMergeThreshold)
1091 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
1092 TriedMerging.insert(MergePotentials[i].getBlock());
1093
1094 // See if we can do any tail merging on those.
1095 if (MergePotentials.size() >= 2)
1096 MadeChange |= TryTailMergeBlocks(nullptr, nullptr, MinCommonTailLength);
1097 }
1098
1099 // Look at blocks (IBB) with multiple predecessors (PBB).
1100 // We change each predecessor to a canonical form, by
1101 // (1) temporarily removing any unconditional branch from the predecessor
1102 // to IBB, and
1103 // (2) alter conditional branches so they branch to the other block
1104 // not IBB; this may require adding back an unconditional branch to IBB
1105 // later, where there wasn't one coming in. E.g.
1106 // Bcc IBB
1107 // fallthrough to QBB
1108 // here becomes
1109 // Bncc QBB
1110 // with a conceptual B to IBB after that, which never actually exists.
1111 // With those changes, we see whether the predecessors' tails match,
1112 // and merge them if so. We change things out of canonical form and
1113 // back to the way they were later in the process. (OptimizeBranches
1114 // would undo some of this, but we can't use it, because we'd get into
1115 // a compile-time infinite loop repeatedly doing and undoing the same
1116 // transformations.)
1117
1118 for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
1119 I != E; ++I) {
1120 if (I->pred_size() < 2) continue;
1121 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
1122 MachineBasicBlock *IBB = &*I;
1123 MachineBasicBlock *PredBB = &*std::prev(I);
1124 MergePotentials.clear();
1125 MachineLoop *ML;
1126
1127 // Bail if merging after placement and IBB is the loop header because
1128 // -- If merging predecessors that belong to the same loop as IBB, the
1129 // common tail of merged predecessors may become the loop top if block
1130 // placement is called again and the predecessors may branch to this common
1131 // tail and require more branches. This can be relaxed if
1132 // MachineBlockPlacement::findBestLoopTop is more flexible.
1133 // --If merging predecessors that do not belong to the same loop as IBB, the
1134 // loop info of IBB's loop and the other loops may be affected. Calling the
1135 // block placement again may make big change to the layout and eliminate the
1136 // reason to do tail merging here.
1137 if (AfterBlockPlacement && MLI) {
1138 ML = MLI->getLoopFor(IBB);
1139 if (ML && IBB == ML->getHeader())
1140 continue;
1141 }
1142
1143 for (MachineBasicBlock *PBB : I->predecessors()) {
1144 if (MergePotentials.size() == TailMergeThreshold)
1145 break;
1146
1147 if (TriedMerging.count(PBB))
1148 continue;
1149
1150 // Skip blocks that loop to themselves, can't tail merge these.
1151 if (PBB == IBB)
1152 continue;
1153
1154 // Visit each predecessor only once.
1155 if (!UniquePreds.insert(PBB).second)
1156 continue;
1157
1158 // Skip blocks which may jump to a landing pad. Can't tail merge these.
1159 if (PBB->hasEHPadSuccessor())
1160 continue;
1161
1162 // After block placement, only consider predecessors that belong to the
1163 // same loop as IBB. The reason is the same as above when skipping loop
1164 // header.
1165 if (AfterBlockPlacement && MLI)
1166 if (ML != MLI->getLoopFor(PBB))
1167 continue;
1168
1169 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
1170 SmallVector<MachineOperand, 4> Cond;
1171 if (!TII->analyzeBranch(*PBB, TBB, FBB, Cond, true)) {
1172 // Failing case: IBB is the target of a cbr, and we cannot reverse the
1173 // branch.
1174 SmallVector<MachineOperand, 4> NewCond(Cond);
1175 if (!Cond.empty() && TBB == IBB) {
1176 if (TII->reverseBranchCondition(NewCond))
1177 continue;
1178 // This is the QBB case described above
1179 if (!FBB) {
1180 auto Next = ++PBB->getIterator();
1181 if (Next != MF.end())
1182 FBB = &*Next;
1183 }
1184 }
1185
1186 // Failing case: the only way IBB can be reached from PBB is via
1187 // exception handling. Happens for landing pads. Would be nice to have
1188 // a bit in the edge so we didn't have to do all this.
1189 if (IBB->isEHPad()) {
1190 MachineFunction::iterator IP = ++PBB->getIterator();
1191 MachineBasicBlock *PredNextBB = nullptr;
1192 if (IP != MF.end())
1193 PredNextBB = &*IP;
1194 if (!TBB) {
1195 if (IBB != PredNextBB) // fallthrough
1196 continue;
1197 } else if (FBB) {
1198 if (TBB != IBB && FBB != IBB) // cbr then ubr
1199 continue;
1200 } else if (Cond.empty()) {
1201 if (TBB != IBB) // ubr
1202 continue;
1203 } else {
1204 if (TBB != IBB && IBB != PredNextBB) // cbr
1205 continue;
1206 }
1207 }
1208
1209 // Remove the unconditional branch at the end, if any.
1210 if (TBB && (Cond.empty() || FBB)) {
1211 DebugLoc dl = PBB->findBranchDebugLoc();
1212 TII->removeBranch(*PBB);
1213 if (!Cond.empty())
1214 // reinsert conditional branch only, for now
1215 TII->insertBranch(*PBB, (TBB == IBB) ? FBB : TBB, nullptr,
1216 NewCond, dl);
1217 }
1218
1219 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(*PBB), PBB));
1220 }
1221 }
1222
1223 // If this is a large problem, avoid visiting the same basic blocks multiple
1224 // times.
1225 if (MergePotentials.size() == TailMergeThreshold)
1226 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
1227 TriedMerging.insert(MergePotentials[i].getBlock());
1228
1229 if (MergePotentials.size() >= 2)
1230 MadeChange |= TryTailMergeBlocks(IBB, PredBB, MinCommonTailLength);
1231
1232 // Reinsert an unconditional branch if needed. The 1 below can occur as a
1233 // result of removing blocks in TryTailMergeBlocks.
1234 PredBB = &*std::prev(I); // this may have been changed in TryTailMergeBlocks
1235 if (MergePotentials.size() == 1 &&
1236 MergePotentials.begin()->getBlock() != PredBB)
1237 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
1238 }
1239
1240 return MadeChange;
1241}
1242
1243void BranchFolder::setCommonTailEdgeWeights(MachineBasicBlock &TailMBB) {
1244 SmallVector<BlockFrequency, 2> EdgeFreqLs(TailMBB.succ_size());
1245 BlockFrequency AccumulatedMBBFreq;
1246
1247 // Aggregate edge frequency of successor edge j:
1248 // edgeFreq(j) = sum (freq(bb) * edgeProb(bb, j)),
1249 // where bb is a basic block that is in SameTails.
1250 for (const auto &Src : SameTails) {
1251 const MachineBasicBlock *SrcMBB = Src.getBlock();
1252 BlockFrequency BlockFreq = MBBFreqInfo.getBlockFreq(SrcMBB);
1253 AccumulatedMBBFreq += BlockFreq;
1254
1255 // It is not necessary to recompute edge weights if TailBB has less than two
1256 // successors.
1257 if (TailMBB.succ_size() <= 1)
1258 continue;
1259
1260 auto EdgeFreq = EdgeFreqLs.begin();
1261
1262 for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
1263 SuccI != SuccE; ++SuccI, ++EdgeFreq)
1264 *EdgeFreq += BlockFreq * MBPI.getEdgeProbability(SrcMBB, *SuccI);
1265 }
1266
1267 MBBFreqInfo.setBlockFreq(&TailMBB, AccumulatedMBBFreq);
1268
1269 if (TailMBB.succ_size() <= 1)
1270 return;
1271
1272 auto SumEdgeFreq =
1273 std::accumulate(EdgeFreqLs.begin(), EdgeFreqLs.end(), BlockFrequency(0))
1274 .getFrequency();
1275 auto EdgeFreq = EdgeFreqLs.begin();
1276
1277 if (SumEdgeFreq > 0) {
1278 for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
1279 SuccI != SuccE; ++SuccI, ++EdgeFreq) {
1280 auto Prob = BranchProbability::getBranchProbability(
1281 EdgeFreq->getFrequency(), SumEdgeFreq);
1282 TailMBB.setSuccProbability(SuccI, Prob);
1283 }
1284 }
1285}
1286
1287//===----------------------------------------------------------------------===//
1288// Branch Optimization
1289//===----------------------------------------------------------------------===//
1290
1291bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
1292 bool MadeChange = false;
1293
1294 // Make sure blocks are numbered in order
1295 MF.RenumberBlocks();
1296 // Renumbering blocks alters EH scope membership, recalculate it.
1297 EHScopeMembership = getEHScopeMembership(MF);
1298
1299 for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
1300 I != E; ) {
1301 MachineBasicBlock *MBB = &*I++;
1302 MadeChange |= OptimizeBlock(MBB);
1303
1304 // If it is dead, remove it.
1305 if (MBB->pred_empty()) {
1306 RemoveDeadBlock(MBB);
1307 MadeChange = true;
1308 ++NumDeadBlocks;
1309 }
1310 }
1311
1312 return MadeChange;
1313}
1314
1315// Blocks should be considered empty if they contain only debug info;
1316// else the debug info would affect codegen.
1317static bool IsEmptyBlock(MachineBasicBlock *MBB) {
1318 return MBB->getFirstNonDebugInstr() == MBB->end();
1319}
1320
1321// Blocks with only debug info and branches should be considered the same
1322// as blocks with only branches.
1323static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
1324 MachineBasicBlock::iterator I = MBB->getFirstNonDebugInstr();
1325 assert(I != MBB->end() && "empty block!")(static_cast <bool> (I != MBB->end() && "empty block!"
) ? void (0) : __assert_fail ("I != MBB->end() && \"empty block!\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 1325, __extension__ __PRETTY_FUNCTION__))
;
1326 return I->isBranch();
1327}
1328
1329/// IsBetterFallthrough - Return true if it would be clearly better to
1330/// fall-through to MBB1 than to fall through into MBB2. This has to return
1331/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
1332/// result in infinite loops.
1333static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
1334 MachineBasicBlock *MBB2) {
1335 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
1336 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
1337 // optimize branches that branch to either a return block or an assert block
1338 // into a fallthrough to the return.
1339 MachineBasicBlock::iterator MBB1I = MBB1->getLastNonDebugInstr();
16
Called C++ object pointer is null
1340 MachineBasicBlock::iterator MBB2I = MBB2->getLastNonDebugInstr();
1341 if (MBB1I == MBB1->end() || MBB2I == MBB2->end())
1342 return false;
1343
1344 // If there is a clear successor ordering we make sure that one block
1345 // will fall through to the next
1346 if (MBB1->isSuccessor(MBB2)) return true;
1347 if (MBB2->isSuccessor(MBB1)) return false;
1348
1349 return MBB2I->isCall() && !MBB1I->isCall();
1350}
1351
1352/// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch
1353/// instructions on the block.
1354static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) {
1355 MachineBasicBlock::iterator I = MBB.getLastNonDebugInstr();
1356 if (I != MBB.end() && I->isBranch())
1357 return I->getDebugLoc();
1358 return DebugLoc();
1359}
1360
1361static void copyDebugInfoToPredecessor(const TargetInstrInfo *TII,
1362 MachineBasicBlock &MBB,
1363 MachineBasicBlock &PredMBB) {
1364 auto InsertBefore = PredMBB.getFirstTerminator();
1365 for (MachineInstr &MI : MBB.instrs())
1366 if (MI.isDebugValue()) {
1367 TII->duplicate(PredMBB, InsertBefore, MI);
1368 LLVM_DEBUG(dbgs() << "Copied debug value from empty block to pred: "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Copied debug value from empty block to pred: "
<< MI; } } while (false)
1369 << MI)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Copied debug value from empty block to pred: "
<< MI; } } while (false)
;
1370 }
1371}
1372
1373static void copyDebugInfoToSuccessor(const TargetInstrInfo *TII,
1374 MachineBasicBlock &MBB,
1375 MachineBasicBlock &SuccMBB) {
1376 auto InsertBefore = SuccMBB.SkipPHIsAndLabels(SuccMBB.begin());
1377 for (MachineInstr &MI : MBB.instrs())
1378 if (MI.isDebugValue()) {
1379 TII->duplicate(SuccMBB, InsertBefore, MI);
1380 LLVM_DEBUG(dbgs() << "Copied debug value from empty block to succ: "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Copied debug value from empty block to succ: "
<< MI; } } while (false)
1381 << MI)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Copied debug value from empty block to succ: "
<< MI; } } while (false)
;
1382 }
1383}
1384
1385// Try to salvage DBG_VALUE instructions from an otherwise empty block. If such
1386// a basic block is removed we would lose the debug information unless we have
1387// copied the information to a predecessor/successor.
1388//
1389// TODO: This function only handles some simple cases. An alternative would be
1390// to run a heavier analysis, such as the LiveDebugValues pass, before we do
1391// branch folding.
1392static void salvageDebugInfoFromEmptyBlock(const TargetInstrInfo *TII,
1393 MachineBasicBlock &MBB) {
1394 assert(IsEmptyBlock(&MBB) && "Expected an empty block (except debug info).")(static_cast <bool> (IsEmptyBlock(&MBB) && "Expected an empty block (except debug info)."
) ? void (0) : __assert_fail ("IsEmptyBlock(&MBB) && \"Expected an empty block (except debug info).\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 1394, __extension__ __PRETTY_FUNCTION__))
;
1395 // If this MBB is the only predecessor of a successor it is legal to copy
1396 // DBG_VALUE instructions to the beginning of the successor.
1397 for (MachineBasicBlock *SuccBB : MBB.successors())
1398 if (SuccBB->pred_size() == 1)
1399 copyDebugInfoToSuccessor(TII, MBB, *SuccBB);
1400 // If this MBB is the only successor of a predecessor it is legal to copy the
1401 // DBG_VALUE instructions to the end of the predecessor (just before the
1402 // terminators, assuming that the terminator isn't affecting the DBG_VALUE).
1403 for (MachineBasicBlock *PredBB : MBB.predecessors())
1404 if (PredBB->succ_size() == 1)
1405 copyDebugInfoToPredecessor(TII, MBB, *PredBB);
1406}
1407
1408bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
1409 bool MadeChange = false;
1410 MachineFunction &MF = *MBB->getParent();
1411ReoptimizeBlock:
1412
1413 MachineFunction::iterator FallThrough = MBB->getIterator();
1414 ++FallThrough;
1415
1416 // Make sure MBB and FallThrough belong to the same EH scope.
1417 bool SameEHScope = true;
1418 if (!EHScopeMembership.empty() && FallThrough != MF.end()) {
1
Assuming the condition is false
2
Taking false branch
1419 auto MBBEHScope = EHScopeMembership.find(MBB);
1420 assert(MBBEHScope != EHScopeMembership.end())(static_cast <bool> (MBBEHScope != EHScopeMembership.end
()) ? void (0) : __assert_fail ("MBBEHScope != EHScopeMembership.end()"
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 1420, __extension__ __PRETTY_FUNCTION__))
;
1421 auto FallThroughEHScope = EHScopeMembership.find(&*FallThrough);
1422 assert(FallThroughEHScope != EHScopeMembership.end())(static_cast <bool> (FallThroughEHScope != EHScopeMembership
.end()) ? void (0) : __assert_fail ("FallThroughEHScope != EHScopeMembership.end()"
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 1422, __extension__ __PRETTY_FUNCTION__))
;
1423 SameEHScope = MBBEHScope->second == FallThroughEHScope->second;
1424 }
1425
1426 // If this block is empty, make everyone use its fall-through, not the block
1427 // explicitly. Landing pads should not do this since the landing-pad table
1428 // points to this block. Blocks with their addresses taken shouldn't be
1429 // optimized away.
1430 if (IsEmptyBlock(MBB) && !MBB->isEHPad() && !MBB->hasAddressTaken() &&
1431 SameEHScope) {
1432 salvageDebugInfoFromEmptyBlock(TII, *MBB);
1433 // Dead block? Leave for cleanup later.
1434 if (MBB->pred_empty()) return MadeChange;
1435
1436 if (FallThrough == MF.end()) {
1437 // TODO: Simplify preds to not branch here if possible!
1438 } else if (FallThrough->isEHPad()) {
1439 // Don't rewrite to a landing pad fallthough. That could lead to the case
1440 // where a BB jumps to more than one landing pad.
1441 // TODO: Is it ever worth rewriting predecessors which don't already
1442 // jump to a landing pad, and so can safely jump to the fallthrough?
1443 } else if (MBB->isSuccessor(&*FallThrough)) {
1444 // Rewrite all predecessors of the old block to go to the fallthrough
1445 // instead.
1446 while (!MBB->pred_empty()) {
1447 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1448 Pred->ReplaceUsesOfBlockWith(MBB, &*FallThrough);
1449 }
1450 // If MBB was the target of a jump table, update jump tables to go to the
1451 // fallthrough instead.
1452 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1453 MJTI->ReplaceMBBInJumpTables(MBB, &*FallThrough);
1454 MadeChange = true;
1455 }
1456 return MadeChange;
1457 }
1458
1459 // Check to see if we can simplify the terminator of the block before this
1460 // one.
1461 MachineBasicBlock &PrevBB = *std::prev(MachineFunction::iterator(MBB));
1462
1463 MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
1464 SmallVector<MachineOperand, 4> PriorCond;
1465 bool PriorUnAnalyzable =
1466 TII->analyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
3
Value assigned to 'PriorTBB'
1467 if (!PriorUnAnalyzable) {
4
Assuming 'PriorUnAnalyzable' is 0
5
Taking true branch
1468 // If the CFG for the prior block has extra edges, remove them.
1469 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1470 !PriorCond.empty());
1471
1472 // If the previous branch is conditional and both conditions go to the same
1473 // destination, remove the branch, replacing it with an unconditional one or
1474 // a fall-through.
1475 if (PriorTBB && PriorTBB == PriorFBB) {
6
Assuming 'PriorTBB' is null
1476 DebugLoc dl = getBranchDebugLoc(PrevBB);
1477 TII->removeBranch(PrevBB);
1478 PriorCond.clear();
1479 if (PriorTBB != MBB)
1480 TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1481 MadeChange = true;
1482 ++NumBranchOpts;
1483 goto ReoptimizeBlock;
1484 }
1485
1486 // If the previous block unconditionally falls through to this block and
1487 // this block has no other predecessors, move the contents of this block
1488 // into the prior block. This doesn't usually happen when SimplifyCFG
1489 // has been used, but it can happen if tail merging splits a fall-through
1490 // predecessor of a block.
1491 // This has to check PrevBB->succ_size() because EH edges are ignored by
1492 // AnalyzeBranch.
1493 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1494 PrevBB.succ_size() == 1 &&
1495 !MBB->hasAddressTaken() && !MBB->isEHPad()) {
1496 LLVM_DEBUG(dbgs() << "\nMerging into block: " << PrevBBdo { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nMerging into block: "
<< PrevBB << "From MBB: " << *MBB; } } while
(false)
1497 << "From MBB: " << *MBB)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nMerging into block: "
<< PrevBB << "From MBB: " << *MBB; } } while
(false)
;
1498 // Remove redundant DBG_VALUEs first.
1499 if (PrevBB.begin() != PrevBB.end()) {
1500 MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
1501 --PrevBBIter;
1502 MachineBasicBlock::iterator MBBIter = MBB->begin();
1503 // Check if DBG_VALUE at the end of PrevBB is identical to the
1504 // DBG_VALUE at the beginning of MBB.
1505 while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
1506 && PrevBBIter->isDebugInstr() && MBBIter->isDebugInstr()) {
1507 if (!MBBIter->isIdenticalTo(*PrevBBIter))
1508 break;
1509 MachineInstr &DuplicateDbg = *MBBIter;
1510 ++MBBIter; -- PrevBBIter;
1511 DuplicateDbg.eraseFromParent();
1512 }
1513 }
1514 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1515 PrevBB.removeSuccessor(PrevBB.succ_begin());
1516 assert(PrevBB.succ_empty())(static_cast <bool> (PrevBB.succ_empty()) ? void (0) : __assert_fail
("PrevBB.succ_empty()", "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 1516, __extension__ __PRETTY_FUNCTION__))
;
1517 PrevBB.transferSuccessors(MBB);
1518 MadeChange = true;
1519 return MadeChange;
1520 }
1521
1522 // If the previous branch *only* branches to *this* block (conditional or
1523 // not) remove the branch.
1524 if (PriorTBB == MBB && !PriorFBB) {
1525 TII->removeBranch(PrevBB);
1526 MadeChange = true;
1527 ++NumBranchOpts;
1528 goto ReoptimizeBlock;
1529 }
1530
1531 // If the prior block branches somewhere else on the condition and here if
1532 // the condition is false, remove the uncond second branch.
1533 if (PriorFBB == MBB) {
7
Assuming 'PriorFBB' is not equal to 'MBB'
8
Taking false branch
1534 DebugLoc dl = getBranchDebugLoc(PrevBB);
1535 TII->removeBranch(PrevBB);
1536 TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1537 MadeChange = true;
1538 ++NumBranchOpts;
1539 goto ReoptimizeBlock;
1540 }
1541
1542 // If the prior block branches here on true and somewhere else on false, and
1543 // if the branch condition is reversible, reverse the branch to create a
1544 // fall-through.
1545 if (PriorTBB == MBB) {
9
Taking false branch
1546 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1547 if (!TII->reverseBranchCondition(NewPriorCond)) {
1548 DebugLoc dl = getBranchDebugLoc(PrevBB);
1549 TII->removeBranch(PrevBB);
1550 TII->insertBranch(PrevBB, PriorFBB, nullptr, NewPriorCond, dl);
1551 MadeChange = true;
1552 ++NumBranchOpts;
1553 goto ReoptimizeBlock;
1554 }
1555 }
1556
1557 // If this block has no successors (e.g. it is a return block or ends with
1558 // a call to a no-return function like abort or __cxa_throw) and if the pred
1559 // falls through into this block, and if it would otherwise fall through
1560 // into the block after this, move this block to the end of the function.
1561 //
1562 // We consider it more likely that execution will stay in the function (e.g.
1563 // due to loops) than it is to exit it. This asserts in loops etc, moving
1564 // the assert condition out of the loop body.
1565 if (MBB->succ_empty() && !PriorCond.empty() && !PriorFBB &&
10
Assuming the condition is true
11
Assuming 'PriorFBB' is null
13
Taking true branch
1566 MachineFunction::iterator(PriorTBB) == FallThrough &&
1567 !MBB->canFallThrough()) {
12
Assuming the condition is true
1568 bool DoTransform = true;
1569
1570 // We have to be careful that the succs of PredBB aren't both no-successor
1571 // blocks. If neither have successors and if PredBB is the second from
1572 // last block in the function, we'd just keep swapping the two blocks for
1573 // last. Only do the swap if one is clearly better to fall through than
1574 // the other.
1575 if (FallThrough == --MF.end() &&
1576 !IsBetterFallthrough(PriorTBB, MBB))
14
Passing null pointer value via 1st parameter 'MBB1'
15
Calling 'IsBetterFallthrough'
1577 DoTransform = false;
1578
1579 if (DoTransform) {
1580 // Reverse the branch so we will fall through on the previous true cond.
1581 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1582 if (!TII->reverseBranchCondition(NewPriorCond)) {
1583 LLVM_DEBUG(dbgs() << "\nMoving MBB: " << *MBBdo { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nMoving MBB: " <<
*MBB << "To make fallthrough to: " << *PriorTBB <<
"\n"; } } while (false)
1584 << "To make fallthrough to: " << *PriorTBB << "\n")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nMoving MBB: " <<
*MBB << "To make fallthrough to: " << *PriorTBB <<
"\n"; } } while (false)
;
1585
1586 DebugLoc dl = getBranchDebugLoc(PrevBB);
1587 TII->removeBranch(PrevBB);
1588 TII->insertBranch(PrevBB, MBB, nullptr, NewPriorCond, dl);
1589
1590 // Move this block to the end of the function.
1591 MBB->moveAfter(&MF.back());
1592 MadeChange = true;
1593 ++NumBranchOpts;
1594 return MadeChange;
1595 }
1596 }
1597 }
1598 }
1599
1600 if (!IsEmptyBlock(MBB) && MBB->pred_size() == 1 &&
1601 MF.getFunction().optForSize()) {
1602 // Changing "Jcc foo; foo: jmp bar;" into "Jcc bar;" might change the branch
1603 // direction, thereby defeating careful block placement and regressing
1604 // performance. Therefore, only consider this for optsize functions.
1605 MachineInstr &TailCall = *MBB->getFirstNonDebugInstr();
1606 if (TII->isUnconditionalTailCall(TailCall)) {
1607 MachineBasicBlock *Pred = *MBB->pred_begin();
1608 MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
1609 SmallVector<MachineOperand, 4> PredCond;
1610 bool PredAnalyzable =
1611 !TII->analyzeBranch(*Pred, PredTBB, PredFBB, PredCond, true);
1612
1613 if (PredAnalyzable && !PredCond.empty() && PredTBB == MBB &&
1614 PredTBB != PredFBB) {
1615 // The predecessor has a conditional branch to this block which consists
1616 // of only a tail call. Try to fold the tail call into the conditional
1617 // branch.
1618 if (TII->canMakeTailCallConditional(PredCond, TailCall)) {
1619 // TODO: It would be nice if analyzeBranch() could provide a pointer
1620 // to the branch instruction so replaceBranchWithTailCall() doesn't
1621 // have to search for it.
1622 TII->replaceBranchWithTailCall(*Pred, PredCond, TailCall);
1623 ++NumTailCalls;
1624 Pred->removeSuccessor(MBB);
1625 MadeChange = true;
1626 return MadeChange;
1627 }
1628 }
1629 // If the predecessor is falling through to this block, we could reverse
1630 // the branch condition and fold the tail call into that. However, after
1631 // that we might have to re-arrange the CFG to fall through to the other
1632 // block and there is a high risk of regressing code size rather than
1633 // improving it.
1634 }
1635 }
1636
1637 // Analyze the branch in the current block.
1638 MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr;
1639 SmallVector<MachineOperand, 4> CurCond;
1640 bool CurUnAnalyzable =
1641 TII->analyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1642 if (!CurUnAnalyzable) {
1643 // If the CFG for the prior block has extra edges, remove them.
1644 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1645
1646 // If this is a two-way branch, and the FBB branches to this block, reverse
1647 // the condition so the single-basic-block loop is faster. Instead of:
1648 // Loop: xxx; jcc Out; jmp Loop
1649 // we want:
1650 // Loop: xxx; jncc Loop; jmp Out
1651 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1652 SmallVector<MachineOperand, 4> NewCond(CurCond);
1653 if (!TII->reverseBranchCondition(NewCond)) {
1654 DebugLoc dl = getBranchDebugLoc(*MBB);
1655 TII->removeBranch(*MBB);
1656 TII->insertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
1657 MadeChange = true;
1658 ++NumBranchOpts;
1659 goto ReoptimizeBlock;
1660 }
1661 }
1662
1663 // If this branch is the only thing in its block, see if we can forward
1664 // other blocks across it.
1665 if (CurTBB && CurCond.empty() && !CurFBB &&
1666 IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
1667 !MBB->hasAddressTaken() && !MBB->isEHPad()) {
1668 DebugLoc dl = getBranchDebugLoc(*MBB);
1669 // This block may contain just an unconditional branch. Because there can
1670 // be 'non-branch terminators' in the block, try removing the branch and
1671 // then seeing if the block is empty.
1672 TII->removeBranch(*MBB);
1673 // If the only things remaining in the block are debug info, remove these
1674 // as well, so this will behave the same as an empty block in non-debug
1675 // mode.
1676 if (IsEmptyBlock(MBB)) {
1677 // Make the block empty, losing the debug info (we could probably
1678 // improve this in some cases.)
1679 MBB->erase(MBB->begin(), MBB->end());
1680 }
1681 // If this block is just an unconditional branch to CurTBB, we can
1682 // usually completely eliminate the block. The only case we cannot
1683 // completely eliminate the block is when the block before this one
1684 // falls through into MBB and we can't understand the prior block's branch
1685 // condition.
1686 if (MBB->empty()) {
1687 bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1688 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1689 !PrevBB.isSuccessor(MBB)) {
1690 // If the prior block falls through into us, turn it into an
1691 // explicit branch to us to make updates simpler.
1692 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1693 PriorTBB != MBB && PriorFBB != MBB) {
1694 if (!PriorTBB) {
1695 assert(PriorCond.empty() && !PriorFBB &&(static_cast <bool> (PriorCond.empty() && !PriorFBB
&& "Bad branch analysis") ? void (0) : __assert_fail
("PriorCond.empty() && !PriorFBB && \"Bad branch analysis\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 1696, __extension__ __PRETTY_FUNCTION__))
1696 "Bad branch analysis")(static_cast <bool> (PriorCond.empty() && !PriorFBB
&& "Bad branch analysis") ? void (0) : __assert_fail
("PriorCond.empty() && !PriorFBB && \"Bad branch analysis\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 1696, __extension__ __PRETTY_FUNCTION__))
;
1697 PriorTBB = MBB;
1698 } else {
1699 assert(!PriorFBB && "Machine CFG out of date!")(static_cast <bool> (!PriorFBB && "Machine CFG out of date!"
) ? void (0) : __assert_fail ("!PriorFBB && \"Machine CFG out of date!\""
, "/build/llvm-toolchain-snapshot-7~svn337490/lib/CodeGen/BranchFolding.cpp"
, 1699, __extension__ __PRETTY_FUNCTION__))
;
1700 PriorFBB = MBB;
1701 }
1702 DebugLoc pdl = getBranchDebugLoc(PrevBB);
1703 TII->removeBranch(PrevBB);
1704 TII->insertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl);
1705 }
1706
1707 // Iterate through all the predecessors, revectoring each in-turn.
1708 size_t PI = 0;
1709 bool DidChange = false;
1710 bool HasBranchToSelf = false;
1711 while(PI != MBB->pred_size()) {
1712 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1713 if (PMBB == MBB) {
1714 // If this block has an uncond branch to itself, leave it.
1715 ++PI;
1716 HasBranchToSelf = true;
1717 } else {
1718 DidChange = true;
1719 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1720 // If this change resulted in PMBB ending in a conditional
1721 // branch where both conditions go to the same destination,
1722 // change this to an unconditional branch (and fix the CFG).
1723 MachineBasicBlock *NewCurTBB = nullptr, *NewCurFBB = nullptr;
1724 SmallVector<MachineOperand, 4> NewCurCond;
1725 bool NewCurUnAnalyzable = TII->analyzeBranch(
1726 *PMBB, NewCurTBB, NewCurFBB, NewCurCond, true);
1727 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1728 DebugLoc pdl = getBranchDebugLoc(*PMBB);
1729 TII->removeBranch(*PMBB);
1730 NewCurCond.clear();
1731 TII->insertBranch(*PMBB, NewCurTBB, nullptr, NewCurCond, pdl);
1732 MadeChange = true;
1733 ++NumBranchOpts;
1734 PMBB->CorrectExtraCFGEdges(NewCurTBB, nullptr, false);
1735 }
1736 }
1737 }
1738
1739 // Change any jumptables to go to the new MBB.
1740 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1741 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1742 if (DidChange) {
1743 ++NumBranchOpts;
1744 MadeChange = true;
1745 if (!HasBranchToSelf) return MadeChange;
1746 }
1747 }
1748 }
1749
1750 // Add the branch back if the block is more than just an uncond branch.
1751 TII->insertBranch(*MBB, CurTBB, nullptr, CurCond, dl);
1752 }
1753 }
1754
1755 // If the prior block doesn't fall through into this block, and if this
1756 // block doesn't fall through into some other block, see if we can find a
1757 // place to move this block where a fall-through will happen.
1758 if (!PrevBB.canFallThrough()) {
1759 // Now we know that there was no fall-through into this block, check to
1760 // see if it has a fall-through into its successor.
1761 bool CurFallsThru = MBB->canFallThrough();
1762
1763 if (!MBB->isEHPad()) {
1764 // Check all the predecessors of this block. If one of them has no fall
1765 // throughs, move this block right after it.
1766 for (MachineBasicBlock *PredBB : MBB->predecessors()) {
1767 // Analyze the branch at the end of the pred.
1768 MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
1769 SmallVector<MachineOperand, 4> PredCond;
1770 if (PredBB != MBB && !PredBB->canFallThrough() &&
1771 !TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) &&
1772 (!CurFallsThru || !CurTBB || !CurFBB) &&
1773 (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1774 // If the current block doesn't fall through, just move it.
1775 // If the current block can fall through and does not end with a
1776 // conditional branch, we need to append an unconditional jump to
1777 // the (current) next block. To avoid a possible compile-time
1778 // infinite loop, move blocks only backward in this case.
1779 // Also, if there are already 2 branches here, we cannot add a third;
1780 // this means we have the case
1781 // Bcc next
1782 // B elsewhere
1783 // next:
1784 if (CurFallsThru) {
1785 MachineBasicBlock *NextBB = &*std::next(MBB->getIterator());
1786 CurCond.clear();
1787 TII->insertBranch(*MBB, NextBB, nullptr, CurCond, DebugLoc());
1788 }
1789 MBB->moveAfter(PredBB);
1790 MadeChange = true;
1791 goto ReoptimizeBlock;
1792 }
1793 }
1794 }
1795
1796 if (!CurFallsThru) {
1797 // Check all successors to see if we can move this block before it.
1798 for (MachineBasicBlock *SuccBB : MBB->successors()) {
1799 // Analyze the branch at the end of the block before the succ.
1800 MachineFunction::iterator SuccPrev = --SuccBB->getIterator();
1801
1802 // If this block doesn't already fall-through to that successor, and if
1803 // the succ doesn't already have a block that can fall through into it,
1804 // and if the successor isn't an EH destination, we can arrange for the
1805 // fallthrough to happen.
1806 if (SuccBB != MBB && &*SuccPrev != MBB &&
1807 !SuccPrev->canFallThrough() && !CurUnAnalyzable &&
1808 !SuccBB->isEHPad()) {
1809 MBB->moveBefore(SuccBB);
1810 MadeChange = true;
1811 goto ReoptimizeBlock;
1812 }
1813 }
1814
1815 // Okay, there is no really great place to put this block. If, however,
1816 // the block before this one would be a fall-through if this block were
1817 // removed, move this block to the end of the function. There is no real
1818 // advantage in "falling through" to an EH block, so we don't want to
1819 // perform this transformation for that case.
1820 //
1821 // Also, Windows EH introduced the possibility of an arbitrary number of
1822 // successors to a given block. The analyzeBranch call does not consider
1823 // exception handling and so we can get in a state where a block
1824 // containing a call is followed by multiple EH blocks that would be
1825 // rotated infinitely at the end of the function if the transformation
1826 // below were performed for EH "FallThrough" blocks. Therefore, even if
1827 // that appears not to be happening anymore, we should assume that it is
1828 // possible and not remove the "!FallThrough()->isEHPad" condition below.
1829 MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr;
1830 SmallVector<MachineOperand, 4> PrevCond;
1831 if (FallThrough != MF.end() &&
1832 !FallThrough->isEHPad() &&
1833 !TII->analyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1834 PrevBB.isSuccessor(&*FallThrough)) {
1835 MBB->moveAfter(&MF.back());
1836 MadeChange = true;
1837 return MadeChange;
1838 }
1839 }
1840 }
1841
1842 return MadeChange;
1843}
1844
1845//===----------------------------------------------------------------------===//
1846// Hoist Common Code
1847//===----------------------------------------------------------------------===//
1848
1849bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
1850 bool MadeChange = false;
1851 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
1852 MachineBasicBlock *MBB = &*I++;
1853 MadeChange |= HoistCommonCodeInSuccs(MBB);
1854 }
1855
1856 return MadeChange;
1857}
1858
1859/// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
1860/// its 'true' successor.
1861static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
1862 MachineBasicBlock *TrueBB) {
1863 for (MachineBasicBlock *SuccBB : BB->successors())
1864 if (SuccBB != TrueBB)
1865 return SuccBB;
1866 return nullptr;
1867}
1868
1869template <class Container>
1870static void addRegAndItsAliases(unsigned Reg, const TargetRegisterInfo *TRI,
1871 Container &Set) {
1872 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
1873 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1874 Set.insert(*AI);
1875 } else {
1876 Set.insert(Reg);
1877 }
1878}
1879
1880/// findHoistingInsertPosAndDeps - Find the location to move common instructions
1881/// in successors to. The location is usually just before the terminator,
1882/// however if the terminator is a conditional branch and its previous
1883/// instruction is the flag setting instruction, the previous instruction is
1884/// the preferred location. This function also gathers uses and defs of the
1885/// instructions from the insertion point to the end of the block. The data is
1886/// used by HoistCommonCodeInSuccs to ensure safety.
1887static
1888MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
1889 const TargetInstrInfo *TII,
1890 const TargetRegisterInfo *TRI,
1891 SmallSet<unsigned,4> &Uses,
1892 SmallSet<unsigned,4> &Defs) {
1893 MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
1894 if (!TII->isUnpredicatedTerminator(*Loc))
1895 return MBB->end();
1896
1897 for (const MachineOperand &MO : Loc->operands()) {
1898 if (!MO.isReg())
1899 continue;
1900 unsigned Reg = MO.getReg();
1901 if (!Reg)
1902 continue;
1903 if (MO.isUse()) {
1904 addRegAndItsAliases(Reg, TRI, Uses);
1905 } else {
1906 if (!MO.isDead())
1907 // Don't try to hoist code in the rare case the terminator defines a
1908 // register that is later used.
1909 return MBB->end();
1910
1911 // If the terminator defines a register, make sure we don't hoist
1912 // the instruction whose def might be clobbered by the terminator.
1913 addRegAndItsAliases(Reg, TRI, Defs);
1914 }
1915 }
1916
1917 if (Uses.empty())
1918 return Loc;
1919 // If the terminator is the only instruction in the block and Uses is not
1920 // empty (or we would have returned above), we can still safely hoist
1921 // instructions just before the terminator as long as the Defs/Uses are not
1922 // violated (which is checked in HoistCommonCodeInSuccs).
1923 if (Loc == MBB->begin())
1924 return Loc;
1925
1926 // The terminator is probably a conditional branch, try not to separate the
1927 // branch from condition setting instruction.
1928 MachineBasicBlock::iterator PI =
1929 skipDebugInstructionsBackward(std::prev(Loc), MBB->begin());
1930
1931 bool IsDef = false;
1932 for (const MachineOperand &MO : PI->operands()) {
1933 // If PI has a regmask operand, it is probably a call. Separate away.
1934 if (MO.isRegMask())
1935 return Loc;
1936 if (!MO.isReg() || MO.isUse())
1937 continue;
1938 unsigned Reg = MO.getReg();
1939 if (!Reg)
1940 continue;
1941 if (Uses.count(Reg)) {
1942 IsDef = true;
1943 break;
1944 }
1945 }
1946 if (!IsDef)
1947 // The condition setting instruction is not just before the conditional
1948 // branch.
1949 return Loc;
1950
1951 // Be conservative, don't insert instruction above something that may have
1952 // side-effects. And since it's potentially bad to separate flag setting
1953 // instruction from the conditional branch, just abort the optimization
1954 // completely.
1955 // Also avoid moving code above predicated instruction since it's hard to
1956 // reason about register liveness with predicated instruction.
1957 bool DontMoveAcrossStore = true;
1958 if (!PI->isSafeToMove(nullptr, DontMoveAcrossStore) || TII->isPredicated(*PI))
1959 return MBB->end();
1960
1961 // Find out what registers are live. Note this routine is ignoring other live
1962 // registers which are only used by instructions in successor blocks.
1963 for (const MachineOperand &MO : PI->operands()) {
1964 if (!MO.isReg())
1965 continue;
1966 unsigned Reg = MO.getReg();
1967 if (!Reg)
1968 continue;
1969 if (MO.isUse()) {
1970 addRegAndItsAliases(Reg, TRI, Uses);
1971 } else {
1972 if (Uses.erase(Reg)) {
1973 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
1974 for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
1975 Uses.erase(*SubRegs); // Use sub-registers to be conservative
1976 }
1977 }
1978 addRegAndItsAliases(Reg, TRI, Defs);
1979 }
1980 }
1981
1982 return PI;
1983}
1984
1985bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
1986 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
1987 SmallVector<MachineOperand, 4> Cond;
1988 if (TII->analyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
1989 return false;
1990
1991 if (!FBB) FBB = findFalseBlock(MBB, TBB);
1992 if (!FBB)
1993 // Malformed bcc? True and false blocks are the same?
1994 return false;
1995
1996 // Restrict the optimization to cases where MBB is the only predecessor,
1997 // it is an obvious win.
1998 if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
1999 return false;
2000
2001 // Find a suitable position to hoist the common instructions to. Also figure
2002 // out which registers are used or defined by instructions from the insertion
2003 // point to the end of the block.
2004 SmallSet<unsigned, 4> Uses, Defs;
2005 MachineBasicBlock::iterator Loc =
2006 findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
2007 if (Loc == MBB->end())
2008 return false;
2009
2010 bool HasDups = false;
2011 SmallSet<unsigned, 4> ActiveDefsSet, AllDefsSet;
2012 MachineBasicBlock::iterator TIB = TBB->begin();
2013 MachineBasicBlock::iterator FIB = FBB->begin();
2014 MachineBasicBlock::iterator TIE = TBB->end();
2015 MachineBasicBlock::iterator FIE = FBB->end();
2016 while (TIB != TIE && FIB != FIE) {
2017 // Skip dbg_value instructions. These do not count.
2018 TIB = skipDebugInstructionsForward(TIB, TIE);
2019 FIB = skipDebugInstructionsForward(FIB, FIE);
2020 if (TIB == TIE || FIB == FIE)
2021 break;
2022
2023 if (!TIB->isIdenticalTo(*FIB, MachineInstr::CheckKillDead))
2024 break;
2025
2026 if (TII->isPredicated(*TIB))
2027 // Hard to reason about register liveness with predicated instruction.
2028 break;
2029
2030 bool IsSafe = true;
2031 for (MachineOperand &MO : TIB->operands()) {
2032 // Don't attempt to hoist instructions with register masks.
2033 if (MO.isRegMask()) {
2034 IsSafe = false;
2035 break;
2036 }
2037 if (!MO.isReg())
2038 continue;
2039 unsigned Reg = MO.getReg();
2040 if (!Reg)
2041 continue;
2042 if (MO.isDef()) {
2043 if (Uses.count(Reg)) {
2044 // Avoid clobbering a register that's used by the instruction at
2045 // the point of insertion.
2046 IsSafe = false;
2047 break;
2048 }
2049
2050 if (Defs.count(Reg) && !MO.isDead()) {
2051 // Don't hoist the instruction if the def would be clobber by the
2052 // instruction at the point insertion. FIXME: This is overly
2053 // conservative. It should be possible to hoist the instructions
2054 // in BB2 in the following example:
2055 // BB1:
2056 // r1, eflag = op1 r2, r3
2057 // brcc eflag
2058 //
2059 // BB2:
2060 // r1 = op2, ...
2061 // = op3, killed r1
2062 IsSafe = false;
2063 break;
2064 }
2065 } else if (!ActiveDefsSet.count(Reg)) {
2066 if (Defs.count(Reg)) {
2067 // Use is defined by the instruction at the point of insertion.
2068 IsSafe = false;
2069 break;
2070 }
2071
2072 if (MO.isKill() && Uses.count(Reg))
2073 // Kills a register that's read by the instruction at the point of
2074 // insertion. Remove the kill marker.
2075 MO.setIsKill(false);
2076 }
2077 }
2078 if (!IsSafe)
2079 break;
2080
2081 bool DontMoveAcrossStore = true;
2082 if (!TIB->isSafeToMove(nullptr, DontMoveAcrossStore))
2083 break;
2084
2085 // Remove kills from ActiveDefsSet, these registers had short live ranges.
2086 for (const MachineOperand &MO : TIB->operands()) {
2087 if (!MO.isReg() || !MO.isUse() || !MO.isKill())
2088 continue;
2089 unsigned Reg = MO.getReg();
2090 if (!Reg)
2091 continue;
2092 if (!AllDefsSet.count(Reg)) {
2093 continue;
2094 }
2095 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
2096 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
2097 ActiveDefsSet.erase(*AI);
2098 } else {
2099 ActiveDefsSet.erase(Reg);
2100 }
2101 }
2102
2103 // Track local defs so we can update liveins.
2104 for (const MachineOperand &MO : TIB->operands()) {
2105 if (!MO.isReg() || !MO.isDef() || MO.isDead())
2106 continue;
2107 unsigned Reg = MO.getReg();
2108 if (!Reg || TargetRegisterInfo::isVirtualRegister(Reg))
2109 continue;
2110 addRegAndItsAliases(Reg, TRI, ActiveDefsSet);
2111 addRegAndItsAliases(Reg, TRI, AllDefsSet);
2112 }
2113
2114 HasDups = true;
2115 ++TIB;
2116 ++FIB;
2117 }
2118
2119 if (!HasDups)
2120 return false;
2121
2122 MBB->splice(Loc, TBB, TBB->begin(), TIB);
2123 FBB->erase(FBB->begin(), FIB);
2124
2125 if (UpdateLiveIns) {
2126 recomputeLiveIns(*TBB);
2127 recomputeLiveIns(*FBB);
2128 }
2129
2130 ++NumHoist;
2131 return true;
2132}