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/IntrinsicInst.h"
39 #include "llvm/Instructions.h"
40 #include "llvm/Target/TargetData.h"
41 #include "llvm/Analysis/LoopInfo.h"
42 #include "llvm/Analysis/LoopPass.h"
43 #include "llvm/Analysis/AliasAnalysis.h"
44 #include "llvm/Analysis/AliasSetTracker.h"
45 #include "llvm/Analysis/Dominators.h"
46 #include "llvm/Analysis/ScalarEvolution.h"
47 #include "llvm/Transforms/Utils/PromoteMemToReg.h"
48 #include "llvm/Support/CFG.h"
49 #include "llvm/Support/CommandLine.h"
50 #include "llvm/Support/raw_ostream.h"
51 #include "llvm/Support/Debug.h"
52 #include "llvm/ADT/Statistic.h"
56 STATISTIC(NumSunk , "Number of instructions sunk out of loop");
57 STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
58 STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
59 STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
60 STATISTIC(NumPromoted , "Number of memory locations promoted to registers");
63 DisablePromotion("disable-licm-promotion", cl::Hidden,
64 cl::desc("Disable memory promotion in LICM pass"));
67 struct LICM : public LoopPass {
68 static char ID; // Pass identification, replacement for typeid
69 LICM() : LoopPass(&ID) {}
71 virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
73 /// This transformation requires natural loop information & requires that
74 /// loop preheaders be inserted into the CFG...
76 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
78 AU.addRequiredID(LoopSimplifyID);
79 AU.addRequired<LoopInfo>();
80 AU.addRequired<DominatorTree>();
81 AU.addRequired<DominanceFrontier>(); // For scalar promotion (mem2reg)
82 AU.addRequired<AliasAnalysis>();
83 AU.addPreserved<ScalarEvolution>();
84 AU.addPreserved<DominanceFrontier>();
85 AU.addPreservedID(LoopSimplifyID);
88 bool doFinalization() {
89 // Free the values stored in the map
90 for (std::map<Loop *, AliasSetTracker *>::iterator
91 I = LoopToAliasMap.begin(), E = LoopToAliasMap.end(); I != E; ++I)
94 LoopToAliasMap.clear();
99 // Various analyses that we use...
100 AliasAnalysis *AA; // Current AliasAnalysis information
101 LoopInfo *LI; // Current LoopInfo
102 DominatorTree *DT; // Dominator Tree for the current Loop...
103 DominanceFrontier *DF; // Current Dominance Frontier
105 // State that is updated as we process loops
106 bool Changed; // Set to true when we change anything.
107 BasicBlock *Preheader; // The preheader block of the current loop...
108 Loop *CurLoop; // The current loop we are working on...
109 AliasSetTracker *CurAST; // AliasSet information for the current loop...
110 std::map<Loop *, AliasSetTracker *> LoopToAliasMap;
112 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
113 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
115 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
117 void deleteAnalysisValue(Value *V, Loop *L);
119 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
120 /// dominated by the specified block, and that are in the current loop) in
121 /// reverse depth first order w.r.t the DominatorTree. This allows us to
122 /// visit uses before definitions, allowing us to sink a loop body in one
123 /// pass without iteration.
125 void SinkRegion(DomTreeNode *N);
127 /// HoistRegion - Walk the specified region of the CFG (defined by all
128 /// blocks dominated by the specified block, and that are in the current
129 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
130 /// visit definitions before uses, allowing us to hoist a loop body in one
131 /// pass without iteration.
133 void HoistRegion(DomTreeNode *N);
135 /// inSubLoop - Little predicate that returns true if the specified basic
136 /// block is in a subloop of the current one, not the current one itself.
138 bool inSubLoop(BasicBlock *BB) {
139 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
140 for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I)
141 if ((*I)->contains(BB))
142 return true; // A subloop actually contains this block!
146 /// isExitBlockDominatedByBlockInLoop - This method checks to see if the
147 /// specified exit block of the loop is dominated by the specified block
148 /// that is in the body of the loop. We use these constraints to
149 /// dramatically limit the amount of the dominator tree that needs to be
151 bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock,
152 BasicBlock *BlockInLoop) const {
153 // If the block in the loop is the loop header, it must be dominated!
154 BasicBlock *LoopHeader = CurLoop->getHeader();
155 if (BlockInLoop == LoopHeader)
158 DomTreeNode *BlockInLoopNode = DT->getNode(BlockInLoop);
159 DomTreeNode *IDom = DT->getNode(ExitBlock);
161 // Because the exit block is not in the loop, we know we have to get _at
162 // least_ its immediate dominator.
163 IDom = IDom->getIDom();
165 while (IDom && IDom != BlockInLoopNode) {
166 // If we have got to the header of the loop, then the instructions block
167 // did not dominate the exit node, so we can't hoist it.
168 if (IDom->getBlock() == LoopHeader)
171 // Get next Immediate Dominator.
172 IDom = IDom->getIDom();
178 /// sink - When an instruction is found to only be used outside of the loop,
179 /// this function moves it to the exit blocks and patches up SSA form as
182 void sink(Instruction &I);
184 /// hoist - When an instruction is found to only use loop invariant operands
185 /// that is safe to hoist, this instruction is called to do the dirty work.
187 void hoist(Instruction &I);
189 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
190 /// is not a trapping instruction or if it is a trapping instruction and is
191 /// guaranteed to execute.
193 bool isSafeToExecuteUnconditionally(Instruction &I);
195 /// pointerInvalidatedByLoop - Return true if the body of this loop may
196 /// store into the memory location pointed to by V.
198 bool pointerInvalidatedByLoop(Value *V, unsigned Size) {
199 // Check to see if any of the basic blocks in CurLoop invalidate *V.
200 return CurAST->getAliasSetForPointer(V, Size).isMod();
203 bool canSinkOrHoistInst(Instruction &I);
204 bool isLoopInvariantInst(Instruction &I);
205 bool isNotUsedInLoop(Instruction &I);
207 /// PromoteValuesInLoop - Look at the stores in the loop and promote as many
208 /// to scalars as we can.
210 void PromoteValuesInLoop();
212 /// FindPromotableValuesInLoop - Check the current loop for stores to
213 /// definite pointers, which are not loaded and stored through may aliases.
214 /// If these are found, create an alloca for the value, add it to the
215 /// PromotedValues list, and keep track of the mapping from value to
218 void FindPromotableValuesInLoop(
219 std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
220 std::map<Value*, AllocaInst*> &Val2AlMap);
225 static RegisterPass<LICM> X("licm", "Loop Invariant Code Motion");
227 Pass *llvm::createLICMPass() { return new LICM(); }
229 /// Hoist expressions out of the specified loop. Note, alias info for inner
230 /// loop is not preserved so it is not a good idea to run LICM multiple
231 /// times on one loop.
233 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
236 // Get our Loop and Alias Analysis information...
237 LI = &getAnalysis<LoopInfo>();
238 AA = &getAnalysis<AliasAnalysis>();
239 DF = &getAnalysis<DominanceFrontier>();
240 DT = &getAnalysis<DominatorTree>();
242 CurAST = new AliasSetTracker(*AA);
243 // Collect Alias info from subloops
244 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
245 LoopItr != LoopItrE; ++LoopItr) {
246 Loop *InnerL = *LoopItr;
247 AliasSetTracker *InnerAST = LoopToAliasMap[InnerL];
248 assert (InnerAST && "Where is my AST?");
250 // What if InnerLoop was modified by other passes ?
251 CurAST->add(*InnerAST);
256 // Get the preheader block to move instructions into...
257 Preheader = L->getLoopPreheader();
259 // Loop over the body of this loop, looking for calls, invokes, and stores.
260 // Because subloops have already been incorporated into AST, we skip blocks in
263 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
266 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops...
267 CurAST->add(*BB); // Incorporate the specified basic block
270 // We want to visit all of the instructions in this loop... that are not parts
271 // of our subloops (they have already had their invariants hoisted out of
272 // their loop, into this loop, so there is no need to process the BODIES of
275 // Traverse the body of the loop in depth first order on the dominator tree so
276 // that we are guaranteed to see definitions before we see uses. This allows
277 // us to sink instructions in one pass, without iteration. After sinking
278 // instructions, we perform another pass to hoist them out of the loop.
280 if (L->hasDedicatedExits())
281 SinkRegion(DT->getNode(L->getHeader()));
283 HoistRegion(DT->getNode(L->getHeader()));
285 // Now that all loop invariants have been removed from the loop, promote any
286 // memory references to scalars that we can...
287 if (!DisablePromotion && Preheader && L->hasDedicatedExits())
288 PromoteValuesInLoop();
290 // Clear out loops state information for the next iteration
294 LoopToAliasMap[L] = CurAST;
298 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
299 /// dominated by the specified block, and that are in the current loop) in
300 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
301 /// uses before definitions, allowing us to sink a loop body in one pass without
304 void LICM::SinkRegion(DomTreeNode *N) {
305 assert(N != 0 && "Null dominator tree node?");
306 BasicBlock *BB = N->getBlock();
308 // If this subregion is not in the top level loop at all, exit.
309 if (!CurLoop->contains(BB)) return;
311 // We are processing blocks in reverse dfo, so process children first...
312 const std::vector<DomTreeNode*> &Children = N->getChildren();
313 for (unsigned i = 0, e = Children.size(); i != e; ++i)
314 SinkRegion(Children[i]);
316 // Only need to process the contents of this block if it is not part of a
317 // subloop (which would already have been processed).
318 if (inSubLoop(BB)) return;
320 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
321 Instruction &I = *--II;
323 // Check to see if we can sink this instruction to the exit blocks
324 // of the loop. We can do this if the all users of the instruction are
325 // outside of the loop. In this case, it doesn't even matter if the
326 // operands of the instruction are loop invariant.
328 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
335 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
336 /// dominated by the specified block, and that are in the current loop) in depth
337 /// first order w.r.t the DominatorTree. This allows us to visit definitions
338 /// before uses, allowing us to hoist a loop body in one pass without iteration.
340 void LICM::HoistRegion(DomTreeNode *N) {
341 assert(N != 0 && "Null dominator tree node?");
342 BasicBlock *BB = N->getBlock();
344 // If this subregion is not in the top level loop at all, exit.
345 if (!CurLoop->contains(BB)) return;
347 // Only need to process the contents of this block if it is not part of a
348 // subloop (which would already have been processed).
350 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
351 Instruction &I = *II++;
353 // Try hoisting the instruction out to the preheader. We can only do this
354 // if all of the operands of the instruction are loop invariant and if it
355 // is safe to hoist the instruction.
357 if (isLoopInvariantInst(I) && canSinkOrHoistInst(I) &&
358 isSafeToExecuteUnconditionally(I))
362 const std::vector<DomTreeNode*> &Children = N->getChildren();
363 for (unsigned i = 0, e = Children.size(); i != e; ++i)
364 HoistRegion(Children[i]);
367 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
370 bool LICM::canSinkOrHoistInst(Instruction &I) {
371 // Loads have extra constraints we have to verify before we can hoist them.
372 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
373 if (LI->isVolatile())
374 return false; // Don't hoist volatile loads!
376 // Loads from constant memory are always safe to move, even if they end up
377 // in the same alias set as something that ends up being modified.
378 if (AA->pointsToConstantMemory(LI->getOperand(0)))
381 // Don't hoist loads which have may-aliased stores in loop.
383 if (LI->getType()->isSized())
384 Size = AA->getTypeStoreSize(LI->getType());
385 return !pointerInvalidatedByLoop(LI->getOperand(0), Size);
386 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
387 // Handle obvious cases efficiently.
388 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
389 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
391 else if (Behavior == AliasAnalysis::OnlyReadsMemory) {
392 // If this call only reads from memory and there are no writes to memory
393 // in the loop, we can hoist or sink the call as appropriate.
394 bool FoundMod = false;
395 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
398 if (!AS.isForwardingAliasSet() && AS.isMod()) {
403 if (!FoundMod) return true;
406 // FIXME: This should use mod/ref information to see if we can hoist or sink
412 // Otherwise these instructions are hoistable/sinkable
413 return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
414 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
415 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
416 isa<ShuffleVectorInst>(I);
419 /// isNotUsedInLoop - Return true if the only users of this instruction are
420 /// outside of the loop. If this is true, we can sink the instruction to the
421 /// exit blocks of the loop.
423 bool LICM::isNotUsedInLoop(Instruction &I) {
424 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
425 Instruction *User = cast<Instruction>(*UI);
426 if (PHINode *PN = dyn_cast<PHINode>(User)) {
427 // PHI node uses occur in predecessor blocks!
428 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
429 if (PN->getIncomingValue(i) == &I)
430 if (CurLoop->contains(PN->getIncomingBlock(i)))
432 } else if (CurLoop->contains(User)) {
440 /// isLoopInvariantInst - Return true if all operands of this instruction are
441 /// loop invariant. We also filter out non-hoistable instructions here just for
444 bool LICM::isLoopInvariantInst(Instruction &I) {
445 // The instruction is loop invariant if all of its operands are loop-invariant
446 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
447 if (!CurLoop->isLoopInvariant(I.getOperand(i)))
450 // If we got this far, the instruction is loop invariant!
454 /// sink - When an instruction is found to only be used outside of the loop,
455 /// this function moves it to the exit blocks and patches up SSA form as needed.
456 /// This method is guaranteed to remove the original instruction from its
457 /// position, and may either delete it or move it to outside of the loop.
459 void LICM::sink(Instruction &I) {
460 DEBUG(dbgs() << "LICM sinking instruction: " << I);
462 SmallVector<BasicBlock*, 8> ExitBlocks;
463 CurLoop->getExitBlocks(ExitBlocks);
465 if (isa<LoadInst>(I)) ++NumMovedLoads;
466 else if (isa<CallInst>(I)) ++NumMovedCalls;
470 // The case where there is only a single exit node of this loop is common
471 // enough that we handle it as a special (more efficient) case. It is more
472 // efficient to handle because there are no PHI nodes that need to be placed.
473 if (ExitBlocks.size() == 1) {
474 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) {
475 // Instruction is not used, just delete it.
476 CurAST->deleteValue(&I);
477 // If I has users in unreachable blocks, eliminate.
478 // If I is not void type then replaceAllUsesWith undef.
479 // This allows ValueHandlers and custom metadata to adjust itself.
480 if (!I.getType()->isVoidTy())
481 I.replaceAllUsesWith(UndefValue::get(I.getType()));
484 // Move the instruction to the start of the exit block, after any PHI
486 I.removeFromParent();
487 BasicBlock::iterator InsertPt = ExitBlocks[0]->getFirstNonPHI();
488 ExitBlocks[0]->getInstList().insert(InsertPt, &I);
490 } else if (ExitBlocks.empty()) {
491 // The instruction is actually dead if there ARE NO exit blocks.
492 CurAST->deleteValue(&I);
493 // If I has users in unreachable blocks, eliminate.
494 // If I is not void type then replaceAllUsesWith undef.
495 // This allows ValueHandlers and custom metadata to adjust itself.
496 if (!I.getType()->isVoidTy())
497 I.replaceAllUsesWith(UndefValue::get(I.getType()));
500 // Otherwise, if we have multiple exits, use the PromoteMem2Reg function to
501 // do all of the hard work of inserting PHI nodes as necessary. We convert
502 // the value into a stack object to get it to do this.
504 // Firstly, we create a stack object to hold the value...
507 if (!I.getType()->isVoidTy()) {
508 AI = new AllocaInst(I.getType(), 0, I.getName(),
509 I.getParent()->getParent()->getEntryBlock().begin());
513 // Secondly, insert load instructions for each use of the instruction
514 // outside of the loop.
515 while (!I.use_empty()) {
516 Instruction *U = cast<Instruction>(I.use_back());
518 // If the user is a PHI Node, we actually have to insert load instructions
519 // in all predecessor blocks, not in the PHI block itself!
520 if (PHINode *UPN = dyn_cast<PHINode>(U)) {
521 // Only insert into each predecessor once, so that we don't have
522 // different incoming values from the same block!
523 std::map<BasicBlock*, Value*> InsertedBlocks;
524 for (unsigned i = 0, e = UPN->getNumIncomingValues(); i != e; ++i)
525 if (UPN->getIncomingValue(i) == &I) {
526 BasicBlock *Pred = UPN->getIncomingBlock(i);
527 Value *&PredVal = InsertedBlocks[Pred];
529 // Insert a new load instruction right before the terminator in
530 // the predecessor block.
531 PredVal = new LoadInst(AI, "", Pred->getTerminator());
532 CurAST->add(cast<LoadInst>(PredVal));
535 UPN->setIncomingValue(i, PredVal);
539 LoadInst *L = new LoadInst(AI, "", U);
540 U->replaceUsesOfWith(&I, L);
545 // Thirdly, insert a copy of the instruction in each exit block of the loop
546 // that is dominated by the instruction, storing the result into the memory
547 // location. Be careful not to insert the instruction into any particular
548 // basic block more than once.
549 std::set<BasicBlock*> InsertedBlocks;
550 BasicBlock *InstOrigBB = I.getParent();
552 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
553 BasicBlock *ExitBlock = ExitBlocks[i];
555 if (isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB)) {
556 // If we haven't already processed this exit block, do so now.
557 if (InsertedBlocks.insert(ExitBlock).second) {
558 // Insert the code after the last PHI node...
559 BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI();
561 // If this is the first exit block processed, just move the original
562 // instruction, otherwise clone the original instruction and insert
565 if (InsertedBlocks.size() == 1) {
566 I.removeFromParent();
567 ExitBlock->getInstList().insert(InsertPt, &I);
571 CurAST->copyValue(&I, New);
572 if (!I.getName().empty())
573 New->setName(I.getName()+".le");
574 ExitBlock->getInstList().insert(InsertPt, New);
577 // Now that we have inserted the instruction, store it into the alloca
578 if (AI) new StoreInst(New, AI, InsertPt);
583 // If the instruction doesn't dominate any exit blocks, it must be dead.
584 if (InsertedBlocks.empty()) {
585 CurAST->deleteValue(&I);
589 // Finally, promote the fine value to SSA form.
591 std::vector<AllocaInst*> Allocas;
592 Allocas.push_back(AI);
593 PromoteMemToReg(Allocas, *DT, *DF, CurAST);
598 /// hoist - When an instruction is found to only use loop invariant operands
599 /// that is safe to hoist, this instruction is called to do the dirty work.
601 void LICM::hoist(Instruction &I) {
602 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
605 // Remove the instruction from its current basic block... but don't delete the
607 I.removeFromParent();
609 // Insert the new node in Preheader, before the terminator.
610 Preheader->getInstList().insert(Preheader->getTerminator(), &I);
612 if (isa<LoadInst>(I)) ++NumMovedLoads;
613 else if (isa<CallInst>(I)) ++NumMovedCalls;
618 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
619 /// not a trapping instruction or if it is a trapping instruction and is
620 /// guaranteed to execute.
622 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
623 // If it is not a trapping instruction, it is always safe to hoist.
624 if (Inst.isSafeToSpeculativelyExecute())
627 // Otherwise we have to check to make sure that the instruction dominates all
628 // of the exit blocks. If it doesn't, then there is a path out of the loop
629 // which does not execute this instruction, so we can't hoist it.
631 // If the instruction is in the header block for the loop (which is very
632 // common), it is always guaranteed to dominate the exit blocks. Since this
633 // is a common case, and can save some work, check it now.
634 if (Inst.getParent() == CurLoop->getHeader())
637 // Get the exit blocks for the current loop.
638 SmallVector<BasicBlock*, 8> ExitBlocks;
639 CurLoop->getExitBlocks(ExitBlocks);
641 // For each exit block, get the DT node and walk up the DT until the
642 // instruction's basic block is found or we exit the loop.
643 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
644 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent()))
651 /// PromoteValuesInLoop - Try to promote memory values to scalars by sinking
652 /// stores out of the loop and moving loads to before the loop. We do this by
653 /// looping over the stores in the loop, looking for stores to Must pointers
654 /// which are loop invariant. We promote these memory locations to use allocas
655 /// instead. These allocas can easily be raised to register values by the
656 /// PromoteMem2Reg functionality.
658 void LICM::PromoteValuesInLoop() {
659 // PromotedValues - List of values that are promoted out of the loop. Each
660 // value has an alloca instruction for it, and a canonical version of the
662 std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
663 std::map<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca
665 FindPromotableValuesInLoop(PromotedValues, ValueToAllocaMap);
666 if (ValueToAllocaMap.empty()) return; // If there are values to promote.
669 NumPromoted += PromotedValues.size();
671 std::vector<Value*> PointerValueNumbers;
673 // Emit a copy from the value into the alloca'd value in the loop preheader
674 TerminatorInst *LoopPredInst = Preheader->getTerminator();
675 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
676 Value *Ptr = PromotedValues[i].second;
678 // If we are promoting a pointer value, update alias information for the
680 Value *LoadValue = 0;
681 if (cast<PointerType>(Ptr->getType())->getElementType()->isPointerTy()) {
682 // Locate a load or store through the pointer, and assign the same value
683 // to LI as we are loading or storing. Since we know that the value is
684 // stored in this loop, this will always succeed.
685 for (Value::use_iterator UI = Ptr->use_begin(), E = Ptr->use_end();
688 if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
691 } else if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
692 if (SI->getOperand(1) == Ptr) {
693 LoadValue = SI->getOperand(0);
698 assert(LoadValue && "No store through the pointer found!");
699 PointerValueNumbers.push_back(LoadValue); // Remember this for later.
702 // Load from the memory we are promoting.
703 LoadInst *LI = new LoadInst(Ptr, Ptr->getName()+".promoted", LoopPredInst);
705 if (LoadValue) CurAST->copyValue(LoadValue, LI);
707 // Store into the temporary alloca.
708 new StoreInst(LI, PromotedValues[i].first, LoopPredInst);
711 // Scan the basic blocks in the loop, replacing uses of our pointers with
712 // uses of the allocas in question.
714 for (Loop::block_iterator I = CurLoop->block_begin(),
715 E = CurLoop->block_end(); I != E; ++I) {
717 // Rewrite all loads and stores in the block of the pointer...
718 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
719 if (LoadInst *L = dyn_cast<LoadInst>(II)) {
720 std::map<Value*, AllocaInst*>::iterator
721 I = ValueToAllocaMap.find(L->getOperand(0));
722 if (I != ValueToAllocaMap.end())
723 L->setOperand(0, I->second); // Rewrite load instruction...
724 } else if (StoreInst *S = dyn_cast<StoreInst>(II)) {
725 std::map<Value*, AllocaInst*>::iterator
726 I = ValueToAllocaMap.find(S->getOperand(1));
727 if (I != ValueToAllocaMap.end())
728 S->setOperand(1, I->second); // Rewrite store instruction...
733 // Now that the body of the loop uses the allocas instead of the original
734 // memory locations, insert code to copy the alloca value back into the
735 // original memory location on all exits from the loop. Note that we only
736 // want to insert one copy of the code in each exit block, though the loop may
737 // exit to the same block more than once.
739 SmallPtrSet<BasicBlock*, 16> ProcessedBlocks;
741 SmallVector<BasicBlock*, 8> ExitBlocks;
742 CurLoop->getExitBlocks(ExitBlocks);
743 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
744 if (!ProcessedBlocks.insert(ExitBlocks[i]))
747 // Copy all of the allocas into their memory locations.
748 BasicBlock::iterator BI = ExitBlocks[i]->getFirstNonPHI();
749 Instruction *InsertPos = BI;
751 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
752 // Load from the alloca.
753 LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
755 // If this is a pointer type, update alias info appropriately.
756 if (LI->getType()->isPointerTy())
757 CurAST->copyValue(PointerValueNumbers[PVN++], LI);
759 // Store into the memory we promoted.
760 new StoreInst(LI, PromotedValues[i].second, InsertPos);
764 // Now that we have done the deed, use the mem2reg functionality to promote
765 // all of the new allocas we just created into real SSA registers.
767 std::vector<AllocaInst*> PromotedAllocas;
768 PromotedAllocas.reserve(PromotedValues.size());
769 for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i)
770 PromotedAllocas.push_back(PromotedValues[i].first);
771 PromoteMemToReg(PromotedAllocas, *DT, *DF, CurAST);
774 /// FindPromotableValuesInLoop - Check the current loop for stores to definite
775 /// pointers, which are not loaded and stored through may aliases and are safe
776 /// for promotion. If these are found, create an alloca for the value, add it
777 /// to the PromotedValues list, and keep track of the mapping from value to
779 void LICM::FindPromotableValuesInLoop(
780 std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
781 std::map<Value*, AllocaInst*> &ValueToAllocaMap) {
782 Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();
784 // Loop over all of the alias sets in the tracker object.
785 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
788 // We can promote this alias set if it has a store, if it is a "Must" alias
789 // set, if the pointer is loop invariant, and if we are not eliminating any
790 // volatile loads or stores.
791 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
792 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
795 assert(!AS.empty() &&
796 "Must alias set should have at least one pointer element in it!");
797 Value *V = AS.begin()->getValue();
799 // Check that all of the pointers in the alias set have the same type. We
800 // cannot (yet) promote a memory location that is loaded and stored in
803 bool PointerOk = true;
804 for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
805 if (V->getType() != I->getValue()->getType()) {
813 // It isn't safe to promote a load/store from the loop if the load/store is
814 // conditional. For example, turning:
816 // for () { if (c) *P += 1; }
820 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
822 // is not safe, because *P may only be valid to access if 'c' is true.
824 // It is safe to promote P if all uses are direct load/stores and if at
825 // least one is guaranteed to be executed.
826 bool GuaranteedToExecute = false;
827 bool InvalidInst = false;
828 for (Value::use_iterator UI = V->use_begin(), UE = V->use_end();
830 // Ignore instructions not in this loop.
831 Instruction *Use = dyn_cast<Instruction>(*UI);
832 if (!Use || !CurLoop->contains(Use))
835 if (!isa<LoadInst>(Use) && !isa<StoreInst>(Use)) {
840 if (!GuaranteedToExecute)
841 GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use);
844 // If there is an non-load/store instruction in the loop, we can't promote
845 // it. If there isn't a guaranteed-to-execute instruction, we can't
847 if (InvalidInst || !GuaranteedToExecute)
850 const Type *Ty = cast<PointerType>(V->getType())->getElementType();
851 AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart);
852 PromotedValues.push_back(std::make_pair(AI, V));
854 // Update the AST and alias analysis.
855 CurAST->copyValue(V, AI);
857 for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
858 ValueToAllocaMap.insert(std::make_pair(I->getValue(), AI));
860 DEBUG(dbgs() << "LICM: Promoting value: " << *V << "\n");
864 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
865 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
866 AliasSetTracker *AST = LoopToAliasMap[L];
870 AST->copyValue(From, To);
873 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
875 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
876 AliasSetTracker *AST = LoopToAliasMap[L];