1 //===- MergedLoadStoreMotion.cpp - merge and hoist/sink load/stores -------===//
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 //! \file
11 //! \brief This pass performs merges of loads and stores on both sides of a
12 // diamond (hammock). It hoists the loads and sinks the stores.
13 //
14 // The algorithm iteratively hoists two loads to the same address out of a
15 // diamond (hammock) and merges them into a single load in the header. Similar
16 // it sinks and merges two stores to the tail block (footer). The algorithm
17 // iterates over the instructions of one side of the diamond and attempts to
18 // find a matching load/store on the other side. It hoists / sinks when it
19 // thinks it safe to do so. This optimization helps with eg. hiding load
20 // latencies, triggering if-conversion, and reducing static code size.
21 //
22 //===----------------------------------------------------------------------===//
23 //
24 //
25 // Example:
26 // Diamond shaped code before merge:
27 //
28 // header:
29 // br %cond, label %if.then, label %if.else
30 // + +
31 // + +
32 // + +
33 // if.then: if.else:
34 // %lt = load %addr_l %le = load %addr_l
35 // <use %lt> <use %le>
36 // <...> <...>
37 // store %st, %addr_s store %se, %addr_s
38 // br label %if.end br label %if.end
39 // + +
40 // + +
41 // + +
42 // if.end ("footer"):
43 // <...>
44 //
45 // Diamond shaped code after merge:
46 //
47 // header:
48 // %l = load %addr_l
49 // br %cond, label %if.then, label %if.else
50 // + +
51 // + +
52 // + +
53 // if.then: if.else:
54 // <use %l> <use %l>
55 // <...> <...>
56 // br label %if.end br label %if.end
57 // + +
58 // + +
59 // + +
60 // if.end ("footer"):
61 // %s.sink = phi [%st, if.then], [%se, if.else]
62 // <...>
63 // store %s.sink, %addr_s
64 // <...>
65 //
66 //
67 //===----------------------- TODO -----------------------------------------===//
68 //
69 // 1) Generalize to regions other than diamonds
70 // 2) Be more aggressive merging memory operations
71 // Note that both changes require register pressure control
72 //
73 //===----------------------------------------------------------------------===//
74
75 #include "llvm/Transforms/Scalar.h"
76 #include "llvm/ADT/SetVector.h"
77 #include "llvm/ADT/SmallPtrSet.h"
78 #include "llvm/ADT/Statistic.h"
79 #include "llvm/Analysis/AliasAnalysis.h"
80 #include "llvm/Analysis/CFG.h"
81 #include "llvm/Analysis/GlobalsModRef.h"
82 #include "llvm/Analysis/Loads.h"
83 #include "llvm/Analysis/MemoryBuiltins.h"
84 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
85 #include "llvm/Analysis/TargetLibraryInfo.h"
86 #include "llvm/IR/Metadata.h"
87 #include "llvm/IR/PatternMatch.h"
88 #include "llvm/Support/Allocator.h"
89 #include "llvm/Support/CommandLine.h"
90 #include "llvm/Support/Debug.h"
91 #include "llvm/Support/raw_ostream.h"
92 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
93 #include "llvm/Transforms/Utils/SSAUpdater.h"
94 #include <vector>
95
96 using namespace llvm;
97
98 #define DEBUG_TYPE "mldst-motion"
99
100 //===----------------------------------------------------------------------===//
101 // MergedLoadStoreMotion Pass
102 //===----------------------------------------------------------------------===//
103
104 namespace {
105 class MergedLoadStoreMotion : public FunctionPass {
106 AliasAnalysis *AA;
107 MemoryDependenceAnalysis *MD;
108
109 public:
110 static char ID; // Pass identification, replacement for typeid
MergedLoadStoreMotion()111 MergedLoadStoreMotion()
112 : FunctionPass(ID), MD(nullptr), MagicCompileTimeControl(250) {
113 initializeMergedLoadStoreMotionPass(*PassRegistry::getPassRegistry());
114 }
115
116 bool runOnFunction(Function &F) override;
117
118 private:
119 // This transformation requires dominator postdominator info
getAnalysisUsage(AnalysisUsage & AU) const120 void getAnalysisUsage(AnalysisUsage &AU) const override {
121 AU.setPreservesCFG();
122 AU.addRequired<TargetLibraryInfoWrapperPass>();
123 AU.addRequired<AAResultsWrapperPass>();
124 AU.addPreserved<GlobalsAAWrapperPass>();
125 AU.addPreserved<MemoryDependenceAnalysis>();
126 }
127
128 // Helper routines
129
130 ///
131 /// \brief Remove instruction from parent and update memory dependence
132 /// analysis.
133 ///
134 void removeInstruction(Instruction *Inst);
135 BasicBlock *getDiamondTail(BasicBlock *BB);
136 bool isDiamondHead(BasicBlock *BB);
137 // Routines for hoisting loads
138 bool isLoadHoistBarrierInRange(const Instruction& Start,
139 const Instruction& End,
140 LoadInst* LI);
141 LoadInst *canHoistFromBlock(BasicBlock *BB, LoadInst *LI);
142 void hoistInstruction(BasicBlock *BB, Instruction *HoistCand,
143 Instruction *ElseInst);
144 bool isSafeToHoist(Instruction *I) const;
145 bool hoistLoad(BasicBlock *BB, LoadInst *HoistCand, LoadInst *ElseInst);
146 bool mergeLoads(BasicBlock *BB);
147 // Routines for sinking stores
148 StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI);
149 PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1);
150 bool isStoreSinkBarrierInRange(const Instruction &Start,
151 const Instruction &End, MemoryLocation Loc);
152 bool sinkStore(BasicBlock *BB, StoreInst *SinkCand, StoreInst *ElseInst);
153 bool mergeStores(BasicBlock *BB);
154 // The mergeLoad/Store algorithms could have Size0 * Size1 complexity,
155 // where Size0 and Size1 are the #instructions on the two sides of
156 // the diamond. The constant chosen here is arbitrary. Compiler Time
157 // Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl.
158 const int MagicCompileTimeControl;
159 };
160
161 char MergedLoadStoreMotion::ID = 0;
162 } // anonymous namespace
163
164 ///
165 /// \brief createMergedLoadStoreMotionPass - The public interface to this file.
166 ///
createMergedLoadStoreMotionPass()167 FunctionPass *llvm::createMergedLoadStoreMotionPass() {
168 return new MergedLoadStoreMotion();
169 }
170
171 INITIALIZE_PASS_BEGIN(MergedLoadStoreMotion, "mldst-motion",
172 "MergedLoadStoreMotion", false, false)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)173 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
174 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
175 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
176 INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
177 INITIALIZE_PASS_END(MergedLoadStoreMotion, "mldst-motion",
178 "MergedLoadStoreMotion", false, false)
179
180 ///
181 /// \brief Remove instruction from parent and update memory dependence analysis.
182 ///
183 void MergedLoadStoreMotion::removeInstruction(Instruction *Inst) {
184 // Notify the memory dependence analysis.
185 if (MD) {
186 MD->removeInstruction(Inst);
187 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
188 MD->invalidateCachedPointerInfo(LI->getPointerOperand());
189 if (Inst->getType()->getScalarType()->isPointerTy()) {
190 MD->invalidateCachedPointerInfo(Inst);
191 }
192 }
193 Inst->eraseFromParent();
194 }
195
196 ///
197 /// \brief Return tail block of a diamond.
198 ///
getDiamondTail(BasicBlock * BB)199 BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) {
200 assert(isDiamondHead(BB) && "Basic block is not head of a diamond");
201 BranchInst *BI = (BranchInst *)(BB->getTerminator());
202 BasicBlock *Succ0 = BI->getSuccessor(0);
203 BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0);
204 return Tail;
205 }
206
207 ///
208 /// \brief True when BB is the head of a diamond (hammock)
209 ///
isDiamondHead(BasicBlock * BB)210 bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) {
211 if (!BB)
212 return false;
213 if (!isa<BranchInst>(BB->getTerminator()))
214 return false;
215 if (BB->getTerminator()->getNumSuccessors() != 2)
216 return false;
217
218 BranchInst *BI = (BranchInst *)(BB->getTerminator());
219 BasicBlock *Succ0 = BI->getSuccessor(0);
220 BasicBlock *Succ1 = BI->getSuccessor(1);
221
222 if (!Succ0->getSinglePredecessor() ||
223 Succ0->getTerminator()->getNumSuccessors() != 1)
224 return false;
225 if (!Succ1->getSinglePredecessor() ||
226 Succ1->getTerminator()->getNumSuccessors() != 1)
227 return false;
228
229 BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0);
230 // Ignore triangles.
231 if (Succ1->getTerminator()->getSuccessor(0) != Tail)
232 return false;
233 return true;
234 }
235
236 ///
237 /// \brief True when instruction is a hoist barrier for a load
238 ///
239 /// Whenever an instruction could possibly modify the value
240 /// being loaded or protect against the load from happening
241 /// it is considered a hoist barrier.
242 ///
isLoadHoistBarrierInRange(const Instruction & Start,const Instruction & End,LoadInst * LI)243 bool MergedLoadStoreMotion::isLoadHoistBarrierInRange(const Instruction& Start,
244 const Instruction& End,
245 LoadInst* LI) {
246 MemoryLocation Loc = MemoryLocation::get(LI);
247 return AA->canInstructionRangeModRef(Start, End, Loc, MRI_Mod);
248 }
249
250 ///
251 /// \brief Decide if a load can be hoisted
252 ///
253 /// When there is a load in \p BB to the same address as \p LI
254 /// and it can be hoisted from \p BB, return that load.
255 /// Otherwise return Null.
256 ///
canHoistFromBlock(BasicBlock * BB1,LoadInst * Load0)257 LoadInst *MergedLoadStoreMotion::canHoistFromBlock(BasicBlock *BB1,
258 LoadInst *Load0) {
259
260 for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end(); BBI != BBE;
261 ++BBI) {
262 Instruction *Inst = &*BBI;
263
264 // Only merge and hoist loads when their result in used only in BB
265 if (!isa<LoadInst>(Inst) || Inst->isUsedOutsideOfBlock(BB1))
266 continue;
267
268 LoadInst *Load1 = dyn_cast<LoadInst>(Inst);
269 BasicBlock *BB0 = Load0->getParent();
270
271 MemoryLocation Loc0 = MemoryLocation::get(Load0);
272 MemoryLocation Loc1 = MemoryLocation::get(Load1);
273 if (AA->isMustAlias(Loc0, Loc1) && Load0->isSameOperationAs(Load1) &&
274 !isLoadHoistBarrierInRange(BB1->front(), *Load1, Load1) &&
275 !isLoadHoistBarrierInRange(BB0->front(), *Load0, Load0)) {
276 return Load1;
277 }
278 }
279 return nullptr;
280 }
281
282 ///
283 /// \brief Merge two equivalent instructions \p HoistCand and \p ElseInst into
284 /// \p BB
285 ///
286 /// BB is the head of a diamond
287 ///
hoistInstruction(BasicBlock * BB,Instruction * HoistCand,Instruction * ElseInst)288 void MergedLoadStoreMotion::hoistInstruction(BasicBlock *BB,
289 Instruction *HoistCand,
290 Instruction *ElseInst) {
291 DEBUG(dbgs() << " Hoist Instruction into BB \n"; BB->dump();
292 dbgs() << "Instruction Left\n"; HoistCand->dump(); dbgs() << "\n";
293 dbgs() << "Instruction Right\n"; ElseInst->dump(); dbgs() << "\n");
294 // Hoist the instruction.
295 assert(HoistCand->getParent() != BB);
296
297 // Intersect optional metadata.
298 HoistCand->intersectOptionalDataWith(ElseInst);
299 HoistCand->dropUnknownNonDebugMetadata();
300
301 // Prepend point for instruction insert
302 Instruction *HoistPt = BB->getTerminator();
303
304 // Merged instruction
305 Instruction *HoistedInst = HoistCand->clone();
306
307 // Hoist instruction.
308 HoistedInst->insertBefore(HoistPt);
309
310 HoistCand->replaceAllUsesWith(HoistedInst);
311 removeInstruction(HoistCand);
312 // Replace the else block instruction.
313 ElseInst->replaceAllUsesWith(HoistedInst);
314 removeInstruction(ElseInst);
315 }
316
317 ///
318 /// \brief Return true if no operand of \p I is defined in I's parent block
319 ///
isSafeToHoist(Instruction * I) const320 bool MergedLoadStoreMotion::isSafeToHoist(Instruction *I) const {
321 BasicBlock *Parent = I->getParent();
322 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
323 Instruction *Instr = dyn_cast<Instruction>(I->getOperand(i));
324 if (Instr && Instr->getParent() == Parent)
325 return false;
326 }
327 return true;
328 }
329
330 ///
331 /// \brief Merge two equivalent loads and GEPs and hoist into diamond head
332 ///
hoistLoad(BasicBlock * BB,LoadInst * L0,LoadInst * L1)333 bool MergedLoadStoreMotion::hoistLoad(BasicBlock *BB, LoadInst *L0,
334 LoadInst *L1) {
335 // Only one definition?
336 Instruction *A0 = dyn_cast<Instruction>(L0->getPointerOperand());
337 Instruction *A1 = dyn_cast<Instruction>(L1->getPointerOperand());
338 if (A0 && A1 && A0->isIdenticalTo(A1) && isSafeToHoist(A0) &&
339 A0->hasOneUse() && (A0->getParent() == L0->getParent()) &&
340 A1->hasOneUse() && (A1->getParent() == L1->getParent()) &&
341 isa<GetElementPtrInst>(A0)) {
342 DEBUG(dbgs() << "Hoist Instruction into BB \n"; BB->dump();
343 dbgs() << "Instruction Left\n"; L0->dump(); dbgs() << "\n";
344 dbgs() << "Instruction Right\n"; L1->dump(); dbgs() << "\n");
345 hoistInstruction(BB, A0, A1);
346 hoistInstruction(BB, L0, L1);
347 return true;
348 } else
349 return false;
350 }
351
352 ///
353 /// \brief Try to hoist two loads to same address into diamond header
354 ///
355 /// Starting from a diamond head block, iterate over the instructions in one
356 /// successor block and try to match a load in the second successor.
357 ///
mergeLoads(BasicBlock * BB)358 bool MergedLoadStoreMotion::mergeLoads(BasicBlock *BB) {
359 bool MergedLoads = false;
360 assert(isDiamondHead(BB));
361 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
362 BasicBlock *Succ0 = BI->getSuccessor(0);
363 BasicBlock *Succ1 = BI->getSuccessor(1);
364 // #Instructions in Succ1 for Compile Time Control
365 int Size1 = Succ1->size();
366 int NLoads = 0;
367 for (BasicBlock::iterator BBI = Succ0->begin(), BBE = Succ0->end();
368 BBI != BBE;) {
369 Instruction *I = &*BBI;
370 ++BBI;
371
372 // Only move non-simple (atomic, volatile) loads.
373 LoadInst *L0 = dyn_cast<LoadInst>(I);
374 if (!L0 || !L0->isSimple() || L0->isUsedOutsideOfBlock(Succ0))
375 continue;
376
377 ++NLoads;
378 if (NLoads * Size1 >= MagicCompileTimeControl)
379 break;
380 if (LoadInst *L1 = canHoistFromBlock(Succ1, L0)) {
381 bool Res = hoistLoad(BB, L0, L1);
382 MergedLoads |= Res;
383 // Don't attempt to hoist above loads that had not been hoisted.
384 if (!Res)
385 break;
386 }
387 }
388 return MergedLoads;
389 }
390
391 ///
392 /// \brief True when instruction is a sink barrier for a store
393 /// located in Loc
394 ///
395 /// Whenever an instruction could possibly read or modify the
396 /// value being stored or protect against the store from
397 /// happening it is considered a sink barrier.
398 ///
isStoreSinkBarrierInRange(const Instruction & Start,const Instruction & End,MemoryLocation Loc)399 bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start,
400 const Instruction &End,
401 MemoryLocation Loc) {
402 return AA->canInstructionRangeModRef(Start, End, Loc, MRI_ModRef);
403 }
404
405 ///
406 /// \brief Check if \p BB contains a store to the same address as \p SI
407 ///
408 /// \return The store in \p when it is safe to sink. Otherwise return Null.
409 ///
canSinkFromBlock(BasicBlock * BB1,StoreInst * Store0)410 StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1,
411 StoreInst *Store0) {
412 DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n");
413 BasicBlock *BB0 = Store0->getParent();
414 for (BasicBlock::reverse_iterator RBI = BB1->rbegin(), RBE = BB1->rend();
415 RBI != RBE; ++RBI) {
416 Instruction *Inst = &*RBI;
417
418 if (!isa<StoreInst>(Inst))
419 continue;
420
421 StoreInst *Store1 = cast<StoreInst>(Inst);
422
423 MemoryLocation Loc0 = MemoryLocation::get(Store0);
424 MemoryLocation Loc1 = MemoryLocation::get(Store1);
425 if (AA->isMustAlias(Loc0, Loc1) && Store0->isSameOperationAs(Store1) &&
426 !isStoreSinkBarrierInRange(*(std::next(BasicBlock::iterator(Store1))),
427 BB1->back(), Loc1) &&
428 !isStoreSinkBarrierInRange(*(std::next(BasicBlock::iterator(Store0))),
429 BB0->back(), Loc0)) {
430 return Store1;
431 }
432 }
433 return nullptr;
434 }
435
436 ///
437 /// \brief Create a PHI node in BB for the operands of S0 and S1
438 ///
getPHIOperand(BasicBlock * BB,StoreInst * S0,StoreInst * S1)439 PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,
440 StoreInst *S1) {
441 // Create a phi if the values mismatch.
442 PHINode *NewPN = nullptr;
443 Value *Opd1 = S0->getValueOperand();
444 Value *Opd2 = S1->getValueOperand();
445 if (Opd1 != Opd2) {
446 NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink",
447 &BB->front());
448 NewPN->addIncoming(Opd1, S0->getParent());
449 NewPN->addIncoming(Opd2, S1->getParent());
450 if (MD && NewPN->getType()->getScalarType()->isPointerTy())
451 MD->invalidateCachedPointerInfo(NewPN);
452 }
453 return NewPN;
454 }
455
456 ///
457 /// \brief Merge two stores to same address and sink into \p BB
458 ///
459 /// Also sinks GEP instruction computing the store address
460 ///
sinkStore(BasicBlock * BB,StoreInst * S0,StoreInst * S1)461 bool MergedLoadStoreMotion::sinkStore(BasicBlock *BB, StoreInst *S0,
462 StoreInst *S1) {
463 // Only one definition?
464 Instruction *A0 = dyn_cast<Instruction>(S0->getPointerOperand());
465 Instruction *A1 = dyn_cast<Instruction>(S1->getPointerOperand());
466 if (A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() &&
467 (A0->getParent() == S0->getParent()) && A1->hasOneUse() &&
468 (A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0)) {
469 DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump();
470 dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n";
471 dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n");
472 // Hoist the instruction.
473 BasicBlock::iterator InsertPt = BB->getFirstInsertionPt();
474 // Intersect optional metadata.
475 S0->intersectOptionalDataWith(S1);
476 S0->dropUnknownNonDebugMetadata();
477
478 // Create the new store to be inserted at the join point.
479 StoreInst *SNew = (StoreInst *)(S0->clone());
480 Instruction *ANew = A0->clone();
481 SNew->insertBefore(&*InsertPt);
482 ANew->insertBefore(SNew);
483
484 assert(S0->getParent() == A0->getParent());
485 assert(S1->getParent() == A1->getParent());
486
487 PHINode *NewPN = getPHIOperand(BB, S0, S1);
488 // New PHI operand? Use it.
489 if (NewPN)
490 SNew->setOperand(0, NewPN);
491 removeInstruction(S0);
492 removeInstruction(S1);
493 A0->replaceAllUsesWith(ANew);
494 removeInstruction(A0);
495 A1->replaceAllUsesWith(ANew);
496 removeInstruction(A1);
497 return true;
498 }
499 return false;
500 }
501
502 ///
503 /// \brief True when two stores are equivalent and can sink into the footer
504 ///
505 /// Starting from a diamond tail block, iterate over the instructions in one
506 /// predecessor block and try to match a store in the second predecessor.
507 ///
mergeStores(BasicBlock * T)508 bool MergedLoadStoreMotion::mergeStores(BasicBlock *T) {
509
510 bool MergedStores = false;
511 assert(T && "Footer of a diamond cannot be empty");
512
513 pred_iterator PI = pred_begin(T), E = pred_end(T);
514 assert(PI != E);
515 BasicBlock *Pred0 = *PI;
516 ++PI;
517 BasicBlock *Pred1 = *PI;
518 ++PI;
519 // tail block of a diamond/hammock?
520 if (Pred0 == Pred1)
521 return false; // No.
522 if (PI != E)
523 return false; // No. More than 2 predecessors.
524
525 // #Instructions in Succ1 for Compile Time Control
526 int Size1 = Pred1->size();
527 int NStores = 0;
528
529 for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend();
530 RBI != RBE;) {
531
532 Instruction *I = &*RBI;
533 ++RBI;
534
535 // Sink move non-simple (atomic, volatile) stores
536 if (!isa<StoreInst>(I))
537 continue;
538 StoreInst *S0 = (StoreInst *)I;
539 if (!S0->isSimple())
540 continue;
541
542 ++NStores;
543 if (NStores * Size1 >= MagicCompileTimeControl)
544 break;
545 if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) {
546 bool Res = sinkStore(T, S0, S1);
547 MergedStores |= Res;
548 // Don't attempt to sink below stores that had to stick around
549 // But after removal of a store and some of its feeding
550 // instruction search again from the beginning since the iterator
551 // is likely stale at this point.
552 if (!Res)
553 break;
554 else {
555 RBI = Pred0->rbegin();
556 RBE = Pred0->rend();
557 DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());
558 }
559 }
560 }
561 return MergedStores;
562 }
563
564 ///
565 /// \brief Run the transformation for each function
566 ///
runOnFunction(Function & F)567 bool MergedLoadStoreMotion::runOnFunction(Function &F) {
568 MD = getAnalysisIfAvailable<MemoryDependenceAnalysis>();
569 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
570
571 bool Changed = false;
572 DEBUG(dbgs() << "Instruction Merger\n");
573
574 // Merge unconditional branches, allowing PRE to catch more
575 // optimization opportunities.
576 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) {
577 BasicBlock *BB = &*FI++;
578
579 // Hoist equivalent loads and sink stores
580 // outside diamonds when possible
581 if (isDiamondHead(BB)) {
582 Changed |= mergeLoads(BB);
583 Changed |= mergeStores(getDiamondTail(BB));
584 }
585 }
586 return Changed;
587 }
588