Bug Summary

File:lib/CodeGen/BranchFolding.cpp
Warning:line 1315, 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-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 -analyzer-config-compatibility-mode=true -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-9/lib/clang/9.0.0 -D _DEBUG -D _GNU_SOURCE -D __STDC_CONSTANT_MACROS -D __STDC_FORMAT_MACROS -D __STDC_LIMIT_MACROS -I /build/llvm-toolchain-snapshot-9~svn361465/build-llvm/lib/CodeGen -I /build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen -I /build/llvm-toolchain-snapshot-9~svn361465/build-llvm/include -I /build/llvm-toolchain-snapshot-9~svn361465/include -U NDEBUG -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/x86_64-linux-gnu/c++/6.3.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/x86_64-linux-gnu/c++/6.3.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/6.3.0/../../../../include/c++/6.3.0/backward -internal-isystem /usr/include/clang/9.0.0/include/ -internal-isystem /usr/local/include -internal-isystem /usr/lib/llvm-9/lib/clang/9.0.0/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-9~svn361465/build-llvm/lib/CodeGen -fdebug-prefix-map=/build/llvm-toolchain-snapshot-9~svn361465=. -ferror-limit 19 -fmessage-length 0 -fvisibility-inlines-hidden -stack-protector 2 -fobjc-runtime=gcc -fdiagnostics-show-option -vectorize-loops -vectorize-slp -analyzer-output=html -analyzer-config stable-report-filename=true -o /tmp/scan-build-2019-05-24-031927-21217-1 -x c++ /build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp -faddrsig
1//===- BranchFolding.cpp - Fold machine code branch instructions ----------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This pass forwards branches to unconditional branches to make them branch
10// directly to the target block. This pass often results in dead MBB's, which
11// it then removes.
12//
13// Note that this pass must be run after register allocation, it cannot handle
14// SSA form. It also must handle virtual registers for targets that emit virtual
15// ISA (e.g. NVPTX).
16//
17//===----------------------------------------------------------------------===//
18
19#include "BranchFolding.h"
20#include "llvm/ADT/BitVector.h"
21#include "llvm/ADT/DenseMap.h"
22#include "llvm/ADT/STLExtras.h"
23#include "llvm/ADT/SmallPtrSet.h"
24#include "llvm/ADT/SmallSet.h"
25#include "llvm/ADT/SmallVector.h"
26#include "llvm/ADT/Statistic.h"
27#include "llvm/CodeGen/Analysis.h"
28#include "llvm/CodeGen/LivePhysRegs.h"
29#include "llvm/CodeGen/MachineBasicBlock.h"
30#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
31#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
32#include "llvm/CodeGen/MachineFunction.h"
33#include "llvm/CodeGen/MachineFunctionPass.h"
34#include "llvm/CodeGen/MachineInstr.h"
35#include "llvm/CodeGen/MachineInstrBuilder.h"
36#include "llvm/CodeGen/MachineJumpTableInfo.h"
37#include "llvm/CodeGen/MachineLoopInfo.h"
38#include "llvm/CodeGen/MachineModuleInfo.h"
39#include "llvm/CodeGen/MachineOperand.h"
40#include "llvm/CodeGen/MachineRegisterInfo.h"
41#include "llvm/CodeGen/TargetInstrInfo.h"
42#include "llvm/CodeGen/TargetOpcodes.h"
43#include "llvm/CodeGen/TargetPassConfig.h"
44#include "llvm/CodeGen/TargetRegisterInfo.h"
45#include "llvm/CodeGen/TargetSubtargetInfo.h"
46#include "llvm/IR/DebugInfoMetadata.h"
47#include "llvm/IR/DebugLoc.h"
48#include "llvm/IR/Function.h"
49#include "llvm/MC/LaneBitmask.h"
50#include "llvm/MC/MCRegisterInfo.h"
51#include "llvm/Pass.h"
52#include "llvm/Support/BlockFrequency.h"
53#include "llvm/Support/BranchProbability.h"
54#include "llvm/Support/CommandLine.h"
55#include "llvm/Support/Debug.h"
56#include "llvm/Support/ErrorHandling.h"
57#include "llvm/Support/raw_ostream.h"
58#include "llvm/Target/TargetMachine.h"
59#include <cassert>
60#include <cstddef>
61#include <iterator>
62#include <numeric>
63#include <vector>
64
65using namespace llvm;
66
67#define DEBUG_TYPE"branch-folder" "branch-folder"
68
69STATISTIC(NumDeadBlocks, "Number of dead blocks removed")static llvm::Statistic NumDeadBlocks = {"branch-folder", "NumDeadBlocks"
, "Number of dead blocks removed", {0}, {false}}
;
70STATISTIC(NumBranchOpts, "Number of branches optimized")static llvm::Statistic NumBranchOpts = {"branch-folder", "NumBranchOpts"
, "Number of branches optimized", {0}, {false}}
;
71STATISTIC(NumTailMerge , "Number of block tails merged")static llvm::Statistic NumTailMerge = {"branch-folder", "NumTailMerge"
, "Number of block tails merged", {0}, {false}}
;
72STATISTIC(NumHoist , "Number of times common instructions are hoisted")static llvm::Statistic NumHoist = {"branch-folder", "NumHoist"
, "Number of times common instructions are hoisted", {0}, {false
}}
;
73STATISTIC(NumTailCalls, "Number of tail calls optimized")static llvm::Statistic NumTailCalls = {"branch-folder", "NumTailCalls"
, "Number of tail calls optimized", {0}, {false}}
;
74
75static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
76 cl::init(cl::BOU_UNSET), cl::Hidden);
77
78// Throttle for huge numbers of predecessors (compile speed problems)
79static cl::opt<unsigned>
80TailMergeThreshold("tail-merge-threshold",
81 cl::desc("Max number of predecessors to consider tail merging"),
82 cl::init(150), cl::Hidden);
83
84// Heuristic for tail merging (and, inversely, tail duplication).
85// TODO: This should be replaced with a target query.
86static cl::opt<unsigned>
87TailMergeSize("tail-merge-size",
88 cl::desc("Min number of instructions to consider tail merging"),
89 cl::init(3), cl::Hidden);
90
91namespace {
92
93 /// BranchFolderPass - Wrap branch folder in a machine function pass.
94 class BranchFolderPass : public MachineFunctionPass {
95 public:
96 static char ID;
97
98 explicit BranchFolderPass(): MachineFunctionPass(ID) {}
99
100 bool runOnMachineFunction(MachineFunction &MF) override;
101
102 void getAnalysisUsage(AnalysisUsage &AU) const override {
103 AU.addRequired<MachineBlockFrequencyInfo>();
104 AU.addRequired<MachineBranchProbabilityInfo>();
105 AU.addRequired<TargetPassConfig>();
106 MachineFunctionPass::getAnalysisUsage(AU);
107 }
108 };
109
110} // end anonymous namespace
111
112char BranchFolderPass::ID = 0;
113
114char &llvm::BranchFolderPassID = BranchFolderPass::ID;
115
116INITIALIZE_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)); }
117 "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)); }
118
119bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
120 if (skipFunction(MF.getFunction()))
121 return false;
122
123 TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
124 // TailMerge can create jump into if branches that make CFG irreducible for
125 // HW that requires structurized CFG.
126 bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
127 PassConfig->getEnableTailMerge();
128 BranchFolder::MBFIWrapper MBBFreqInfo(
129 getAnalysis<MachineBlockFrequencyInfo>());
130 BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true, MBBFreqInfo,
131 getAnalysis<MachineBranchProbabilityInfo>());
132 return Folder.OptimizeFunction(MF, MF.getSubtarget().getInstrInfo(),
133 MF.getSubtarget().getRegisterInfo(),
134 getAnalysisIfAvailable<MachineModuleInfo>());
135}
136
137BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist,
138 MBFIWrapper &FreqInfo,
139 const MachineBranchProbabilityInfo &ProbInfo,
140 unsigned MinTailLength)
141 : EnableHoistCommonCode(CommonHoist), MinCommonTailLength(MinTailLength),
142 MBBFreqInfo(FreqInfo), MBPI(ProbInfo) {
143 if (MinCommonTailLength == 0)
144 MinCommonTailLength = TailMergeSize;
145 switch (FlagEnableTailMerge) {
146 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
147 case cl::BOU_TRUE: EnableTailMerge = true; break;
148 case cl::BOU_FALSE: EnableTailMerge = false; break;
149 }
150}
151
152void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
153 assert(MBB->pred_empty() && "MBB must be dead!")((MBB->pred_empty() && "MBB must be dead!") ? static_cast
<void> (0) : __assert_fail ("MBB->pred_empty() && \"MBB must be dead!\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 153, __PRETTY_FUNCTION__))
;
154 LLVM_DEBUG(dbgs() << "\nRemoving MBB: " << *MBB)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nRemoving MBB: " <<
*MBB; } } while (false)
;
155
156 MachineFunction *MF = MBB->getParent();
157 // drop all successors.
158 while (!MBB->succ_empty())
159 MBB->removeSuccessor(MBB->succ_end()-1);
160
161 // Avoid matching if this pointer gets reused.
162 TriedMerging.erase(MBB);
163
164 // Remove the block.
165 MF->erase(MBB);
166 EHScopeMembership.erase(MBB);
167 if (MLI)
168 MLI->removeBlock(MBB);
169}
170
171bool BranchFolder::OptimizeFunction(MachineFunction &MF,
172 const TargetInstrInfo *tii,
173 const TargetRegisterInfo *tri,
174 MachineModuleInfo *mmi,
175 MachineLoopInfo *mli, bool AfterPlacement) {
176 if (!tii) return false;
177
178 TriedMerging.clear();
179
180 MachineRegisterInfo &MRI = MF.getRegInfo();
181 AfterBlockPlacement = AfterPlacement;
182 TII = tii;
183 TRI = tri;
184 MMI = mmi;
185 MLI = mli;
186 this->MRI = &MRI;
187
188 UpdateLiveIns = MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF);
189 if (!UpdateLiveIns)
190 MRI.invalidateLiveness();
191
192 // Fix CFG. The later algorithms expect it to be right.
193 bool MadeChange = false;
194 for (MachineBasicBlock &MBB : MF) {
195 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
196 SmallVector<MachineOperand, 4> Cond;
197 if (!TII->analyzeBranch(MBB, TBB, FBB, Cond, true))
198 MadeChange |= MBB.CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
199 }
200
201 // Recalculate EH scope membership.
202 EHScopeMembership = getEHScopeMembership(MF);
203
204 bool MadeChangeThisIteration = true;
205 while (MadeChangeThisIteration) {
206 MadeChangeThisIteration = TailMergeBlocks(MF);
207 // No need to clean up if tail merging does not change anything after the
208 // block placement.
209 if (!AfterBlockPlacement || MadeChangeThisIteration)
210 MadeChangeThisIteration |= OptimizeBranches(MF);
211 if (EnableHoistCommonCode)
212 MadeChangeThisIteration |= HoistCommonCode(MF);
213 MadeChange |= MadeChangeThisIteration;
214 }
215
216 // See if any jump tables have become dead as the code generator
217 // did its thing.
218 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
219 if (!JTI)
220 return MadeChange;
221
222 // Walk the function to find jump tables that are live.
223 BitVector JTIsLive(JTI->getJumpTables().size());
224 for (const MachineBasicBlock &BB : MF) {
225 for (const MachineInstr &I : BB)
226 for (const MachineOperand &Op : I.operands()) {
227 if (!Op.isJTI()) continue;
228
229 // Remember that this JT is live.
230 JTIsLive.set(Op.getIndex());
231 }
232 }
233
234 // Finally, remove dead jump tables. This happens when the
235 // indirect jump was unreachable (and thus deleted).
236 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
237 if (!JTIsLive.test(i)) {
238 JTI->RemoveJumpTable(i);
239 MadeChange = true;
240 }
241
242 return MadeChange;
243}
244
245//===----------------------------------------------------------------------===//
246// Tail Merging of Blocks
247//===----------------------------------------------------------------------===//
248
249/// HashMachineInstr - Compute a hash value for MI and its operands.
250static unsigned HashMachineInstr(const MachineInstr &MI) {
251 unsigned Hash = MI.getOpcode();
252 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
253 const MachineOperand &Op = MI.getOperand(i);
254
255 // Merge in bits from the operand if easy. We can't use MachineOperand's
256 // hash_code here because it's not deterministic and we sort by hash value
257 // later.
258 unsigned OperandHash = 0;
259 switch (Op.getType()) {
260 case MachineOperand::MO_Register:
261 OperandHash = Op.getReg();
262 break;
263 case MachineOperand::MO_Immediate:
264 OperandHash = Op.getImm();
265 break;
266 case MachineOperand::MO_MachineBasicBlock:
267 OperandHash = Op.getMBB()->getNumber();
268 break;
269 case MachineOperand::MO_FrameIndex:
270 case MachineOperand::MO_ConstantPoolIndex:
271 case MachineOperand::MO_JumpTableIndex:
272 OperandHash = Op.getIndex();
273 break;
274 case MachineOperand::MO_GlobalAddress:
275 case MachineOperand::MO_ExternalSymbol:
276 // Global address / external symbol are too hard, don't bother, but do
277 // pull in the offset.
278 OperandHash = Op.getOffset();
279 break;
280 default:
281 break;
282 }
283
284 Hash += ((OperandHash << 3) | Op.getType()) << (i & 31);
285 }
286 return Hash;
287}
288
289/// HashEndOfMBB - Hash the last instruction in the MBB.
290static unsigned HashEndOfMBB(const MachineBasicBlock &MBB) {
291 MachineBasicBlock::const_iterator I = MBB.getLastNonDebugInstr();
292 if (I == MBB.end())
293 return 0;
294
295 return HashMachineInstr(*I);
296}
297
298/// Whether MI should be counted as an instruction when calculating common tail.
299static bool countsAsInstruction(const MachineInstr &MI) {
300 return !(MI.isDebugInstr() || MI.isCFIInstruction());
301}
302
303/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
304/// of instructions they actually have in common together at their end. Return
305/// iterators for the first shared instruction in each block.
306static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
307 MachineBasicBlock *MBB2,
308 MachineBasicBlock::iterator &I1,
309 MachineBasicBlock::iterator &I2) {
310 I1 = MBB1->end();
311 I2 = MBB2->end();
312
313 unsigned TailLen = 0;
314 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
315 --I1; --I2;
316 // Skip debugging pseudos; necessary to avoid changing the code.
317 while (!countsAsInstruction(*I1)) {
318 if (I1==MBB1->begin()) {
319 while (!countsAsInstruction(*I2)) {
320 if (I2==MBB2->begin()) {
321 // I1==DBG at begin; I2==DBG at begin
322 goto SkipTopCFIAndReturn;
323 }
324 --I2;
325 }
326 ++I2;
327 // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
328 goto SkipTopCFIAndReturn;
329 }
330 --I1;
331 }
332 // I1==first (untested) non-DBG preceding known match
333 while (!countsAsInstruction(*I2)) {
334 if (I2==MBB2->begin()) {
335 ++I1;
336 // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
337 goto SkipTopCFIAndReturn;
338 }
339 --I2;
340 }
341 // I1, I2==first (untested) non-DBGs preceding known match
342 if (!I1->isIdenticalTo(*I2) ||
343 // FIXME: This check is dubious. It's used to get around a problem where
344 // people incorrectly expect inline asm directives to remain in the same
345 // relative order. This is untenable because normal compiler
346 // optimizations (like this one) may reorder and/or merge these
347 // directives.
348 I1->isInlineAsm()) {
349 ++I1; ++I2;
350 break;
351 }
352 ++TailLen;
353 }
354 // Back past possible debugging pseudos at beginning of block. This matters
355 // when one block differs from the other only by whether debugging pseudos
356 // are present at the beginning. (This way, the various checks later for
357 // I1==MBB1->begin() work as expected.)
358 if (I1 == MBB1->begin() && I2 != MBB2->begin()) {
359 --I2;
360 while (I2->isDebugInstr()) {
361 if (I2 == MBB2->begin())
362 return TailLen;
363 --I2;
364 }
365 ++I2;
366 }
367 if (I2 == MBB2->begin() && I1 != MBB1->begin()) {
368 --I1;
369 while (I1->isDebugInstr()) {
370 if (I1 == MBB1->begin())
371 return TailLen;
372 --I1;
373 }
374 ++I1;
375 }
376
377SkipTopCFIAndReturn:
378 // Ensure that I1 and I2 do not point to a CFI_INSTRUCTION. This can happen if
379 // I1 and I2 are non-identical when compared and then one or both of them ends
380 // up pointing to a CFI instruction after being incremented. For example:
381 /*
382 BB1:
383 ...
384 INSTRUCTION_A
385 ADD32ri8 <- last common instruction
386 ...
387 BB2:
388 ...
389 INSTRUCTION_B
390 CFI_INSTRUCTION
391 ADD32ri8 <- last common instruction
392 ...
393 */
394 // When INSTRUCTION_A and INSTRUCTION_B are compared as not equal, after
395 // incrementing the iterators, I1 will point to ADD, however I2 will point to
396 // the CFI instruction. Later on, this leads to BB2 being 'hacked off' at the
397 // wrong place (in ReplaceTailWithBranchTo()) which results in losing this CFI
398 // instruction.
399 while (I1 != MBB1->end() && I1->isCFIInstruction()) {
400 ++I1;
401 }
402
403 while (I2 != MBB2->end() && I2->isCFIInstruction()) {
404 ++I2;
405 }
406
407 return TailLen;
408}
409
410void BranchFolder::replaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
411 MachineBasicBlock &NewDest) {
412 if (UpdateLiveIns) {
413 // OldInst should always point to an instruction.
414 MachineBasicBlock &OldMBB = *OldInst->getParent();
415 LiveRegs.clear();
416 LiveRegs.addLiveOuts(OldMBB);
417 // Move backward to the place where will insert the jump.
418 MachineBasicBlock::iterator I = OldMBB.end();
419 do {
420 --I;
421 LiveRegs.stepBackward(*I);
422 } while (I != OldInst);
423
424 // Merging the tails may have switched some undef operand to non-undef ones.
425 // Add IMPLICIT_DEFS into OldMBB as necessary to have a definition of the
426 // register.
427 for (MachineBasicBlock::RegisterMaskPair P : NewDest.liveins()) {
428 // We computed the liveins with computeLiveIn earlier and should only see
429 // full registers:
430 assert(P.LaneMask == LaneBitmask::getAll() &&((P.LaneMask == LaneBitmask::getAll() && "Can only handle full register."
) ? static_cast<void> (0) : __assert_fail ("P.LaneMask == LaneBitmask::getAll() && \"Can only handle full register.\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 431, __PRETTY_FUNCTION__))
431 "Can only handle full register.")((P.LaneMask == LaneBitmask::getAll() && "Can only handle full register."
) ? static_cast<void> (0) : __assert_fail ("P.LaneMask == LaneBitmask::getAll() && \"Can only handle full register.\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 431, __PRETTY_FUNCTION__))
;
432 MCPhysReg Reg = P.PhysReg;
433 if (!LiveRegs.available(*MRI, Reg))
434 continue;
435 DebugLoc DL;
436 BuildMI(OldMBB, OldInst, DL, TII->get(TargetOpcode::IMPLICIT_DEF), Reg);
437 }
438 }
439
440 TII->ReplaceTailWithBranchTo(OldInst, &NewDest);
441 ++NumTailMerge;
442}
443
444MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
445 MachineBasicBlock::iterator BBI1,
446 const BasicBlock *BB) {
447 if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
448 return nullptr;
449
450 MachineFunction &MF = *CurMBB.getParent();
451
452 // Create the fall-through block.
453 MachineFunction::iterator MBBI = CurMBB.getIterator();
454 MachineBasicBlock *NewMBB = MF.CreateMachineBasicBlock(BB);
455 CurMBB.getParent()->insert(++MBBI, NewMBB);
456
457 // Move all the successors of this block to the specified block.
458 NewMBB->transferSuccessors(&CurMBB);
459
460 // Add an edge from CurMBB to NewMBB for the fall-through.
461 CurMBB.addSuccessor(NewMBB);
462
463 // Splice the code over.
464 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
465
466 // NewMBB belongs to the same loop as CurMBB.
467 if (MLI)
468 if (MachineLoop *ML = MLI->getLoopFor(&CurMBB))
469 ML->addBasicBlockToLoop(NewMBB, MLI->getBase());
470
471 // NewMBB inherits CurMBB's block frequency.
472 MBBFreqInfo.setBlockFreq(NewMBB, MBBFreqInfo.getBlockFreq(&CurMBB));
473
474 if (UpdateLiveIns)
475 computeAndAddLiveIns(LiveRegs, *NewMBB);
476
477 // Add the new block to the EH scope.
478 const auto &EHScopeI = EHScopeMembership.find(&CurMBB);
479 if (EHScopeI != EHScopeMembership.end()) {
480 auto n = EHScopeI->second;
481 EHScopeMembership[NewMBB] = n;
482 }
483
484 return NewMBB;
485}
486
487/// EstimateRuntime - Make a rough estimate for how long it will take to run
488/// the specified code.
489static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
490 MachineBasicBlock::iterator E) {
491 unsigned Time = 0;
492 for (; I != E; ++I) {
493 if (!countsAsInstruction(*I))
494 continue;
495 if (I->isCall())
496 Time += 10;
497 else if (I->mayLoad() || I->mayStore())
498 Time += 2;
499 else
500 ++Time;
501 }
502 return Time;
503}
504
505// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
506// branches temporarily for tail merging). In the case where CurMBB ends
507// with a conditional branch to the next block, optimize by reversing the
508// test and conditionally branching to SuccMBB instead.
509static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
510 const TargetInstrInfo *TII) {
511 MachineFunction *MF = CurMBB->getParent();
512 MachineFunction::iterator I = std::next(MachineFunction::iterator(CurMBB));
513 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
514 SmallVector<MachineOperand, 4> Cond;
515 DebugLoc dl = CurMBB->findBranchDebugLoc();
516 if (I != MF->end() && !TII->analyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
517 MachineBasicBlock *NextBB = &*I;
518 if (TBB == NextBB && !Cond.empty() && !FBB) {
519 if (!TII->reverseBranchCondition(Cond)) {
520 TII->removeBranch(*CurMBB);
521 TII->insertBranch(*CurMBB, SuccBB, nullptr, Cond, dl);
522 return;
523 }
524 }
525 }
526 TII->insertBranch(*CurMBB, SuccBB, nullptr,
527 SmallVector<MachineOperand, 0>(), dl);
528}
529
530bool
531BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
532 if (getHash() < o.getHash())
533 return true;
534 if (getHash() > o.getHash())
535 return false;
536 if (getBlock()->getNumber() < o.getBlock()->getNumber())
537 return true;
538 if (getBlock()->getNumber() > o.getBlock()->getNumber())
539 return false;
540 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
541 // an object with itself.
542#ifndef _GLIBCXX_DEBUG
543 llvm_unreachable("Predecessor appears twice")::llvm::llvm_unreachable_internal("Predecessor appears twice"
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 543)
;
544#else
545 return false;
546#endif
547}
548
549BlockFrequency
550BranchFolder::MBFIWrapper::getBlockFreq(const MachineBasicBlock *MBB) const {
551 auto I = MergedBBFreq.find(MBB);
552
553 if (I != MergedBBFreq.end())
554 return I->second;
555
556 return MBFI.getBlockFreq(MBB);
557}
558
559void BranchFolder::MBFIWrapper::setBlockFreq(const MachineBasicBlock *MBB,
560 BlockFrequency F) {
561 MergedBBFreq[MBB] = F;
562}
563
564raw_ostream &
565BranchFolder::MBFIWrapper::printBlockFreq(raw_ostream &OS,
566 const MachineBasicBlock *MBB) const {
567 return MBFI.printBlockFreq(OS, getBlockFreq(MBB));
568}
569
570raw_ostream &
571BranchFolder::MBFIWrapper::printBlockFreq(raw_ostream &OS,
572 const BlockFrequency Freq) const {
573 return MBFI.printBlockFreq(OS, Freq);
574}
575
576void BranchFolder::MBFIWrapper::view(const Twine &Name, bool isSimple) {
577 MBFI.view(Name, isSimple);
578}
579
580uint64_t
581BranchFolder::MBFIWrapper::getEntryFreq() const {
582 return MBFI.getEntryFreq();
583}
584
585/// CountTerminators - Count the number of terminators in the given
586/// block and set I to the position of the first non-terminator, if there
587/// is one, or MBB->end() otherwise.
588static unsigned CountTerminators(MachineBasicBlock *MBB,
589 MachineBasicBlock::iterator &I) {
590 I = MBB->end();
591 unsigned NumTerms = 0;
592 while (true) {
593 if (I == MBB->begin()) {
594 I = MBB->end();
595 break;
596 }
597 --I;
598 if (!I->isTerminator()) break;
599 ++NumTerms;
600 }
601 return NumTerms;
602}
603
604/// A no successor, non-return block probably ends in unreachable and is cold.
605/// Also consider a block that ends in an indirect branch to be a return block,
606/// since many targets use plain indirect branches to return.
607static bool blockEndsInUnreachable(const MachineBasicBlock *MBB) {
608 if (!MBB->succ_empty())
609 return false;
610 if (MBB->empty())
611 return true;
612 return !(MBB->back().isReturn() || MBB->back().isIndirectBranch());
613}
614
615/// ProfitableToMerge - Check if two machine basic blocks have a common tail
616/// and decide if it would be profitable to merge those tails. Return the
617/// length of the common tail and iterators to the first common instruction
618/// in each block.
619/// MBB1, MBB2 The blocks to check
620/// MinCommonTailLength Minimum size of tail block to be merged.
621/// CommonTailLen Out parameter to record the size of the shared tail between
622/// MBB1 and MBB2
623/// I1, I2 Iterator references that will be changed to point to the first
624/// instruction in the common tail shared by MBB1,MBB2
625/// SuccBB A common successor of MBB1, MBB2 which are in a canonical form
626/// relative to SuccBB
627/// PredBB The layout predecessor of SuccBB, if any.
628/// EHScopeMembership map from block to EH scope #.
629/// AfterPlacement True if we are merging blocks after layout. Stricter
630/// thresholds apply to prevent undoing tail-duplication.
631static bool
632ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
633 unsigned MinCommonTailLength, unsigned &CommonTailLen,
634 MachineBasicBlock::iterator &I1,
635 MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB,
636 MachineBasicBlock *PredBB,
637 DenseMap<const MachineBasicBlock *, int> &EHScopeMembership,
638 bool AfterPlacement) {
639 // It is never profitable to tail-merge blocks from two different EH scopes.
640 if (!EHScopeMembership.empty()) {
641 auto EHScope1 = EHScopeMembership.find(MBB1);
642 assert(EHScope1 != EHScopeMembership.end())((EHScope1 != EHScopeMembership.end()) ? static_cast<void>
(0) : __assert_fail ("EHScope1 != EHScopeMembership.end()", "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 642, __PRETTY_FUNCTION__))
;
643 auto EHScope2 = EHScopeMembership.find(MBB2);
644 assert(EHScope2 != EHScopeMembership.end())((EHScope2 != EHScopeMembership.end()) ? static_cast<void>
(0) : __assert_fail ("EHScope2 != EHScopeMembership.end()", "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 644, __PRETTY_FUNCTION__))
;
645 if (EHScope1->second != EHScope2->second)
646 return false;
647 }
648
649 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
650 if (CommonTailLen == 0)
651 return false;
652 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)
653 << " 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)
654 << 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)
;
655
656 // It's almost always profitable to merge any number of non-terminator
657 // instructions with the block that falls through into the common successor.
658 // This is true only for a single successor. For multiple successors, we are
659 // trading a conditional branch for an unconditional one.
660 // TODO: Re-visit successor size for non-layout tail merging.
661 if ((MBB1 == PredBB || MBB2 == PredBB) &&
662 (!AfterPlacement || MBB1->succ_size() == 1)) {
663 MachineBasicBlock::iterator I;
664 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
665 if (CommonTailLen > NumTerms)
666 return true;
667 }
668
669 // If these are identical non-return blocks with no successors, merge them.
670 // Such blocks are typically cold calls to noreturn functions like abort, and
671 // are unlikely to become a fallthrough target after machine block placement.
672 // Tail merging these blocks is unlikely to create additional unconditional
673 // branches, and will reduce the size of this cold code.
674 if (I1 == MBB1->begin() && I2 == MBB2->begin() &&
675 blockEndsInUnreachable(MBB1) && blockEndsInUnreachable(MBB2))
676 return true;
677
678 // If one of the blocks can be completely merged and happens to be in
679 // a position where the other could fall through into it, merge any number
680 // of instructions, because it can be done without a branch.
681 // TODO: If the blocks are not adjacent, move one of them so that they are?
682 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
683 return true;
684 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
685 return true;
686
687 // If both blocks are identical and end in a branch, merge them unless they
688 // both have a fallthrough predecessor and successor.
689 // We can only do this after block placement because it depends on whether
690 // there are fallthroughs, and we don't know until after layout.
691 if (AfterPlacement && I1 == MBB1->begin() && I2 == MBB2->begin()) {
692 auto BothFallThrough = [](MachineBasicBlock *MBB) {
693 if (MBB->succ_size() != 0 && !MBB->canFallThrough())
694 return false;
695 MachineFunction::iterator I(MBB);
696 MachineFunction *MF = MBB->getParent();
697 return (MBB != &*MF->begin()) && std::prev(I)->canFallThrough();
698 };
699 if (!BothFallThrough(MBB1) || !BothFallThrough(MBB2))
700 return true;
701 }
702
703 // If both blocks have an unconditional branch temporarily stripped out,
704 // count that as an additional common instruction for the following
705 // heuristics. This heuristic is only accurate for single-succ blocks, so to
706 // make sure that during layout merging and duplicating don't crash, we check
707 // for that when merging during layout.
708 unsigned EffectiveTailLen = CommonTailLen;
709 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
710 (MBB1->succ_size() == 1 || !AfterPlacement) &&
711 !MBB1->back().isBarrier() &&
712 !MBB2->back().isBarrier())
713 ++EffectiveTailLen;
714
715 // Check if the common tail is long enough to be worthwhile.
716 if (EffectiveTailLen >= MinCommonTailLength)
717 return true;
718
719 // If we are optimizing for code size, 2 instructions in common is enough if
720 // we don't have to split a block. At worst we will be introducing 1 new
721 // branch instruction, which is likely to be smaller than the 2
722 // instructions that would be deleted in the merge.
723 MachineFunction *MF = MBB1->getParent();
724 return EffectiveTailLen >= 2 && MF->getFunction().hasOptSize() &&
725 (I1 == MBB1->begin() || I2 == MBB2->begin());
726}
727
728unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
729 unsigned MinCommonTailLength,
730 MachineBasicBlock *SuccBB,
731 MachineBasicBlock *PredBB) {
732 unsigned maxCommonTailLength = 0U;
733 SameTails.clear();
734 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
735 MPIterator HighestMPIter = std::prev(MergePotentials.end());
736 for (MPIterator CurMPIter = std::prev(MergePotentials.end()),
737 B = MergePotentials.begin();
738 CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) {
739 for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) {
740 unsigned CommonTailLen;
741 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
742 MinCommonTailLength,
743 CommonTailLen, TrialBBI1, TrialBBI2,
744 SuccBB, PredBB,
745 EHScopeMembership,
746 AfterBlockPlacement)) {
747 if (CommonTailLen > maxCommonTailLength) {
748 SameTails.clear();
749 maxCommonTailLength = CommonTailLen;
750 HighestMPIter = CurMPIter;
751 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
752 }
753 if (HighestMPIter == CurMPIter &&
754 CommonTailLen == maxCommonTailLength)
755 SameTails.push_back(SameTailElt(I, TrialBBI2));
756 }
757 if (I == B)
758 break;
759 }
760 }
761 return maxCommonTailLength;
762}
763
764void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
765 MachineBasicBlock *SuccBB,
766 MachineBasicBlock *PredBB) {
767 MPIterator CurMPIter, B;
768 for (CurMPIter = std::prev(MergePotentials.end()),
769 B = MergePotentials.begin();
770 CurMPIter->getHash() == CurHash; --CurMPIter) {
771 // Put the unconditional branch back, if we need one.
772 MachineBasicBlock *CurMBB = CurMPIter->getBlock();
773 if (SuccBB && CurMBB != PredBB)
774 FixTail(CurMBB, SuccBB, TII);
775 if (CurMPIter == B)
776 break;
777 }
778 if (CurMPIter->getHash() != CurHash)
779 CurMPIter++;
780 MergePotentials.erase(CurMPIter, MergePotentials.end());
781}
782
783bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
784 MachineBasicBlock *SuccBB,
785 unsigned maxCommonTailLength,
786 unsigned &commonTailIndex) {
787 commonTailIndex = 0;
788 unsigned TimeEstimate = ~0U;
789 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
790 // Use PredBB if possible; that doesn't require a new branch.
791 if (SameTails[i].getBlock() == PredBB) {
792 commonTailIndex = i;
793 break;
794 }
795 // Otherwise, make a (fairly bogus) choice based on estimate of
796 // how long it will take the various blocks to execute.
797 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
798 SameTails[i].getTailStartPos());
799 if (t <= TimeEstimate) {
800 TimeEstimate = t;
801 commonTailIndex = i;
802 }
803 }
804
805 MachineBasicBlock::iterator BBI =
806 SameTails[commonTailIndex].getTailStartPos();
807 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
808
809 LLVM_DEBUG(dbgs() << "\nSplitting " << printMBBReference(*MBB) << ", size "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nSplitting " << printMBBReference
(*MBB) << ", size " << maxCommonTailLength; } } while
(false)
810 << maxCommonTailLength)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nSplitting " << printMBBReference
(*MBB) << ", size " << maxCommonTailLength; } } while
(false)
;
811
812 // If the split block unconditionally falls-thru to SuccBB, it will be
813 // merged. In control flow terms it should then take SuccBB's name. e.g. If
814 // SuccBB is an inner loop, the common tail is still part of the inner loop.
815 const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ?
816 SuccBB->getBasicBlock() : MBB->getBasicBlock();
817 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB);
818 if (!newMBB) {
819 LLVM_DEBUG(dbgs() << "... failed!")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "... failed!"; } } while
(false)
;
820 return false;
821 }
822
823 SameTails[commonTailIndex].setBlock(newMBB);
824 SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
825
826 // If we split PredBB, newMBB is the new predecessor.
827 if (PredBB == MBB)
828 PredBB = newMBB;
829
830 return true;
831}
832
833static void
834mergeOperations(MachineBasicBlock::iterator MBBIStartPos,
835 MachineBasicBlock &MBBCommon) {
836 MachineBasicBlock *MBB = MBBIStartPos->getParent();
837 // Note CommonTailLen does not necessarily matches the size of
838 // the common BB nor all its instructions because of debug
839 // instructions differences.
840 unsigned CommonTailLen = 0;
841 for (auto E = MBB->end(); MBBIStartPos != E; ++MBBIStartPos)
842 ++CommonTailLen;
843
844 MachineBasicBlock::reverse_iterator MBBI = MBB->rbegin();
845 MachineBasicBlock::reverse_iterator MBBIE = MBB->rend();
846 MachineBasicBlock::reverse_iterator MBBICommon = MBBCommon.rbegin();
847 MachineBasicBlock::reverse_iterator MBBIECommon = MBBCommon.rend();
848
849 while (CommonTailLen--) {
850 assert(MBBI != MBBIE && "Reached BB end within common tail length!")((MBBI != MBBIE && "Reached BB end within common tail length!"
) ? static_cast<void> (0) : __assert_fail ("MBBI != MBBIE && \"Reached BB end within common tail length!\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 850, __PRETTY_FUNCTION__))
;
851 (void)MBBIE;
852
853 if (!countsAsInstruction(*MBBI)) {
854 ++MBBI;
855 continue;
856 }
857
858 while ((MBBICommon != MBBIECommon) && !countsAsInstruction(*MBBICommon))
859 ++MBBICommon;
860
861 assert(MBBICommon != MBBIECommon &&((MBBICommon != MBBIECommon && "Reached BB end within common tail length!"
) ? static_cast<void> (0) : __assert_fail ("MBBICommon != MBBIECommon && \"Reached BB end within common tail length!\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 862, __PRETTY_FUNCTION__))
862 "Reached BB end within common tail length!")((MBBICommon != MBBIECommon && "Reached BB end within common tail length!"
) ? static_cast<void> (0) : __assert_fail ("MBBICommon != MBBIECommon && \"Reached BB end within common tail length!\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 862, __PRETTY_FUNCTION__))
;
863 assert(MBBICommon->isIdenticalTo(*MBBI) && "Expected matching MIIs!")((MBBICommon->isIdenticalTo(*MBBI) && "Expected matching MIIs!"
) ? static_cast<void> (0) : __assert_fail ("MBBICommon->isIdenticalTo(*MBBI) && \"Expected matching MIIs!\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 863, __PRETTY_FUNCTION__))
;
864
865 // Merge MMOs from memory operations in the common block.
866 if (MBBICommon->mayLoad() || MBBICommon->mayStore())
867 MBBICommon->cloneMergedMemRefs(*MBB->getParent(), {&*MBBICommon, &*MBBI});
868 // Drop undef flags if they aren't present in all merged instructions.
869 for (unsigned I = 0, E = MBBICommon->getNumOperands(); I != E; ++I) {
870 MachineOperand &MO = MBBICommon->getOperand(I);
871 if (MO.isReg() && MO.isUndef()) {
872 const MachineOperand &OtherMO = MBBI->getOperand(I);
873 if (!OtherMO.isUndef())
874 MO.setIsUndef(false);
875 }
876 }
877
878 ++MBBI;
879 ++MBBICommon;
880 }
881}
882
883void BranchFolder::mergeCommonTails(unsigned commonTailIndex) {
884 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
885
886 std::vector<MachineBasicBlock::iterator> NextCommonInsts(SameTails.size());
887 for (unsigned int i = 0 ; i != SameTails.size() ; ++i) {
888 if (i != commonTailIndex) {
889 NextCommonInsts[i] = SameTails[i].getTailStartPos();
890 mergeOperations(SameTails[i].getTailStartPos(), *MBB);
891 } else {
892 assert(SameTails[i].getTailStartPos() == MBB->begin() &&((SameTails[i].getTailStartPos() == MBB->begin() &&
"MBB is not a common tail only block") ? static_cast<void
> (0) : __assert_fail ("SameTails[i].getTailStartPos() == MBB->begin() && \"MBB is not a common tail only block\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 893, __PRETTY_FUNCTION__))
893 "MBB is not a common tail only block")((SameTails[i].getTailStartPos() == MBB->begin() &&
"MBB is not a common tail only block") ? static_cast<void
> (0) : __assert_fail ("SameTails[i].getTailStartPos() == MBB->begin() && \"MBB is not a common tail only block\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 893, __PRETTY_FUNCTION__))
;
894 }
895 }
896
897 for (auto &MI : *MBB) {
898 if (!countsAsInstruction(MI))
899 continue;
900 DebugLoc DL = MI.getDebugLoc();
901 for (unsigned int i = 0 ; i < NextCommonInsts.size() ; i++) {
902 if (i == commonTailIndex)
903 continue;
904
905 auto &Pos = NextCommonInsts[i];
906 assert(Pos != SameTails[i].getBlock()->end() &&((Pos != SameTails[i].getBlock()->end() && "Reached BB end within common tail"
) ? static_cast<void> (0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 907, __PRETTY_FUNCTION__))
907 "Reached BB end within common tail")((Pos != SameTails[i].getBlock()->end() && "Reached BB end within common tail"
) ? static_cast<void> (0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 907, __PRETTY_FUNCTION__))
;
908 while (!countsAsInstruction(*Pos)) {
909 ++Pos;
910 assert(Pos != SameTails[i].getBlock()->end() &&((Pos != SameTails[i].getBlock()->end() && "Reached BB end within common tail"
) ? static_cast<void> (0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 911, __PRETTY_FUNCTION__))
911 "Reached BB end within common tail")((Pos != SameTails[i].getBlock()->end() && "Reached BB end within common tail"
) ? static_cast<void> (0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 911, __PRETTY_FUNCTION__))
;
912 }
913 assert(MI.isIdenticalTo(*Pos) && "Expected matching MIIs!")((MI.isIdenticalTo(*Pos) && "Expected matching MIIs!"
) ? static_cast<void> (0) : __assert_fail ("MI.isIdenticalTo(*Pos) && \"Expected matching MIIs!\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 913, __PRETTY_FUNCTION__))
;
914 DL = DILocation::getMergedLocation(DL, Pos->getDebugLoc());
915 NextCommonInsts[i] = ++Pos;
916 }
917 MI.setDebugLoc(DL);
918 }
919
920 if (UpdateLiveIns) {
921 LivePhysRegs NewLiveIns(*TRI);
922 computeLiveIns(NewLiveIns, *MBB);
923 LiveRegs.init(*TRI);
924
925 // The flag merging may lead to some register uses no longer using the
926 // <undef> flag, add IMPLICIT_DEFs in the predecessors as necessary.
927 for (MachineBasicBlock *Pred : MBB->predecessors()) {
928 LiveRegs.clear();
929 LiveRegs.addLiveOuts(*Pred);
930 MachineBasicBlock::iterator InsertBefore = Pred->getFirstTerminator();
931 for (unsigned Reg : NewLiveIns) {
932 if (!LiveRegs.available(*MRI, Reg))
933 continue;
934 DebugLoc DL;
935 BuildMI(*Pred, InsertBefore, DL, TII->get(TargetOpcode::IMPLICIT_DEF),
936 Reg);
937 }
938 }
939
940 MBB->clearLiveIns();
941 addLiveIns(*MBB, NewLiveIns);
942 }
943}
944
945// See if any of the blocks in MergePotentials (which all have SuccBB as a
946// successor, or all have no successor if it is null) can be tail-merged.
947// If there is a successor, any blocks in MergePotentials that are not
948// tail-merged and are not immediately before Succ must have an unconditional
949// branch to Succ added (but the predecessor/successor lists need no
950// adjustment). The lone predecessor of Succ that falls through into Succ,
951// if any, is given in PredBB.
952// MinCommonTailLength - Except for the special cases below, tail-merge if
953// there are at least this many instructions in common.
954bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
955 MachineBasicBlock *PredBB,
956 unsigned MinCommonTailLength) {
957 bool MadeChange = false;
958
959 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)
960 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)
961 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)
962 << 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)
963 << (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)
964 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)
965 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)
966 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)
967 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)
968 << 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)
969 } 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)
970 << 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)
971 << (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)
;
972
973 // Sort by hash value so that blocks with identical end sequences sort
974 // together.
975 array_pod_sort(MergePotentials.begin(), MergePotentials.end());
976
977 // Walk through equivalence sets looking for actual exact matches.
978 while (MergePotentials.size() > 1) {
979 unsigned CurHash = MergePotentials.back().getHash();
980
981 // Build SameTails, identifying the set of blocks with this hash code
982 // and with the maximum number of instructions in common.
983 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
984 MinCommonTailLength,
985 SuccBB, PredBB);
986
987 // If we didn't find any pair that has at least MinCommonTailLength
988 // instructions in common, remove all blocks with this hash code and retry.
989 if (SameTails.empty()) {
990 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
991 continue;
992 }
993
994 // If one of the blocks is the entire common tail (and not the entry
995 // block, which we can't jump to), we can treat all blocks with this same
996 // tail at once. Use PredBB if that is one of the possibilities, as that
997 // will not introduce any extra branches.
998 MachineBasicBlock *EntryBB =
999 &MergePotentials.front().getBlock()->getParent()->front();
1000 unsigned commonTailIndex = SameTails.size();
1001 // If there are two blocks, check to see if one can be made to fall through
1002 // into the other.
1003 if (SameTails.size() == 2 &&
1004 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
1005 SameTails[1].tailIsWholeBlock())
1006 commonTailIndex = 1;
1007 else if (SameTails.size() == 2 &&
1008 SameTails[1].getBlock()->isLayoutSuccessor(
1009 SameTails[0].getBlock()) &&
1010 SameTails[0].tailIsWholeBlock())
1011 commonTailIndex = 0;
1012 else {
1013 // Otherwise just pick one, favoring the fall-through predecessor if
1014 // there is one.
1015 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
1016 MachineBasicBlock *MBB = SameTails[i].getBlock();
1017 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
1018 continue;
1019 if (MBB == PredBB) {
1020 commonTailIndex = i;
1021 break;
1022 }
1023 if (SameTails[i].tailIsWholeBlock())
1024 commonTailIndex = i;
1025 }
1026 }
1027
1028 if (commonTailIndex == SameTails.size() ||
1029 (SameTails[commonTailIndex].getBlock() == PredBB &&
1030 !SameTails[commonTailIndex].tailIsWholeBlock())) {
1031 // None of the blocks consist entirely of the common tail.
1032 // Split a block so that one does.
1033 if (!CreateCommonTailOnlyBlock(PredBB, SuccBB,
1034 maxCommonTailLength, commonTailIndex)) {
1035 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
1036 continue;
1037 }
1038 }
1039
1040 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
1041
1042 // Recompute common tail MBB's edge weights and block frequency.
1043 setCommonTailEdgeWeights(*MBB);
1044
1045 // Merge debug locations, MMOs and undef flags across identical instructions
1046 // for common tail.
1047 mergeCommonTails(commonTailIndex);
1048
1049 // MBB is common tail. Adjust all other BB's to jump to this one.
1050 // Traversal must be forwards so erases work.
1051 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)
1052 << " for ")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nUsing common tail in "
<< printMBBReference(*MBB) << " for "; } } while
(false)
;
1053 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
1054 if (commonTailIndex == i)
1055 continue;
1056 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)
1057 << (i == e - 1 ? "" : ", "))do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << printMBBReference(*SameTails
[i].getBlock()) << (i == e - 1 ? "" : ", "); } } while (
false)
;
1058 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
1059 replaceTailWithBranchTo(SameTails[i].getTailStartPos(), *MBB);
1060 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
1061 MergePotentials.erase(SameTails[i].getMPIter());
1062 }
1063 LLVM_DEBUG(dbgs() << "\n")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\n"; } } while (false)
;
1064 // We leave commonTailIndex in the worklist in case there are other blocks
1065 // that match it with a smaller number of instructions.
1066 MadeChange = true;
1067 }
1068 return MadeChange;
1069}
1070
1071bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
1072 bool MadeChange = false;
1073 if (!EnableTailMerge) return MadeChange;
1074
1075 // First find blocks with no successors.
1076 // Block placement does not create new tail merging opportunities for these
1077 // blocks.
1078 if (!AfterBlockPlacement) {
1079 MergePotentials.clear();
1080 for (MachineBasicBlock &MBB : MF) {
1081 if (MergePotentials.size() == TailMergeThreshold)
1082 break;
1083 if (!TriedMerging.count(&MBB) && MBB.succ_empty())
1084 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(MBB), &MBB));
1085 }
1086
1087 // If this is a large problem, avoid visiting the same basic blocks
1088 // multiple times.
1089 if (MergePotentials.size() == TailMergeThreshold)
1090 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
1091 TriedMerging.insert(MergePotentials[i].getBlock());
1092
1093 // See if we can do any tail merging on those.
1094 if (MergePotentials.size() >= 2)
1095 MadeChange |= TryTailMergeBlocks(nullptr, nullptr, MinCommonTailLength);
1096 }
1097
1098 // Look at blocks (IBB) with multiple predecessors (PBB).
1099 // We change each predecessor to a canonical form, by
1100 // (1) temporarily removing any unconditional branch from the predecessor
1101 // to IBB, and
1102 // (2) alter conditional branches so they branch to the other block
1103 // not IBB; this may require adding back an unconditional branch to IBB
1104 // later, where there wasn't one coming in. E.g.
1105 // Bcc IBB
1106 // fallthrough to QBB
1107 // here becomes
1108 // Bncc QBB
1109 // with a conceptual B to IBB after that, which never actually exists.
1110 // With those changes, we see whether the predecessors' tails match,
1111 // and merge them if so. We change things out of canonical form and
1112 // back to the way they were later in the process. (OptimizeBranches
1113 // would undo some of this, but we can't use it, because we'd get into
1114 // a compile-time infinite loop repeatedly doing and undoing the same
1115 // transformations.)
1116
1117 for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
1118 I != E; ++I) {
1119 if (I->pred_size() < 2) continue;
1120 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
1121 MachineBasicBlock *IBB = &*I;
1122 MachineBasicBlock *PredBB = &*std::prev(I);
1123 MergePotentials.clear();
1124 MachineLoop *ML;
1125
1126 // Bail if merging after placement and IBB is the loop header because
1127 // -- If merging predecessors that belong to the same loop as IBB, the
1128 // common tail of merged predecessors may become the loop top if block
1129 // placement is called again and the predecessors may branch to this common
1130 // tail and require more branches. This can be relaxed if
1131 // MachineBlockPlacement::findBestLoopTop is more flexible.
1132 // --If merging predecessors that do not belong to the same loop as IBB, the
1133 // loop info of IBB's loop and the other loops may be affected. Calling the
1134 // block placement again may make big change to the layout and eliminate the
1135 // reason to do tail merging here.
1136 if (AfterBlockPlacement && MLI) {
1137 ML = MLI->getLoopFor(IBB);
1138 if (ML && IBB == ML->getHeader())
1139 continue;
1140 }
1141
1142 for (MachineBasicBlock *PBB : I->predecessors()) {
1143 if (MergePotentials.size() == TailMergeThreshold)
1144 break;
1145
1146 if (TriedMerging.count(PBB))
1147 continue;
1148
1149 // Skip blocks that loop to themselves, can't tail merge these.
1150 if (PBB == IBB)
1151 continue;
1152
1153 // Visit each predecessor only once.
1154 if (!UniquePreds.insert(PBB).second)
1155 continue;
1156
1157 // Skip blocks which may jump to a landing pad. Can't tail merge these.
1158 if (PBB->hasEHPadSuccessor())
1159 continue;
1160
1161 // After block placement, only consider predecessors that belong to the
1162 // same loop as IBB. The reason is the same as above when skipping loop
1163 // header.
1164 if (AfterBlockPlacement && MLI)
1165 if (ML != MLI->getLoopFor(PBB))
1166 continue;
1167
1168 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
1169 SmallVector<MachineOperand, 4> Cond;
1170 if (!TII->analyzeBranch(*PBB, TBB, FBB, Cond, true)) {
1171 // Failing case: IBB is the target of a cbr, and we cannot reverse the
1172 // branch.
1173 SmallVector<MachineOperand, 4> NewCond(Cond);
1174 if (!Cond.empty() && TBB == IBB) {
1175 if (TII->reverseBranchCondition(NewCond))
1176 continue;
1177 // This is the QBB case described above
1178 if (!FBB) {
1179 auto Next = ++PBB->getIterator();
1180 if (Next != MF.end())
1181 FBB = &*Next;
1182 }
1183 }
1184
1185 // Remove the unconditional branch at the end, if any.
1186 if (TBB && (Cond.empty() || FBB)) {
1187 DebugLoc dl = PBB->findBranchDebugLoc();
1188 TII->removeBranch(*PBB);
1189 if (!Cond.empty())
1190 // reinsert conditional branch only, for now
1191 TII->insertBranch(*PBB, (TBB == IBB) ? FBB : TBB, nullptr,
1192 NewCond, dl);
1193 }
1194
1195 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(*PBB), PBB));
1196 }
1197 }
1198
1199 // If this is a large problem, avoid visiting the same basic blocks multiple
1200 // times.
1201 if (MergePotentials.size() == TailMergeThreshold)
1202 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
1203 TriedMerging.insert(MergePotentials[i].getBlock());
1204
1205 if (MergePotentials.size() >= 2)
1206 MadeChange |= TryTailMergeBlocks(IBB, PredBB, MinCommonTailLength);
1207
1208 // Reinsert an unconditional branch if needed. The 1 below can occur as a
1209 // result of removing blocks in TryTailMergeBlocks.
1210 PredBB = &*std::prev(I); // this may have been changed in TryTailMergeBlocks
1211 if (MergePotentials.size() == 1 &&
1212 MergePotentials.begin()->getBlock() != PredBB)
1213 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
1214 }
1215
1216 return MadeChange;
1217}
1218
1219void BranchFolder::setCommonTailEdgeWeights(MachineBasicBlock &TailMBB) {
1220 SmallVector<BlockFrequency, 2> EdgeFreqLs(TailMBB.succ_size());
1221 BlockFrequency AccumulatedMBBFreq;
1222
1223 // Aggregate edge frequency of successor edge j:
1224 // edgeFreq(j) = sum (freq(bb) * edgeProb(bb, j)),
1225 // where bb is a basic block that is in SameTails.
1226 for (const auto &Src : SameTails) {
1227 const MachineBasicBlock *SrcMBB = Src.getBlock();
1228 BlockFrequency BlockFreq = MBBFreqInfo.getBlockFreq(SrcMBB);
1229 AccumulatedMBBFreq += BlockFreq;
1230
1231 // It is not necessary to recompute edge weights if TailBB has less than two
1232 // successors.
1233 if (TailMBB.succ_size() <= 1)
1234 continue;
1235
1236 auto EdgeFreq = EdgeFreqLs.begin();
1237
1238 for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
1239 SuccI != SuccE; ++SuccI, ++EdgeFreq)
1240 *EdgeFreq += BlockFreq * MBPI.getEdgeProbability(SrcMBB, *SuccI);
1241 }
1242
1243 MBBFreqInfo.setBlockFreq(&TailMBB, AccumulatedMBBFreq);
1244
1245 if (TailMBB.succ_size() <= 1)
1246 return;
1247
1248 auto SumEdgeFreq =
1249 std::accumulate(EdgeFreqLs.begin(), EdgeFreqLs.end(), BlockFrequency(0))
1250 .getFrequency();
1251 auto EdgeFreq = EdgeFreqLs.begin();
1252
1253 if (SumEdgeFreq > 0) {
1254 for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
1255 SuccI != SuccE; ++SuccI, ++EdgeFreq) {
1256 auto Prob = BranchProbability::getBranchProbability(
1257 EdgeFreq->getFrequency(), SumEdgeFreq);
1258 TailMBB.setSuccProbability(SuccI, Prob);
1259 }
1260 }
1261}
1262
1263//===----------------------------------------------------------------------===//
1264// Branch Optimization
1265//===----------------------------------------------------------------------===//
1266
1267bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
1268 bool MadeChange = false;
1269
1270 // Make sure blocks are numbered in order
1271 MF.RenumberBlocks();
1272 // Renumbering blocks alters EH scope membership, recalculate it.
1273 EHScopeMembership = getEHScopeMembership(MF);
1274
1275 for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
1276 I != E; ) {
1277 MachineBasicBlock *MBB = &*I++;
1278 MadeChange |= OptimizeBlock(MBB);
1279
1280 // If it is dead, remove it.
1281 if (MBB->pred_empty()) {
1282 RemoveDeadBlock(MBB);
1283 MadeChange = true;
1284 ++NumDeadBlocks;
1285 }
1286 }
1287
1288 return MadeChange;
1289}
1290
1291// Blocks should be considered empty if they contain only debug info;
1292// else the debug info would affect codegen.
1293static bool IsEmptyBlock(MachineBasicBlock *MBB) {
1294 return MBB->getFirstNonDebugInstr() == MBB->end();
1295}
1296
1297// Blocks with only debug info and branches should be considered the same
1298// as blocks with only branches.
1299static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
1300 MachineBasicBlock::iterator I = MBB->getFirstNonDebugInstr();
1301 assert(I != MBB->end() && "empty block!")((I != MBB->end() && "empty block!") ? static_cast
<void> (0) : __assert_fail ("I != MBB->end() && \"empty block!\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 1301, __PRETTY_FUNCTION__))
;
1302 return I->isBranch();
1303}
1304
1305/// IsBetterFallthrough - Return true if it would be clearly better to
1306/// fall-through to MBB1 than to fall through into MBB2. This has to return
1307/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
1308/// result in infinite loops.
1309static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
1310 MachineBasicBlock *MBB2) {
1311 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
1312 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
1313 // optimize branches that branch to either a return block or an assert block
1314 // into a fallthrough to the return.
1315 MachineBasicBlock::iterator MBB1I = MBB1->getLastNonDebugInstr();
16
Called C++ object pointer is null
1316 MachineBasicBlock::iterator MBB2I = MBB2->getLastNonDebugInstr();
1317 if (MBB1I == MBB1->end() || MBB2I == MBB2->end())
1318 return false;
1319
1320 // If there is a clear successor ordering we make sure that one block
1321 // will fall through to the next
1322 if (MBB1->isSuccessor(MBB2)) return true;
1323 if (MBB2->isSuccessor(MBB1)) return false;
1324
1325 return MBB2I->isCall() && !MBB1I->isCall();
1326}
1327
1328/// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch
1329/// instructions on the block.
1330static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) {
1331 MachineBasicBlock::iterator I = MBB.getLastNonDebugInstr();
1332 if (I != MBB.end() && I->isBranch())
1333 return I->getDebugLoc();
1334 return DebugLoc();
1335}
1336
1337static void copyDebugInfoToPredecessor(const TargetInstrInfo *TII,
1338 MachineBasicBlock &MBB,
1339 MachineBasicBlock &PredMBB) {
1340 auto InsertBefore = PredMBB.getFirstTerminator();
1341 for (MachineInstr &MI : MBB.instrs())
1342 if (MI.isDebugInstr()) {
1343 TII->duplicate(PredMBB, InsertBefore, MI);
1344 LLVM_DEBUG(dbgs() << "Copied debug entity from empty block to pred: "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Copied debug entity from empty block to pred: "
<< MI; } } while (false)
1345 << MI)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Copied debug entity from empty block to pred: "
<< MI; } } while (false)
;
1346 }
1347}
1348
1349static void copyDebugInfoToSuccessor(const TargetInstrInfo *TII,
1350 MachineBasicBlock &MBB,
1351 MachineBasicBlock &SuccMBB) {
1352 auto InsertBefore = SuccMBB.SkipPHIsAndLabels(SuccMBB.begin());
1353 for (MachineInstr &MI : MBB.instrs())
1354 if (MI.isDebugInstr()) {
1355 TII->duplicate(SuccMBB, InsertBefore, MI);
1356 LLVM_DEBUG(dbgs() << "Copied debug entity from empty block to succ: "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Copied debug entity from empty block to succ: "
<< MI; } } while (false)
1357 << MI)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Copied debug entity from empty block to succ: "
<< MI; } } while (false)
;
1358 }
1359}
1360
1361// Try to salvage DBG_VALUE instructions from an otherwise empty block. If such
1362// a basic block is removed we would lose the debug information unless we have
1363// copied the information to a predecessor/successor.
1364//
1365// TODO: This function only handles some simple cases. An alternative would be
1366// to run a heavier analysis, such as the LiveDebugValues pass, before we do
1367// branch folding.
1368static void salvageDebugInfoFromEmptyBlock(const TargetInstrInfo *TII,
1369 MachineBasicBlock &MBB) {
1370 assert(IsEmptyBlock(&MBB) && "Expected an empty block (except debug info).")((IsEmptyBlock(&MBB) && "Expected an empty block (except debug info)."
) ? static_cast<void> (0) : __assert_fail ("IsEmptyBlock(&MBB) && \"Expected an empty block (except debug info).\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 1370, __PRETTY_FUNCTION__))
;
1371 // If this MBB is the only predecessor of a successor it is legal to copy
1372 // DBG_VALUE instructions to the beginning of the successor.
1373 for (MachineBasicBlock *SuccBB : MBB.successors())
1374 if (SuccBB->pred_size() == 1)
1375 copyDebugInfoToSuccessor(TII, MBB, *SuccBB);
1376 // If this MBB is the only successor of a predecessor it is legal to copy the
1377 // DBG_VALUE instructions to the end of the predecessor (just before the
1378 // terminators, assuming that the terminator isn't affecting the DBG_VALUE).
1379 for (MachineBasicBlock *PredBB : MBB.predecessors())
1380 if (PredBB->succ_size() == 1)
1381 copyDebugInfoToPredecessor(TII, MBB, *PredBB);
1382}
1383
1384bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
1385 bool MadeChange = false;
1386 MachineFunction &MF = *MBB->getParent();
1387ReoptimizeBlock:
1388
1389 MachineFunction::iterator FallThrough = MBB->getIterator();
1390 ++FallThrough;
1391
1392 // Make sure MBB and FallThrough belong to the same EH scope.
1393 bool SameEHScope = true;
1394 if (!EHScopeMembership.empty() && FallThrough != MF.end()) {
1
Assuming the condition is false
2
Taking false branch
1395 auto MBBEHScope = EHScopeMembership.find(MBB);
1396 assert(MBBEHScope != EHScopeMembership.end())((MBBEHScope != EHScopeMembership.end()) ? static_cast<void
> (0) : __assert_fail ("MBBEHScope != EHScopeMembership.end()"
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 1396, __PRETTY_FUNCTION__))
;
1397 auto FallThroughEHScope = EHScopeMembership.find(&*FallThrough);
1398 assert(FallThroughEHScope != EHScopeMembership.end())((FallThroughEHScope != EHScopeMembership.end()) ? static_cast
<void> (0) : __assert_fail ("FallThroughEHScope != EHScopeMembership.end()"
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 1398, __PRETTY_FUNCTION__))
;
1399 SameEHScope = MBBEHScope->second == FallThroughEHScope->second;
1400 }
1401
1402 // If this block is empty, make everyone use its fall-through, not the block
1403 // explicitly. Landing pads should not do this since the landing-pad table
1404 // points to this block. Blocks with their addresses taken shouldn't be
1405 // optimized away.
1406 if (IsEmptyBlock(MBB) && !MBB->isEHPad() && !MBB->hasAddressTaken() &&
1407 SameEHScope) {
1408 salvageDebugInfoFromEmptyBlock(TII, *MBB);
1409 // Dead block? Leave for cleanup later.
1410 if (MBB->pred_empty()) return MadeChange;
1411
1412 if (FallThrough == MF.end()) {
1413 // TODO: Simplify preds to not branch here if possible!
1414 } else if (FallThrough->isEHPad()) {
1415 // Don't rewrite to a landing pad fallthough. That could lead to the case
1416 // where a BB jumps to more than one landing pad.
1417 // TODO: Is it ever worth rewriting predecessors which don't already
1418 // jump to a landing pad, and so can safely jump to the fallthrough?
1419 } else if (MBB->isSuccessor(&*FallThrough)) {
1420 // Rewrite all predecessors of the old block to go to the fallthrough
1421 // instead.
1422 while (!MBB->pred_empty()) {
1423 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1424 Pred->ReplaceUsesOfBlockWith(MBB, &*FallThrough);
1425 }
1426 // If MBB was the target of a jump table, update jump tables to go to the
1427 // fallthrough instead.
1428 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1429 MJTI->ReplaceMBBInJumpTables(MBB, &*FallThrough);
1430 MadeChange = true;
1431 }
1432 return MadeChange;
1433 }
1434
1435 // Check to see if we can simplify the terminator of the block before this
1436 // one.
1437 MachineBasicBlock &PrevBB = *std::prev(MachineFunction::iterator(MBB));
1438
1439 MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
1440 SmallVector<MachineOperand, 4> PriorCond;
1441 bool PriorUnAnalyzable =
1442 TII->analyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
3
Value assigned to 'PriorTBB'
1443 if (!PriorUnAnalyzable) {
4
Assuming 'PriorUnAnalyzable' is 0
5
Taking true branch
1444 // If the CFG for the prior block has extra edges, remove them.
1445 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1446 !PriorCond.empty());
1447
1448 // If the previous branch is conditional and both conditions go to the same
1449 // destination, remove the branch, replacing it with an unconditional one or
1450 // a fall-through.
1451 if (PriorTBB && PriorTBB == PriorFBB) {
6
Assuming 'PriorTBB' is null
1452 DebugLoc dl = getBranchDebugLoc(PrevBB);
1453 TII->removeBranch(PrevBB);
1454 PriorCond.clear();
1455 if (PriorTBB != MBB)
1456 TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1457 MadeChange = true;
1458 ++NumBranchOpts;
1459 goto ReoptimizeBlock;
1460 }
1461
1462 // If the previous block unconditionally falls through to this block and
1463 // this block has no other predecessors, move the contents of this block
1464 // into the prior block. This doesn't usually happen when SimplifyCFG
1465 // has been used, but it can happen if tail merging splits a fall-through
1466 // predecessor of a block.
1467 // This has to check PrevBB->succ_size() because EH edges are ignored by
1468 // AnalyzeBranch.
1469 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1470 PrevBB.succ_size() == 1 &&
1471 !MBB->hasAddressTaken() && !MBB->isEHPad()) {
1472 LLVM_DEBUG(dbgs() << "\nMerging into block: " << PrevBBdo { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nMerging into block: "
<< PrevBB << "From MBB: " << *MBB; } } while
(false)
1473 << "From MBB: " << *MBB)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nMerging into block: "
<< PrevBB << "From MBB: " << *MBB; } } while
(false)
;
1474 // Remove redundant DBG_VALUEs first.
1475 if (PrevBB.begin() != PrevBB.end()) {
1476 MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
1477 --PrevBBIter;
1478 MachineBasicBlock::iterator MBBIter = MBB->begin();
1479 // Check if DBG_VALUE at the end of PrevBB is identical to the
1480 // DBG_VALUE at the beginning of MBB.
1481 while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
1482 && PrevBBIter->isDebugInstr() && MBBIter->isDebugInstr()) {
1483 if (!MBBIter->isIdenticalTo(*PrevBBIter))
1484 break;
1485 MachineInstr &DuplicateDbg = *MBBIter;
1486 ++MBBIter; -- PrevBBIter;
1487 DuplicateDbg.eraseFromParent();
1488 }
1489 }
1490 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1491 PrevBB.removeSuccessor(PrevBB.succ_begin());
1492 assert(PrevBB.succ_empty())((PrevBB.succ_empty()) ? static_cast<void> (0) : __assert_fail
("PrevBB.succ_empty()", "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 1492, __PRETTY_FUNCTION__))
;
1493 PrevBB.transferSuccessors(MBB);
1494 MadeChange = true;
1495 return MadeChange;
1496 }
1497
1498 // If the previous branch *only* branches to *this* block (conditional or
1499 // not) remove the branch.
1500 if (PriorTBB == MBB && !PriorFBB) {
1501 TII->removeBranch(PrevBB);
1502 MadeChange = true;
1503 ++NumBranchOpts;
1504 goto ReoptimizeBlock;
1505 }
1506
1507 // If the prior block branches somewhere else on the condition and here if
1508 // the condition is false, remove the uncond second branch.
1509 if (PriorFBB == MBB) {
7
Assuming 'PriorFBB' is not equal to 'MBB'
8
Taking false branch
1510 DebugLoc dl = getBranchDebugLoc(PrevBB);
1511 TII->removeBranch(PrevBB);
1512 TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1513 MadeChange = true;
1514 ++NumBranchOpts;
1515 goto ReoptimizeBlock;
1516 }
1517
1518 // If the prior block branches here on true and somewhere else on false, and
1519 // if the branch condition is reversible, reverse the branch to create a
1520 // fall-through.
1521 if (PriorTBB == MBB) {
9
Taking false branch
1522 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1523 if (!TII->reverseBranchCondition(NewPriorCond)) {
1524 DebugLoc dl = getBranchDebugLoc(PrevBB);
1525 TII->removeBranch(PrevBB);
1526 TII->insertBranch(PrevBB, PriorFBB, nullptr, NewPriorCond, dl);
1527 MadeChange = true;
1528 ++NumBranchOpts;
1529 goto ReoptimizeBlock;
1530 }
1531 }
1532
1533 // If this block has no successors (e.g. it is a return block or ends with
1534 // a call to a no-return function like abort or __cxa_throw) and if the pred
1535 // falls through into this block, and if it would otherwise fall through
1536 // into the block after this, move this block to the end of the function.
1537 //
1538 // We consider it more likely that execution will stay in the function (e.g.
1539 // due to loops) than it is to exit it. This asserts in loops etc, moving
1540 // the assert condition out of the loop body.
1541 if (MBB->succ_empty() && !PriorCond.empty() && !PriorFBB &&
10
Assuming the condition is true
11
Assuming 'PriorFBB' is null
13
Taking true branch
1542 MachineFunction::iterator(PriorTBB) == FallThrough &&
1543 !MBB->canFallThrough()) {
12
Assuming the condition is true
1544 bool DoTransform = true;
1545
1546 // We have to be careful that the succs of PredBB aren't both no-successor
1547 // blocks. If neither have successors and if PredBB is the second from
1548 // last block in the function, we'd just keep swapping the two blocks for
1549 // last. Only do the swap if one is clearly better to fall through than
1550 // the other.
1551 if (FallThrough == --MF.end() &&
1552 !IsBetterFallthrough(PriorTBB, MBB))
14
Passing null pointer value via 1st parameter 'MBB1'
15
Calling 'IsBetterFallthrough'
1553 DoTransform = false;
1554
1555 if (DoTransform) {
1556 // Reverse the branch so we will fall through on the previous true cond.
1557 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1558 if (!TII->reverseBranchCondition(NewPriorCond)) {
1559 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)
1560 << "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)
;
1561
1562 DebugLoc dl = getBranchDebugLoc(PrevBB);
1563 TII->removeBranch(PrevBB);
1564 TII->insertBranch(PrevBB, MBB, nullptr, NewPriorCond, dl);
1565
1566 // Move this block to the end of the function.
1567 MBB->moveAfter(&MF.back());
1568 MadeChange = true;
1569 ++NumBranchOpts;
1570 return MadeChange;
1571 }
1572 }
1573 }
1574 }
1575
1576 if (!IsEmptyBlock(MBB) && MBB->pred_size() == 1 &&
1577 MF.getFunction().hasOptSize()) {
1578 // Changing "Jcc foo; foo: jmp bar;" into "Jcc bar;" might change the branch
1579 // direction, thereby defeating careful block placement and regressing
1580 // performance. Therefore, only consider this for optsize functions.
1581 MachineInstr &TailCall = *MBB->getFirstNonDebugInstr();
1582 if (TII->isUnconditionalTailCall(TailCall)) {
1583 MachineBasicBlock *Pred = *MBB->pred_begin();
1584 MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
1585 SmallVector<MachineOperand, 4> PredCond;
1586 bool PredAnalyzable =
1587 !TII->analyzeBranch(*Pred, PredTBB, PredFBB, PredCond, true);
1588
1589 if (PredAnalyzable && !PredCond.empty() && PredTBB == MBB &&
1590 PredTBB != PredFBB) {
1591 // The predecessor has a conditional branch to this block which consists
1592 // of only a tail call. Try to fold the tail call into the conditional
1593 // branch.
1594 if (TII->canMakeTailCallConditional(PredCond, TailCall)) {
1595 // TODO: It would be nice if analyzeBranch() could provide a pointer
1596 // to the branch instruction so replaceBranchWithTailCall() doesn't
1597 // have to search for it.
1598 TII->replaceBranchWithTailCall(*Pred, PredCond, TailCall);
1599 ++NumTailCalls;
1600 Pred->removeSuccessor(MBB);
1601 MadeChange = true;
1602 return MadeChange;
1603 }
1604 }
1605 // If the predecessor is falling through to this block, we could reverse
1606 // the branch condition and fold the tail call into that. However, after
1607 // that we might have to re-arrange the CFG to fall through to the other
1608 // block and there is a high risk of regressing code size rather than
1609 // improving it.
1610 }
1611 }
1612
1613 // Analyze the branch in the current block.
1614 MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr;
1615 SmallVector<MachineOperand, 4> CurCond;
1616 bool CurUnAnalyzable =
1617 TII->analyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1618 if (!CurUnAnalyzable) {
1619 // If the CFG for the prior block has extra edges, remove them.
1620 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1621
1622 // If this is a two-way branch, and the FBB branches to this block, reverse
1623 // the condition so the single-basic-block loop is faster. Instead of:
1624 // Loop: xxx; jcc Out; jmp Loop
1625 // we want:
1626 // Loop: xxx; jncc Loop; jmp Out
1627 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1628 SmallVector<MachineOperand, 4> NewCond(CurCond);
1629 if (!TII->reverseBranchCondition(NewCond)) {
1630 DebugLoc dl = getBranchDebugLoc(*MBB);
1631 TII->removeBranch(*MBB);
1632 TII->insertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
1633 MadeChange = true;
1634 ++NumBranchOpts;
1635 goto ReoptimizeBlock;
1636 }
1637 }
1638
1639 // If this branch is the only thing in its block, see if we can forward
1640 // other blocks across it.
1641 if (CurTBB && CurCond.empty() && !CurFBB &&
1642 IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
1643 !MBB->hasAddressTaken() && !MBB->isEHPad()) {
1644 DebugLoc dl = getBranchDebugLoc(*MBB);
1645 // This block may contain just an unconditional branch. Because there can
1646 // be 'non-branch terminators' in the block, try removing the branch and
1647 // then seeing if the block is empty.
1648 TII->removeBranch(*MBB);
1649 // If the only things remaining in the block are debug info, remove these
1650 // as well, so this will behave the same as an empty block in non-debug
1651 // mode.
1652 if (IsEmptyBlock(MBB)) {
1653 // Make the block empty, losing the debug info (we could probably
1654 // improve this in some cases.)
1655 MBB->erase(MBB->begin(), MBB->end());
1656 }
1657 // If this block is just an unconditional branch to CurTBB, we can
1658 // usually completely eliminate the block. The only case we cannot
1659 // completely eliminate the block is when the block before this one
1660 // falls through into MBB and we can't understand the prior block's branch
1661 // condition.
1662 if (MBB->empty()) {
1663 bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1664 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1665 !PrevBB.isSuccessor(MBB)) {
1666 // If the prior block falls through into us, turn it into an
1667 // explicit branch to us to make updates simpler.
1668 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1669 PriorTBB != MBB && PriorFBB != MBB) {
1670 if (!PriorTBB) {
1671 assert(PriorCond.empty() && !PriorFBB &&((PriorCond.empty() && !PriorFBB && "Bad branch analysis"
) ? static_cast<void> (0) : __assert_fail ("PriorCond.empty() && !PriorFBB && \"Bad branch analysis\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 1672, __PRETTY_FUNCTION__))
1672 "Bad branch analysis")((PriorCond.empty() && !PriorFBB && "Bad branch analysis"
) ? static_cast<void> (0) : __assert_fail ("PriorCond.empty() && !PriorFBB && \"Bad branch analysis\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 1672, __PRETTY_FUNCTION__))
;
1673 PriorTBB = MBB;
1674 } else {
1675 assert(!PriorFBB && "Machine CFG out of date!")((!PriorFBB && "Machine CFG out of date!") ? static_cast
<void> (0) : __assert_fail ("!PriorFBB && \"Machine CFG out of date!\""
, "/build/llvm-toolchain-snapshot-9~svn361465/lib/CodeGen/BranchFolding.cpp"
, 1675, __PRETTY_FUNCTION__))
;
1676 PriorFBB = MBB;
1677 }
1678 DebugLoc pdl = getBranchDebugLoc(PrevBB);
1679 TII->removeBranch(PrevBB);
1680 TII->insertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl);
1681 }
1682
1683 // Iterate through all the predecessors, revectoring each in-turn.
1684 size_t PI = 0;
1685 bool DidChange = false;
1686 bool HasBranchToSelf = false;
1687 while(PI != MBB->pred_size()) {
1688 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1689 if (PMBB == MBB) {
1690 // If this block has an uncond branch to itself, leave it.
1691 ++PI;
1692 HasBranchToSelf = true;
1693 } else {
1694 DidChange = true;
1695 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1696 // If this change resulted in PMBB ending in a conditional
1697 // branch where both conditions go to the same destination,
1698 // change this to an unconditional branch (and fix the CFG).
1699 MachineBasicBlock *NewCurTBB = nullptr, *NewCurFBB = nullptr;
1700 SmallVector<MachineOperand, 4> NewCurCond;
1701 bool NewCurUnAnalyzable = TII->analyzeBranch(
1702 *PMBB, NewCurTBB, NewCurFBB, NewCurCond, true);
1703 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1704 DebugLoc pdl = getBranchDebugLoc(*PMBB);
1705 TII->removeBranch(*PMBB);
1706 NewCurCond.clear();
1707 TII->insertBranch(*PMBB, NewCurTBB, nullptr, NewCurCond, pdl);
1708 MadeChange = true;
1709 ++NumBranchOpts;
1710 PMBB->CorrectExtraCFGEdges(NewCurTBB, nullptr, false);
1711 }
1712 }
1713 }
1714
1715 // Change any jumptables to go to the new MBB.
1716 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1717 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1718 if (DidChange) {
1719 ++NumBranchOpts;
1720 MadeChange = true;
1721 if (!HasBranchToSelf) return MadeChange;
1722 }
1723 }
1724 }
1725
1726 // Add the branch back if the block is more than just an uncond branch.
1727 TII->insertBranch(*MBB, CurTBB, nullptr, CurCond, dl);
1728 }
1729 }
1730
1731 // If the prior block doesn't fall through into this block, and if this
1732 // block doesn't fall through into some other block, see if we can find a
1733 // place to move this block where a fall-through will happen.
1734 if (!PrevBB.canFallThrough()) {
1735 // Now we know that there was no fall-through into this block, check to
1736 // see if it has a fall-through into its successor.
1737 bool CurFallsThru = MBB->canFallThrough();
1738
1739 if (!MBB->isEHPad()) {
1740 // Check all the predecessors of this block. If one of them has no fall
1741 // throughs, move this block right after it.
1742 for (MachineBasicBlock *PredBB : MBB->predecessors()) {
1743 // Analyze the branch at the end of the pred.
1744 MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
1745 SmallVector<MachineOperand, 4> PredCond;
1746 if (PredBB != MBB && !PredBB->canFallThrough() &&
1747 !TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) &&
1748 (!CurFallsThru || !CurTBB || !CurFBB) &&
1749 (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1750 // If the current block doesn't fall through, just move it.
1751 // If the current block can fall through and does not end with a
1752 // conditional branch, we need to append an unconditional jump to
1753 // the (current) next block. To avoid a possible compile-time
1754 // infinite loop, move blocks only backward in this case.
1755 // Also, if there are already 2 branches here, we cannot add a third;
1756 // this means we have the case
1757 // Bcc next
1758 // B elsewhere
1759 // next:
1760 if (CurFallsThru) {
1761 MachineBasicBlock *NextBB = &*std::next(MBB->getIterator());
1762 CurCond.clear();
1763 TII->insertBranch(*MBB, NextBB, nullptr, CurCond, DebugLoc());
1764 }
1765 MBB->moveAfter(PredBB);
1766 MadeChange = true;
1767 goto ReoptimizeBlock;
1768 }
1769 }
1770 }
1771
1772 if (!CurFallsThru) {
1773 // Check all successors to see if we can move this block before it.
1774 for (MachineBasicBlock *SuccBB : MBB->successors()) {
1775 // Analyze the branch at the end of the block before the succ.
1776 MachineFunction::iterator SuccPrev = --SuccBB->getIterator();
1777
1778 // If this block doesn't already fall-through to that successor, and if
1779 // the succ doesn't already have a block that can fall through into it,
1780 // and if the successor isn't an EH destination, we can arrange for the
1781 // fallthrough to happen.
1782 if (SuccBB != MBB && &*SuccPrev != MBB &&
1783 !SuccPrev->canFallThrough() && !CurUnAnalyzable &&
1784 !SuccBB->isEHPad()) {
1785 MBB->moveBefore(SuccBB);
1786 MadeChange = true;
1787 goto ReoptimizeBlock;
1788 }
1789 }
1790
1791 // Okay, there is no really great place to put this block. If, however,
1792 // the block before this one would be a fall-through if this block were
1793 // removed, move this block to the end of the function. There is no real
1794 // advantage in "falling through" to an EH block, so we don't want to
1795 // perform this transformation for that case.
1796 //
1797 // Also, Windows EH introduced the possibility of an arbitrary number of
1798 // successors to a given block. The analyzeBranch call does not consider
1799 // exception handling and so we can get in a state where a block
1800 // containing a call is followed by multiple EH blocks that would be
1801 // rotated infinitely at the end of the function if the transformation
1802 // below were performed for EH "FallThrough" blocks. Therefore, even if
1803 // that appears not to be happening anymore, we should assume that it is
1804 // possible and not remove the "!FallThrough()->isEHPad" condition below.
1805 MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr;
1806 SmallVector<MachineOperand, 4> PrevCond;
1807 if (FallThrough != MF.end() &&
1808 !FallThrough->isEHPad() &&
1809 !TII->analyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1810 PrevBB.isSuccessor(&*FallThrough)) {
1811 MBB->moveAfter(&MF.back());
1812 MadeChange = true;
1813 return MadeChange;
1814 }
1815 }
1816 }
1817
1818 return MadeChange;
1819}
1820
1821//===----------------------------------------------------------------------===//
1822// Hoist Common Code
1823//===----------------------------------------------------------------------===//
1824
1825bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
1826 bool MadeChange = false;
1827 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
1828 MachineBasicBlock *MBB = &*I++;
1829 MadeChange |= HoistCommonCodeInSuccs(MBB);
1830 }
1831
1832 return MadeChange;
1833}
1834
1835/// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
1836/// its 'true' successor.
1837static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
1838 MachineBasicBlock *TrueBB) {
1839 for (MachineBasicBlock *SuccBB : BB->successors())
1840 if (SuccBB != TrueBB)
1841 return SuccBB;
1842 return nullptr;
1843}
1844
1845template <class Container>
1846static void addRegAndItsAliases(unsigned Reg, const TargetRegisterInfo *TRI,
1847 Container &Set) {
1848 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
1849 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1850 Set.insert(*AI);
1851 } else {
1852 Set.insert(Reg);
1853 }
1854}
1855
1856/// findHoistingInsertPosAndDeps - Find the location to move common instructions
1857/// in successors to. The location is usually just before the terminator,
1858/// however if the terminator is a conditional branch and its previous
1859/// instruction is the flag setting instruction, the previous instruction is
1860/// the preferred location. This function also gathers uses and defs of the
1861/// instructions from the insertion point to the end of the block. The data is
1862/// used by HoistCommonCodeInSuccs to ensure safety.
1863static
1864MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
1865 const TargetInstrInfo *TII,
1866 const TargetRegisterInfo *TRI,
1867 SmallSet<unsigned,4> &Uses,
1868 SmallSet<unsigned,4> &Defs) {
1869 MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
1870 if (!TII->isUnpredicatedTerminator(*Loc))
1871 return MBB->end();
1872
1873 for (const MachineOperand &MO : Loc->operands()) {
1874 if (!MO.isReg())
1875 continue;
1876 unsigned Reg = MO.getReg();
1877 if (!Reg)
1878 continue;
1879 if (MO.isUse()) {
1880 addRegAndItsAliases(Reg, TRI, Uses);
1881 } else {
1882 if (!MO.isDead())
1883 // Don't try to hoist code in the rare case the terminator defines a
1884 // register that is later used.
1885 return MBB->end();
1886
1887 // If the terminator defines a register, make sure we don't hoist
1888 // the instruction whose def might be clobbered by the terminator.
1889 addRegAndItsAliases(Reg, TRI, Defs);
1890 }
1891 }
1892
1893 if (Uses.empty())
1894 return Loc;
1895 // If the terminator is the only instruction in the block and Uses is not
1896 // empty (or we would have returned above), we can still safely hoist
1897 // instructions just before the terminator as long as the Defs/Uses are not
1898 // violated (which is checked in HoistCommonCodeInSuccs).
1899 if (Loc == MBB->begin())
1900 return Loc;
1901
1902 // The terminator is probably a conditional branch, try not to separate the
1903 // branch from condition setting instruction.
1904 MachineBasicBlock::iterator PI =
1905 skipDebugInstructionsBackward(std::prev(Loc), MBB->begin());
1906
1907 bool IsDef = false;
1908 for (const MachineOperand &MO : PI->operands()) {
1909 // If PI has a regmask operand, it is probably a call. Separate away.
1910 if (MO.isRegMask())
1911 return Loc;
1912 if (!MO.isReg() || MO.isUse())
1913 continue;
1914 unsigned Reg = MO.getReg();
1915 if (!Reg)
1916 continue;
1917 if (Uses.count(Reg)) {
1918 IsDef = true;
1919 break;
1920 }
1921 }
1922 if (!IsDef)
1923 // The condition setting instruction is not just before the conditional
1924 // branch.
1925 return Loc;
1926
1927 // Be conservative, don't insert instruction above something that may have
1928 // side-effects. And since it's potentially bad to separate flag setting
1929 // instruction from the conditional branch, just abort the optimization
1930 // completely.
1931 // Also avoid moving code above predicated instruction since it's hard to
1932 // reason about register liveness with predicated instruction.
1933 bool DontMoveAcrossStore = true;
1934 if (!PI->isSafeToMove(nullptr, DontMoveAcrossStore) || TII->isPredicated(*PI))
1935 return MBB->end();
1936
1937 // Find out what registers are live. Note this routine is ignoring other live
1938 // registers which are only used by instructions in successor blocks.
1939 for (const MachineOperand &MO : PI->operands()) {
1940 if (!MO.isReg())
1941 continue;
1942 unsigned Reg = MO.getReg();
1943 if (!Reg)
1944 continue;
1945 if (MO.isUse()) {
1946 addRegAndItsAliases(Reg, TRI, Uses);
1947 } else {
1948 if (Uses.erase(Reg)) {
1949 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
1950 for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
1951 Uses.erase(*SubRegs); // Use sub-registers to be conservative
1952 }
1953 }
1954 addRegAndItsAliases(Reg, TRI, Defs);
1955 }
1956 }
1957
1958 return PI;
1959}
1960
1961bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
1962 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
1963 SmallVector<MachineOperand, 4> Cond;
1964 if (TII->analyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
1965 return false;
1966
1967 if (!FBB) FBB = findFalseBlock(MBB, TBB);
1968 if (!FBB)
1969 // Malformed bcc? True and false blocks are the same?
1970 return false;
1971
1972 // Restrict the optimization to cases where MBB is the only predecessor,
1973 // it is an obvious win.
1974 if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
1975 return false;
1976
1977 // Find a suitable position to hoist the common instructions to. Also figure
1978 // out which registers are used or defined by instructions from the insertion
1979 // point to the end of the block.
1980 SmallSet<unsigned, 4> Uses, Defs;
1981 MachineBasicBlock::iterator Loc =
1982 findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
1983 if (Loc == MBB->end())
1984 return false;
1985
1986 bool HasDups = false;
1987 SmallSet<unsigned, 4> ActiveDefsSet, AllDefsSet;
1988 MachineBasicBlock::iterator TIB = TBB->begin();
1989 MachineBasicBlock::iterator FIB = FBB->begin();
1990 MachineBasicBlock::iterator TIE = TBB->end();
1991 MachineBasicBlock::iterator FIE = FBB->end();
1992 while (TIB != TIE && FIB != FIE) {
1993 // Skip dbg_value instructions. These do not count.
1994 TIB = skipDebugInstructionsForward(TIB, TIE);
1995 FIB = skipDebugInstructionsForward(FIB, FIE);
1996 if (TIB == TIE || FIB == FIE)
1997 break;
1998
1999 if (!TIB->isIdenticalTo(*FIB, MachineInstr::CheckKillDead))
2000 break;
2001
2002 if (TII->isPredicated(*TIB))
2003 // Hard to reason about register liveness with predicated instruction.
2004 break;
2005
2006 bool IsSafe = true;
2007 for (MachineOperand &MO : TIB->operands()) {
2008 // Don't attempt to hoist instructions with register masks.
2009 if (MO.isRegMask()) {
2010 IsSafe = false;
2011 break;
2012 }
2013 if (!MO.isReg())
2014 continue;
2015 unsigned Reg = MO.getReg();
2016 if (!Reg)
2017 continue;
2018 if (MO.isDef()) {
2019 if (Uses.count(Reg)) {
2020 // Avoid clobbering a register that's used by the instruction at
2021 // the point of insertion.
2022 IsSafe = false;
2023 break;
2024 }
2025
2026 if (Defs.count(Reg) && !MO.isDead()) {
2027 // Don't hoist the instruction if the def would be clobber by the
2028 // instruction at the point insertion. FIXME: This is overly
2029 // conservative. It should be possible to hoist the instructions
2030 // in BB2 in the following example:
2031 // BB1:
2032 // r1, eflag = op1 r2, r3
2033 // brcc eflag
2034 //
2035 // BB2:
2036 // r1 = op2, ...
2037 // = op3, killed r1
2038 IsSafe = false;
2039 break;
2040 }
2041 } else if (!ActiveDefsSet.count(Reg)) {
2042 if (Defs.count(Reg)) {
2043 // Use is defined by the instruction at the point of insertion.
2044 IsSafe = false;
2045 break;
2046 }
2047
2048 if (MO.isKill() && Uses.count(Reg))
2049 // Kills a register that's read by the instruction at the point of
2050 // insertion. Remove the kill marker.
2051 MO.setIsKill(false);
2052 }
2053 }
2054 if (!IsSafe)
2055 break;
2056
2057 bool DontMoveAcrossStore = true;
2058 if (!TIB->isSafeToMove(nullptr, DontMoveAcrossStore))
2059 break;
2060
2061 // Remove kills from ActiveDefsSet, these registers had short live ranges.
2062 for (const MachineOperand &MO : TIB->operands()) {
2063 if (!MO.isReg() || !MO.isUse() || !MO.isKill())
2064 continue;
2065 unsigned Reg = MO.getReg();
2066 if (!Reg)
2067 continue;
2068 if (!AllDefsSet.count(Reg)) {
2069 continue;
2070 }
2071 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
2072 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
2073 ActiveDefsSet.erase(*AI);
2074 } else {
2075 ActiveDefsSet.erase(Reg);
2076 }
2077 }
2078
2079 // Track local defs so we can update liveins.
2080 for (const MachineOperand &MO : TIB->operands()) {
2081 if (!MO.isReg() || !MO.isDef() || MO.isDead())
2082 continue;
2083 unsigned Reg = MO.getReg();
2084 if (!Reg || TargetRegisterInfo::isVirtualRegister(Reg))
2085 continue;
2086 addRegAndItsAliases(Reg, TRI, ActiveDefsSet);
2087 addRegAndItsAliases(Reg, TRI, AllDefsSet);
2088 }
2089
2090 HasDups = true;
2091 ++TIB;
2092 ++FIB;
2093 }
2094
2095 if (!HasDups)
2096 return false;
2097
2098 MBB->splice(Loc, TBB, TBB->begin(), TIB);
2099 FBB->erase(FBB->begin(), FIB);
2100
2101 if (UpdateLiveIns) {
2102 recomputeLiveIns(*TBB);
2103 recomputeLiveIns(*FBB);
2104 }
2105
2106 ++NumHoist;
2107 return true;
2108}