Bug Summary

File:lib/CodeGen/BranchFolding.cpp
Warning:line 1300, column 39
Called C++ object pointer is null

Annotated Source Code

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clang -cc1 -triple x86_64-pc-linux-gnu -analyze -disable-free -disable-llvm-verifier -discard-value-names -main-file-name BranchFolding.cpp -analyzer-store=region -analyzer-opt-analyze-nested-blocks -analyzer-eagerly-assume -analyzer-checker=core -analyzer-checker=apiModeling -analyzer-checker=unix -analyzer-checker=deadcode -analyzer-checker=cplusplus -analyzer-checker=security.insecureAPI.UncheckedReturn -analyzer-checker=security.insecureAPI.getpw -analyzer-checker=security.insecureAPI.gets -analyzer-checker=security.insecureAPI.mktemp -analyzer-checker=security.insecureAPI.mkstemp -analyzer-checker=security.insecureAPI.vfork -analyzer-checker=nullability.NullPassedToNonnull -analyzer-checker=nullability.NullReturnedFromNonnull -analyzer-output plist -w -mrelocation-model pic -pic-level 2 -mthread-model posix -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb -momit-leaf-frame-pointer -ffunction-sections -fdata-sections -resource-dir /usr/lib/llvm-7/lib/clang/7.0.0 -D _DEBUG -D _GNU_SOURCE -D __STDC_CONSTANT_MACROS -D __STDC_FORMAT_MACROS -D __STDC_LIMIT_MACROS -I /build/llvm-toolchain-snapshot-7~svn329677/build-llvm/lib/CodeGen -I /build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen -I /build/llvm-toolchain-snapshot-7~svn329677/build-llvm/include -I /build/llvm-toolchain-snapshot-7~svn329677/include -U NDEBUG -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/x86_64-linux-gnu/c++/7.3.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/x86_64-linux-gnu/c++/7.3.0 -internal-isystem /usr/lib/gcc/x86_64-linux-gnu/7.3.0/../../../../include/c++/7.3.0/backward -internal-isystem /usr/include/clang/7.0.0/include/ -internal-isystem /usr/local/include -internal-isystem /usr/lib/llvm-7/lib/clang/7.0.0/include -internal-externc-isystem /usr/include/x86_64-linux-gnu -internal-externc-isystem /include -internal-externc-isystem /usr/include -O2 -Wno-unused-parameter -Wwrite-strings -Wno-missing-field-initializers -Wno-long-long -Wno-maybe-uninitialized -Wno-comment -std=c++11 -fdeprecated-macro -fdebug-compilation-dir /build/llvm-toolchain-snapshot-7~svn329677/build-llvm/lib/CodeGen -ferror-limit 19 -fmessage-length 0 -fvisibility-inlines-hidden -fobjc-runtime=gcc -fdiagnostics-show-option -vectorize-loops -vectorize-slp -analyzer-checker optin.performance.Padding -analyzer-output=html -analyzer-config stable-report-filename=true -o /tmp/scan-build-2018-04-11-031539-24776-1 -x c++ /build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp
1//===- BranchFolding.cpp - Fold machine code branch instructions ----------===//
2//
3// The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This pass forwards branches to unconditional branches to make them branch
11// directly to the target block. This pass often results in dead MBB's, which
12// it then removes.
13//
14// Note that this pass must be run after register allocation, it cannot handle
15// SSA form. It also must handle virtual registers for targets that emit virtual
16// ISA (e.g. NVPTX).
17//
18//===----------------------------------------------------------------------===//
19
20#include "BranchFolding.h"
21#include "llvm/ADT/BitVector.h"
22#include "llvm/ADT/DenseMap.h"
23#include "llvm/ADT/STLExtras.h"
24#include "llvm/ADT/SmallPtrSet.h"
25#include "llvm/ADT/SmallSet.h"
26#include "llvm/ADT/SmallVector.h"
27#include "llvm/ADT/Statistic.h"
28#include "llvm/CodeGen/Analysis.h"
29#include "llvm/CodeGen/LivePhysRegs.h"
30#include "llvm/CodeGen/MachineBasicBlock.h"
31#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
32#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
33#include "llvm/CodeGen/MachineFunction.h"
34#include "llvm/CodeGen/MachineFunctionPass.h"
35#include "llvm/CodeGen/MachineInstr.h"
36#include "llvm/CodeGen/MachineInstrBuilder.h"
37#include "llvm/CodeGen/MachineJumpTableInfo.h"
38#include "llvm/CodeGen/MachineLoopInfo.h"
39#include "llvm/CodeGen/MachineModuleInfo.h"
40#include "llvm/CodeGen/MachineOperand.h"
41#include "llvm/CodeGen/MachineRegisterInfo.h"
42#include "llvm/CodeGen/TargetInstrInfo.h"
43#include "llvm/CodeGen/TargetOpcodes.h"
44#include "llvm/CodeGen/TargetPassConfig.h"
45#include "llvm/CodeGen/TargetRegisterInfo.h"
46#include "llvm/CodeGen/TargetSubtargetInfo.h"
47#include "llvm/IR/DebugInfoMetadata.h"
48#include "llvm/IR/DebugLoc.h"
49#include "llvm/IR/Function.h"
50#include "llvm/MC/LaneBitmask.h"
51#include "llvm/MC/MCRegisterInfo.h"
52#include "llvm/Pass.h"
53#include "llvm/Support/BlockFrequency.h"
54#include "llvm/Support/BranchProbability.h"
55#include "llvm/Support/CommandLine.h"
56#include "llvm/Support/Debug.h"
57#include "llvm/Support/ErrorHandling.h"
58#include "llvm/Support/raw_ostream.h"
59#include "llvm/Target/TargetMachine.h"
60#include <cassert>
61#include <cstddef>
62#include <iterator>
63#include <numeric>
64#include <vector>
65
66using namespace llvm;
67
68#define DEBUG_TYPE"branch-folder" "branch-folder"
69
70STATISTIC(NumDeadBlocks, "Number of dead blocks removed")static llvm::Statistic NumDeadBlocks = {"branch-folder", "NumDeadBlocks"
, "Number of dead blocks removed", {0}, {false}}
;
71STATISTIC(NumBranchOpts, "Number of branches optimized")static llvm::Statistic NumBranchOpts = {"branch-folder", "NumBranchOpts"
, "Number of branches optimized", {0}, {false}}
;
72STATISTIC(NumTailMerge , "Number of block tails merged")static llvm::Statistic NumTailMerge = {"branch-folder", "NumTailMerge"
, "Number of block tails merged", {0}, {false}}
;
73STATISTIC(NumHoist , "Number of times common instructions are hoisted")static llvm::Statistic NumHoist = {"branch-folder", "NumHoist"
, "Number of times common instructions are hoisted", {0}, {false
}}
;
74STATISTIC(NumTailCalls, "Number of tail calls optimized")static llvm::Statistic NumTailCalls = {"branch-folder", "NumTailCalls"
, "Number of tail calls optimized", {0}, {false}}
;
75
76static cl::opt<cl::boolOrDefault> FlagEnableTailMerge("enable-tail-merge",
77 cl::init(cl::BOU_UNSET), cl::Hidden);
78
79// Throttle for huge numbers of predecessors (compile speed problems)
80static cl::opt<unsigned>
81TailMergeThreshold("tail-merge-threshold",
82 cl::desc("Max number of predecessors to consider tail merging"),
83 cl::init(150), cl::Hidden);
84
85// Heuristic for tail merging (and, inversely, tail duplication).
86// TODO: This should be replaced with a target query.
87static cl::opt<unsigned>
88TailMergeSize("tail-merge-size",
89 cl::desc("Min number of instructions to consider tail merging"),
90 cl::init(3), cl::Hidden);
91
92namespace {
93
94 /// BranchFolderPass - Wrap branch folder in a machine function pass.
95 class BranchFolderPass : public MachineFunctionPass {
96 public:
97 static char ID;
98
99 explicit BranchFolderPass(): MachineFunctionPass(ID) {}
100
101 bool runOnMachineFunction(MachineFunction &MF) override;
102
103 void getAnalysisUsage(AnalysisUsage &AU) const override {
104 AU.addRequired<MachineBlockFrequencyInfo>();
105 AU.addRequired<MachineBranchProbabilityInfo>();
106 AU.addRequired<TargetPassConfig>();
107 MachineFunctionPass::getAnalysisUsage(AU);
108 }
109 };
110
111} // end anonymous namespace
112
113char BranchFolderPass::ID = 0;
114
115char &llvm::BranchFolderPassID = BranchFolderPass::ID;
116
117INITIALIZE_PASS(BranchFolderPass, DEBUG_TYPE,static void *initializeBranchFolderPassPassOnce(PassRegistry &
Registry) { PassInfo *PI = new PassInfo( "Control Flow Optimizer"
, "branch-folder", &BranchFolderPass::ID, PassInfo::NormalCtor_t
(callDefaultCtor<BranchFolderPass>), false, false); Registry
.registerPass(*PI, true); return PI; } static llvm::once_flag
InitializeBranchFolderPassPassFlag; void llvm::initializeBranchFolderPassPass
(PassRegistry &Registry) { llvm::call_once(InitializeBranchFolderPassPassFlag
, initializeBranchFolderPassPassOnce, std::ref(Registry)); }
118 "Control Flow Optimizer", false, false)static void *initializeBranchFolderPassPassOnce(PassRegistry &
Registry) { PassInfo *PI = new PassInfo( "Control Flow Optimizer"
, "branch-folder", &BranchFolderPass::ID, PassInfo::NormalCtor_t
(callDefaultCtor<BranchFolderPass>), false, false); Registry
.registerPass(*PI, true); return PI; } static llvm::once_flag
InitializeBranchFolderPassPassFlag; void llvm::initializeBranchFolderPassPass
(PassRegistry &Registry) { llvm::call_once(InitializeBranchFolderPassPassFlag
, initializeBranchFolderPassPassOnce, std::ref(Registry)); }
119
120bool BranchFolderPass::runOnMachineFunction(MachineFunction &MF) {
121 if (skipFunction(MF.getFunction()))
122 return false;
123
124 TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
125 // TailMerge can create jump into if branches that make CFG irreducible for
126 // HW that requires structurized CFG.
127 bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
128 PassConfig->getEnableTailMerge();
129 BranchFolder::MBFIWrapper MBBFreqInfo(
130 getAnalysis<MachineBlockFrequencyInfo>());
131 BranchFolder Folder(EnableTailMerge, /*CommonHoist=*/true, MBBFreqInfo,
132 getAnalysis<MachineBranchProbabilityInfo>());
133 return Folder.OptimizeFunction(MF, MF.getSubtarget().getInstrInfo(),
134 MF.getSubtarget().getRegisterInfo(),
135 getAnalysisIfAvailable<MachineModuleInfo>());
136}
137
138BranchFolder::BranchFolder(bool defaultEnableTailMerge, bool CommonHoist,
139 MBFIWrapper &FreqInfo,
140 const MachineBranchProbabilityInfo &ProbInfo,
141 unsigned MinTailLength)
142 : EnableHoistCommonCode(CommonHoist), MinCommonTailLength(MinTailLength),
143 MBBFreqInfo(FreqInfo), MBPI(ProbInfo) {
144 if (MinCommonTailLength == 0)
145 MinCommonTailLength = TailMergeSize;
146 switch (FlagEnableTailMerge) {
147 case cl::BOU_UNSET: EnableTailMerge = defaultEnableTailMerge; break;
148 case cl::BOU_TRUE: EnableTailMerge = true; break;
149 case cl::BOU_FALSE: EnableTailMerge = false; break;
150 }
151}
152
153void BranchFolder::RemoveDeadBlock(MachineBasicBlock *MBB) {
154 assert(MBB->pred_empty() && "MBB must be dead!")(static_cast <bool> (MBB->pred_empty() && "MBB must be dead!"
) ? void (0) : __assert_fail ("MBB->pred_empty() && \"MBB must be dead!\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 154, __extension__ __PRETTY_FUNCTION__))
;
155 DEBUG(dbgs() << "\nRemoving MBB: " << *MBB)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nRemoving MBB: " <<
*MBB; } } while (false)
;
156
157 MachineFunction *MF = MBB->getParent();
158 // drop all successors.
159 while (!MBB->succ_empty())
160 MBB->removeSuccessor(MBB->succ_end()-1);
161
162 // Avoid matching if this pointer gets reused.
163 TriedMerging.erase(MBB);
164
165 // Remove the block.
166 MF->erase(MBB);
167 FuncletMembership.erase(MBB);
168 if (MLI)
169 MLI->removeBlock(MBB);
170}
171
172bool BranchFolder::OptimizeFunction(MachineFunction &MF,
173 const TargetInstrInfo *tii,
174 const TargetRegisterInfo *tri,
175 MachineModuleInfo *mmi,
176 MachineLoopInfo *mli, bool AfterPlacement) {
177 if (!tii) return false;
178
179 TriedMerging.clear();
180
181 MachineRegisterInfo &MRI = MF.getRegInfo();
182 AfterBlockPlacement = AfterPlacement;
183 TII = tii;
184 TRI = tri;
185 MMI = mmi;
186 MLI = mli;
187 this->MRI = &MRI;
188
189 UpdateLiveIns = MRI.tracksLiveness() && TRI->trackLivenessAfterRegAlloc(MF);
190 if (!UpdateLiveIns)
191 MRI.invalidateLiveness();
192
193 // Fix CFG. The later algorithms expect it to be right.
194 bool MadeChange = false;
195 for (MachineBasicBlock &MBB : MF) {
196 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
197 SmallVector<MachineOperand, 4> Cond;
198 if (!TII->analyzeBranch(MBB, TBB, FBB, Cond, true))
199 MadeChange |= MBB.CorrectExtraCFGEdges(TBB, FBB, !Cond.empty());
200 }
201
202 // Recalculate funclet membership.
203 FuncletMembership = getFuncletMembership(MF);
204
205 bool MadeChangeThisIteration = true;
206 while (MadeChangeThisIteration) {
207 MadeChangeThisIteration = TailMergeBlocks(MF);
208 // No need to clean up if tail merging does not change anything after the
209 // block placement.
210 if (!AfterBlockPlacement || MadeChangeThisIteration)
211 MadeChangeThisIteration |= OptimizeBranches(MF);
212 if (EnableHoistCommonCode)
213 MadeChangeThisIteration |= HoistCommonCode(MF);
214 MadeChange |= MadeChangeThisIteration;
215 }
216
217 // See if any jump tables have become dead as the code generator
218 // did its thing.
219 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
220 if (!JTI)
221 return MadeChange;
222
223 // Walk the function to find jump tables that are live.
224 BitVector JTIsLive(JTI->getJumpTables().size());
225 for (const MachineBasicBlock &BB : MF) {
226 for (const MachineInstr &I : BB)
227 for (const MachineOperand &Op : I.operands()) {
228 if (!Op.isJTI()) continue;
229
230 // Remember that this JT is live.
231 JTIsLive.set(Op.getIndex());
232 }
233 }
234
235 // Finally, remove dead jump tables. This happens when the
236 // indirect jump was unreachable (and thus deleted).
237 for (unsigned i = 0, e = JTIsLive.size(); i != e; ++i)
238 if (!JTIsLive.test(i)) {
239 JTI->RemoveJumpTable(i);
240 MadeChange = true;
241 }
242
243 return MadeChange;
244}
245
246//===----------------------------------------------------------------------===//
247// Tail Merging of Blocks
248//===----------------------------------------------------------------------===//
249
250/// HashMachineInstr - Compute a hash value for MI and its operands.
251static unsigned HashMachineInstr(const MachineInstr &MI) {
252 unsigned Hash = MI.getOpcode();
253 for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
254 const MachineOperand &Op = MI.getOperand(i);
255
256 // Merge in bits from the operand if easy. We can't use MachineOperand's
257 // hash_code here because it's not deterministic and we sort by hash value
258 // later.
259 unsigned OperandHash = 0;
260 switch (Op.getType()) {
261 case MachineOperand::MO_Register:
262 OperandHash = Op.getReg();
263 break;
264 case MachineOperand::MO_Immediate:
265 OperandHash = Op.getImm();
266 break;
267 case MachineOperand::MO_MachineBasicBlock:
268 OperandHash = Op.getMBB()->getNumber();
269 break;
270 case MachineOperand::MO_FrameIndex:
271 case MachineOperand::MO_ConstantPoolIndex:
272 case MachineOperand::MO_JumpTableIndex:
273 OperandHash = Op.getIndex();
274 break;
275 case MachineOperand::MO_GlobalAddress:
276 case MachineOperand::MO_ExternalSymbol:
277 // Global address / external symbol are too hard, don't bother, but do
278 // pull in the offset.
279 OperandHash = Op.getOffset();
280 break;
281 default:
282 break;
283 }
284
285 Hash += ((OperandHash << 3) | Op.getType()) << (i & 31);
286 }
287 return Hash;
288}
289
290/// HashEndOfMBB - Hash the last instruction in the MBB.
291static unsigned HashEndOfMBB(const MachineBasicBlock &MBB) {
292 MachineBasicBlock::const_iterator I = MBB.getLastNonDebugInstr();
293 if (I == MBB.end())
294 return 0;
295
296 return HashMachineInstr(*I);
297}
298
299/// ComputeCommonTailLength - Given two machine basic blocks, compute the number
300/// of instructions they actually have in common together at their end. Return
301/// iterators for the first shared instruction in each block.
302static unsigned ComputeCommonTailLength(MachineBasicBlock *MBB1,
303 MachineBasicBlock *MBB2,
304 MachineBasicBlock::iterator &I1,
305 MachineBasicBlock::iterator &I2) {
306 I1 = MBB1->end();
307 I2 = MBB2->end();
308
309 unsigned TailLen = 0;
310 while (I1 != MBB1->begin() && I2 != MBB2->begin()) {
311 --I1; --I2;
312 // Skip debugging pseudos; necessary to avoid changing the code.
313 while (I1->isDebugValue()) {
314 if (I1==MBB1->begin()) {
315 while (I2->isDebugValue()) {
316 if (I2==MBB2->begin())
317 // I1==DBG at begin; I2==DBG at begin
318 return TailLen;
319 --I2;
320 }
321 ++I2;
322 // I1==DBG at begin; I2==non-DBG, or first of DBGs not at begin
323 return TailLen;
324 }
325 --I1;
326 }
327 // I1==first (untested) non-DBG preceding known match
328 while (I2->isDebugValue()) {
329 if (I2==MBB2->begin()) {
330 ++I1;
331 // I1==non-DBG, or first of DBGs not at begin; I2==DBG at begin
332 return TailLen;
333 }
334 --I2;
335 }
336 // I1, I2==first (untested) non-DBGs preceding known match
337 if (!I1->isIdenticalTo(*I2) ||
338 // FIXME: This check is dubious. It's used to get around a problem where
339 // people incorrectly expect inline asm directives to remain in the same
340 // relative order. This is untenable because normal compiler
341 // optimizations (like this one) may reorder and/or merge these
342 // directives.
343 I1->isInlineAsm()) {
344 ++I1; ++I2;
345 break;
346 }
347 ++TailLen;
348 }
349 // Back past possible debugging pseudos at beginning of block. This matters
350 // when one block differs from the other only by whether debugging pseudos
351 // are present at the beginning. (This way, the various checks later for
352 // I1==MBB1->begin() work as expected.)
353 if (I1 == MBB1->begin() && I2 != MBB2->begin()) {
354 --I2;
355 while (I2->isDebugValue()) {
356 if (I2 == MBB2->begin())
357 return TailLen;
358 --I2;
359 }
360 ++I2;
361 }
362 if (I2 == MBB2->begin() && I1 != MBB1->begin()) {
363 --I1;
364 while (I1->isDebugValue()) {
365 if (I1 == MBB1->begin())
366 return TailLen;
367 --I1;
368 }
369 ++I1;
370 }
371 return TailLen;
372}
373
374void BranchFolder::replaceTailWithBranchTo(MachineBasicBlock::iterator OldInst,
375 MachineBasicBlock &NewDest) {
376 if (UpdateLiveIns) {
377 // OldInst should always point to an instruction.
378 MachineBasicBlock &OldMBB = *OldInst->getParent();
379 LiveRegs.clear();
380 LiveRegs.addLiveOuts(OldMBB);
381 // Move backward to the place where will insert the jump.
382 MachineBasicBlock::iterator I = OldMBB.end();
383 do {
384 --I;
385 LiveRegs.stepBackward(*I);
386 } while (I != OldInst);
387
388 // Merging the tails may have switched some undef operand to non-undef ones.
389 // Add IMPLICIT_DEFS into OldMBB as necessary to have a definition of the
390 // register.
391 for (MachineBasicBlock::RegisterMaskPair P : NewDest.liveins()) {
392 // We computed the liveins with computeLiveIn earlier and should only see
393 // full registers:
394 assert(P.LaneMask == LaneBitmask::getAll() &&(static_cast <bool> (P.LaneMask == LaneBitmask::getAll(
) && "Can only handle full register.") ? void (0) : __assert_fail
("P.LaneMask == LaneBitmask::getAll() && \"Can only handle full register.\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 395, __extension__ __PRETTY_FUNCTION__))
395 "Can only handle full register.")(static_cast <bool> (P.LaneMask == LaneBitmask::getAll(
) && "Can only handle full register.") ? void (0) : __assert_fail
("P.LaneMask == LaneBitmask::getAll() && \"Can only handle full register.\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 395, __extension__ __PRETTY_FUNCTION__))
;
396 MCPhysReg Reg = P.PhysReg;
397 if (!LiveRegs.available(*MRI, Reg))
398 continue;
399 DebugLoc DL;
400 BuildMI(OldMBB, OldInst, DL, TII->get(TargetOpcode::IMPLICIT_DEF), Reg);
401 }
402 }
403
404 TII->ReplaceTailWithBranchTo(OldInst, &NewDest);
405 ++NumTailMerge;
406}
407
408MachineBasicBlock *BranchFolder::SplitMBBAt(MachineBasicBlock &CurMBB,
409 MachineBasicBlock::iterator BBI1,
410 const BasicBlock *BB) {
411 if (!TII->isLegalToSplitMBBAt(CurMBB, BBI1))
412 return nullptr;
413
414 MachineFunction &MF = *CurMBB.getParent();
415
416 // Create the fall-through block.
417 MachineFunction::iterator MBBI = CurMBB.getIterator();
418 MachineBasicBlock *NewMBB = MF.CreateMachineBasicBlock(BB);
419 CurMBB.getParent()->insert(++MBBI, NewMBB);
420
421 // Move all the successors of this block to the specified block.
422 NewMBB->transferSuccessors(&CurMBB);
423
424 // Add an edge from CurMBB to NewMBB for the fall-through.
425 CurMBB.addSuccessor(NewMBB);
426
427 // Splice the code over.
428 NewMBB->splice(NewMBB->end(), &CurMBB, BBI1, CurMBB.end());
429
430 // NewMBB belongs to the same loop as CurMBB.
431 if (MLI)
432 if (MachineLoop *ML = MLI->getLoopFor(&CurMBB))
433 ML->addBasicBlockToLoop(NewMBB, MLI->getBase());
434
435 // NewMBB inherits CurMBB's block frequency.
436 MBBFreqInfo.setBlockFreq(NewMBB, MBBFreqInfo.getBlockFreq(&CurMBB));
437
438 if (UpdateLiveIns)
439 computeAndAddLiveIns(LiveRegs, *NewMBB);
440
441 // Add the new block to the funclet.
442 const auto &FuncletI = FuncletMembership.find(&CurMBB);
443 if (FuncletI != FuncletMembership.end()) {
444 auto n = FuncletI->second;
445 FuncletMembership[NewMBB] = n;
446 }
447
448 return NewMBB;
449}
450
451/// EstimateRuntime - Make a rough estimate for how long it will take to run
452/// the specified code.
453static unsigned EstimateRuntime(MachineBasicBlock::iterator I,
454 MachineBasicBlock::iterator E) {
455 unsigned Time = 0;
456 for (; I != E; ++I) {
457 if (I->isDebugValue())
458 continue;
459 if (I->isCall())
460 Time += 10;
461 else if (I->mayLoad() || I->mayStore())
462 Time += 2;
463 else
464 ++Time;
465 }
466 return Time;
467}
468
469// CurMBB needs to add an unconditional branch to SuccMBB (we removed these
470// branches temporarily for tail merging). In the case where CurMBB ends
471// with a conditional branch to the next block, optimize by reversing the
472// test and conditionally branching to SuccMBB instead.
473static void FixTail(MachineBasicBlock *CurMBB, MachineBasicBlock *SuccBB,
474 const TargetInstrInfo *TII) {
475 MachineFunction *MF = CurMBB->getParent();
476 MachineFunction::iterator I = std::next(MachineFunction::iterator(CurMBB));
477 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
478 SmallVector<MachineOperand, 4> Cond;
479 DebugLoc dl = CurMBB->findBranchDebugLoc();
480 if (I != MF->end() && !TII->analyzeBranch(*CurMBB, TBB, FBB, Cond, true)) {
481 MachineBasicBlock *NextBB = &*I;
482 if (TBB == NextBB && !Cond.empty() && !FBB) {
483 if (!TII->reverseBranchCondition(Cond)) {
484 TII->removeBranch(*CurMBB);
485 TII->insertBranch(*CurMBB, SuccBB, nullptr, Cond, dl);
486 return;
487 }
488 }
489 }
490 TII->insertBranch(*CurMBB, SuccBB, nullptr,
491 SmallVector<MachineOperand, 0>(), dl);
492}
493
494bool
495BranchFolder::MergePotentialsElt::operator<(const MergePotentialsElt &o) const {
496 if (getHash() < o.getHash())
497 return true;
498 if (getHash() > o.getHash())
499 return false;
500 if (getBlock()->getNumber() < o.getBlock()->getNumber())
501 return true;
502 if (getBlock()->getNumber() > o.getBlock()->getNumber())
503 return false;
504 // _GLIBCXX_DEBUG checks strict weak ordering, which involves comparing
505 // an object with itself.
506#ifndef _GLIBCXX_DEBUG
507 llvm_unreachable("Predecessor appears twice")::llvm::llvm_unreachable_internal("Predecessor appears twice"
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 507)
;
508#else
509 return false;
510#endif
511}
512
513BlockFrequency
514BranchFolder::MBFIWrapper::getBlockFreq(const MachineBasicBlock *MBB) const {
515 auto I = MergedBBFreq.find(MBB);
516
517 if (I != MergedBBFreq.end())
518 return I->second;
519
520 return MBFI.getBlockFreq(MBB);
521}
522
523void BranchFolder::MBFIWrapper::setBlockFreq(const MachineBasicBlock *MBB,
524 BlockFrequency F) {
525 MergedBBFreq[MBB] = F;
526}
527
528raw_ostream &
529BranchFolder::MBFIWrapper::printBlockFreq(raw_ostream &OS,
530 const MachineBasicBlock *MBB) const {
531 return MBFI.printBlockFreq(OS, getBlockFreq(MBB));
532}
533
534raw_ostream &
535BranchFolder::MBFIWrapper::printBlockFreq(raw_ostream &OS,
536 const BlockFrequency Freq) const {
537 return MBFI.printBlockFreq(OS, Freq);
538}
539
540void BranchFolder::MBFIWrapper::view(const Twine &Name, bool isSimple) {
541 MBFI.view(Name, isSimple);
542}
543
544uint64_t
545BranchFolder::MBFIWrapper::getEntryFreq() const {
546 return MBFI.getEntryFreq();
547}
548
549/// CountTerminators - Count the number of terminators in the given
550/// block and set I to the position of the first non-terminator, if there
551/// is one, or MBB->end() otherwise.
552static unsigned CountTerminators(MachineBasicBlock *MBB,
553 MachineBasicBlock::iterator &I) {
554 I = MBB->end();
555 unsigned NumTerms = 0;
556 while (true) {
557 if (I == MBB->begin()) {
558 I = MBB->end();
559 break;
560 }
561 --I;
562 if (!I->isTerminator()) break;
563 ++NumTerms;
564 }
565 return NumTerms;
566}
567
568/// A no successor, non-return block probably ends in unreachable and is cold.
569/// Also consider a block that ends in an indirect branch to be a return block,
570/// since many targets use plain indirect branches to return.
571static bool blockEndsInUnreachable(const MachineBasicBlock *MBB) {
572 if (!MBB->succ_empty())
573 return false;
574 if (MBB->empty())
575 return true;
576 return !(MBB->back().isReturn() || MBB->back().isIndirectBranch());
577}
578
579/// ProfitableToMerge - Check if two machine basic blocks have a common tail
580/// and decide if it would be profitable to merge those tails. Return the
581/// length of the common tail and iterators to the first common instruction
582/// in each block.
583/// MBB1, MBB2 The blocks to check
584/// MinCommonTailLength Minimum size of tail block to be merged.
585/// CommonTailLen Out parameter to record the size of the shared tail between
586/// MBB1 and MBB2
587/// I1, I2 Iterator references that will be changed to point to the first
588/// instruction in the common tail shared by MBB1,MBB2
589/// SuccBB A common successor of MBB1, MBB2 which are in a canonical form
590/// relative to SuccBB
591/// PredBB The layout predecessor of SuccBB, if any.
592/// FuncletMembership map from block to funclet #.
593/// AfterPlacement True if we are merging blocks after layout. Stricter
594/// thresholds apply to prevent undoing tail-duplication.
595static bool
596ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
597 unsigned MinCommonTailLength, unsigned &CommonTailLen,
598 MachineBasicBlock::iterator &I1,
599 MachineBasicBlock::iterator &I2, MachineBasicBlock *SuccBB,
600 MachineBasicBlock *PredBB,
601 DenseMap<const MachineBasicBlock *, int> &FuncletMembership,
602 bool AfterPlacement) {
603 // It is never profitable to tail-merge blocks from two different funclets.
604 if (!FuncletMembership.empty()) {
605 auto Funclet1 = FuncletMembership.find(MBB1);
606 assert(Funclet1 != FuncletMembership.end())(static_cast <bool> (Funclet1 != FuncletMembership.end(
)) ? void (0) : __assert_fail ("Funclet1 != FuncletMembership.end()"
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 606, __extension__ __PRETTY_FUNCTION__))
;
607 auto Funclet2 = FuncletMembership.find(MBB2);
608 assert(Funclet2 != FuncletMembership.end())(static_cast <bool> (Funclet2 != FuncletMembership.end(
)) ? void (0) : __assert_fail ("Funclet2 != FuncletMembership.end()"
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 608, __extension__ __PRETTY_FUNCTION__))
;
609 if (Funclet1->second != Funclet2->second)
610 return false;
611 }
612
613 CommonTailLen = ComputeCommonTailLength(MBB1, MBB2, I1, I2);
614 if (CommonTailLen == 0)
615 return false;
616 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)
617 << " and " << printMBBReference(*MBB2) << " is " << CommonTailLendo { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Common tail length of "
<< printMBBReference(*MBB1) << " and " << printMBBReference
(*MBB2) << " is " << CommonTailLen << '\n';
} } while (false)
618 << '\n')do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "Common tail length of "
<< printMBBReference(*MBB1) << " and " << printMBBReference
(*MBB2) << " is " << CommonTailLen << '\n';
} } while (false)
;
619
620 // It's almost always profitable to merge any number of non-terminator
621 // instructions with the block that falls through into the common successor.
622 // This is true only for a single successor. For multiple successors, we are
623 // trading a conditional branch for an unconditional one.
624 // TODO: Re-visit successor size for non-layout tail merging.
625 if ((MBB1 == PredBB || MBB2 == PredBB) &&
626 (!AfterPlacement || MBB1->succ_size() == 1)) {
627 MachineBasicBlock::iterator I;
628 unsigned NumTerms = CountTerminators(MBB1 == PredBB ? MBB2 : MBB1, I);
629 if (CommonTailLen > NumTerms)
630 return true;
631 }
632
633 // If these are identical non-return blocks with no successors, merge them.
634 // Such blocks are typically cold calls to noreturn functions like abort, and
635 // are unlikely to become a fallthrough target after machine block placement.
636 // Tail merging these blocks is unlikely to create additional unconditional
637 // branches, and will reduce the size of this cold code.
638 if (I1 == MBB1->begin() && I2 == MBB2->begin() &&
639 blockEndsInUnreachable(MBB1) && blockEndsInUnreachable(MBB2))
640 return true;
641
642 // If one of the blocks can be completely merged and happens to be in
643 // a position where the other could fall through into it, merge any number
644 // of instructions, because it can be done without a branch.
645 // TODO: If the blocks are not adjacent, move one of them so that they are?
646 if (MBB1->isLayoutSuccessor(MBB2) && I2 == MBB2->begin())
647 return true;
648 if (MBB2->isLayoutSuccessor(MBB1) && I1 == MBB1->begin())
649 return true;
650
651 // If both blocks are identical and end in a branch, merge them unless they
652 // both have a fallthrough predecessor and successor.
653 // We can only do this after block placement because it depends on whether
654 // there are fallthroughs, and we don't know until after layout.
655 if (AfterPlacement && I1 == MBB1->begin() && I2 == MBB2->begin()) {
656 auto BothFallThrough = [](MachineBasicBlock *MBB) {
657 if (MBB->succ_size() != 0 && !MBB->canFallThrough())
658 return false;
659 MachineFunction::iterator I(MBB);
660 MachineFunction *MF = MBB->getParent();
661 return (MBB != &*MF->begin()) && std::prev(I)->canFallThrough();
662 };
663 if (!BothFallThrough(MBB1) || !BothFallThrough(MBB2))
664 return true;
665 }
666
667 // If both blocks have an unconditional branch temporarily stripped out,
668 // count that as an additional common instruction for the following
669 // heuristics. This heuristic is only accurate for single-succ blocks, so to
670 // make sure that during layout merging and duplicating don't crash, we check
671 // for that when merging during layout.
672 unsigned EffectiveTailLen = CommonTailLen;
673 if (SuccBB && MBB1 != PredBB && MBB2 != PredBB &&
674 (MBB1->succ_size() == 1 || !AfterPlacement) &&
675 !MBB1->back().isBarrier() &&
676 !MBB2->back().isBarrier())
677 ++EffectiveTailLen;
678
679 // Check if the common tail is long enough to be worthwhile.
680 if (EffectiveTailLen >= MinCommonTailLength)
681 return true;
682
683 // If we are optimizing for code size, 2 instructions in common is enough if
684 // we don't have to split a block. At worst we will be introducing 1 new
685 // branch instruction, which is likely to be smaller than the 2
686 // instructions that would be deleted in the merge.
687 MachineFunction *MF = MBB1->getParent();
688 return EffectiveTailLen >= 2 && MF->getFunction().optForSize() &&
689 (I1 == MBB1->begin() || I2 == MBB2->begin());
690}
691
692unsigned BranchFolder::ComputeSameTails(unsigned CurHash,
693 unsigned MinCommonTailLength,
694 MachineBasicBlock *SuccBB,
695 MachineBasicBlock *PredBB) {
696 unsigned maxCommonTailLength = 0U;
697 SameTails.clear();
698 MachineBasicBlock::iterator TrialBBI1, TrialBBI2;
699 MPIterator HighestMPIter = std::prev(MergePotentials.end());
700 for (MPIterator CurMPIter = std::prev(MergePotentials.end()),
701 B = MergePotentials.begin();
702 CurMPIter != B && CurMPIter->getHash() == CurHash; --CurMPIter) {
703 for (MPIterator I = std::prev(CurMPIter); I->getHash() == CurHash; --I) {
704 unsigned CommonTailLen;
705 if (ProfitableToMerge(CurMPIter->getBlock(), I->getBlock(),
706 MinCommonTailLength,
707 CommonTailLen, TrialBBI1, TrialBBI2,
708 SuccBB, PredBB,
709 FuncletMembership,
710 AfterBlockPlacement)) {
711 if (CommonTailLen > maxCommonTailLength) {
712 SameTails.clear();
713 maxCommonTailLength = CommonTailLen;
714 HighestMPIter = CurMPIter;
715 SameTails.push_back(SameTailElt(CurMPIter, TrialBBI1));
716 }
717 if (HighestMPIter == CurMPIter &&
718 CommonTailLen == maxCommonTailLength)
719 SameTails.push_back(SameTailElt(I, TrialBBI2));
720 }
721 if (I == B)
722 break;
723 }
724 }
725 return maxCommonTailLength;
726}
727
728void BranchFolder::RemoveBlocksWithHash(unsigned CurHash,
729 MachineBasicBlock *SuccBB,
730 MachineBasicBlock *PredBB) {
731 MPIterator CurMPIter, B;
732 for (CurMPIter = std::prev(MergePotentials.end()),
733 B = MergePotentials.begin();
734 CurMPIter->getHash() == CurHash; --CurMPIter) {
735 // Put the unconditional branch back, if we need one.
736 MachineBasicBlock *CurMBB = CurMPIter->getBlock();
737 if (SuccBB && CurMBB != PredBB)
738 FixTail(CurMBB, SuccBB, TII);
739 if (CurMPIter == B)
740 break;
741 }
742 if (CurMPIter->getHash() != CurHash)
743 CurMPIter++;
744 MergePotentials.erase(CurMPIter, MergePotentials.end());
745}
746
747bool BranchFolder::CreateCommonTailOnlyBlock(MachineBasicBlock *&PredBB,
748 MachineBasicBlock *SuccBB,
749 unsigned maxCommonTailLength,
750 unsigned &commonTailIndex) {
751 commonTailIndex = 0;
752 unsigned TimeEstimate = ~0U;
753 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
754 // Use PredBB if possible; that doesn't require a new branch.
755 if (SameTails[i].getBlock() == PredBB) {
756 commonTailIndex = i;
757 break;
758 }
759 // Otherwise, make a (fairly bogus) choice based on estimate of
760 // how long it will take the various blocks to execute.
761 unsigned t = EstimateRuntime(SameTails[i].getBlock()->begin(),
762 SameTails[i].getTailStartPos());
763 if (t <= TimeEstimate) {
764 TimeEstimate = t;
765 commonTailIndex = i;
766 }
767 }
768
769 MachineBasicBlock::iterator BBI =
770 SameTails[commonTailIndex].getTailStartPos();
771 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
772
773 DEBUG(dbgs() << "\nSplitting " << printMBBReference(*MBB) << ", size "do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nSplitting " << printMBBReference
(*MBB) << ", size " << maxCommonTailLength; } } while
(false)
774 << maxCommonTailLength)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nSplitting " << printMBBReference
(*MBB) << ", size " << maxCommonTailLength; } } while
(false)
;
775
776 // If the split block unconditionally falls-thru to SuccBB, it will be
777 // merged. In control flow terms it should then take SuccBB's name. e.g. If
778 // SuccBB is an inner loop, the common tail is still part of the inner loop.
779 const BasicBlock *BB = (SuccBB && MBB->succ_size() == 1) ?
780 SuccBB->getBasicBlock() : MBB->getBasicBlock();
781 MachineBasicBlock *newMBB = SplitMBBAt(*MBB, BBI, BB);
782 if (!newMBB) {
783 DEBUG(dbgs() << "... failed!")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "... failed!"; } } while
(false)
;
784 return false;
785 }
786
787 SameTails[commonTailIndex].setBlock(newMBB);
788 SameTails[commonTailIndex].setTailStartPos(newMBB->begin());
789
790 // If we split PredBB, newMBB is the new predecessor.
791 if (PredBB == MBB)
792 PredBB = newMBB;
793
794 return true;
795}
796
797static void
798mergeOperations(MachineBasicBlock::iterator MBBIStartPos,
799 MachineBasicBlock &MBBCommon) {
800 MachineBasicBlock *MBB = MBBIStartPos->getParent();
801 // Note CommonTailLen does not necessarily matches the size of
802 // the common BB nor all its instructions because of debug
803 // instructions differences.
804 unsigned CommonTailLen = 0;
805 for (auto E = MBB->end(); MBBIStartPos != E; ++MBBIStartPos)
806 ++CommonTailLen;
807
808 MachineBasicBlock::reverse_iterator MBBI = MBB->rbegin();
809 MachineBasicBlock::reverse_iterator MBBIE = MBB->rend();
810 MachineBasicBlock::reverse_iterator MBBICommon = MBBCommon.rbegin();
811 MachineBasicBlock::reverse_iterator MBBIECommon = MBBCommon.rend();
812
813 while (CommonTailLen--) {
814 assert(MBBI != MBBIE && "Reached BB end within common tail length!")(static_cast <bool> (MBBI != MBBIE && "Reached BB end within common tail length!"
) ? void (0) : __assert_fail ("MBBI != MBBIE && \"Reached BB end within common tail length!\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 814, __extension__ __PRETTY_FUNCTION__))
;
815 (void)MBBIE;
816
817 if (MBBI->isDebugValue()) {
818 ++MBBI;
819 continue;
820 }
821
822 while ((MBBICommon != MBBIECommon) && MBBICommon->isDebugValue())
823 ++MBBICommon;
824
825 assert(MBBICommon != MBBIECommon &&(static_cast <bool> (MBBICommon != MBBIECommon &&
"Reached BB end within common tail length!") ? void (0) : __assert_fail
("MBBICommon != MBBIECommon && \"Reached BB end within common tail length!\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 826, __extension__ __PRETTY_FUNCTION__))
826 "Reached BB end within common tail length!")(static_cast <bool> (MBBICommon != MBBIECommon &&
"Reached BB end within common tail length!") ? void (0) : __assert_fail
("MBBICommon != MBBIECommon && \"Reached BB end within common tail length!\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 826, __extension__ __PRETTY_FUNCTION__))
;
827 assert(MBBICommon->isIdenticalTo(*MBBI) && "Expected matching MIIs!")(static_cast <bool> (MBBICommon->isIdenticalTo(*MBBI
) && "Expected matching MIIs!") ? void (0) : __assert_fail
("MBBICommon->isIdenticalTo(*MBBI) && \"Expected matching MIIs!\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 827, __extension__ __PRETTY_FUNCTION__))
;
828
829 // Merge MMOs from memory operations in the common block.
830 if (MBBICommon->mayLoad() || MBBICommon->mayStore())
831 MBBICommon->setMemRefs(MBBICommon->mergeMemRefsWith(*MBBI));
832 // Drop undef flags if they aren't present in all merged instructions.
833 for (unsigned I = 0, E = MBBICommon->getNumOperands(); I != E; ++I) {
834 MachineOperand &MO = MBBICommon->getOperand(I);
835 if (MO.isReg() && MO.isUndef()) {
836 const MachineOperand &OtherMO = MBBI->getOperand(I);
837 if (!OtherMO.isUndef())
838 MO.setIsUndef(false);
839 }
840 }
841
842 ++MBBI;
843 ++MBBICommon;
844 }
845}
846
847void BranchFolder::mergeCommonTails(unsigned commonTailIndex) {
848 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
849
850 std::vector<MachineBasicBlock::iterator> NextCommonInsts(SameTails.size());
851 for (unsigned int i = 0 ; i != SameTails.size() ; ++i) {
852 if (i != commonTailIndex) {
853 NextCommonInsts[i] = SameTails[i].getTailStartPos();
854 mergeOperations(SameTails[i].getTailStartPos(), *MBB);
855 } else {
856 assert(SameTails[i].getTailStartPos() == MBB->begin() &&(static_cast <bool> (SameTails[i].getTailStartPos() == MBB
->begin() && "MBB is not a common tail only block"
) ? void (0) : __assert_fail ("SameTails[i].getTailStartPos() == MBB->begin() && \"MBB is not a common tail only block\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 857, __extension__ __PRETTY_FUNCTION__))
857 "MBB is not a common tail only block")(static_cast <bool> (SameTails[i].getTailStartPos() == MBB
->begin() && "MBB is not a common tail only block"
) ? void (0) : __assert_fail ("SameTails[i].getTailStartPos() == MBB->begin() && \"MBB is not a common tail only block\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 857, __extension__ __PRETTY_FUNCTION__))
;
858 }
859 }
860
861 for (auto &MI : *MBB) {
862 if (MI.isDebugValue())
863 continue;
864 DebugLoc DL = MI.getDebugLoc();
865 for (unsigned int i = 0 ; i < NextCommonInsts.size() ; i++) {
866 if (i == commonTailIndex)
867 continue;
868
869 auto &Pos = NextCommonInsts[i];
870 assert(Pos != SameTails[i].getBlock()->end() &&(static_cast <bool> (Pos != SameTails[i].getBlock()->
end() && "Reached BB end within common tail") ? void (
0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 871, __extension__ __PRETTY_FUNCTION__))
871 "Reached BB end within common tail")(static_cast <bool> (Pos != SameTails[i].getBlock()->
end() && "Reached BB end within common tail") ? void (
0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 871, __extension__ __PRETTY_FUNCTION__))
;
872 while (Pos->isDebugValue()) {
873 ++Pos;
874 assert(Pos != SameTails[i].getBlock()->end() &&(static_cast <bool> (Pos != SameTails[i].getBlock()->
end() && "Reached BB end within common tail") ? void (
0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 875, __extension__ __PRETTY_FUNCTION__))
875 "Reached BB end within common tail")(static_cast <bool> (Pos != SameTails[i].getBlock()->
end() && "Reached BB end within common tail") ? void (
0) : __assert_fail ("Pos != SameTails[i].getBlock()->end() && \"Reached BB end within common tail\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 875, __extension__ __PRETTY_FUNCTION__))
;
876 }
877 assert(MI.isIdenticalTo(*Pos) && "Expected matching MIIs!")(static_cast <bool> (MI.isIdenticalTo(*Pos) && "Expected matching MIIs!"
) ? void (0) : __assert_fail ("MI.isIdenticalTo(*Pos) && \"Expected matching MIIs!\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 877, __extension__ __PRETTY_FUNCTION__))
;
878 DL = DILocation::getMergedLocation(DL, Pos->getDebugLoc());
879 NextCommonInsts[i] = ++Pos;
880 }
881 MI.setDebugLoc(DL);
882 }
883
884 if (UpdateLiveIns) {
885 LivePhysRegs NewLiveIns(*TRI);
886 computeLiveIns(NewLiveIns, *MBB);
887
888 // The flag merging may lead to some register uses no longer using the
889 // <undef> flag, add IMPLICIT_DEFs in the predecessors as necessary.
890 for (MachineBasicBlock *Pred : MBB->predecessors()) {
891 LiveRegs.init(*TRI);
892 LiveRegs.addLiveOuts(*Pred);
893 MachineBasicBlock::iterator InsertBefore = Pred->getFirstTerminator();
894 for (unsigned Reg : NewLiveIns) {
895 if (!LiveRegs.available(*MRI, Reg))
896 continue;
897 DebugLoc DL;
898 BuildMI(*Pred, InsertBefore, DL, TII->get(TargetOpcode::IMPLICIT_DEF),
899 Reg);
900 }
901 }
902
903 MBB->clearLiveIns();
904 addLiveIns(*MBB, NewLiveIns);
905 }
906}
907
908// See if any of the blocks in MergePotentials (which all have SuccBB as a
909// successor, or all have no successor if it is null) can be tail-merged.
910// If there is a successor, any blocks in MergePotentials that are not
911// tail-merged and are not immediately before Succ must have an unconditional
912// branch to Succ added (but the predecessor/successor lists need no
913// adjustment). The lone predecessor of Succ that falls through into Succ,
914// if any, is given in PredBB.
915// MinCommonTailLength - Except for the special cases below, tail-merge if
916// there are at least this many instructions in common.
917bool BranchFolder::TryTailMergeBlocks(MachineBasicBlock *SuccBB,
918 MachineBasicBlock *PredBB,
919 unsigned MinCommonTailLength) {
920 bool MadeChange = false;
921
922 DEBUG(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)
923 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)
924 << 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)
925 << (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)
926 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)
927 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)
928 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)
929 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)
930 << 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)
931 } 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)
932 << 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)
933 << (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)
;
934
935 // Sort by hash value so that blocks with identical end sequences sort
936 // together.
937 array_pod_sort(MergePotentials.begin(), MergePotentials.end());
938
939 // Walk through equivalence sets looking for actual exact matches.
940 while (MergePotentials.size() > 1) {
941 unsigned CurHash = MergePotentials.back().getHash();
942
943 // Build SameTails, identifying the set of blocks with this hash code
944 // and with the maximum number of instructions in common.
945 unsigned maxCommonTailLength = ComputeSameTails(CurHash,
946 MinCommonTailLength,
947 SuccBB, PredBB);
948
949 // If we didn't find any pair that has at least MinCommonTailLength
950 // instructions in common, remove all blocks with this hash code and retry.
951 if (SameTails.empty()) {
952 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
953 continue;
954 }
955
956 // If one of the blocks is the entire common tail (and not the entry
957 // block, which we can't jump to), we can treat all blocks with this same
958 // tail at once. Use PredBB if that is one of the possibilities, as that
959 // will not introduce any extra branches.
960 MachineBasicBlock *EntryBB =
961 &MergePotentials.front().getBlock()->getParent()->front();
962 unsigned commonTailIndex = SameTails.size();
963 // If there are two blocks, check to see if one can be made to fall through
964 // into the other.
965 if (SameTails.size() == 2 &&
966 SameTails[0].getBlock()->isLayoutSuccessor(SameTails[1].getBlock()) &&
967 SameTails[1].tailIsWholeBlock())
968 commonTailIndex = 1;
969 else if (SameTails.size() == 2 &&
970 SameTails[1].getBlock()->isLayoutSuccessor(
971 SameTails[0].getBlock()) &&
972 SameTails[0].tailIsWholeBlock())
973 commonTailIndex = 0;
974 else {
975 // Otherwise just pick one, favoring the fall-through predecessor if
976 // there is one.
977 for (unsigned i = 0, e = SameTails.size(); i != e; ++i) {
978 MachineBasicBlock *MBB = SameTails[i].getBlock();
979 if (MBB == EntryBB && SameTails[i].tailIsWholeBlock())
980 continue;
981 if (MBB == PredBB) {
982 commonTailIndex = i;
983 break;
984 }
985 if (SameTails[i].tailIsWholeBlock())
986 commonTailIndex = i;
987 }
988 }
989
990 if (commonTailIndex == SameTails.size() ||
991 (SameTails[commonTailIndex].getBlock() == PredBB &&
992 !SameTails[commonTailIndex].tailIsWholeBlock())) {
993 // None of the blocks consist entirely of the common tail.
994 // Split a block so that one does.
995 if (!CreateCommonTailOnlyBlock(PredBB, SuccBB,
996 maxCommonTailLength, commonTailIndex)) {
997 RemoveBlocksWithHash(CurHash, SuccBB, PredBB);
998 continue;
999 }
1000 }
1001
1002 MachineBasicBlock *MBB = SameTails[commonTailIndex].getBlock();
1003
1004 // Recompute common tail MBB's edge weights and block frequency.
1005 setCommonTailEdgeWeights(*MBB);
1006
1007 // Merge debug locations, MMOs and undef flags across identical instructions
1008 // for common tail.
1009 mergeCommonTails(commonTailIndex);
1010
1011 // MBB is common tail. Adjust all other BB's to jump to this one.
1012 // Traversal must be forwards so erases work.
1013 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)
1014 << " for ")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nUsing common tail in "
<< printMBBReference(*MBB) << " for "; } } while
(false)
;
1015 for (unsigned int i=0, e = SameTails.size(); i != e; ++i) {
1016 if (commonTailIndex == i)
1017 continue;
1018 DEBUG(dbgs() << printMBBReference(*SameTails[i].getBlock())do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << printMBBReference(*SameTails
[i].getBlock()) << (i == e - 1 ? "" : ", "); } } while (
false)
1019 << (i == e - 1 ? "" : ", "))do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << printMBBReference(*SameTails
[i].getBlock()) << (i == e - 1 ? "" : ", "); } } while (
false)
;
1020 // Hack the end off BB i, making it jump to BB commonTailIndex instead.
1021 replaceTailWithBranchTo(SameTails[i].getTailStartPos(), *MBB);
1022 // BB i is no longer a predecessor of SuccBB; remove it from the worklist.
1023 MergePotentials.erase(SameTails[i].getMPIter());
1024 }
1025 DEBUG(dbgs() << "\n")do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\n"; } } while (false)
;
1026 // We leave commonTailIndex in the worklist in case there are other blocks
1027 // that match it with a smaller number of instructions.
1028 MadeChange = true;
1029 }
1030 return MadeChange;
1031}
1032
1033bool BranchFolder::TailMergeBlocks(MachineFunction &MF) {
1034 bool MadeChange = false;
1035 if (!EnableTailMerge) return MadeChange;
1036
1037 // First find blocks with no successors.
1038 // Block placement does not create new tail merging opportunities for these
1039 // blocks.
1040 if (!AfterBlockPlacement) {
1041 MergePotentials.clear();
1042 for (MachineBasicBlock &MBB : MF) {
1043 if (MergePotentials.size() == TailMergeThreshold)
1044 break;
1045 if (!TriedMerging.count(&MBB) && MBB.succ_empty())
1046 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(MBB), &MBB));
1047 }
1048
1049 // If this is a large problem, avoid visiting the same basic blocks
1050 // multiple times.
1051 if (MergePotentials.size() == TailMergeThreshold)
1052 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
1053 TriedMerging.insert(MergePotentials[i].getBlock());
1054
1055 // See if we can do any tail merging on those.
1056 if (MergePotentials.size() >= 2)
1057 MadeChange |= TryTailMergeBlocks(nullptr, nullptr, MinCommonTailLength);
1058 }
1059
1060 // Look at blocks (IBB) with multiple predecessors (PBB).
1061 // We change each predecessor to a canonical form, by
1062 // (1) temporarily removing any unconditional branch from the predecessor
1063 // to IBB, and
1064 // (2) alter conditional branches so they branch to the other block
1065 // not IBB; this may require adding back an unconditional branch to IBB
1066 // later, where there wasn't one coming in. E.g.
1067 // Bcc IBB
1068 // fallthrough to QBB
1069 // here becomes
1070 // Bncc QBB
1071 // with a conceptual B to IBB after that, which never actually exists.
1072 // With those changes, we see whether the predecessors' tails match,
1073 // and merge them if so. We change things out of canonical form and
1074 // back to the way they were later in the process. (OptimizeBranches
1075 // would undo some of this, but we can't use it, because we'd get into
1076 // a compile-time infinite loop repeatedly doing and undoing the same
1077 // transformations.)
1078
1079 for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
1080 I != E; ++I) {
1081 if (I->pred_size() < 2) continue;
1082 SmallPtrSet<MachineBasicBlock *, 8> UniquePreds;
1083 MachineBasicBlock *IBB = &*I;
1084 MachineBasicBlock *PredBB = &*std::prev(I);
1085 MergePotentials.clear();
1086 MachineLoop *ML;
1087
1088 // Bail if merging after placement and IBB is the loop header because
1089 // -- If merging predecessors that belong to the same loop as IBB, the
1090 // common tail of merged predecessors may become the loop top if block
1091 // placement is called again and the predecessors may branch to this common
1092 // tail and require more branches. This can be relaxed if
1093 // MachineBlockPlacement::findBestLoopTop is more flexible.
1094 // --If merging predecessors that do not belong to the same loop as IBB, the
1095 // loop info of IBB's loop and the other loops may be affected. Calling the
1096 // block placement again may make big change to the layout and eliminate the
1097 // reason to do tail merging here.
1098 if (AfterBlockPlacement && MLI) {
1099 ML = MLI->getLoopFor(IBB);
1100 if (ML && IBB == ML->getHeader())
1101 continue;
1102 }
1103
1104 for (MachineBasicBlock *PBB : I->predecessors()) {
1105 if (MergePotentials.size() == TailMergeThreshold)
1106 break;
1107
1108 if (TriedMerging.count(PBB))
1109 continue;
1110
1111 // Skip blocks that loop to themselves, can't tail merge these.
1112 if (PBB == IBB)
1113 continue;
1114
1115 // Visit each predecessor only once.
1116 if (!UniquePreds.insert(PBB).second)
1117 continue;
1118
1119 // Skip blocks which may jump to a landing pad. Can't tail merge these.
1120 if (PBB->hasEHPadSuccessor())
1121 continue;
1122
1123 // After block placement, only consider predecessors that belong to the
1124 // same loop as IBB. The reason is the same as above when skipping loop
1125 // header.
1126 if (AfterBlockPlacement && MLI)
1127 if (ML != MLI->getLoopFor(PBB))
1128 continue;
1129
1130 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
1131 SmallVector<MachineOperand, 4> Cond;
1132 if (!TII->analyzeBranch(*PBB, TBB, FBB, Cond, true)) {
1133 // Failing case: IBB is the target of a cbr, and we cannot reverse the
1134 // branch.
1135 SmallVector<MachineOperand, 4> NewCond(Cond);
1136 if (!Cond.empty() && TBB == IBB) {
1137 if (TII->reverseBranchCondition(NewCond))
1138 continue;
1139 // This is the QBB case described above
1140 if (!FBB) {
1141 auto Next = ++PBB->getIterator();
1142 if (Next != MF.end())
1143 FBB = &*Next;
1144 }
1145 }
1146
1147 // Failing case: the only way IBB can be reached from PBB is via
1148 // exception handling. Happens for landing pads. Would be nice to have
1149 // a bit in the edge so we didn't have to do all this.
1150 if (IBB->isEHPad()) {
1151 MachineFunction::iterator IP = ++PBB->getIterator();
1152 MachineBasicBlock *PredNextBB = nullptr;
1153 if (IP != MF.end())
1154 PredNextBB = &*IP;
1155 if (!TBB) {
1156 if (IBB != PredNextBB) // fallthrough
1157 continue;
1158 } else if (FBB) {
1159 if (TBB != IBB && FBB != IBB) // cbr then ubr
1160 continue;
1161 } else if (Cond.empty()) {
1162 if (TBB != IBB) // ubr
1163 continue;
1164 } else {
1165 if (TBB != IBB && IBB != PredNextBB) // cbr
1166 continue;
1167 }
1168 }
1169
1170 // Remove the unconditional branch at the end, if any.
1171 if (TBB && (Cond.empty() || FBB)) {
1172 DebugLoc dl = PBB->findBranchDebugLoc();
1173 TII->removeBranch(*PBB);
1174 if (!Cond.empty())
1175 // reinsert conditional branch only, for now
1176 TII->insertBranch(*PBB, (TBB == IBB) ? FBB : TBB, nullptr,
1177 NewCond, dl);
1178 }
1179
1180 MergePotentials.push_back(MergePotentialsElt(HashEndOfMBB(*PBB), PBB));
1181 }
1182 }
1183
1184 // If this is a large problem, avoid visiting the same basic blocks multiple
1185 // times.
1186 if (MergePotentials.size() == TailMergeThreshold)
1187 for (unsigned i = 0, e = MergePotentials.size(); i != e; ++i)
1188 TriedMerging.insert(MergePotentials[i].getBlock());
1189
1190 if (MergePotentials.size() >= 2)
1191 MadeChange |= TryTailMergeBlocks(IBB, PredBB, MinCommonTailLength);
1192
1193 // Reinsert an unconditional branch if needed. The 1 below can occur as a
1194 // result of removing blocks in TryTailMergeBlocks.
1195 PredBB = &*std::prev(I); // this may have been changed in TryTailMergeBlocks
1196 if (MergePotentials.size() == 1 &&
1197 MergePotentials.begin()->getBlock() != PredBB)
1198 FixTail(MergePotentials.begin()->getBlock(), IBB, TII);
1199 }
1200
1201 return MadeChange;
1202}
1203
1204void BranchFolder::setCommonTailEdgeWeights(MachineBasicBlock &TailMBB) {
1205 SmallVector<BlockFrequency, 2> EdgeFreqLs(TailMBB.succ_size());
1206 BlockFrequency AccumulatedMBBFreq;
1207
1208 // Aggregate edge frequency of successor edge j:
1209 // edgeFreq(j) = sum (freq(bb) * edgeProb(bb, j)),
1210 // where bb is a basic block that is in SameTails.
1211 for (const auto &Src : SameTails) {
1212 const MachineBasicBlock *SrcMBB = Src.getBlock();
1213 BlockFrequency BlockFreq = MBBFreqInfo.getBlockFreq(SrcMBB);
1214 AccumulatedMBBFreq += BlockFreq;
1215
1216 // It is not necessary to recompute edge weights if TailBB has less than two
1217 // successors.
1218 if (TailMBB.succ_size() <= 1)
1219 continue;
1220
1221 auto EdgeFreq = EdgeFreqLs.begin();
1222
1223 for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
1224 SuccI != SuccE; ++SuccI, ++EdgeFreq)
1225 *EdgeFreq += BlockFreq * MBPI.getEdgeProbability(SrcMBB, *SuccI);
1226 }
1227
1228 MBBFreqInfo.setBlockFreq(&TailMBB, AccumulatedMBBFreq);
1229
1230 if (TailMBB.succ_size() <= 1)
1231 return;
1232
1233 auto SumEdgeFreq =
1234 std::accumulate(EdgeFreqLs.begin(), EdgeFreqLs.end(), BlockFrequency(0))
1235 .getFrequency();
1236 auto EdgeFreq = EdgeFreqLs.begin();
1237
1238 if (SumEdgeFreq > 0) {
1239 for (auto SuccI = TailMBB.succ_begin(), SuccE = TailMBB.succ_end();
1240 SuccI != SuccE; ++SuccI, ++EdgeFreq) {
1241 auto Prob = BranchProbability::getBranchProbability(
1242 EdgeFreq->getFrequency(), SumEdgeFreq);
1243 TailMBB.setSuccProbability(SuccI, Prob);
1244 }
1245 }
1246}
1247
1248//===----------------------------------------------------------------------===//
1249// Branch Optimization
1250//===----------------------------------------------------------------------===//
1251
1252bool BranchFolder::OptimizeBranches(MachineFunction &MF) {
1253 bool MadeChange = false;
1254
1255 // Make sure blocks are numbered in order
1256 MF.RenumberBlocks();
1257 // Renumbering blocks alters funclet membership, recalculate it.
1258 FuncletMembership = getFuncletMembership(MF);
1259
1260 for (MachineFunction::iterator I = std::next(MF.begin()), E = MF.end();
1261 I != E; ) {
1262 MachineBasicBlock *MBB = &*I++;
1263 MadeChange |= OptimizeBlock(MBB);
1264
1265 // If it is dead, remove it.
1266 if (MBB->pred_empty()) {
1267 RemoveDeadBlock(MBB);
1268 MadeChange = true;
1269 ++NumDeadBlocks;
1270 }
1271 }
1272
1273 return MadeChange;
1274}
1275
1276// Blocks should be considered empty if they contain only debug info;
1277// else the debug info would affect codegen.
1278static bool IsEmptyBlock(MachineBasicBlock *MBB) {
1279 return MBB->getFirstNonDebugInstr() == MBB->end();
1280}
1281
1282// Blocks with only debug info and branches should be considered the same
1283// as blocks with only branches.
1284static bool IsBranchOnlyBlock(MachineBasicBlock *MBB) {
1285 MachineBasicBlock::iterator I = MBB->getFirstNonDebugInstr();
1286 assert(I != MBB->end() && "empty block!")(static_cast <bool> (I != MBB->end() && "empty block!"
) ? void (0) : __assert_fail ("I != MBB->end() && \"empty block!\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 1286, __extension__ __PRETTY_FUNCTION__))
;
1287 return I->isBranch();
1288}
1289
1290/// IsBetterFallthrough - Return true if it would be clearly better to
1291/// fall-through to MBB1 than to fall through into MBB2. This has to return
1292/// a strict ordering, returning true for both (MBB1,MBB2) and (MBB2,MBB1) will
1293/// result in infinite loops.
1294static bool IsBetterFallthrough(MachineBasicBlock *MBB1,
1295 MachineBasicBlock *MBB2) {
1296 // Right now, we use a simple heuristic. If MBB2 ends with a call, and
1297 // MBB1 doesn't, we prefer to fall through into MBB1. This allows us to
1298 // optimize branches that branch to either a return block or an assert block
1299 // into a fallthrough to the return.
1300 MachineBasicBlock::iterator MBB1I = MBB1->getLastNonDebugInstr();
16
Called C++ object pointer is null
1301 MachineBasicBlock::iterator MBB2I = MBB2->getLastNonDebugInstr();
1302 if (MBB1I == MBB1->end() || MBB2I == MBB2->end())
1303 return false;
1304
1305 // If there is a clear successor ordering we make sure that one block
1306 // will fall through to the next
1307 if (MBB1->isSuccessor(MBB2)) return true;
1308 if (MBB2->isSuccessor(MBB1)) return false;
1309
1310 return MBB2I->isCall() && !MBB1I->isCall();
1311}
1312
1313/// getBranchDebugLoc - Find and return, if any, the DebugLoc of the branch
1314/// instructions on the block.
1315static DebugLoc getBranchDebugLoc(MachineBasicBlock &MBB) {
1316 MachineBasicBlock::iterator I = MBB.getLastNonDebugInstr();
1317 if (I != MBB.end() && I->isBranch())
1318 return I->getDebugLoc();
1319 return DebugLoc();
1320}
1321
1322bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
1323 bool MadeChange = false;
1324 MachineFunction &MF = *MBB->getParent();
1325ReoptimizeBlock:
1326
1327 MachineFunction::iterator FallThrough = MBB->getIterator();
1328 ++FallThrough;
1329
1330 // Make sure MBB and FallThrough belong to the same funclet.
1331 bool SameFunclet = true;
1332 if (!FuncletMembership.empty() && FallThrough != MF.end()) {
1
Assuming the condition is false
2
Taking false branch
1333 auto MBBFunclet = FuncletMembership.find(MBB);
1334 assert(MBBFunclet != FuncletMembership.end())(static_cast <bool> (MBBFunclet != FuncletMembership.end
()) ? void (0) : __assert_fail ("MBBFunclet != FuncletMembership.end()"
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 1334, __extension__ __PRETTY_FUNCTION__))
;
1335 auto FallThroughFunclet = FuncletMembership.find(&*FallThrough);
1336 assert(FallThroughFunclet != FuncletMembership.end())(static_cast <bool> (FallThroughFunclet != FuncletMembership
.end()) ? void (0) : __assert_fail ("FallThroughFunclet != FuncletMembership.end()"
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 1336, __extension__ __PRETTY_FUNCTION__))
;
1337 SameFunclet = MBBFunclet->second == FallThroughFunclet->second;
1338 }
1339
1340 // If this block is empty, make everyone use its fall-through, not the block
1341 // explicitly. Landing pads should not do this since the landing-pad table
1342 // points to this block. Blocks with their addresses taken shouldn't be
1343 // optimized away.
1344 if (IsEmptyBlock(MBB) && !MBB->isEHPad() && !MBB->hasAddressTaken() &&
1345 SameFunclet) {
1346 // Dead block? Leave for cleanup later.
1347 if (MBB->pred_empty()) return MadeChange;
1348
1349 if (FallThrough == MF.end()) {
1350 // TODO: Simplify preds to not branch here if possible!
1351 } else if (FallThrough->isEHPad()) {
1352 // Don't rewrite to a landing pad fallthough. That could lead to the case
1353 // where a BB jumps to more than one landing pad.
1354 // TODO: Is it ever worth rewriting predecessors which don't already
1355 // jump to a landing pad, and so can safely jump to the fallthrough?
1356 } else if (MBB->isSuccessor(&*FallThrough)) {
1357 // Rewrite all predecessors of the old block to go to the fallthrough
1358 // instead.
1359 while (!MBB->pred_empty()) {
1360 MachineBasicBlock *Pred = *(MBB->pred_end()-1);
1361 Pred->ReplaceUsesOfBlockWith(MBB, &*FallThrough);
1362 }
1363 // If MBB was the target of a jump table, update jump tables to go to the
1364 // fallthrough instead.
1365 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1366 MJTI->ReplaceMBBInJumpTables(MBB, &*FallThrough);
1367 MadeChange = true;
1368 }
1369 return MadeChange;
1370 }
1371
1372 // Check to see if we can simplify the terminator of the block before this
1373 // one.
1374 MachineBasicBlock &PrevBB = *std::prev(MachineFunction::iterator(MBB));
1375
1376 MachineBasicBlock *PriorTBB = nullptr, *PriorFBB = nullptr;
1377 SmallVector<MachineOperand, 4> PriorCond;
1378 bool PriorUnAnalyzable =
1379 TII->analyzeBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, true);
3
Value assigned to 'PriorTBB'
1380 if (!PriorUnAnalyzable) {
4
Assuming 'PriorUnAnalyzable' is 0
5
Taking true branch
1381 // If the CFG for the prior block has extra edges, remove them.
1382 MadeChange |= PrevBB.CorrectExtraCFGEdges(PriorTBB, PriorFBB,
1383 !PriorCond.empty());
1384
1385 // If the previous branch is conditional and both conditions go to the same
1386 // destination, remove the branch, replacing it with an unconditional one or
1387 // a fall-through.
1388 if (PriorTBB && PriorTBB == PriorFBB) {
6
Assuming 'PriorTBB' is null
1389 DebugLoc dl = getBranchDebugLoc(PrevBB);
1390 TII->removeBranch(PrevBB);
1391 PriorCond.clear();
1392 if (PriorTBB != MBB)
1393 TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1394 MadeChange = true;
1395 ++NumBranchOpts;
1396 goto ReoptimizeBlock;
1397 }
1398
1399 // If the previous block unconditionally falls through to this block and
1400 // this block has no other predecessors, move the contents of this block
1401 // into the prior block. This doesn't usually happen when SimplifyCFG
1402 // has been used, but it can happen if tail merging splits a fall-through
1403 // predecessor of a block.
1404 // This has to check PrevBB->succ_size() because EH edges are ignored by
1405 // AnalyzeBranch.
1406 if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
1407 PrevBB.succ_size() == 1 &&
1408 !MBB->hasAddressTaken() && !MBB->isEHPad()) {
1409 DEBUG(dbgs() << "\nMerging into block: " << PrevBBdo { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nMerging into block: "
<< PrevBB << "From MBB: " << *MBB; } } while
(false)
1410 << "From MBB: " << *MBB)do { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nMerging into block: "
<< PrevBB << "From MBB: " << *MBB; } } while
(false)
;
1411 // Remove redundant DBG_VALUEs first.
1412 if (PrevBB.begin() != PrevBB.end()) {
1413 MachineBasicBlock::iterator PrevBBIter = PrevBB.end();
1414 --PrevBBIter;
1415 MachineBasicBlock::iterator MBBIter = MBB->begin();
1416 // Check if DBG_VALUE at the end of PrevBB is identical to the
1417 // DBG_VALUE at the beginning of MBB.
1418 while (PrevBBIter != PrevBB.begin() && MBBIter != MBB->end()
1419 && PrevBBIter->isDebugValue() && MBBIter->isDebugValue()) {
1420 if (!MBBIter->isIdenticalTo(*PrevBBIter))
1421 break;
1422 MachineInstr &DuplicateDbg = *MBBIter;
1423 ++MBBIter; -- PrevBBIter;
1424 DuplicateDbg.eraseFromParent();
1425 }
1426 }
1427 PrevBB.splice(PrevBB.end(), MBB, MBB->begin(), MBB->end());
1428 PrevBB.removeSuccessor(PrevBB.succ_begin());
1429 assert(PrevBB.succ_empty())(static_cast <bool> (PrevBB.succ_empty()) ? void (0) : __assert_fail
("PrevBB.succ_empty()", "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 1429, __extension__ __PRETTY_FUNCTION__))
;
1430 PrevBB.transferSuccessors(MBB);
1431 MadeChange = true;
1432 return MadeChange;
1433 }
1434
1435 // If the previous branch *only* branches to *this* block (conditional or
1436 // not) remove the branch.
1437 if (PriorTBB == MBB && !PriorFBB) {
1438 TII->removeBranch(PrevBB);
1439 MadeChange = true;
1440 ++NumBranchOpts;
1441 goto ReoptimizeBlock;
1442 }
1443
1444 // If the prior block branches somewhere else on the condition and here if
1445 // the condition is false, remove the uncond second branch.
1446 if (PriorFBB == MBB) {
7
Assuming 'PriorFBB' is not equal to 'MBB'
8
Taking false branch
1447 DebugLoc dl = getBranchDebugLoc(PrevBB);
1448 TII->removeBranch(PrevBB);
1449 TII->insertBranch(PrevBB, PriorTBB, nullptr, PriorCond, dl);
1450 MadeChange = true;
1451 ++NumBranchOpts;
1452 goto ReoptimizeBlock;
1453 }
1454
1455 // If the prior block branches here on true and somewhere else on false, and
1456 // if the branch condition is reversible, reverse the branch to create a
1457 // fall-through.
1458 if (PriorTBB == MBB) {
9
Taking false branch
1459 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1460 if (!TII->reverseBranchCondition(NewPriorCond)) {
1461 DebugLoc dl = getBranchDebugLoc(PrevBB);
1462 TII->removeBranch(PrevBB);
1463 TII->insertBranch(PrevBB, PriorFBB, nullptr, NewPriorCond, dl);
1464 MadeChange = true;
1465 ++NumBranchOpts;
1466 goto ReoptimizeBlock;
1467 }
1468 }
1469
1470 // If this block has no successors (e.g. it is a return block or ends with
1471 // a call to a no-return function like abort or __cxa_throw) and if the pred
1472 // falls through into this block, and if it would otherwise fall through
1473 // into the block after this, move this block to the end of the function.
1474 //
1475 // We consider it more likely that execution will stay in the function (e.g.
1476 // due to loops) than it is to exit it. This asserts in loops etc, moving
1477 // the assert condition out of the loop body.
1478 if (MBB->succ_empty() && !PriorCond.empty() && !PriorFBB &&
10
Assuming the condition is true
11
Assuming 'PriorFBB' is null
13
Taking true branch
1479 MachineFunction::iterator(PriorTBB) == FallThrough &&
1480 !MBB->canFallThrough()) {
12
Assuming the condition is true
1481 bool DoTransform = true;
1482
1483 // We have to be careful that the succs of PredBB aren't both no-successor
1484 // blocks. If neither have successors and if PredBB is the second from
1485 // last block in the function, we'd just keep swapping the two blocks for
1486 // last. Only do the swap if one is clearly better to fall through than
1487 // the other.
1488 if (FallThrough == --MF.end() &&
1489 !IsBetterFallthrough(PriorTBB, MBB))
14
Passing null pointer value via 1st parameter 'MBB1'
15
Calling 'IsBetterFallthrough'
1490 DoTransform = false;
1491
1492 if (DoTransform) {
1493 // Reverse the branch so we will fall through on the previous true cond.
1494 SmallVector<MachineOperand, 4> NewPriorCond(PriorCond);
1495 if (!TII->reverseBranchCondition(NewPriorCond)) {
1496 DEBUG(dbgs() << "\nMoving MBB: " << *MBBdo { if (::llvm::DebugFlag && ::llvm::isCurrentDebugType
("branch-folder")) { dbgs() << "\nMoving MBB: " <<
*MBB << "To make fallthrough to: " << *PriorTBB <<
"\n"; } } while (false)
1497 << "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)
;
1498
1499 DebugLoc dl = getBranchDebugLoc(PrevBB);
1500 TII->removeBranch(PrevBB);
1501 TII->insertBranch(PrevBB, MBB, nullptr, NewPriorCond, dl);
1502
1503 // Move this block to the end of the function.
1504 MBB->moveAfter(&MF.back());
1505 MadeChange = true;
1506 ++NumBranchOpts;
1507 return MadeChange;
1508 }
1509 }
1510 }
1511 }
1512
1513 if (!IsEmptyBlock(MBB) && MBB->pred_size() == 1 &&
1514 MF.getFunction().optForSize()) {
1515 // Changing "Jcc foo; foo: jmp bar;" into "Jcc bar;" might change the branch
1516 // direction, thereby defeating careful block placement and regressing
1517 // performance. Therefore, only consider this for optsize functions.
1518 MachineInstr &TailCall = *MBB->getFirstNonDebugInstr();
1519 if (TII->isUnconditionalTailCall(TailCall)) {
1520 MachineBasicBlock *Pred = *MBB->pred_begin();
1521 MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
1522 SmallVector<MachineOperand, 4> PredCond;
1523 bool PredAnalyzable =
1524 !TII->analyzeBranch(*Pred, PredTBB, PredFBB, PredCond, true);
1525
1526 if (PredAnalyzable && !PredCond.empty() && PredTBB == MBB &&
1527 PredTBB != PredFBB) {
1528 // The predecessor has a conditional branch to this block which consists
1529 // of only a tail call. Try to fold the tail call into the conditional
1530 // branch.
1531 if (TII->canMakeTailCallConditional(PredCond, TailCall)) {
1532 // TODO: It would be nice if analyzeBranch() could provide a pointer
1533 // to the branch instruction so replaceBranchWithTailCall() doesn't
1534 // have to search for it.
1535 TII->replaceBranchWithTailCall(*Pred, PredCond, TailCall);
1536 ++NumTailCalls;
1537 Pred->removeSuccessor(MBB);
1538 MadeChange = true;
1539 return MadeChange;
1540 }
1541 }
1542 // If the predecessor is falling through to this block, we could reverse
1543 // the branch condition and fold the tail call into that. However, after
1544 // that we might have to re-arrange the CFG to fall through to the other
1545 // block and there is a high risk of regressing code size rather than
1546 // improving it.
1547 }
1548 }
1549
1550 // Analyze the branch in the current block.
1551 MachineBasicBlock *CurTBB = nullptr, *CurFBB = nullptr;
1552 SmallVector<MachineOperand, 4> CurCond;
1553 bool CurUnAnalyzable =
1554 TII->analyzeBranch(*MBB, CurTBB, CurFBB, CurCond, true);
1555 if (!CurUnAnalyzable) {
1556 // If the CFG for the prior block has extra edges, remove them.
1557 MadeChange |= MBB->CorrectExtraCFGEdges(CurTBB, CurFBB, !CurCond.empty());
1558
1559 // If this is a two-way branch, and the FBB branches to this block, reverse
1560 // the condition so the single-basic-block loop is faster. Instead of:
1561 // Loop: xxx; jcc Out; jmp Loop
1562 // we want:
1563 // Loop: xxx; jncc Loop; jmp Out
1564 if (CurTBB && CurFBB && CurFBB == MBB && CurTBB != MBB) {
1565 SmallVector<MachineOperand, 4> NewCond(CurCond);
1566 if (!TII->reverseBranchCondition(NewCond)) {
1567 DebugLoc dl = getBranchDebugLoc(*MBB);
1568 TII->removeBranch(*MBB);
1569 TII->insertBranch(*MBB, CurFBB, CurTBB, NewCond, dl);
1570 MadeChange = true;
1571 ++NumBranchOpts;
1572 goto ReoptimizeBlock;
1573 }
1574 }
1575
1576 // If this branch is the only thing in its block, see if we can forward
1577 // other blocks across it.
1578 if (CurTBB && CurCond.empty() && !CurFBB &&
1579 IsBranchOnlyBlock(MBB) && CurTBB != MBB &&
1580 !MBB->hasAddressTaken() && !MBB->isEHPad()) {
1581 DebugLoc dl = getBranchDebugLoc(*MBB);
1582 // This block may contain just an unconditional branch. Because there can
1583 // be 'non-branch terminators' in the block, try removing the branch and
1584 // then seeing if the block is empty.
1585 TII->removeBranch(*MBB);
1586 // If the only things remaining in the block are debug info, remove these
1587 // as well, so this will behave the same as an empty block in non-debug
1588 // mode.
1589 if (IsEmptyBlock(MBB)) {
1590 // Make the block empty, losing the debug info (we could probably
1591 // improve this in some cases.)
1592 MBB->erase(MBB->begin(), MBB->end());
1593 }
1594 // If this block is just an unconditional branch to CurTBB, we can
1595 // usually completely eliminate the block. The only case we cannot
1596 // completely eliminate the block is when the block before this one
1597 // falls through into MBB and we can't understand the prior block's branch
1598 // condition.
1599 if (MBB->empty()) {
1600 bool PredHasNoFallThrough = !PrevBB.canFallThrough();
1601 if (PredHasNoFallThrough || !PriorUnAnalyzable ||
1602 !PrevBB.isSuccessor(MBB)) {
1603 // If the prior block falls through into us, turn it into an
1604 // explicit branch to us to make updates simpler.
1605 if (!PredHasNoFallThrough && PrevBB.isSuccessor(MBB) &&
1606 PriorTBB != MBB && PriorFBB != MBB) {
1607 if (!PriorTBB) {
1608 assert(PriorCond.empty() && !PriorFBB &&(static_cast <bool> (PriorCond.empty() && !PriorFBB
&& "Bad branch analysis") ? void (0) : __assert_fail
("PriorCond.empty() && !PriorFBB && \"Bad branch analysis\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 1609, __extension__ __PRETTY_FUNCTION__))
1609 "Bad branch analysis")(static_cast <bool> (PriorCond.empty() && !PriorFBB
&& "Bad branch analysis") ? void (0) : __assert_fail
("PriorCond.empty() && !PriorFBB && \"Bad branch analysis\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 1609, __extension__ __PRETTY_FUNCTION__))
;
1610 PriorTBB = MBB;
1611 } else {
1612 assert(!PriorFBB && "Machine CFG out of date!")(static_cast <bool> (!PriorFBB && "Machine CFG out of date!"
) ? void (0) : __assert_fail ("!PriorFBB && \"Machine CFG out of date!\""
, "/build/llvm-toolchain-snapshot-7~svn329677/lib/CodeGen/BranchFolding.cpp"
, 1612, __extension__ __PRETTY_FUNCTION__))
;
1613 PriorFBB = MBB;
1614 }
1615 DebugLoc pdl = getBranchDebugLoc(PrevBB);
1616 TII->removeBranch(PrevBB);
1617 TII->insertBranch(PrevBB, PriorTBB, PriorFBB, PriorCond, pdl);
1618 }
1619
1620 // Iterate through all the predecessors, revectoring each in-turn.
1621 size_t PI = 0;
1622 bool DidChange = false;
1623 bool HasBranchToSelf = false;
1624 while(PI != MBB->pred_size()) {
1625 MachineBasicBlock *PMBB = *(MBB->pred_begin() + PI);
1626 if (PMBB == MBB) {
1627 // If this block has an uncond branch to itself, leave it.
1628 ++PI;
1629 HasBranchToSelf = true;
1630 } else {
1631 DidChange = true;
1632 PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
1633 // If this change resulted in PMBB ending in a conditional
1634 // branch where both conditions go to the same destination,
1635 // change this to an unconditional branch (and fix the CFG).
1636 MachineBasicBlock *NewCurTBB = nullptr, *NewCurFBB = nullptr;
1637 SmallVector<MachineOperand, 4> NewCurCond;
1638 bool NewCurUnAnalyzable = TII->analyzeBranch(
1639 *PMBB, NewCurTBB, NewCurFBB, NewCurCond, true);
1640 if (!NewCurUnAnalyzable && NewCurTBB && NewCurTBB == NewCurFBB) {
1641 DebugLoc pdl = getBranchDebugLoc(*PMBB);
1642 TII->removeBranch(*PMBB);
1643 NewCurCond.clear();
1644 TII->insertBranch(*PMBB, NewCurTBB, nullptr, NewCurCond, pdl);
1645 MadeChange = true;
1646 ++NumBranchOpts;
1647 PMBB->CorrectExtraCFGEdges(NewCurTBB, nullptr, false);
1648 }
1649 }
1650 }
1651
1652 // Change any jumptables to go to the new MBB.
1653 if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
1654 MJTI->ReplaceMBBInJumpTables(MBB, CurTBB);
1655 if (DidChange) {
1656 ++NumBranchOpts;
1657 MadeChange = true;
1658 if (!HasBranchToSelf) return MadeChange;
1659 }
1660 }
1661 }
1662
1663 // Add the branch back if the block is more than just an uncond branch.
1664 TII->insertBranch(*MBB, CurTBB, nullptr, CurCond, dl);
1665 }
1666 }
1667
1668 // If the prior block doesn't fall through into this block, and if this
1669 // block doesn't fall through into some other block, see if we can find a
1670 // place to move this block where a fall-through will happen.
1671 if (!PrevBB.canFallThrough()) {
1672 // Now we know that there was no fall-through into this block, check to
1673 // see if it has a fall-through into its successor.
1674 bool CurFallsThru = MBB->canFallThrough();
1675
1676 if (!MBB->isEHPad()) {
1677 // Check all the predecessors of this block. If one of them has no fall
1678 // throughs, move this block right after it.
1679 for (MachineBasicBlock *PredBB : MBB->predecessors()) {
1680 // Analyze the branch at the end of the pred.
1681 MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
1682 SmallVector<MachineOperand, 4> PredCond;
1683 if (PredBB != MBB && !PredBB->canFallThrough() &&
1684 !TII->analyzeBranch(*PredBB, PredTBB, PredFBB, PredCond, true) &&
1685 (!CurFallsThru || !CurTBB || !CurFBB) &&
1686 (!CurFallsThru || MBB->getNumber() >= PredBB->getNumber())) {
1687 // If the current block doesn't fall through, just move it.
1688 // If the current block can fall through and does not end with a
1689 // conditional branch, we need to append an unconditional jump to
1690 // the (current) next block. To avoid a possible compile-time
1691 // infinite loop, move blocks only backward in this case.
1692 // Also, if there are already 2 branches here, we cannot add a third;
1693 // this means we have the case
1694 // Bcc next
1695 // B elsewhere
1696 // next:
1697 if (CurFallsThru) {
1698 MachineBasicBlock *NextBB = &*std::next(MBB->getIterator());
1699 CurCond.clear();
1700 TII->insertBranch(*MBB, NextBB, nullptr, CurCond, DebugLoc());
1701 }
1702 MBB->moveAfter(PredBB);
1703 MadeChange = true;
1704 goto ReoptimizeBlock;
1705 }
1706 }
1707 }
1708
1709 if (!CurFallsThru) {
1710 // Check all successors to see if we can move this block before it.
1711 for (MachineBasicBlock *SuccBB : MBB->successors()) {
1712 // Analyze the branch at the end of the block before the succ.
1713 MachineFunction::iterator SuccPrev = --SuccBB->getIterator();
1714
1715 // If this block doesn't already fall-through to that successor, and if
1716 // the succ doesn't already have a block that can fall through into it,
1717 // and if the successor isn't an EH destination, we can arrange for the
1718 // fallthrough to happen.
1719 if (SuccBB != MBB && &*SuccPrev != MBB &&
1720 !SuccPrev->canFallThrough() && !CurUnAnalyzable &&
1721 !SuccBB->isEHPad()) {
1722 MBB->moveBefore(SuccBB);
1723 MadeChange = true;
1724 goto ReoptimizeBlock;
1725 }
1726 }
1727
1728 // Okay, there is no really great place to put this block. If, however,
1729 // the block before this one would be a fall-through if this block were
1730 // removed, move this block to the end of the function. There is no real
1731 // advantage in "falling through" to an EH block, so we don't want to
1732 // perform this transformation for that case.
1733 //
1734 // Also, Windows EH introduced the possibility of an arbitrary number of
1735 // successors to a given block. The analyzeBranch call does not consider
1736 // exception handling and so we can get in a state where a block
1737 // containing a call is followed by multiple EH blocks that would be
1738 // rotated infinitely at the end of the function if the transformation
1739 // below were performed for EH "FallThrough" blocks. Therefore, even if
1740 // that appears not to be happening anymore, we should assume that it is
1741 // possible and not remove the "!FallThrough()->isEHPad" condition below.
1742 MachineBasicBlock *PrevTBB = nullptr, *PrevFBB = nullptr;
1743 SmallVector<MachineOperand, 4> PrevCond;
1744 if (FallThrough != MF.end() &&
1745 !FallThrough->isEHPad() &&
1746 !TII->analyzeBranch(PrevBB, PrevTBB, PrevFBB, PrevCond, true) &&
1747 PrevBB.isSuccessor(&*FallThrough)) {
1748 MBB->moveAfter(&MF.back());
1749 MadeChange = true;
1750 return MadeChange;
1751 }
1752 }
1753 }
1754
1755 return MadeChange;
1756}
1757
1758//===----------------------------------------------------------------------===//
1759// Hoist Common Code
1760//===----------------------------------------------------------------------===//
1761
1762bool BranchFolder::HoistCommonCode(MachineFunction &MF) {
1763 bool MadeChange = false;
1764 for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ) {
1765 MachineBasicBlock *MBB = &*I++;
1766 MadeChange |= HoistCommonCodeInSuccs(MBB);
1767 }
1768
1769 return MadeChange;
1770}
1771
1772/// findFalseBlock - BB has a fallthrough. Find its 'false' successor given
1773/// its 'true' successor.
1774static MachineBasicBlock *findFalseBlock(MachineBasicBlock *BB,
1775 MachineBasicBlock *TrueBB) {
1776 for (MachineBasicBlock *SuccBB : BB->successors())
1777 if (SuccBB != TrueBB)
1778 return SuccBB;
1779 return nullptr;
1780}
1781
1782template <class Container>
1783static void addRegAndItsAliases(unsigned Reg, const TargetRegisterInfo *TRI,
1784 Container &Set) {
1785 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
1786 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
1787 Set.insert(*AI);
1788 } else {
1789 Set.insert(Reg);
1790 }
1791}
1792
1793/// findHoistingInsertPosAndDeps - Find the location to move common instructions
1794/// in successors to. The location is usually just before the terminator,
1795/// however if the terminator is a conditional branch and its previous
1796/// instruction is the flag setting instruction, the previous instruction is
1797/// the preferred location. This function also gathers uses and defs of the
1798/// instructions from the insertion point to the end of the block. The data is
1799/// used by HoistCommonCodeInSuccs to ensure safety.
1800static
1801MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
1802 const TargetInstrInfo *TII,
1803 const TargetRegisterInfo *TRI,
1804 SmallSet<unsigned,4> &Uses,
1805 SmallSet<unsigned,4> &Defs) {
1806 MachineBasicBlock::iterator Loc = MBB->getFirstTerminator();
1807 if (!TII->isUnpredicatedTerminator(*Loc))
1808 return MBB->end();
1809
1810 for (const MachineOperand &MO : Loc->operands()) {
1811 if (!MO.isReg())
1812 continue;
1813 unsigned Reg = MO.getReg();
1814 if (!Reg)
1815 continue;
1816 if (MO.isUse()) {
1817 addRegAndItsAliases(Reg, TRI, Uses);
1818 } else {
1819 if (!MO.isDead())
1820 // Don't try to hoist code in the rare case the terminator defines a
1821 // register that is later used.
1822 return MBB->end();
1823
1824 // If the terminator defines a register, make sure we don't hoist
1825 // the instruction whose def might be clobbered by the terminator.
1826 addRegAndItsAliases(Reg, TRI, Defs);
1827 }
1828 }
1829
1830 if (Uses.empty())
1831 return Loc;
1832 if (Loc == MBB->begin())
1833 return MBB->end();
1834
1835 // The terminator is probably a conditional branch, try not to separate the
1836 // branch from condition setting instruction.
1837 MachineBasicBlock::iterator PI =
1838 skipDebugInstructionsBackward(std::prev(Loc), MBB->begin());
1839
1840 bool IsDef = false;
1841 for (const MachineOperand &MO : PI->operands()) {
1842 // If PI has a regmask operand, it is probably a call. Separate away.
1843 if (MO.isRegMask())
1844 return Loc;
1845 if (!MO.isReg() || MO.isUse())
1846 continue;
1847 unsigned Reg = MO.getReg();
1848 if (!Reg)
1849 continue;
1850 if (Uses.count(Reg)) {
1851 IsDef = true;
1852 break;
1853 }
1854 }
1855 if (!IsDef)
1856 // The condition setting instruction is not just before the conditional
1857 // branch.
1858 return Loc;
1859
1860 // Be conservative, don't insert instruction above something that may have
1861 // side-effects. And since it's potentially bad to separate flag setting
1862 // instruction from the conditional branch, just abort the optimization
1863 // completely.
1864 // Also avoid moving code above predicated instruction since it's hard to
1865 // reason about register liveness with predicated instruction.
1866 bool DontMoveAcrossStore = true;
1867 if (!PI->isSafeToMove(nullptr, DontMoveAcrossStore) || TII->isPredicated(*PI))
1868 return MBB->end();
1869
1870 // Find out what registers are live. Note this routine is ignoring other live
1871 // registers which are only used by instructions in successor blocks.
1872 for (const MachineOperand &MO : PI->operands()) {
1873 if (!MO.isReg())
1874 continue;
1875 unsigned Reg = MO.getReg();
1876 if (!Reg)
1877 continue;
1878 if (MO.isUse()) {
1879 addRegAndItsAliases(Reg, TRI, Uses);
1880 } else {
1881 if (Uses.erase(Reg)) {
1882 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
1883 for (MCSubRegIterator SubRegs(Reg, TRI); SubRegs.isValid(); ++SubRegs)
1884 Uses.erase(*SubRegs); // Use sub-registers to be conservative
1885 }
1886 }
1887 addRegAndItsAliases(Reg, TRI, Defs);
1888 }
1889 }
1890
1891 return PI;
1892}
1893
1894bool BranchFolder::HoistCommonCodeInSuccs(MachineBasicBlock *MBB) {
1895 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
1896 SmallVector<MachineOperand, 4> Cond;
1897 if (TII->analyzeBranch(*MBB, TBB, FBB, Cond, true) || !TBB || Cond.empty())
1898 return false;
1899
1900 if (!FBB) FBB = findFalseBlock(MBB, TBB);
1901 if (!FBB)
1902 // Malformed bcc? True and false blocks are the same?
1903 return false;
1904
1905 // Restrict the optimization to cases where MBB is the only predecessor,
1906 // it is an obvious win.
1907 if (TBB->pred_size() > 1 || FBB->pred_size() > 1)
1908 return false;
1909
1910 // Find a suitable position to hoist the common instructions to. Also figure
1911 // out which registers are used or defined by instructions from the insertion
1912 // point to the end of the block.
1913 SmallSet<unsigned, 4> Uses, Defs;
1914 MachineBasicBlock::iterator Loc =
1915 findHoistingInsertPosAndDeps(MBB, TII, TRI, Uses, Defs);
1916 if (Loc == MBB->end())
1917 return false;
1918
1919 bool HasDups = false;
1920 SmallVector<unsigned, 4> LocalDefs, LocalKills;
1921 SmallSet<unsigned, 4> ActiveDefsSet, AllDefsSet;
1922 MachineBasicBlock::iterator TIB = TBB->begin();
1923 MachineBasicBlock::iterator FIB = FBB->begin();
1924 MachineBasicBlock::iterator TIE = TBB->end();
1925 MachineBasicBlock::iterator FIE = FBB->end();
1926 while (TIB != TIE && FIB != FIE) {
1927 // Skip dbg_value instructions. These do not count.
1928 TIB = skipDebugInstructionsForward(TIB, TIE);
1929 FIB = skipDebugInstructionsForward(FIB, FIE);
1930 if (TIB == TIE || FIB == FIE)
1931 break;
1932
1933 if (!TIB->isIdenticalTo(*FIB, MachineInstr::CheckKillDead))
1934 break;
1935
1936 if (TII->isPredicated(*TIB))
1937 // Hard to reason about register liveness with predicated instruction.
1938 break;
1939
1940 bool IsSafe = true;
1941 for (MachineOperand &MO : TIB->operands()) {
1942 // Don't attempt to hoist instructions with register masks.
1943 if (MO.isRegMask()) {
1944 IsSafe = false;
1945 break;
1946 }
1947 if (!MO.isReg())
1948 continue;
1949 unsigned Reg = MO.getReg();
1950 if (!Reg)
1951 continue;
1952 if (MO.isDef()) {
1953 if (Uses.count(Reg)) {
1954 // Avoid clobbering a register that's used by the instruction at
1955 // the point of insertion.
1956 IsSafe = false;
1957 break;
1958 }
1959
1960 if (Defs.count(Reg) && !MO.isDead()) {
1961 // Don't hoist the instruction if the def would be clobber by the
1962 // instruction at the point insertion. FIXME: This is overly
1963 // conservative. It should be possible to hoist the instructions
1964 // in BB2 in the following example:
1965 // BB1:
1966 // r1, eflag = op1 r2, r3
1967 // brcc eflag
1968 //
1969 // BB2:
1970 // r1 = op2, ...
1971 // = op3, killed r1
1972 IsSafe = false;
1973 break;
1974 }
1975 } else if (!ActiveDefsSet.count(Reg)) {
1976 if (Defs.count(Reg)) {
1977 // Use is defined by the instruction at the point of insertion.
1978 IsSafe = false;
1979 break;
1980 }
1981
1982 if (MO.isKill() && Uses.count(Reg))
1983 // Kills a register that's read by the instruction at the point of
1984 // insertion. Remove the kill marker.
1985 MO.setIsKill(false);
1986 }
1987 }
1988 if (!IsSafe)
1989 break;
1990
1991 bool DontMoveAcrossStore = true;
1992 if (!TIB->isSafeToMove(nullptr, DontMoveAcrossStore))
1993 break;
1994
1995 // Remove kills from ActiveDefsSet, these registers had short live ranges.
1996 for (const MachineOperand &MO : TIB->operands()) {
1997 if (!MO.isReg() || !MO.isUse() || !MO.isKill())
1998 continue;
1999 unsigned Reg = MO.getReg();
2000 if (!Reg)
2001 continue;
2002 if (!AllDefsSet.count(Reg)) {
2003 LocalKills.push_back(Reg);
2004 continue;
2005 }
2006 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
2007 for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
2008 ActiveDefsSet.erase(*AI);
2009 } else {
2010 ActiveDefsSet.erase(Reg);
2011 }
2012 }
2013
2014 // Track local defs so we can update liveins.
2015 for (const MachineOperand &MO : TIB->operands()) {
2016 if (!MO.isReg() || !MO.isDef() || MO.isDead())
2017 continue;
2018 unsigned Reg = MO.getReg();
2019 if (!Reg || TargetRegisterInfo::isVirtualRegister(Reg))
2020 continue;
2021 LocalDefs.push_back(Reg);
2022 addRegAndItsAliases(Reg, TRI, ActiveDefsSet);
2023 addRegAndItsAliases(Reg, TRI, AllDefsSet);
2024 }
2025
2026 HasDups = true;
2027 ++TIB;
2028 ++FIB;
2029 }
2030
2031 if (!HasDups)
2032 return false;
2033
2034 MBB->splice(Loc, TBB, TBB->begin(), TIB);
2035 FBB->erase(FBB->begin(), FIB);
2036
2037 // Update livein's.
2038 bool ChangedLiveIns = false;
2039 for (unsigned i = 0, e = LocalDefs.size(); i != e; ++i) {
2040 unsigned Def = LocalDefs[i];
2041 if (ActiveDefsSet.count(Def)) {
2042 TBB->addLiveIn(Def);
2043 FBB->addLiveIn(Def);
2044 ChangedLiveIns = true;
2045 }
2046 }
2047 for (unsigned K : LocalKills) {
2048 TBB->removeLiveIn(K);
2049 FBB->removeLiveIn(K);
2050 ChangedLiveIns = true;
2051 }
2052
2053 if (ChangedLiveIns) {
2054 TBB->sortUniqueLiveIns();
2055 FBB->sortUniqueLiveIns();
2056 }
2057
2058 ++NumHoist;
2059 return true;
2060}