1 //===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // 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/Analysis/ScalarEvolution.h"
46 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
47 #include "llvm/Support/CFG.h"
48 #include "llvm/Support/Compiler.h"
49 #include "llvm/Support/CommandLine.h"
50 #include "llvm/Support/Debug.h"
51 #include "llvm/ADT/Statistic.h"
55 STATISTIC(NumSunk , "Number of instructions sunk out of loop");
56 STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
57 STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
58 STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
59 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"));
66 struct VISIBILITY_HIDDEN LICM : public LoopPass {
67 static char ID; // Pass identification, replacement for typeid
68 LICM() : LoopPass((intptr_t)&ID) {}
70 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
72 /// This transformation requires natural loop information & requires that
73 /// loop preheaders be inserted into the CFG...
75 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
77 AU.addRequiredID(LoopSimplifyID);
78 AU.addRequired<LoopInfo>();
79 AU.addRequired<DominatorTree>();
80 AU.addRequired<DominanceFrontier>(); // For scalar promotion (mem2reg)
81 AU.addRequired<AliasAnalysis>();
82 AU.addPreserved<ScalarEvolution>();
83 AU.addPreserved<DominanceFrontier>();
86 bool doFinalization() {
87 // Free the values stored in the map
88 for (std::map<Loop *, AliasSetTracker *>::iterator
89 I = LoopToAliasMap.begin(), E = LoopToAliasMap.end(); I != E; ++I)
92 LoopToAliasMap.clear();
97 // Various analyses that we use...
98 AliasAnalysis *AA; // Current AliasAnalysis information
99 LoopInfo *LI; // Current LoopInfo
100 DominatorTree *DT; // Dominator Tree for the current Loop...
101 DominanceFrontier *DF; // Current Dominance Frontier
103 // State that is updated as we process loops
104 bool Changed; // Set to true when we change anything.
105 BasicBlock *Preheader; // The preheader block of the current loop...
106 Loop *CurLoop; // The current loop we are working on...
107 AliasSetTracker *CurAST; // AliasSet information for the current loop...
108 std::map<Loop *, AliasSetTracker *> LoopToAliasMap;
110 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
111 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
113 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
115 void deleteAnalysisValue(Value *V, Loop *L);
117 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
118 /// dominated by the specified block, and that are in the current loop) in
119 /// reverse depth first order w.r.t the DominatorTree. This allows us to
120 /// visit uses before definitions, allowing us to sink a loop body in one
121 /// pass without iteration.
123 void SinkRegion(DomTreeNode *N);
125 /// HoistRegion - Walk the specified region of the CFG (defined by all
126 /// blocks dominated by the specified block, and that are in the current
127 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
128 /// visit definitions before uses, allowing us to hoist a loop body in one
129 /// pass without iteration.
131 void HoistRegion(DomTreeNode *N);
133 /// inSubLoop - Little predicate that returns true if the specified basic
134 /// block is in a subloop of the current one, not the current one itself.
136 bool inSubLoop(BasicBlock *BB) {
137 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
138 for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I)
139 if ((*I)->contains(BB))
140 return true; // A subloop actually contains this block!
144 /// isExitBlockDominatedByBlockInLoop - This method checks to see if the
145 /// specified exit block of the loop is dominated by the specified block
146 /// that is in the body of the loop. We use these constraints to
147 /// dramatically limit the amount of the dominator tree that needs to be
149 bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock,
150 BasicBlock *BlockInLoop) const {
151 // If the block in the loop is the loop header, it must be dominated!
152 BasicBlock *LoopHeader = CurLoop->getHeader();
153 if (BlockInLoop == LoopHeader)
156 DomTreeNode *BlockInLoopNode = DT->getNode(BlockInLoop);
157 DomTreeNode *IDom = DT->getNode(ExitBlock);
159 // Because the exit block is not in the loop, we know we have to get _at
160 // least_ its immediate dominator.
162 // Get next Immediate Dominator.
163 IDom = IDom->getIDom();
165 // If we have got to the header of the loop, then the instructions block
166 // did not dominate the exit node, so we can't hoist it.
167 if (IDom->getBlock() == LoopHeader)
170 } while (IDom != BlockInLoopNode);
175 /// sink - When an instruction is found to only be used outside of the loop,
176 /// this function moves it to the exit blocks and patches up SSA form as
179 void sink(Instruction &I);
181 /// hoist - When an instruction is found to only use loop invariant operands
182 /// that is safe to hoist, this instruction is called to do the dirty work.
184 void hoist(Instruction &I);
186 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
187 /// is not a trapping instruction or if it is a trapping instruction and is
188 /// guaranteed to execute.
190 bool isSafeToExecuteUnconditionally(Instruction &I);
192 /// pointerInvalidatedByLoop - Return true if the body of this loop may
193 /// store into the memory location pointed to by V.
195 bool pointerInvalidatedByLoop(Value *V, unsigned Size) {
196 // Check to see if any of the basic blocks in CurLoop invalidate *V.
197 return CurAST->getAliasSetForPointer(V, Size).isMod();
200 bool canSinkOrHoistInst(Instruction &I);
201 bool isLoopInvariantInst(Instruction &I);
202 bool isNotUsedInLoop(Instruction &I);
204 /// PromoteValuesInLoop - Look at the stores in the loop and promote as many
205 /// to scalars as we can.
207 void PromoteValuesInLoop();
209 /// FindPromotableValuesInLoop - Check the current loop for stores to
210 /// definite pointers, which are not loaded and stored through may aliases.
211 /// If these are found, create an alloca for the value, add it to the
212 /// PromotedValues list, and keep track of the mapping from value to
215 void FindPromotableValuesInLoop(
216 std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
217 std::map<Value*, AllocaInst*> &Val2AlMap);
222 static RegisterPass<LICM> X("licm", "Loop Invariant Code Motion");
224 LoopPass *llvm::createLICMPass() { return new LICM(); }
226 /// Hoist expressions out of the specified loop. Note, alias info for inner
227 /// loop is not preserved so it is not a good idea to run LICM multiple
228 /// times on one loop.
230 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
233 // Get our Loop and Alias Analysis information...
234 LI = &getAnalysis<LoopInfo>();
235 AA = &getAnalysis<AliasAnalysis>();
236 DF = &getAnalysis<DominanceFrontier>();
237 DT = &getAnalysis<DominatorTree>();
239 CurAST = new AliasSetTracker(*AA);
240 // Collect Alias info from subloops
241 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
242 LoopItr != LoopItrE; ++LoopItr) {
243 Loop *InnerL = *LoopItr;
244 AliasSetTracker *InnerAST = LoopToAliasMap[InnerL];
245 assert (InnerAST && "Where is my AST?");
247 // What if InnerLoop was modified by other passes ?
248 CurAST->add(*InnerAST);
253 // Get the preheader block to move instructions into...
254 Preheader = L->getLoopPreheader();
255 assert(Preheader&&"Preheader insertion pass guarantees we have a preheader!");
257 // Loop over the body of this loop, looking for calls, invokes, and stores.
258 // Because subloops have already been incorporated into AST, we skip blocks in
261 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
264 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops...
265 CurAST->add(*BB); // Incorporate the specified basic block
268 // We want to visit all of the instructions in this loop... that are not parts
269 // of our subloops (they have already had their invariants hoisted out of
270 // their loop, into this loop, so there is no need to process the BODIES of
273 // Traverse the body of the loop in depth first order on the dominator tree so
274 // that we are guaranteed to see definitions before we see uses. This allows
275 // us to sink instructions in one pass, without iteration. After sinking
276 // instructions, we perform another pass to hoist them out of the loop.
278 SinkRegion(DT->getNode(L->getHeader()));
279 HoistRegion(DT->getNode(L->getHeader()));
281 // Now that all loop invariants have been removed from the loop, promote any
282 // memory references to scalars that we can...
283 if (!DisablePromotion)
284 PromoteValuesInLoop();
286 // Clear out loops state information for the next iteration
290 LoopToAliasMap[L] = CurAST;
294 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
295 /// dominated by the specified block, and that are in the current loop) in
296 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
297 /// uses before definitions, allowing us to sink a loop body in one pass without
300 void LICM::SinkRegion(DomTreeNode *N) {
301 assert(N != 0 && "Null dominator tree node?");
302 BasicBlock *BB = N->getBlock();
304 // If this subregion is not in the top level loop at all, exit.
305 if (!CurLoop->contains(BB)) return;
307 // We are processing blocks in reverse dfo, so process children first...
308 const std::vector<DomTreeNode*> &Children = N->getChildren();
309 for (unsigned i = 0, e = Children.size(); i != e; ++i)
310 SinkRegion(Children[i]);
312 // Only need to process the contents of this block if it is not part of a
313 // subloop (which would already have been processed).
314 if (inSubLoop(BB)) return;
316 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
317 Instruction &I = *--II;
319 // Check to see if we can sink this instruction to the exit blocks
320 // of the loop. We can do this if the all users of the instruction are
321 // outside of the loop. In this case, it doesn't even matter if the
322 // operands of the instruction are loop invariant.
324 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
332 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
333 /// dominated by the specified block, and that are in the current loop) in depth
334 /// first order w.r.t the DominatorTree. This allows us to visit definitions
335 /// before uses, allowing us to hoist a loop body in one pass without iteration.
337 void LICM::HoistRegion(DomTreeNode *N) {
338 assert(N != 0 && "Null dominator tree node?");
339 BasicBlock *BB = N->getBlock();
341 // If this subregion is not in the top level loop at all, exit.
342 if (!CurLoop->contains(BB)) return;
344 // Only need to process the contents of this block if it is not part of a
345 // subloop (which would already have been processed).
347 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
348 Instruction &I = *II++;
350 // Try hoisting the instruction out to the preheader. We can only do this
351 // if all of the operands of the instruction are loop invariant and if it
352 // is safe to hoist the instruction.
354 if (isLoopInvariantInst(I) && canSinkOrHoistInst(I) &&
355 isSafeToExecuteUnconditionally(I))
359 const std::vector<DomTreeNode*> &Children = N->getChildren();
360 for (unsigned i = 0, e = Children.size(); i != e; ++i)
361 HoistRegion(Children[i]);
364 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
367 bool LICM::canSinkOrHoistInst(Instruction &I) {
368 // Loads have extra constraints we have to verify before we can hoist them.
369 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
370 if (LI->isVolatile())
371 return false; // Don't hoist volatile loads!
373 // Don't hoist loads which have may-aliased stores in loop.
375 if (LI->getType()->isSized())
376 Size = AA->getTargetData().getTypeStoreSize(LI->getType());
377 return !pointerInvalidatedByLoop(LI->getOperand(0), Size);
378 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
379 // Handle obvious cases efficiently.
380 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
381 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
383 else if (Behavior == AliasAnalysis::OnlyReadsMemory) {
384 // If this call only reads from memory and there are no writes to memory
385 // in the loop, we can hoist or sink the call as appropriate.
386 bool FoundMod = false;
387 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
390 if (!AS.isForwardingAliasSet() && AS.isMod()) {
395 if (!FoundMod) return true;
398 // FIXME: This should use mod/ref information to see if we can hoist or sink
404 // Otherwise these instructions are hoistable/sinkable
405 return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
406 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
407 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
408 isa<ShuffleVectorInst>(I);
411 /// isNotUsedInLoop - Return true if the only users of this instruction are
412 /// outside of the loop. If this is true, we can sink the instruction to the
413 /// exit blocks of the loop.
415 bool LICM::isNotUsedInLoop(Instruction &I) {
416 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
417 Instruction *User = cast<Instruction>(*UI);
418 if (PHINode *PN = dyn_cast<PHINode>(User)) {
419 // PHI node uses occur in predecessor blocks!
420 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
421 if (PN->getIncomingValue(i) == &I)
422 if (CurLoop->contains(PN->getIncomingBlock(i)))
424 } else if (CurLoop->contains(User->getParent())) {
432 /// isLoopInvariantInst - Return true if all operands of this instruction are
433 /// loop invariant. We also filter out non-hoistable instructions here just for
436 bool LICM::isLoopInvariantInst(Instruction &I) {
437 // The instruction is loop invariant if all of its operands are loop-invariant
438 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
439 if (!CurLoop->isLoopInvariant(I.getOperand(i)))
442 // If we got this far, the instruction is loop invariant!
446 /// sink - When an instruction is found to only be used outside of the loop,
447 /// this function moves it to the exit blocks and patches up SSA form as needed.
448 /// This method is guaranteed to remove the original instruction from its
449 /// position, and may either delete it or move it to outside of the loop.
451 void LICM::sink(Instruction &I) {
452 DOUT << "LICM sinking instruction: " << I;
454 SmallVector<BasicBlock*, 8> ExitBlocks;
455 CurLoop->getExitBlocks(ExitBlocks);
457 if (isa<LoadInst>(I)) ++NumMovedLoads;
458 else if (isa<CallInst>(I)) ++NumMovedCalls;
462 // The case where there is only a single exit node of this loop is common
463 // enough that we handle it as a special (more efficient) case. It is more
464 // efficient to handle because there are no PHI nodes that need to be placed.
465 if (ExitBlocks.size() == 1) {
466 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) {
467 // Instruction is not used, just delete it.
468 CurAST->deleteValue(&I);
469 if (!I.use_empty()) // If I has users in unreachable blocks, eliminate.
470 I.replaceAllUsesWith(UndefValue::get(I.getType()));
473 // Move the instruction to the start of the exit block, after any PHI
475 I.removeFromParent();
477 BasicBlock::iterator InsertPt = ExitBlocks[0]->getFirstNonPHI();
478 ExitBlocks[0]->getInstList().insert(InsertPt, &I);
480 } else if (ExitBlocks.empty()) {
481 // The instruction is actually dead if there ARE NO exit blocks.
482 CurAST->deleteValue(&I);
483 if (!I.use_empty()) // If I has users in unreachable blocks, eliminate.
484 I.replaceAllUsesWith(UndefValue::get(I.getType()));
487 // Otherwise, if we have multiple exits, use the PromoteMem2Reg function to
488 // do all of the hard work of inserting PHI nodes as necessary. We convert
489 // the value into a stack object to get it to do this.
491 // Firstly, we create a stack object to hold the value...
494 if (I.getType() != Type::VoidTy) {
495 AI = new AllocaInst(I.getType(), 0, I.getName(),
496 I.getParent()->getParent()->getEntryBlock().begin());
500 // Secondly, insert load instructions for each use of the instruction
501 // outside of the loop.
502 while (!I.use_empty()) {
503 Instruction *U = cast<Instruction>(I.use_back());
505 // If the user is a PHI Node, we actually have to insert load instructions
506 // in all predecessor blocks, not in the PHI block itself!
507 if (PHINode *UPN = dyn_cast<PHINode>(U)) {
508 // Only insert into each predecessor once, so that we don't have
509 // different incoming values from the same block!
510 std::map<BasicBlock*, Value*> InsertedBlocks;
511 for (unsigned i = 0, e = UPN->getNumIncomingValues(); i != e; ++i)
512 if (UPN->getIncomingValue(i) == &I) {
513 BasicBlock *Pred = UPN->getIncomingBlock(i);
514 Value *&PredVal = InsertedBlocks[Pred];
516 // Insert a new load instruction right before the terminator in
517 // the predecessor block.
518 PredVal = new LoadInst(AI, "", Pred->getTerminator());
519 CurAST->add(cast<LoadInst>(PredVal));
522 UPN->setIncomingValue(i, PredVal);
526 LoadInst *L = new LoadInst(AI, "", U);
527 U->replaceUsesOfWith(&I, L);
532 // Thirdly, insert a copy of the instruction in each exit block of the loop
533 // that is dominated by the instruction, storing the result into the memory
534 // location. Be careful not to insert the instruction into any particular
535 // basic block more than once.
536 std::set<BasicBlock*> InsertedBlocks;
537 BasicBlock *InstOrigBB = I.getParent();
539 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
540 BasicBlock *ExitBlock = ExitBlocks[i];
542 if (isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB)) {
543 // If we haven't already processed this exit block, do so now.
544 if (InsertedBlocks.insert(ExitBlock).second) {
545 // Insert the code after the last PHI node...
546 BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI();
548 // If this is the first exit block processed, just move the original
549 // instruction, otherwise clone the original instruction and insert
552 if (InsertedBlocks.size() == 1) {
553 I.removeFromParent();
554 ExitBlock->getInstList().insert(InsertPt, &I);
558 CurAST->copyValue(&I, New);
559 if (!I.getName().empty())
560 New->setName(I.getName()+".le");
561 ExitBlock->getInstList().insert(InsertPt, New);
564 // Now that we have inserted the instruction, store it into the alloca
565 if (AI) new StoreInst(New, AI, InsertPt);
570 // If the instruction doesn't dominate any exit blocks, it must be dead.
571 if (InsertedBlocks.empty()) {
572 CurAST->deleteValue(&I);
576 // Finally, promote the fine value to SSA form.
578 std::vector<AllocaInst*> Allocas;
579 Allocas.push_back(AI);
580 PromoteMemToReg(Allocas, *DT, *DF, CurAST);
585 /// hoist - When an instruction is found to only use loop invariant operands
586 /// that is safe to hoist, this instruction is called to do the dirty work.
588 void LICM::hoist(Instruction &I) {
589 DOUT << "LICM hoisting to " << Preheader->getName() << ": " << I;
591 // Remove the instruction from its current basic block... but don't delete the
593 I.removeFromParent();
595 // Insert the new node in Preheader, before the terminator.
596 Preheader->getInstList().insert(Preheader->getTerminator(), &I);
598 if (isa<LoadInst>(I)) ++NumMovedLoads;
599 else if (isa<CallInst>(I)) ++NumMovedCalls;
604 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
605 /// not a trapping instruction or if it is a trapping instruction and is
606 /// guaranteed to execute.
608 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
609 // If it is not a trapping instruction, it is always safe to hoist.
610 if (!Inst.isTrapping()) return true;
612 // Otherwise we have to check to make sure that the instruction dominates all
613 // of the exit blocks. If it doesn't, then there is a path out of the loop
614 // which does not execute this instruction, so we can't hoist it.
616 // If the instruction is in the header block for the loop (which is very
617 // common), it is always guaranteed to dominate the exit blocks. Since this
618 // is a common case, and can save some work, check it now.
619 if (Inst.getParent() == CurLoop->getHeader())
622 // It's always safe to load from a global or alloca.
623 if (isa<LoadInst>(Inst))
624 if (isa<AllocationInst>(Inst.getOperand(0)) ||
625 isa<GlobalVariable>(Inst.getOperand(0)))
628 // Get the exit blocks for the current loop.
629 SmallVector<BasicBlock*, 8> ExitBlocks;
630 CurLoop->getExitBlocks(ExitBlocks);
632 // For each exit block, get the DT node and walk up the DT until the
633 // instruction's basic block is found or we exit the loop.
634 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
635 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent()))
642 /// PromoteValuesInLoop - Try to promote memory values to scalars by sinking
643 /// stores out of the loop and moving loads to before the loop. We do this by
644 /// looping over the stores in the loop, looking for stores to Must pointers
645 /// which are loop invariant. We promote these memory locations to use allocas
646 /// instead. These allocas can easily be raised to register values by the
647 /// PromoteMem2Reg functionality.
649 void LICM::PromoteValuesInLoop() {
650 // PromotedValues - List of values that are promoted out of the loop. Each
651 // value has an alloca instruction for it, and a canonical version of the
653 std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
654 std::map<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca
656 FindPromotableValuesInLoop(PromotedValues, ValueToAllocaMap);
657 if (ValueToAllocaMap.empty()) return; // If there are values to promote.
660 NumPromoted += PromotedValues.size();
662 std::vector<Value*> PointerValueNumbers;
664 // Emit a copy from the value into the alloca'd value in the loop preheader
665 TerminatorInst *LoopPredInst = Preheader->getTerminator();
666 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
667 Value *Ptr = PromotedValues[i].second;
669 // If we are promoting a pointer value, update alias information for the
671 Value *LoadValue = 0;
672 if (isa<PointerType>(cast<PointerType>(Ptr->getType())->getElementType())) {
673 // Locate a load or store through the pointer, and assign the same value
674 // to LI as we are loading or storing. Since we know that the value is
675 // stored in this loop, this will always succeed.
676 for (Value::use_iterator UI = Ptr->use_begin(), E = Ptr->use_end();
678 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
681 } else if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
682 if (SI->getOperand(1) == Ptr) {
683 LoadValue = SI->getOperand(0);
687 assert(LoadValue && "No store through the pointer found!");
688 PointerValueNumbers.push_back(LoadValue); // Remember this for later.
691 // Load from the memory we are promoting.
692 LoadInst *LI = new LoadInst(Ptr, Ptr->getName()+".promoted", LoopPredInst);
694 if (LoadValue) CurAST->copyValue(LoadValue, LI);
696 // Store into the temporary alloca.
697 new StoreInst(LI, PromotedValues[i].first, LoopPredInst);
700 // Scan the basic blocks in the loop, replacing uses of our pointers with
701 // uses of the allocas in question.
703 for (Loop::block_iterator I = CurLoop->block_begin(),
704 E = CurLoop->block_end(); I != E; ++I) {
706 // Rewrite all loads and stores in the block of the pointer...
707 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
708 if (LoadInst *L = dyn_cast<LoadInst>(II)) {
709 std::map<Value*, AllocaInst*>::iterator
710 I = ValueToAllocaMap.find(L->getOperand(0));
711 if (I != ValueToAllocaMap.end())
712 L->setOperand(0, I->second); // Rewrite load instruction...
713 } else if (StoreInst *S = dyn_cast<StoreInst>(II)) {
714 std::map<Value*, AllocaInst*>::iterator
715 I = ValueToAllocaMap.find(S->getOperand(1));
716 if (I != ValueToAllocaMap.end())
717 S->setOperand(1, I->second); // Rewrite store instruction...
722 // Now that the body of the loop uses the allocas instead of the original
723 // memory locations, insert code to copy the alloca value back into the
724 // original memory location on all exits from the loop. Note that we only
725 // want to insert one copy of the code in each exit block, though the loop may
726 // exit to the same block more than once.
728 SmallPtrSet<BasicBlock*, 16> ProcessedBlocks;
730 SmallVector<BasicBlock*, 8> ExitBlocks;
731 CurLoop->getExitBlocks(ExitBlocks);
732 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
733 if (!ProcessedBlocks.insert(ExitBlocks[i]))
736 // Copy all of the allocas into their memory locations.
737 BasicBlock::iterator BI = ExitBlocks[i]->getFirstNonPHI();
738 Instruction *InsertPos = BI;
740 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
741 // Load from the alloca.
742 LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
744 // If this is a pointer type, update alias info appropriately.
745 if (isa<PointerType>(LI->getType()))
746 CurAST->copyValue(PointerValueNumbers[PVN++], LI);
748 // Store into the memory we promoted.
749 new StoreInst(LI, PromotedValues[i].second, InsertPos);
753 // Now that we have done the deed, use the mem2reg functionality to promote
754 // all of the new allocas we just created into real SSA registers.
756 std::vector<AllocaInst*> PromotedAllocas;
757 PromotedAllocas.reserve(PromotedValues.size());
758 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i)
759 PromotedAllocas.push_back(PromotedValues[i].first);
760 PromoteMemToReg(PromotedAllocas, *DT, *DF, CurAST);
763 /// FindPromotableValuesInLoop - Check the current loop for stores to definite
764 /// pointers, which are not loaded and stored through may aliases and are safe
765 /// for promotion. If these are found, create an alloca for the value, add it
766 /// to the PromotedValues list, and keep track of the mapping from value to
768 void LICM::FindPromotableValuesInLoop(
769 std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
770 std::map<Value*, AllocaInst*> &ValueToAllocaMap) {
771 Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();
773 SmallVector<BasicBlock*, 4> ExitingBlocks;
774 CurLoop->getExitingBlocks(ExitingBlocks);
776 // Loop over all of the alias sets in the tracker object.
777 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
780 // We can promote this alias set if it has a store, if it is a "Must" alias
781 // set, if the pointer is loop invariant, and if we are not eliminating any
782 // volatile loads or stores.
783 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
784 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->first))
787 assert(!AS.empty() &&
788 "Must alias set should have at least one pointer element in it!");
789 Value *V = AS.begin()->first;
791 // Check that all of the pointers in the alias set have the same type. We
792 // cannot (yet) promote a memory location that is loaded and stored in
795 bool PointerOk = true;
796 for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
797 if (V->getType() != I->first->getType()) {
805 // It isn't safe to promote a load/store from the loop if the load/store is
806 // conditional. For example, turning:
808 // for () { if (c) *P += 1; }
812 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
814 // is not safe, because *P may only be valid to access if 'c' is true.
816 // It is safe to promote P if all uses are direct load/stores and if at
817 // least one is guaranteed to be executed.
818 bool GuaranteedToExecute = false;
819 bool InvalidInst = false;
820 for (Value::use_iterator UI = V->use_begin(), UE = V->use_end();
822 // Ignore instructions not in this loop.
823 Instruction *Use = dyn_cast<Instruction>(*UI);
824 if (!Use || !CurLoop->contains(Use->getParent()))
827 if (!isa<LoadInst>(Use) && !isa<StoreInst>(Use)) {
832 if (!GuaranteedToExecute)
833 GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use);
836 // If there is an non-load/store instruction in the loop, we can't promote
837 // it. If there isn't a guaranteed-to-execute instruction, we can't
839 if (InvalidInst || !GuaranteedToExecute)
842 const Type *Ty = cast<PointerType>(V->getType())->getElementType();
843 AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart);
844 PromotedValues.push_back(std::make_pair(AI, V));
846 // Update the AST and alias analysis.
847 CurAST->copyValue(V, AI);
849 for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
850 ValueToAllocaMap.insert(std::make_pair(I->first, AI));
852 DOUT << "LICM: Promoting value: " << *V << "\n";
856 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
857 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
858 AliasSetTracker *AST = LoopToAliasMap[L];
862 AST->copyValue(From, To);
865 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
867 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
868 AliasSetTracker *AST = LoopToAliasMap[L];