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/Loads.h"
82 #include "llvm/Analysis/MemoryBuiltins.h"
83 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
84 #include "llvm/Analysis/TargetLibraryInfo.h"
85 #include "llvm/IR/Metadata.h"
86 #include "llvm/IR/PatternMatch.h"
87 #include "llvm/Support/Allocator.h"
88 #include "llvm/Support/CommandLine.h"
89 #include "llvm/Support/Debug.h"
90 #include "llvm/Support/raw_ostream.h"
91 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
92 #include "llvm/Transforms/Utils/SSAUpdater.h"
93 #include <vector>
94 using namespace llvm;
95
96 #define DEBUG_TYPE "mldst-motion"
97
98 //===----------------------------------------------------------------------===//
99 // MergedLoadStoreMotion Pass
100 //===----------------------------------------------------------------------===//
101
102 namespace {
103 class MergedLoadStoreMotion : public FunctionPass {
104 AliasAnalysis *AA;
105 MemoryDependenceAnalysis *MD;
106
107 public:
108 static char ID; // Pass identification, replacement for typeid
MergedLoadStoreMotion(void)109 explicit MergedLoadStoreMotion(void)
110 : FunctionPass(ID), MD(nullptr), MagicCompileTimeControl(250) {
111 initializeMergedLoadStoreMotionPass(*PassRegistry::getPassRegistry());
112 }
113
114 bool runOnFunction(Function &F) override;
115
116 private:
117 // This transformation requires dominator postdominator info
getAnalysisUsage(AnalysisUsage & AU) const118 void getAnalysisUsage(AnalysisUsage &AU) const override {
119 AU.addRequired<TargetLibraryInfoWrapperPass>();
120 AU.addRequired<AliasAnalysis>();
121 AU.addPreserved<MemoryDependenceAnalysis>();
122 AU.addPreserved<AliasAnalysis>();
123 }
124
125 // Helper routines
126
127 ///
128 /// \brief Remove instruction from parent and update memory dependence
129 /// analysis.
130 ///
131 void removeInstruction(Instruction *Inst);
132 BasicBlock *getDiamondTail(BasicBlock *BB);
133 bool isDiamondHead(BasicBlock *BB);
134 // Routines for hoisting loads
135 bool isLoadHoistBarrierInRange(const Instruction& Start,
136 const Instruction& End,
137 LoadInst* LI);
138 LoadInst *canHoistFromBlock(BasicBlock *BB, LoadInst *LI);
139 void hoistInstruction(BasicBlock *BB, Instruction *HoistCand,
140 Instruction *ElseInst);
141 bool isSafeToHoist(Instruction *I) const;
142 bool hoistLoad(BasicBlock *BB, LoadInst *HoistCand, LoadInst *ElseInst);
143 bool mergeLoads(BasicBlock *BB);
144 // Routines for sinking stores
145 StoreInst *canSinkFromBlock(BasicBlock *BB, StoreInst *SI);
146 PHINode *getPHIOperand(BasicBlock *BB, StoreInst *S0, StoreInst *S1);
147 bool isStoreSinkBarrierInRange(const Instruction &Start,
148 const Instruction &End, MemoryLocation Loc);
149 bool sinkStore(BasicBlock *BB, StoreInst *SinkCand, StoreInst *ElseInst);
150 bool mergeStores(BasicBlock *BB);
151 // The mergeLoad/Store algorithms could have Size0 * Size1 complexity,
152 // where Size0 and Size1 are the #instructions on the two sides of
153 // the diamond. The constant chosen here is arbitrary. Compiler Time
154 // Control is enforced by the check Size0 * Size1 < MagicCompileTimeControl.
155 const int MagicCompileTimeControl;
156 };
157
158 char MergedLoadStoreMotion::ID = 0;
159 }
160
161 ///
162 /// \brief createMergedLoadStoreMotionPass - The public interface to this file.
163 ///
createMergedLoadStoreMotionPass()164 FunctionPass *llvm::createMergedLoadStoreMotionPass() {
165 return new MergedLoadStoreMotion();
166 }
167
168 INITIALIZE_PASS_BEGIN(MergedLoadStoreMotion, "mldst-motion",
169 "MergedLoadStoreMotion", false, false)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)170 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
171 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
172 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
173 INITIALIZE_PASS_END(MergedLoadStoreMotion, "mldst-motion",
174 "MergedLoadStoreMotion", false, false)
175
176 ///
177 /// \brief Remove instruction from parent and update memory dependence analysis.
178 ///
179 void MergedLoadStoreMotion::removeInstruction(Instruction *Inst) {
180 // Notify the memory dependence analysis.
181 if (MD) {
182 MD->removeInstruction(Inst);
183 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
184 MD->invalidateCachedPointerInfo(LI->getPointerOperand());
185 if (Inst->getType()->getScalarType()->isPointerTy()) {
186 MD->invalidateCachedPointerInfo(Inst);
187 }
188 }
189 Inst->eraseFromParent();
190 }
191
192 ///
193 /// \brief Return tail block of a diamond.
194 ///
getDiamondTail(BasicBlock * BB)195 BasicBlock *MergedLoadStoreMotion::getDiamondTail(BasicBlock *BB) {
196 assert(isDiamondHead(BB) && "Basic block is not head of a diamond");
197 BranchInst *BI = (BranchInst *)(BB->getTerminator());
198 BasicBlock *Succ0 = BI->getSuccessor(0);
199 BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0);
200 return Tail;
201 }
202
203 ///
204 /// \brief True when BB is the head of a diamond (hammock)
205 ///
isDiamondHead(BasicBlock * BB)206 bool MergedLoadStoreMotion::isDiamondHead(BasicBlock *BB) {
207 if (!BB)
208 return false;
209 if (!isa<BranchInst>(BB->getTerminator()))
210 return false;
211 if (BB->getTerminator()->getNumSuccessors() != 2)
212 return false;
213
214 BranchInst *BI = (BranchInst *)(BB->getTerminator());
215 BasicBlock *Succ0 = BI->getSuccessor(0);
216 BasicBlock *Succ1 = BI->getSuccessor(1);
217
218 if (!Succ0->getSinglePredecessor() ||
219 Succ0->getTerminator()->getNumSuccessors() != 1)
220 return false;
221 if (!Succ1->getSinglePredecessor() ||
222 Succ1->getTerminator()->getNumSuccessors() != 1)
223 return false;
224
225 BasicBlock *Tail = Succ0->getTerminator()->getSuccessor(0);
226 // Ignore triangles.
227 if (Succ1->getTerminator()->getSuccessor(0) != Tail)
228 return false;
229 return true;
230 }
231
232 ///
233 /// \brief True when instruction is a hoist barrier for a load
234 ///
235 /// Whenever an instruction could possibly modify the value
236 /// being loaded or protect against the load from happening
237 /// it is considered a hoist barrier.
238 ///
239
isLoadHoistBarrierInRange(const Instruction & Start,const Instruction & End,LoadInst * LI)240 bool MergedLoadStoreMotion::isLoadHoistBarrierInRange(const Instruction& Start,
241 const Instruction& End,
242 LoadInst* LI) {
243 MemoryLocation Loc = MemoryLocation::get(LI);
244 return AA->canInstructionRangeModRef(Start, End, Loc, AliasAnalysis::Mod);
245 }
246
247 ///
248 /// \brief Decide if a load can be hoisted
249 ///
250 /// When there is a load in \p BB to the same address as \p LI
251 /// and it can be hoisted from \p BB, return that load.
252 /// Otherwise return Null.
253 ///
canHoistFromBlock(BasicBlock * BB1,LoadInst * Load0)254 LoadInst *MergedLoadStoreMotion::canHoistFromBlock(BasicBlock *BB1,
255 LoadInst *Load0) {
256
257 for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end(); BBI != BBE;
258 ++BBI) {
259 Instruction *Inst = BBI;
260
261 // Only merge and hoist loads when their result in used only in BB
262 if (!isa<LoadInst>(Inst) || Inst->isUsedOutsideOfBlock(BB1))
263 continue;
264
265 LoadInst *Load1 = dyn_cast<LoadInst>(Inst);
266 BasicBlock *BB0 = Load0->getParent();
267
268 MemoryLocation Loc0 = MemoryLocation::get(Load0);
269 MemoryLocation Loc1 = MemoryLocation::get(Load1);
270 if (AA->isMustAlias(Loc0, Loc1) && Load0->isSameOperationAs(Load1) &&
271 !isLoadHoistBarrierInRange(BB1->front(), *Load1, Load1) &&
272 !isLoadHoistBarrierInRange(BB0->front(), *Load0, Load0)) {
273 return Load1;
274 }
275 }
276 return nullptr;
277 }
278
279 ///
280 /// \brief Merge two equivalent instructions \p HoistCand and \p ElseInst into
281 /// \p BB
282 ///
283 /// BB is the head of a diamond
284 ///
hoistInstruction(BasicBlock * BB,Instruction * HoistCand,Instruction * ElseInst)285 void MergedLoadStoreMotion::hoistInstruction(BasicBlock *BB,
286 Instruction *HoistCand,
287 Instruction *ElseInst) {
288 DEBUG(dbgs() << " Hoist Instruction into BB \n"; BB->dump();
289 dbgs() << "Instruction Left\n"; HoistCand->dump(); dbgs() << "\n";
290 dbgs() << "Instruction Right\n"; ElseInst->dump(); dbgs() << "\n");
291 // Hoist the instruction.
292 assert(HoistCand->getParent() != BB);
293
294 // Intersect optional metadata.
295 HoistCand->intersectOptionalDataWith(ElseInst);
296 HoistCand->dropUnknownMetadata();
297
298 // Prepend point for instruction insert
299 Instruction *HoistPt = BB->getTerminator();
300
301 // Merged instruction
302 Instruction *HoistedInst = HoistCand->clone();
303
304 // Hoist instruction.
305 HoistedInst->insertBefore(HoistPt);
306
307 HoistCand->replaceAllUsesWith(HoistedInst);
308 removeInstruction(HoistCand);
309 // Replace the else block instruction.
310 ElseInst->replaceAllUsesWith(HoistedInst);
311 removeInstruction(ElseInst);
312 }
313
314 ///
315 /// \brief Return true if no operand of \p I is defined in I's parent block
316 ///
isSafeToHoist(Instruction * I) const317 bool MergedLoadStoreMotion::isSafeToHoist(Instruction *I) const {
318 BasicBlock *Parent = I->getParent();
319 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
320 Instruction *Instr = dyn_cast<Instruction>(I->getOperand(i));
321 if (Instr && Instr->getParent() == Parent)
322 return false;
323 }
324 return true;
325 }
326
327 ///
328 /// \brief Merge two equivalent loads and GEPs and hoist into diamond head
329 ///
hoistLoad(BasicBlock * BB,LoadInst * L0,LoadInst * L1)330 bool MergedLoadStoreMotion::hoistLoad(BasicBlock *BB, LoadInst *L0,
331 LoadInst *L1) {
332 // Only one definition?
333 Instruction *A0 = dyn_cast<Instruction>(L0->getPointerOperand());
334 Instruction *A1 = dyn_cast<Instruction>(L1->getPointerOperand());
335 if (A0 && A1 && A0->isIdenticalTo(A1) && isSafeToHoist(A0) &&
336 A0->hasOneUse() && (A0->getParent() == L0->getParent()) &&
337 A1->hasOneUse() && (A1->getParent() == L1->getParent()) &&
338 isa<GetElementPtrInst>(A0)) {
339 DEBUG(dbgs() << "Hoist Instruction into BB \n"; BB->dump();
340 dbgs() << "Instruction Left\n"; L0->dump(); dbgs() << "\n";
341 dbgs() << "Instruction Right\n"; L1->dump(); dbgs() << "\n");
342 hoistInstruction(BB, A0, A1);
343 hoistInstruction(BB, L0, L1);
344 return true;
345 } else
346 return false;
347 }
348
349 ///
350 /// \brief Try to hoist two loads to same address into diamond header
351 ///
352 /// Starting from a diamond head block, iterate over the instructions in one
353 /// successor block and try to match a load in the second successor.
354 ///
mergeLoads(BasicBlock * BB)355 bool MergedLoadStoreMotion::mergeLoads(BasicBlock *BB) {
356 bool MergedLoads = false;
357 assert(isDiamondHead(BB));
358 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
359 BasicBlock *Succ0 = BI->getSuccessor(0);
360 BasicBlock *Succ1 = BI->getSuccessor(1);
361 // #Instructions in Succ1 for Compile Time Control
362 int Size1 = Succ1->size();
363 int NLoads = 0;
364 for (BasicBlock::iterator BBI = Succ0->begin(), BBE = Succ0->end();
365 BBI != BBE;) {
366
367 Instruction *I = BBI;
368 ++BBI;
369
370 // Only move non-simple (atomic, volatile) loads.
371 LoadInst *L0 = dyn_cast<LoadInst>(I);
372 if (!L0 || !L0->isSimple() || L0->isUsedOutsideOfBlock(Succ0))
373 continue;
374
375 ++NLoads;
376 if (NLoads * Size1 >= MagicCompileTimeControl)
377 break;
378 if (LoadInst *L1 = canHoistFromBlock(Succ1, L0)) {
379 bool Res = hoistLoad(BB, L0, L1);
380 MergedLoads |= Res;
381 // Don't attempt to hoist above loads that had not been hoisted.
382 if (!Res)
383 break;
384 }
385 }
386 return MergedLoads;
387 }
388
389 ///
390 /// \brief True when instruction is a sink barrier for a store
391 /// located in Loc
392 ///
393 /// Whenever an instruction could possibly read or modify the
394 /// value being stored or protect against the store from
395 /// happening it is considered a sink barrier.
396 ///
397
isStoreSinkBarrierInRange(const Instruction & Start,const Instruction & End,MemoryLocation Loc)398 bool MergedLoadStoreMotion::isStoreSinkBarrierInRange(const Instruction &Start,
399 const Instruction &End,
400 MemoryLocation Loc) {
401 return AA->canInstructionRangeModRef(Start, End, Loc, AliasAnalysis::ModRef);
402 }
403
404 ///
405 /// \brief Check if \p BB contains a store to the same address as \p SI
406 ///
407 /// \return The store in \p when it is safe to sink. Otherwise return Null.
408 ///
canSinkFromBlock(BasicBlock * BB1,StoreInst * Store0)409 StoreInst *MergedLoadStoreMotion::canSinkFromBlock(BasicBlock *BB1,
410 StoreInst *Store0) {
411 DEBUG(dbgs() << "can Sink? : "; Store0->dump(); dbgs() << "\n");
412 BasicBlock *BB0 = Store0->getParent();
413 for (BasicBlock::reverse_iterator RBI = BB1->rbegin(), RBE = BB1->rend();
414 RBI != RBE; ++RBI) {
415 Instruction *Inst = &*RBI;
416
417 if (!isa<StoreInst>(Inst))
418 continue;
419
420 StoreInst *Store1 = cast<StoreInst>(Inst);
421
422 MemoryLocation Loc0 = MemoryLocation::get(Store0);
423 MemoryLocation Loc1 = MemoryLocation::get(Store1);
424 if (AA->isMustAlias(Loc0, Loc1) && Store0->isSameOperationAs(Store1) &&
425 !isStoreSinkBarrierInRange(*(std::next(BasicBlock::iterator(Store1))),
426 BB1->back(), Loc1) &&
427 !isStoreSinkBarrierInRange(*(std::next(BasicBlock::iterator(Store0))),
428 BB0->back(), Loc0)) {
429 return Store1;
430 }
431 }
432 return nullptr;
433 }
434
435 ///
436 /// \brief Create a PHI node in BB for the operands of S0 and S1
437 ///
getPHIOperand(BasicBlock * BB,StoreInst * S0,StoreInst * S1)438 PHINode *MergedLoadStoreMotion::getPHIOperand(BasicBlock *BB, StoreInst *S0,
439 StoreInst *S1) {
440 // Create a phi if the values mismatch.
441 PHINode *NewPN = 0;
442 Value *Opd1 = S0->getValueOperand();
443 Value *Opd2 = S1->getValueOperand();
444 if (Opd1 != Opd2) {
445 NewPN = PHINode::Create(Opd1->getType(), 2, Opd2->getName() + ".sink",
446 BB->begin());
447 NewPN->addIncoming(Opd1, S0->getParent());
448 NewPN->addIncoming(Opd2, S1->getParent());
449 if (NewPN->getType()->getScalarType()->isPointerTy()) {
450 // AA needs to be informed when a PHI-use of the pointer value is added
451 for (unsigned I = 0, E = NewPN->getNumIncomingValues(); I != E; ++I) {
452 unsigned J = PHINode::getOperandNumForIncomingValue(I);
453 AA->addEscapingUse(NewPN->getOperandUse(J));
454 }
455 if (MD)
456 MD->invalidateCachedPointerInfo(NewPN);
457 }
458 }
459 return NewPN;
460 }
461
462 ///
463 /// \brief Merge two stores to same address and sink into \p BB
464 ///
465 /// Also sinks GEP instruction computing the store address
466 ///
sinkStore(BasicBlock * BB,StoreInst * S0,StoreInst * S1)467 bool MergedLoadStoreMotion::sinkStore(BasicBlock *BB, StoreInst *S0,
468 StoreInst *S1) {
469 // Only one definition?
470 Instruction *A0 = dyn_cast<Instruction>(S0->getPointerOperand());
471 Instruction *A1 = dyn_cast<Instruction>(S1->getPointerOperand());
472 if (A0 && A1 && A0->isIdenticalTo(A1) && A0->hasOneUse() &&
473 (A0->getParent() == S0->getParent()) && A1->hasOneUse() &&
474 (A1->getParent() == S1->getParent()) && isa<GetElementPtrInst>(A0)) {
475 DEBUG(dbgs() << "Sink Instruction into BB \n"; BB->dump();
476 dbgs() << "Instruction Left\n"; S0->dump(); dbgs() << "\n";
477 dbgs() << "Instruction Right\n"; S1->dump(); dbgs() << "\n");
478 // Hoist the instruction.
479 BasicBlock::iterator InsertPt = BB->getFirstInsertionPt();
480 // Intersect optional metadata.
481 S0->intersectOptionalDataWith(S1);
482 S0->dropUnknownMetadata();
483
484 // Create the new store to be inserted at the join point.
485 StoreInst *SNew = (StoreInst *)(S0->clone());
486 Instruction *ANew = A0->clone();
487 SNew->insertBefore(InsertPt);
488 ANew->insertBefore(SNew);
489
490 assert(S0->getParent() == A0->getParent());
491 assert(S1->getParent() == A1->getParent());
492
493 PHINode *NewPN = getPHIOperand(BB, S0, S1);
494 // New PHI operand? Use it.
495 if (NewPN)
496 SNew->setOperand(0, NewPN);
497 removeInstruction(S0);
498 removeInstruction(S1);
499 A0->replaceAllUsesWith(ANew);
500 removeInstruction(A0);
501 A1->replaceAllUsesWith(ANew);
502 removeInstruction(A1);
503 return true;
504 }
505 return false;
506 }
507
508 ///
509 /// \brief True when two stores are equivalent and can sink into the footer
510 ///
511 /// Starting from a diamond tail block, iterate over the instructions in one
512 /// predecessor block and try to match a store in the second predecessor.
513 ///
mergeStores(BasicBlock * T)514 bool MergedLoadStoreMotion::mergeStores(BasicBlock *T) {
515
516 bool MergedStores = false;
517 assert(T && "Footer of a diamond cannot be empty");
518
519 pred_iterator PI = pred_begin(T), E = pred_end(T);
520 assert(PI != E);
521 BasicBlock *Pred0 = *PI;
522 ++PI;
523 BasicBlock *Pred1 = *PI;
524 ++PI;
525 // tail block of a diamond/hammock?
526 if (Pred0 == Pred1)
527 return false; // No.
528 if (PI != E)
529 return false; // No. More than 2 predecessors.
530
531 // #Instructions in Succ1 for Compile Time Control
532 int Size1 = Pred1->size();
533 int NStores = 0;
534
535 for (BasicBlock::reverse_iterator RBI = Pred0->rbegin(), RBE = Pred0->rend();
536 RBI != RBE;) {
537
538 Instruction *I = &*RBI;
539 ++RBI;
540
541 // Sink move non-simple (atomic, volatile) stores
542 if (!isa<StoreInst>(I))
543 continue;
544 StoreInst *S0 = (StoreInst *)I;
545 if (!S0->isSimple())
546 continue;
547
548 ++NStores;
549 if (NStores * Size1 >= MagicCompileTimeControl)
550 break;
551 if (StoreInst *S1 = canSinkFromBlock(Pred1, S0)) {
552 bool Res = sinkStore(T, S0, S1);
553 MergedStores |= Res;
554 // Don't attempt to sink below stores that had to stick around
555 // But after removal of a store and some of its feeding
556 // instruction search again from the beginning since the iterator
557 // is likely stale at this point.
558 if (!Res)
559 break;
560 else {
561 RBI = Pred0->rbegin();
562 RBE = Pred0->rend();
563 DEBUG(dbgs() << "Search again\n"; Instruction *I = &*RBI; I->dump());
564 }
565 }
566 }
567 return MergedStores;
568 }
569 ///
570 /// \brief Run the transformation for each function
571 ///
runOnFunction(Function & F)572 bool MergedLoadStoreMotion::runOnFunction(Function &F) {
573 MD = getAnalysisIfAvailable<MemoryDependenceAnalysis>();
574 AA = &getAnalysis<AliasAnalysis>();
575
576 bool Changed = false;
577 DEBUG(dbgs() << "Instruction Merger\n");
578
579 // Merge unconditional branches, allowing PRE to catch more
580 // optimization opportunities.
581 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE;) {
582 BasicBlock *BB = FI++;
583
584 // Hoist equivalent loads and sink stores
585 // outside diamonds when possible
586 if (isDiamondHead(BB)) {
587 Changed |= mergeLoads(BB);
588 Changed |= mergeStores(getDiamondTail(BB));
589 }
590 }
591 return Changed;
592 }
593