1 //===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass performs loop invariant code motion, attempting to remove as much
11 // code from the body of a loop as possible. It does this by either hoisting
12 // code into the preheader block, or by sinking code to the exit blocks if it is
13 // safe. This pass also promotes must-aliased memory locations in the loop to
14 // live in registers, thus hoisting and sinking "invariant" loads and stores.
16 // This pass uses alias analysis for two purposes:
18 // 1. Moving loop invariant loads and calls out of loops. If we can determine
19 // that a load or call inside of a loop never aliases anything stored to,
20 // we can hoist it or sink it like any other instruction.
21 // 2. Scalar Promotion of Memory - If there is a store instruction inside of
22 // the loop, we try to move the store to happen AFTER the loop instead of
23 // inside of the loop. This can only happen if a few conditions are true:
24 // A. The pointer stored through is loop invariant
25 // B. There are no stores or loads in the loop which _may_ alias the
26 // pointer. There are no calls in the loop which mod/ref the pointer.
27 // If these conditions are true, we can promote the loads and stores in the
28 // loop of the pointer to use a temporary alloca'd variable. We then use
29 // the mem2reg functionality to construct the appropriate SSA form for the
32 //===----------------------------------------------------------------------===//
34 #define DEBUG_TYPE "licm"
35 #include "llvm/Transforms/Scalar.h"
36 #include "llvm/Constants.h"
37 #include "llvm/DerivedTypes.h"
38 #include "llvm/Instructions.h"
39 #include "llvm/Target/TargetData.h"
40 #include "llvm/Analysis/LoopInfo.h"
41 #include "llvm/Analysis/LoopPass.h"
42 #include "llvm/Analysis/AliasAnalysis.h"
43 #include "llvm/Analysis/AliasSetTracker.h"
44 #include "llvm/Analysis/Dominators.h"
45 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
46 #include "llvm/Support/CFG.h"
47 #include "llvm/Support/Compiler.h"
48 #include "llvm/Support/CommandLine.h"
49 #include "llvm/Support/Debug.h"
50 #include "llvm/ADT/Statistic.h"
54 STATISTIC(NumSunk , "Number of instructions sunk out of loop");
55 STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
56 STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
57 STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
58 STATISTIC(NumPromoted , "Number of memory locations promoted to registers");
62 DisablePromotion("disable-licm-promotion", cl::Hidden,
63 cl::desc("Disable memory promotion in LICM pass"));
65 struct VISIBILITY_HIDDEN LICM : public LoopPass {
66 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
68 /// This transformation requires natural loop information & requires that
69 /// loop preheaders be inserted into the CFG...
71 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
73 AU.addRequiredID(LoopSimplifyID);
74 AU.addRequired<LoopInfo>();
75 AU.addRequired<ETForest>();
76 AU.addRequired<DominanceFrontier>(); // For scalar promotion (mem2reg)
77 AU.addRequired<AliasAnalysis>();
80 bool doFinalization() {
81 LoopToAliasMap.clear();
86 // Various analyses that we use...
87 AliasAnalysis *AA; // Current AliasAnalysis information
88 LoopInfo *LI; // Current LoopInfo
89 ETForest *ET; // ETForest for the current Loop...
90 DominanceFrontier *DF; // Current Dominance Frontier
92 // State that is updated as we process loops
93 bool Changed; // Set to true when we change anything.
94 BasicBlock *Preheader; // The preheader block of the current loop...
95 Loop *CurLoop; // The current loop we are working on...
96 AliasSetTracker *CurAST; // AliasSet information for the current loop...
97 std::map<Loop *, AliasSetTracker *> LoopToAliasMap;
99 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
100 /// dominated by the specified block, and that are in the current loop) in
101 /// reverse depth first order w.r.t the ETForest. This allows us to
102 /// visit uses before definitions, allowing us to sink a loop body in one
103 /// pass without iteration.
105 void SinkRegion(BasicBlock *BB);
107 /// HoistRegion - Walk the specified region of the CFG (defined by all
108 /// blocks dominated by the specified block, and that are in the current
109 /// loop) in depth first order w.r.t the ETForest. This allows us to
110 /// visit definitions before uses, allowing us to hoist a loop body in one
111 /// pass without iteration.
113 void HoistRegion(BasicBlock *BB);
115 /// inSubLoop - Little predicate that returns true if the specified basic
116 /// block is in a subloop of the current one, not the current one itself.
118 bool inSubLoop(BasicBlock *BB) {
119 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
120 for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I)
121 if ((*I)->contains(BB))
122 return true; // A subloop actually contains this block!
126 /// isExitBlockDominatedByBlockInLoop - This method checks to see if the
127 /// specified exit block of the loop is dominated by the specified block
128 /// that is in the body of the loop. We use these constraints to
129 /// dramatically limit the amount of the dominator tree that needs to be
131 bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock,
132 BasicBlock *BlockInLoop) const {
133 // If the block in the loop is the loop header, it must be dominated!
134 BasicBlock *LoopHeader = CurLoop->getHeader();
135 if (BlockInLoop == LoopHeader)
138 BasicBlock *IDom = ExitBlock;
140 // Because the exit block is not in the loop, we know we have to get _at
141 // least_ its immediate dominator.
143 // Get next Immediate Dominator.
144 IDom = ET->getIDom(IDom);
146 // If we have got to the header of the loop, then the instructions block
147 // did not dominate the exit node, so we can't hoist it.
148 if (IDom == LoopHeader)
151 } while (IDom != BlockInLoop);
156 /// sink - When an instruction is found to only be used outside of the loop,
157 /// this function moves it to the exit blocks and patches up SSA form as
160 void sink(Instruction &I);
162 /// hoist - When an instruction is found to only use loop invariant operands
163 /// that is safe to hoist, this instruction is called to do the dirty work.
165 void hoist(Instruction &I);
167 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
168 /// is not a trapping instruction or if it is a trapping instruction and is
169 /// guaranteed to execute.
171 bool isSafeToExecuteUnconditionally(Instruction &I);
173 /// pointerInvalidatedByLoop - Return true if the body of this loop may
174 /// store into the memory location pointed to by V.
176 bool pointerInvalidatedByLoop(Value *V, unsigned Size) {
177 // Check to see if any of the basic blocks in CurLoop invalidate *V.
178 return CurAST->getAliasSetForPointer(V, Size).isMod();
181 bool canSinkOrHoistInst(Instruction &I);
182 bool isLoopInvariantInst(Instruction &I);
183 bool isNotUsedInLoop(Instruction &I);
185 /// PromoteValuesInLoop - Look at the stores in the loop and promote as many
186 /// to scalars as we can.
188 void PromoteValuesInLoop();
190 /// FindPromotableValuesInLoop - Check the current loop for stores to
191 /// definite pointers, which are not loaded and stored through may aliases.
192 /// If these are found, create an alloca for the value, add it to the
193 /// PromotedValues list, and keep track of the mapping from value to
196 void FindPromotableValuesInLoop(
197 std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
198 std::map<Value*, AllocaInst*> &Val2AlMap);
201 RegisterPass<LICM> X("licm", "Loop Invariant Code Motion");
204 LoopPass *llvm::createLICMPass() { return new LICM(); }
206 /// Hoist expressions out of the specified loop...
208 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
211 // Get our Loop and Alias Analysis information...
212 LI = &getAnalysis<LoopInfo>();
213 AA = &getAnalysis<AliasAnalysis>();
214 DF = &getAnalysis<DominanceFrontier>();
215 ET = &getAnalysis<ETForest>();
217 CurAST = new AliasSetTracker(*AA);
218 // Collect Alias info frmo subloops
219 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
220 LoopItr != LoopItrE; ++LoopItr) {
221 Loop *InnerL = *LoopItr;
222 AliasSetTracker *InnerAST = LoopToAliasMap[InnerL];
223 assert (InnerAST && "Where is my AST?");
225 // What if InnerLoop was modified by other passes ?
226 CurAST->add(*InnerAST);
231 // Get the preheader block to move instructions into...
232 Preheader = L->getLoopPreheader();
233 assert(Preheader&&"Preheader insertion pass guarantees we have a preheader!");
235 // Loop over the body of this loop, looking for calls, invokes, and stores.
236 // Because subloops have already been incorporated into AST, we skip blocks in
239 for (std::vector<BasicBlock*>::const_iterator I = L->getBlocks().begin(),
240 E = L->getBlocks().end(); I != E; ++I)
241 if (LI->getLoopFor(*I) == L) // Ignore blocks in subloops...
242 CurAST->add(**I); // Incorporate the specified basic block
244 // We want to visit all of the instructions in this loop... that are not parts
245 // of our subloops (they have already had their invariants hoisted out of
246 // their loop, into this loop, so there is no need to process the BODIES of
249 // Traverse the body of the loop in depth first order on the dominator tree so
250 // that we are guaranteed to see definitions before we see uses. This allows
251 // us to sink instructions in one pass, without iteration. AFter sinking
252 // instructions, we perform another pass to hoist them out of the loop.
254 SinkRegion(L->getHeader());
255 HoistRegion(L->getHeader());
257 // Now that all loop invariants have been removed from the loop, promote any
258 // memory references to scalars that we can...
259 if (!DisablePromotion)
260 PromoteValuesInLoop();
262 // Clear out loops state information for the next iteration
266 LoopToAliasMap[L] = CurAST;
270 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
271 /// dominated by the specified block, and that are in the current loop) in
272 /// reverse depth first order w.r.t the ETForest. This allows us to visit
273 /// uses before definitions, allowing us to sink a loop body in one pass without
276 void LICM::SinkRegion(BasicBlock *BB) {
277 assert(BB != 0 && "Null sink block?");
279 // If this subregion is not in the top level loop at all, exit.
280 if (!CurLoop->contains(BB)) return;
282 // We are processing blocks in reverse dfo, so process children first...
283 std::vector<BasicBlock*> Children;
284 ET->getChildren(BB, Children);
285 for (unsigned i = 0, e = Children.size(); i != e; ++i)
286 SinkRegion(Children[i]);
288 // Only need to process the contents of this block if it is not part of a
289 // subloop (which would already have been processed).
290 if (inSubLoop(BB)) return;
292 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
293 Instruction &I = *--II;
295 // Check to see if we can sink this instruction to the exit blocks
296 // of the loop. We can do this if the all users of the instruction are
297 // outside of the loop. In this case, it doesn't even matter if the
298 // operands of the instruction are loop invariant.
300 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
308 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
309 /// dominated by the specified block, and that are in the current loop) in depth
310 /// first order w.r.t the ETForest. This allows us to visit definitions
311 /// before uses, allowing us to hoist a loop body in one pass without iteration.
313 void LICM::HoistRegion(BasicBlock *BB) {
314 assert(BB != 0 && "Null hoist block?");
316 // If this subregion is not in the top level loop at all, exit.
317 if (!CurLoop->contains(BB)) return;
319 // Only need to process the contents of this block if it is not part of a
320 // subloop (which would already have been processed).
322 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
323 Instruction &I = *II++;
325 // Try hoisting the instruction out to the preheader. We can only do this
326 // if all of the operands of the instruction are loop invariant and if it
327 // is safe to hoist the instruction.
329 if (isLoopInvariantInst(I) && canSinkOrHoistInst(I) &&
330 isSafeToExecuteUnconditionally(I))
334 std::vector<BasicBlock*> Children;
335 ET->getChildren(BB, Children);
336 for (unsigned i = 0, e = Children.size(); i != e; ++i)
337 HoistRegion(Children[i]);
340 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
343 bool LICM::canSinkOrHoistInst(Instruction &I) {
344 // Loads have extra constraints we have to verify before we can hoist them.
345 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
346 if (LI->isVolatile())
347 return false; // Don't hoist volatile loads!
349 // Don't hoist loads which have may-aliased stores in loop.
351 if (LI->getType()->isSized())
352 Size = AA->getTargetData().getTypeSize(LI->getType());
353 return !pointerInvalidatedByLoop(LI->getOperand(0), Size);
354 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
355 // Handle obvious cases efficiently.
356 if (Function *Callee = CI->getCalledFunction()) {
357 AliasAnalysis::ModRefBehavior Behavior =AA->getModRefBehavior(Callee, CI);
358 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
360 else if (Behavior == AliasAnalysis::OnlyReadsMemory) {
361 // If this call only reads from memory and there are no writes to memory
362 // in the loop, we can hoist or sink the call as appropriate.
363 bool FoundMod = false;
364 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
367 if (!AS.isForwardingAliasSet() && AS.isMod()) {
372 if (!FoundMod) return true;
376 // FIXME: This should use mod/ref information to see if we can hoist or sink
382 // Otherwise these instructions are hoistable/sinkable
383 return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
384 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I);
387 /// isNotUsedInLoop - Return true if the only users of this instruction are
388 /// outside of the loop. If this is true, we can sink the instruction to the
389 /// exit blocks of the loop.
391 bool LICM::isNotUsedInLoop(Instruction &I) {
392 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
393 Instruction *User = cast<Instruction>(*UI);
394 if (PHINode *PN = dyn_cast<PHINode>(User)) {
395 // PHI node uses occur in predecessor blocks!
396 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
397 if (PN->getIncomingValue(i) == &I)
398 if (CurLoop->contains(PN->getIncomingBlock(i)))
400 } else if (CurLoop->contains(User->getParent())) {
408 /// isLoopInvariantInst - Return true if all operands of this instruction are
409 /// loop invariant. We also filter out non-hoistable instructions here just for
412 bool LICM::isLoopInvariantInst(Instruction &I) {
413 // The instruction is loop invariant if all of its operands are loop-invariant
414 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
415 if (!CurLoop->isLoopInvariant(I.getOperand(i)))
418 // If we got this far, the instruction is loop invariant!
422 /// sink - When an instruction is found to only be used outside of the loop,
423 /// this function moves it to the exit blocks and patches up SSA form as needed.
424 /// This method is guaranteed to remove the original instruction from its
425 /// position, and may either delete it or move it to outside of the loop.
427 void LICM::sink(Instruction &I) {
428 DOUT << "LICM sinking instruction: " << I;
430 std::vector<BasicBlock*> ExitBlocks;
431 CurLoop->getExitBlocks(ExitBlocks);
433 if (isa<LoadInst>(I)) ++NumMovedLoads;
434 else if (isa<CallInst>(I)) ++NumMovedCalls;
438 // The case where there is only a single exit node of this loop is common
439 // enough that we handle it as a special (more efficient) case. It is more
440 // efficient to handle because there are no PHI nodes that need to be placed.
441 if (ExitBlocks.size() == 1) {
442 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) {
443 // Instruction is not used, just delete it.
444 CurAST->deleteValue(&I);
445 if (!I.use_empty()) // If I has users in unreachable blocks, eliminate.
446 I.replaceAllUsesWith(UndefValue::get(I.getType()));
449 // Move the instruction to the start of the exit block, after any PHI
451 I.removeFromParent();
453 BasicBlock::iterator InsertPt = ExitBlocks[0]->begin();
454 while (isa<PHINode>(InsertPt)) ++InsertPt;
455 ExitBlocks[0]->getInstList().insert(InsertPt, &I);
457 } else if (ExitBlocks.size() == 0) {
458 // The instruction is actually dead if there ARE NO exit blocks.
459 CurAST->deleteValue(&I);
460 if (!I.use_empty()) // If I has users in unreachable blocks, eliminate.
461 I.replaceAllUsesWith(UndefValue::get(I.getType()));
464 // Otherwise, if we have multiple exits, use the PromoteMem2Reg function to
465 // do all of the hard work of inserting PHI nodes as necessary. We convert
466 // the value into a stack object to get it to do this.
468 // Firstly, we create a stack object to hold the value...
471 if (I.getType() != Type::VoidTy)
472 AI = new AllocaInst(I.getType(), 0, I.getName(),
473 I.getParent()->getParent()->getEntryBlock().begin());
475 // Secondly, insert load instructions for each use of the instruction
476 // outside of the loop.
477 while (!I.use_empty()) {
478 Instruction *U = cast<Instruction>(I.use_back());
480 // If the user is a PHI Node, we actually have to insert load instructions
481 // in all predecessor blocks, not in the PHI block itself!
482 if (PHINode *UPN = dyn_cast<PHINode>(U)) {
483 // Only insert into each predecessor once, so that we don't have
484 // different incoming values from the same block!
485 std::map<BasicBlock*, Value*> InsertedBlocks;
486 for (unsigned i = 0, e = UPN->getNumIncomingValues(); i != e; ++i)
487 if (UPN->getIncomingValue(i) == &I) {
488 BasicBlock *Pred = UPN->getIncomingBlock(i);
489 Value *&PredVal = InsertedBlocks[Pred];
491 // Insert a new load instruction right before the terminator in
492 // the predecessor block.
493 PredVal = new LoadInst(AI, "", Pred->getTerminator());
496 UPN->setIncomingValue(i, PredVal);
500 LoadInst *L = new LoadInst(AI, "", U);
501 U->replaceUsesOfWith(&I, L);
505 // Thirdly, insert a copy of the instruction in each exit block of the loop
506 // that is dominated by the instruction, storing the result into the memory
507 // location. Be careful not to insert the instruction into any particular
508 // basic block more than once.
509 std::set<BasicBlock*> InsertedBlocks;
510 BasicBlock *InstOrigBB = I.getParent();
512 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
513 BasicBlock *ExitBlock = ExitBlocks[i];
515 if (isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB)) {
516 // If we haven't already processed this exit block, do so now.
517 if (InsertedBlocks.insert(ExitBlock).second) {
518 // Insert the code after the last PHI node...
519 BasicBlock::iterator InsertPt = ExitBlock->begin();
520 while (isa<PHINode>(InsertPt)) ++InsertPt;
522 // If this is the first exit block processed, just move the original
523 // instruction, otherwise clone the original instruction and insert
526 if (InsertedBlocks.size() == 1) {
527 I.removeFromParent();
528 ExitBlock->getInstList().insert(InsertPt, &I);
532 CurAST->copyValue(&I, New);
533 if (!I.getName().empty())
534 New->setName(I.getName()+".le");
535 ExitBlock->getInstList().insert(InsertPt, New);
538 // Now that we have inserted the instruction, store it into the alloca
539 if (AI) new StoreInst(New, AI, InsertPt);
544 // If the instruction doesn't dominate any exit blocks, it must be dead.
545 if (InsertedBlocks.empty()) {
546 CurAST->deleteValue(&I);
550 // Finally, promote the fine value to SSA form.
552 std::vector<AllocaInst*> Allocas;
553 Allocas.push_back(AI);
554 PromoteMemToReg(Allocas, *ET, *DF, AA->getTargetData(), CurAST);
559 /// hoist - When an instruction is found to only use loop invariant operands
560 /// that is safe to hoist, this instruction is called to do the dirty work.
562 void LICM::hoist(Instruction &I) {
563 DOUT << "LICM hoisting to " << Preheader->getName() << ": " << I;
565 // Remove the instruction from its current basic block... but don't delete the
567 I.removeFromParent();
569 // Insert the new node in Preheader, before the terminator.
570 Preheader->getInstList().insert(Preheader->getTerminator(), &I);
572 if (isa<LoadInst>(I)) ++NumMovedLoads;
573 else if (isa<CallInst>(I)) ++NumMovedCalls;
578 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
579 /// not a trapping instruction or if it is a trapping instruction and is
580 /// guaranteed to execute.
582 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
583 // If it is not a trapping instruction, it is always safe to hoist.
584 if (!Inst.isTrapping()) return true;
586 // Otherwise we have to check to make sure that the instruction dominates all
587 // of the exit blocks. If it doesn't, then there is a path out of the loop
588 // which does not execute this instruction, so we can't hoist it.
590 // If the instruction is in the header block for the loop (which is very
591 // common), it is always guaranteed to dominate the exit blocks. Since this
592 // is a common case, and can save some work, check it now.
593 if (Inst.getParent() == CurLoop->getHeader())
596 // It's always safe to load from a global or alloca.
597 if (isa<LoadInst>(Inst))
598 if (isa<AllocationInst>(Inst.getOperand(0)) ||
599 isa<GlobalVariable>(Inst.getOperand(0)))
602 // Get the exit blocks for the current loop.
603 std::vector<BasicBlock*> ExitBlocks;
604 CurLoop->getExitBlocks(ExitBlocks);
606 // For each exit block, walk up the ET until the
607 // instruction's basic block is found or we exit the loop.
608 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
609 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent()))
616 /// PromoteValuesInLoop - Try to promote memory values to scalars by sinking
617 /// stores out of the loop and moving loads to before the loop. We do this by
618 /// looping over the stores in the loop, looking for stores to Must pointers
619 /// which are loop invariant. We promote these memory locations to use allocas
620 /// instead. These allocas can easily be raised to register values by the
621 /// PromoteMem2Reg functionality.
623 void LICM::PromoteValuesInLoop() {
624 // PromotedValues - List of values that are promoted out of the loop. Each
625 // value has an alloca instruction for it, and a canonical version of the
627 std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
628 std::map<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca
630 FindPromotableValuesInLoop(PromotedValues, ValueToAllocaMap);
631 if (ValueToAllocaMap.empty()) return; // If there are values to promote.
634 NumPromoted += PromotedValues.size();
636 std::vector<Value*> PointerValueNumbers;
638 // Emit a copy from the value into the alloca'd value in the loop preheader
639 TerminatorInst *LoopPredInst = Preheader->getTerminator();
640 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
641 Value *Ptr = PromotedValues[i].second;
643 // If we are promoting a pointer value, update alias information for the
645 Value *LoadValue = 0;
646 if (isa<PointerType>(cast<PointerType>(Ptr->getType())->getElementType())) {
647 // Locate a load or store through the pointer, and assign the same value
648 // to LI as we are loading or storing. Since we know that the value is
649 // stored in this loop, this will always succeed.
650 for (Value::use_iterator UI = Ptr->use_begin(), E = Ptr->use_end();
652 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
655 } else if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
656 if (SI->getOperand(1) == Ptr) {
657 LoadValue = SI->getOperand(0);
661 assert(LoadValue && "No store through the pointer found!");
662 PointerValueNumbers.push_back(LoadValue); // Remember this for later.
665 // Load from the memory we are promoting.
666 LoadInst *LI = new LoadInst(Ptr, Ptr->getName()+".promoted", LoopPredInst);
668 if (LoadValue) CurAST->copyValue(LoadValue, LI);
670 // Store into the temporary alloca.
671 new StoreInst(LI, PromotedValues[i].first, LoopPredInst);
674 // Scan the basic blocks in the loop, replacing uses of our pointers with
675 // uses of the allocas in question.
677 const std::vector<BasicBlock*> &LoopBBs = CurLoop->getBlocks();
678 for (std::vector<BasicBlock*>::const_iterator I = LoopBBs.begin(),
679 E = LoopBBs.end(); I != E; ++I) {
680 // Rewrite all loads and stores in the block of the pointer...
681 for (BasicBlock::iterator II = (*I)->begin(), E = (*I)->end();
683 if (LoadInst *L = dyn_cast<LoadInst>(II)) {
684 std::map<Value*, AllocaInst*>::iterator
685 I = ValueToAllocaMap.find(L->getOperand(0));
686 if (I != ValueToAllocaMap.end())
687 L->setOperand(0, I->second); // Rewrite load instruction...
688 } else if (StoreInst *S = dyn_cast<StoreInst>(II)) {
689 std::map<Value*, AllocaInst*>::iterator
690 I = ValueToAllocaMap.find(S->getOperand(1));
691 if (I != ValueToAllocaMap.end())
692 S->setOperand(1, I->second); // Rewrite store instruction...
697 // Now that the body of the loop uses the allocas instead of the original
698 // memory locations, insert code to copy the alloca value back into the
699 // original memory location on all exits from the loop. Note that we only
700 // want to insert one copy of the code in each exit block, though the loop may
701 // exit to the same block more than once.
703 std::set<BasicBlock*> ProcessedBlocks;
705 std::vector<BasicBlock*> ExitBlocks;
706 CurLoop->getExitBlocks(ExitBlocks);
707 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
708 if (ProcessedBlocks.insert(ExitBlocks[i]).second) {
709 // Copy all of the allocas into their memory locations.
710 BasicBlock::iterator BI = ExitBlocks[i]->begin();
711 while (isa<PHINode>(*BI))
712 ++BI; // Skip over all of the phi nodes in the block.
713 Instruction *InsertPos = BI;
715 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
716 // Load from the alloca.
717 LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
719 // If this is a pointer type, update alias info appropriately.
720 if (isa<PointerType>(LI->getType()))
721 CurAST->copyValue(PointerValueNumbers[PVN++], LI);
723 // Store into the memory we promoted.
724 new StoreInst(LI, PromotedValues[i].second, InsertPos);
728 // Now that we have done the deed, use the mem2reg functionality to promote
729 // all of the new allocas we just created into real SSA registers.
731 std::vector<AllocaInst*> PromotedAllocas;
732 PromotedAllocas.reserve(PromotedValues.size());
733 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i)
734 PromotedAllocas.push_back(PromotedValues[i].first);
735 PromoteMemToReg(PromotedAllocas, *ET, *DF, AA->getTargetData(), CurAST);
738 /// FindPromotableValuesInLoop - Check the current loop for stores to definite
739 /// pointers, which are not loaded and stored through may aliases. If these are
740 /// found, create an alloca for the value, add it to the PromotedValues list,
741 /// and keep track of the mapping from value to alloca.
743 void LICM::FindPromotableValuesInLoop(
744 std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
745 std::map<Value*, AllocaInst*> &ValueToAllocaMap) {
746 Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();
748 // Loop over all of the alias sets in the tracker object.
749 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
752 // We can promote this alias set if it has a store, if it is a "Must" alias
753 // set, if the pointer is loop invariant, and if we are not eliminating any
754 // volatile loads or stores.
755 if (!AS.isForwardingAliasSet() && AS.isMod() && AS.isMustAlias() &&
756 !AS.isVolatile() && CurLoop->isLoopInvariant(AS.begin()->first)) {
757 assert(AS.begin() != AS.end() &&
758 "Must alias set should have at least one pointer element in it!");
759 Value *V = AS.begin()->first;
761 // Check that all of the pointers in the alias set have the same type. We
762 // cannot (yet) promote a memory location that is loaded and stored in
764 bool PointerOk = true;
765 for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
766 if (V->getType() != I->first->getType()) {
772 const Type *Ty = cast<PointerType>(V->getType())->getElementType();
773 AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart);
774 PromotedValues.push_back(std::make_pair(AI, V));
776 // Update the AST and alias analysis.
777 CurAST->copyValue(V, AI);
779 for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
780 ValueToAllocaMap.insert(std::make_pair(I->first, AI));
782 DOUT << "LICM: Promoting value: " << *V << "\n";