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 SSAUpdater to construct the appropriate SSA form for the value.
31 //===----------------------------------------------------------------------===//
33 #define DEBUG_TYPE "licm"
34 #include "llvm/Transforms/Scalar.h"
35 #include "llvm/Constants.h"
36 #include "llvm/DerivedTypes.h"
37 #include "llvm/IntrinsicInst.h"
38 #include "llvm/Instructions.h"
39 #include "llvm/Analysis/AliasAnalysis.h"
40 #include "llvm/Analysis/AliasSetTracker.h"
41 #include "llvm/Analysis/ConstantFolding.h"
42 #include "llvm/Analysis/LoopInfo.h"
43 #include "llvm/Analysis/LoopPass.h"
44 #include "llvm/Analysis/Dominators.h"
45 #include "llvm/Analysis/ScalarEvolution.h"
46 #include "llvm/Transforms/Utils/Local.h"
47 #include "llvm/Transforms/Utils/SSAUpdater.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.addRequired<DominatorTree>();
79 AU.addRequired<LoopInfo>();
80 AU.addRequiredID(LoopSimplifyID);
81 AU.addRequired<AliasAnalysis>();
82 AU.addPreserved<AliasAnalysis>();
83 AU.addPreserved<ScalarEvolution>();
84 AU.addPreservedID(LoopSimplifyID);
87 bool doFinalization() {
88 assert(LoopToAliasSetMap.empty() && "Didn't free loop alias sets");
93 AliasAnalysis *AA; // Current AliasAnalysis information
94 LoopInfo *LI; // Current LoopInfo
95 DominatorTree *DT; // Dominator Tree for the current Loop.
97 // State that is updated as we process loops.
98 bool Changed; // Set to true when we change anything.
99 BasicBlock *Preheader; // The preheader block of the current loop...
100 Loop *CurLoop; // The current loop we are working on...
101 AliasSetTracker *CurAST; // AliasSet information for the current loop...
102 DenseMap<Loop*, AliasSetTracker*> LoopToAliasSetMap;
104 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
105 void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
107 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
109 void deleteAnalysisValue(Value *V, Loop *L);
111 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
112 /// dominated by the specified block, and that are in the current loop) in
113 /// reverse depth first order w.r.t the DominatorTree. This allows us to
114 /// visit uses before definitions, allowing us to sink a loop body in one
115 /// pass without iteration.
117 void SinkRegion(DomTreeNode *N);
119 /// HoistRegion - Walk the specified region of the CFG (defined by all
120 /// blocks dominated by the specified block, and that are in the current
121 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
122 /// visit definitions before uses, allowing us to hoist a loop body in one
123 /// pass without iteration.
125 void HoistRegion(DomTreeNode *N);
127 /// inSubLoop - Little predicate that returns true if the specified basic
128 /// block is in a subloop of the current one, not the current one itself.
130 bool inSubLoop(BasicBlock *BB) {
131 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
132 for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I)
133 if ((*I)->contains(BB))
134 return true; // A subloop actually contains this block!
138 /// isExitBlockDominatedByBlockInLoop - This method checks to see if the
139 /// specified exit block of the loop is dominated by the specified block
140 /// that is in the body of the loop. We use these constraints to
141 /// dramatically limit the amount of the dominator tree that needs to be
143 bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock,
144 BasicBlock *BlockInLoop) const {
145 // If the block in the loop is the loop header, it must be dominated!
146 BasicBlock *LoopHeader = CurLoop->getHeader();
147 if (BlockInLoop == LoopHeader)
150 DomTreeNode *BlockInLoopNode = DT->getNode(BlockInLoop);
151 DomTreeNode *IDom = DT->getNode(ExitBlock);
153 // Because the exit block is not in the loop, we know we have to get _at
154 // least_ its immediate dominator.
155 IDom = IDom->getIDom();
157 while (IDom && IDom != BlockInLoopNode) {
158 // If we have got to the header of the loop, then the instructions block
159 // did not dominate the exit node, so we can't hoist it.
160 if (IDom->getBlock() == LoopHeader)
163 // Get next Immediate Dominator.
164 IDom = IDom->getIDom();
170 /// sink - When an instruction is found to only be used outside of the loop,
171 /// this function moves it to the exit blocks and patches up SSA form as
174 void sink(Instruction &I);
176 /// hoist - When an instruction is found to only use loop invariant operands
177 /// that is safe to hoist, this instruction is called to do the dirty work.
179 void hoist(Instruction &I);
181 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
182 /// is not a trapping instruction or if it is a trapping instruction and is
183 /// guaranteed to execute.
185 bool isSafeToExecuteUnconditionally(Instruction &I);
187 /// pointerInvalidatedByLoop - Return true if the body of this loop may
188 /// store into the memory location pointed to by V.
190 bool pointerInvalidatedByLoop(Value *V, unsigned Size) {
191 // Check to see if any of the basic blocks in CurLoop invalidate *V.
192 return CurAST->getAliasSetForPointer(V, Size).isMod();
195 bool canSinkOrHoistInst(Instruction &I);
196 bool isLoopInvariantInst(Instruction &I);
197 bool isNotUsedInLoop(Instruction &I);
199 void PromoteAliasSet(AliasSet &AS);
204 INITIALIZE_PASS(LICM, "licm", "Loop Invariant Code Motion", false, false);
206 Pass *llvm::createLICMPass() { return new LICM(); }
208 /// Hoist expressions out of the specified loop. Note, alias info for inner
209 /// loop is not preserved so it is not a good idea to run LICM multiple
210 /// times on one loop.
212 bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
215 // Get our Loop and Alias Analysis information...
216 LI = &getAnalysis<LoopInfo>();
217 AA = &getAnalysis<AliasAnalysis>();
218 DT = &getAnalysis<DominatorTree>();
220 CurAST = new AliasSetTracker(*AA);
221 // Collect Alias info from subloops.
222 for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
223 LoopItr != LoopItrE; ++LoopItr) {
224 Loop *InnerL = *LoopItr;
225 AliasSetTracker *InnerAST = LoopToAliasSetMap[InnerL];
226 assert(InnerAST && "Where is my AST?");
228 // What if InnerLoop was modified by other passes ?
229 CurAST->add(*InnerAST);
231 // Once we've incorporated the inner loop's AST into ours, we don't need the
232 // subloop's anymore.
234 LoopToAliasSetMap.erase(InnerL);
239 // Get the preheader block to move instructions into...
240 Preheader = L->getLoopPreheader();
242 // Loop over the body of this loop, looking for calls, invokes, and stores.
243 // Because subloops have already been incorporated into AST, we skip blocks in
246 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
249 if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops.
250 CurAST->add(*BB); // Incorporate the specified basic block
253 // We want to visit all of the instructions in this loop... that are not parts
254 // of our subloops (they have already had their invariants hoisted out of
255 // their loop, into this loop, so there is no need to process the BODIES of
258 // Traverse the body of the loop in depth first order on the dominator tree so
259 // that we are guaranteed to see definitions before we see uses. This allows
260 // us to sink instructions in one pass, without iteration. After sinking
261 // instructions, we perform another pass to hoist them out of the loop.
263 if (L->hasDedicatedExits())
264 SinkRegion(DT->getNode(L->getHeader()));
266 HoistRegion(DT->getNode(L->getHeader()));
268 // Now that all loop invariants have been removed from the loop, promote any
269 // memory references to scalars that we can.
270 if (!DisablePromotion && Preheader && L->hasDedicatedExits()) {
271 // Loop over all of the alias sets in the tracker object.
272 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
277 // Clear out loops state information for the next iteration
281 // If this loop is nested inside of another one, save the alias information
282 // for when we process the outer loop.
283 if (L->getParentLoop())
284 LoopToAliasSetMap[L] = CurAST;
290 /// SinkRegion - Walk the specified region of the CFG (defined by all blocks
291 /// dominated by the specified block, and that are in the current loop) in
292 /// reverse depth first order w.r.t the DominatorTree. This allows us to visit
293 /// uses before definitions, allowing us to sink a loop body in one pass without
296 void LICM::SinkRegion(DomTreeNode *N) {
297 assert(N != 0 && "Null dominator tree node?");
298 BasicBlock *BB = N->getBlock();
300 // If this subregion is not in the top level loop at all, exit.
301 if (!CurLoop->contains(BB)) return;
303 // We are processing blocks in reverse dfo, so process children first.
304 const std::vector<DomTreeNode*> &Children = N->getChildren();
305 for (unsigned i = 0, e = Children.size(); i != e; ++i)
306 SinkRegion(Children[i]);
308 // Only need to process the contents of this block if it is not part of a
309 // subloop (which would already have been processed).
310 if (inSubLoop(BB)) return;
312 for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
313 Instruction &I = *--II;
315 // If the instruction is dead, we would try to sink it because it isn't used
316 // in the loop, instead, just delete it.
317 if (isInstructionTriviallyDead(&I)) {
318 DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
320 CurAST->deleteValue(&I);
326 // Check to see if we can sink this instruction to the exit blocks
327 // of the loop. We can do this if the all users of the instruction are
328 // outside of the loop. In this case, it doesn't even matter if the
329 // operands of the instruction are loop invariant.
331 if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
338 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
339 /// dominated by the specified block, and that are in the current loop) in depth
340 /// first order w.r.t the DominatorTree. This allows us to visit definitions
341 /// before uses, allowing us to hoist a loop body in one pass without iteration.
343 void LICM::HoistRegion(DomTreeNode *N) {
344 assert(N != 0 && "Null dominator tree node?");
345 BasicBlock *BB = N->getBlock();
347 // If this subregion is not in the top level loop at all, exit.
348 if (!CurLoop->contains(BB)) return;
350 // Only need to process the contents of this block if it is not part of a
351 // subloop (which would already have been processed).
353 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
354 Instruction &I = *II++;
356 // Try constant folding this instruction. If all the operands are
357 // constants, it is technically hoistable, but it would be better to just
359 if (Constant *C = ConstantFoldInstruction(&I)) {
360 DEBUG(dbgs() << "LICM folding inst: " << I << " --> " << *C << '\n');
361 CurAST->copyValue(&I, C);
362 CurAST->deleteValue(&I);
363 I.replaceAllUsesWith(C);
368 // Try hoisting the instruction out to the preheader. We can only do this
369 // if all of the operands of the instruction are loop invariant and if it
370 // is safe to hoist the instruction.
372 if (isLoopInvariantInst(I) && canSinkOrHoistInst(I) &&
373 isSafeToExecuteUnconditionally(I))
377 const std::vector<DomTreeNode*> &Children = N->getChildren();
378 for (unsigned i = 0, e = Children.size(); i != e; ++i)
379 HoistRegion(Children[i]);
382 /// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
385 bool LICM::canSinkOrHoistInst(Instruction &I) {
386 // Loads have extra constraints we have to verify before we can hoist them.
387 if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
388 if (LI->isVolatile())
389 return false; // Don't hoist volatile loads!
391 // Loads from constant memory are always safe to move, even if they end up
392 // in the same alias set as something that ends up being modified.
393 if (AA->pointsToConstantMemory(LI->getOperand(0)))
396 // Don't hoist loads which have may-aliased stores in loop.
398 if (LI->getType()->isSized())
399 Size = AA->getTypeStoreSize(LI->getType());
400 return !pointerInvalidatedByLoop(LI->getOperand(0), Size);
401 } else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
402 // Handle obvious cases efficiently.
403 AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
404 if (Behavior == AliasAnalysis::DoesNotAccessMemory)
406 else if (Behavior == AliasAnalysis::OnlyReadsMemory) {
407 // If this call only reads from memory and there are no writes to memory
408 // in the loop, we can hoist or sink the call as appropriate.
409 bool FoundMod = false;
410 for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
413 if (!AS.isForwardingAliasSet() && AS.isMod()) {
418 if (!FoundMod) return true;
421 // FIXME: This should use mod/ref information to see if we can hoist or sink
427 // Otherwise these instructions are hoistable/sinkable
428 return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
429 isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
430 isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
431 isa<ShuffleVectorInst>(I);
434 /// isNotUsedInLoop - Return true if the only users of this instruction are
435 /// outside of the loop. If this is true, we can sink the instruction to the
436 /// exit blocks of the loop.
438 bool LICM::isNotUsedInLoop(Instruction &I) {
439 for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
440 Instruction *User = cast<Instruction>(*UI);
441 if (PHINode *PN = dyn_cast<PHINode>(User)) {
442 // PHI node uses occur in predecessor blocks!
443 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
444 if (PN->getIncomingValue(i) == &I)
445 if (CurLoop->contains(PN->getIncomingBlock(i)))
447 } else if (CurLoop->contains(User)) {
455 /// isLoopInvariantInst - Return true if all operands of this instruction are
456 /// loop invariant. We also filter out non-hoistable instructions here just for
459 bool LICM::isLoopInvariantInst(Instruction &I) {
460 // The instruction is loop invariant if all of its operands are loop-invariant
461 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
462 if (!CurLoop->isLoopInvariant(I.getOperand(i)))
465 // If we got this far, the instruction is loop invariant!
469 /// sink - When an instruction is found to only be used outside of the loop,
470 /// this function moves it to the exit blocks and patches up SSA form as needed.
471 /// This method is guaranteed to remove the original instruction from its
472 /// position, and may either delete it or move it to outside of the loop.
474 void LICM::sink(Instruction &I) {
475 DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
477 SmallVector<BasicBlock*, 8> ExitBlocks;
478 CurLoop->getUniqueExitBlocks(ExitBlocks);
480 if (isa<LoadInst>(I)) ++NumMovedLoads;
481 else if (isa<CallInst>(I)) ++NumMovedCalls;
485 // The case where there is only a single exit node of this loop is common
486 // enough that we handle it as a special (more efficient) case. It is more
487 // efficient to handle because there are no PHI nodes that need to be placed.
488 if (ExitBlocks.size() == 1) {
489 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) {
490 // Instruction is not used, just delete it.
491 CurAST->deleteValue(&I);
492 // If I has users in unreachable blocks, eliminate.
493 // If I is not void type then replaceAllUsesWith undef.
494 // This allows ValueHandlers and custom metadata to adjust itself.
496 I.replaceAllUsesWith(UndefValue::get(I.getType()));
499 // Move the instruction to the start of the exit block, after any PHI
501 I.moveBefore(ExitBlocks[0]->getFirstNonPHI());
503 // This instruction is no longer in the AST for the current loop, because
504 // we just sunk it out of the loop. If we just sunk it into an outer
505 // loop, we will rediscover the operation when we process it.
506 CurAST->deleteValue(&I);
511 if (ExitBlocks.empty()) {
512 // The instruction is actually dead if there ARE NO exit blocks.
513 CurAST->deleteValue(&I);
514 // If I has users in unreachable blocks, eliminate.
515 // If I is not void type then replaceAllUsesWith undef.
516 // This allows ValueHandlers and custom metadata to adjust itself.
518 I.replaceAllUsesWith(UndefValue::get(I.getType()));
523 // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
524 // hard work of inserting PHI nodes as necessary.
525 SmallVector<PHINode*, 8> NewPHIs;
526 SSAUpdater SSA(&NewPHIs);
529 SSA.Initialize(I.getType(), I.getName());
531 // Insert a copy of the instruction in each exit block of the loop that is
532 // dominated by the instruction. Each exit block is known to only be in the
533 // ExitBlocks list once.
534 BasicBlock *InstOrigBB = I.getParent();
535 unsigned NumInserted = 0;
537 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
538 BasicBlock *ExitBlock = ExitBlocks[i];
540 if (!isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB))
543 // Insert the code after the last PHI node.
544 BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI();
546 // If this is the first exit block processed, just move the original
547 // instruction, otherwise clone the original instruction and insert
550 if (NumInserted++ == 0) {
551 I.moveBefore(InsertPt);
555 if (!I.getName().empty())
556 New->setName(I.getName()+".le");
557 ExitBlock->getInstList().insert(InsertPt, New);
560 // Now that we have inserted the instruction, inform SSAUpdater.
562 SSA.AddAvailableValue(ExitBlock, New);
565 // If the instruction doesn't dominate any exit blocks, it must be dead.
566 if (NumInserted == 0) {
567 CurAST->deleteValue(&I);
569 I.replaceAllUsesWith(UndefValue::get(I.getType()));
574 // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
575 for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
576 // Grab the use before incrementing the iterator.
577 Use &U = UI.getUse();
578 // Increment the iterator before removing the use from the list.
580 SSA.RewriteUseAfterInsertions(U);
583 // Update CurAST for NewPHIs if I had pointer type.
584 if (I.getType()->isPointerTy())
585 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
586 CurAST->copyValue(&I, NewPHIs[i]);
588 // Finally, remove the instruction from CurAST. It is no longer in the loop.
589 CurAST->deleteValue(&I);
592 /// hoist - When an instruction is found to only use loop invariant operands
593 /// that is safe to hoist, this instruction is called to do the dirty work.
595 void LICM::hoist(Instruction &I) {
596 DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
599 // Move the new node to the Preheader, before its terminator.
600 I.moveBefore(Preheader->getTerminator());
602 if (isa<LoadInst>(I)) ++NumMovedLoads;
603 else if (isa<CallInst>(I)) ++NumMovedCalls;
608 /// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
609 /// not a trapping instruction or if it is a trapping instruction and is
610 /// guaranteed to execute.
612 bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
613 // If it is not a trapping instruction, it is always safe to hoist.
614 if (Inst.isSafeToSpeculativelyExecute())
617 // Otherwise we have to check to make sure that the instruction dominates all
618 // of the exit blocks. If it doesn't, then there is a path out of the loop
619 // which does not execute this instruction, so we can't hoist it.
621 // If the instruction is in the header block for the loop (which is very
622 // common), it is always guaranteed to dominate the exit blocks. Since this
623 // is a common case, and can save some work, check it now.
624 if (Inst.getParent() == CurLoop->getHeader())
627 // Get the exit blocks for the current loop.
628 SmallVector<BasicBlock*, 8> ExitBlocks;
629 CurLoop->getExitBlocks(ExitBlocks);
631 // For each exit block, get the DT node and walk up the DT until the
632 // instruction's basic block is found or we exit the loop.
633 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
634 if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent()))
640 /// PromoteAliasSet - Try to promote memory values to scalars by sinking
641 /// stores out of the loop and moving loads to before the loop. We do this by
642 /// looping over the stores in the loop, looking for stores to Must pointers
643 /// which are loop invariant.
645 void LICM::PromoteAliasSet(AliasSet &AS) {
646 // We can promote this alias set if it has a store, if it is a "Must" alias
647 // set, if the pointer is loop invariant, and if we are not eliminating any
648 // volatile loads or stores.
649 if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
650 AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
653 assert(!AS.empty() &&
654 "Must alias set should have at least one pointer element in it!");
655 Value *SomePtr = AS.begin()->getValue();
657 // It isn't safe to promote a load/store from the loop if the load/store is
658 // conditional. For example, turning:
660 // for () { if (c) *P += 1; }
664 // tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
666 // is not safe, because *P may only be valid to access if 'c' is true.
668 // It is safe to promote P if all uses are direct load/stores and if at
669 // least one is guaranteed to be executed.
670 bool GuaranteedToExecute = false;
672 SmallVector<Instruction*, 64> LoopUses;
673 SmallPtrSet<Value*, 4> PointerMustAliases;
675 // Check that all of the pointers in the alias set have the same type. We
676 // cannot (yet) promote a memory location that is loaded and stored in
678 for (AliasSet::iterator ASI = AS.begin(), E = AS.end(); ASI != E; ++ASI) {
679 Value *ASIV = ASI->getValue();
680 PointerMustAliases.insert(ASIV);
682 // Check that all of the pointers in the alias set have the same type. We
683 // cannot (yet) promote a memory location that is loaded and stored in
685 if (SomePtr->getType() != ASIV->getType())
688 for (Value::use_iterator UI = ASIV->use_begin(), UE = ASIV->use_end();
690 // Ignore instructions that are outside the loop.
691 Instruction *Use = dyn_cast<Instruction>(*UI);
692 if (!Use || !CurLoop->contains(Use))
695 // If there is an non-load/store instruction in the loop, we can't promote
697 if (isa<LoadInst>(Use))
698 assert(!cast<LoadInst>(Use)->isVolatile() && "AST broken");
699 else if (isa<StoreInst>(Use))
700 assert(!cast<StoreInst>(Use)->isVolatile() &&
701 Use->getOperand(0) != ASIV && "AST broken");
703 return; // Not a load or store.
705 if (!GuaranteedToExecute)
706 GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use);
708 LoopUses.push_back(Use);
712 // If there isn't a guaranteed-to-execute instruction, we can't promote.
713 if (!GuaranteedToExecute)
716 // Otherwise, this is safe to promote, lets do it!
717 DEBUG(dbgs() << "LICM: Promoting value stored to in loop: " <<*SomePtr<<'\n');
721 // We use the SSAUpdater interface to insert phi nodes as required.
722 SmallVector<PHINode*, 16> NewPHIs;
723 SSAUpdater SSA(&NewPHIs);
725 // It wants to know some value of the same type as what we'll be inserting.
727 if (isa<LoadInst>(LoopUses[0]))
728 SomeValue = LoopUses[0];
730 SomeValue = cast<StoreInst>(LoopUses[0])->getOperand(0);
731 SSA.Initialize(SomeValue->getType(), SomeValue->getName());
733 // First step: bucket up uses of the pointers by the block they occur in.
734 // This is important because we have to handle multiple defs/uses in a block
735 // ourselves: SSAUpdater is purely for cross-block references.
736 // FIXME: Want a TinyVector<Instruction*> since there is usually 0/1 element.
737 DenseMap<BasicBlock*, std::vector<Instruction*> > UsesByBlock;
738 for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
739 Instruction *User = LoopUses[i];
740 UsesByBlock[User->getParent()].push_back(User);
743 // Okay, now we can iterate over all the blocks in the loop with uses,
744 // processing them. Keep track of which loads are loading a live-in value.
745 SmallVector<LoadInst*, 32> LiveInLoads;
746 DenseMap<Value*, Value*> ReplacedLoads;
748 for (unsigned LoopUse = 0, e = LoopUses.size(); LoopUse != e; ++LoopUse) {
749 Instruction *User = LoopUses[LoopUse];
750 std::vector<Instruction*> &BlockUses = UsesByBlock[User->getParent()];
752 // If this block has already been processed, ignore this repeat use.
753 if (BlockUses.empty()) continue;
755 // Okay, this is the first use in the block. If this block just has a
756 // single user in it, we can rewrite it trivially.
757 if (BlockUses.size() == 1) {
758 // If it is a store, it is a trivial def of the value in the block.
759 if (isa<StoreInst>(User)) {
760 SSA.AddAvailableValue(User->getParent(),
761 cast<StoreInst>(User)->getOperand(0));
763 // Otherwise it is a load, queue it to rewrite as a live-in load.
764 LiveInLoads.push_back(cast<LoadInst>(User));
770 // Otherwise, check to see if this block is all loads. If so, we can queue
771 // them all as live in loads.
772 bool HasStore = false;
773 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i) {
774 if (isa<StoreInst>(BlockUses[i])) {
781 for (unsigned i = 0, e = BlockUses.size(); i != e; ++i)
782 LiveInLoads.push_back(cast<LoadInst>(BlockUses[i]));
787 // Otherwise, we have mixed loads and stores (or just a bunch of stores).
788 // Since SSAUpdater is purely for cross-block values, we need to determine
789 // the order of these instructions in the block. If the first use in the
790 // block is a load, then it uses the live in value. The last store defines
791 // the live out value. We handle this by doing a linear scan of the block.
792 BasicBlock *BB = User->getParent();
793 Value *StoredValue = 0;
794 for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
795 if (LoadInst *L = dyn_cast<LoadInst>(II)) {
796 // If this is a load from an unrelated pointer, ignore it.
797 if (!PointerMustAliases.count(L->getOperand(0))) continue;
799 // If we haven't seen a store yet, this is a live in use, otherwise
800 // use the stored value.
802 L->replaceAllUsesWith(StoredValue);
803 ReplacedLoads[L] = StoredValue;
805 LiveInLoads.push_back(L);
810 if (StoreInst *S = dyn_cast<StoreInst>(II)) {
811 // If this is a store to an unrelated pointer, ignore it.
812 if (!PointerMustAliases.count(S->getOperand(1))) continue;
814 // Remember that this is the active value in the block.
815 StoredValue = S->getOperand(0);
819 // The last stored value that happened is the live-out for the block.
820 assert(StoredValue && "Already checked that there is a store in block");
821 SSA.AddAvailableValue(BB, StoredValue);
825 // Now that all the intra-loop values are classified, set up the preheader.
826 // It gets a load of the pointer we're promoting, and it is the live-out value
827 // from the preheader.
828 LoadInst *PreheaderLoad = new LoadInst(SomePtr,SomePtr->getName()+".promoted",
829 Preheader->getTerminator());
830 SSA.AddAvailableValue(Preheader, PreheaderLoad);
832 // Now that the preheader is good to go, set up the exit blocks. Each exit
833 // block gets a store of the live-out values that feed them. Since we've
834 // already told the SSA updater about the defs in the loop and the preheader
835 // definition, it is all set and we can start using it.
836 SmallVector<BasicBlock*, 8> ExitBlocks;
837 CurLoop->getUniqueExitBlocks(ExitBlocks);
838 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
839 BasicBlock *ExitBlock = ExitBlocks[i];
840 Value *LiveInValue = SSA.GetValueInMiddleOfBlock(ExitBlock);
841 Instruction *InsertPos = ExitBlock->getFirstNonPHI();
842 new StoreInst(LiveInValue, SomePtr, InsertPos);
845 // Okay, now we rewrite all loads that use live-in values in the loop,
846 // inserting PHI nodes as necessary.
847 for (unsigned i = 0, e = LiveInLoads.size(); i != e; ++i) {
848 LoadInst *ALoad = LiveInLoads[i];
849 Value *NewVal = SSA.GetValueInMiddleOfBlock(ALoad->getParent());
850 ALoad->replaceAllUsesWith(NewVal);
851 CurAST->copyValue(ALoad, NewVal);
852 ReplacedLoads[ALoad] = NewVal;
855 // Now that everything is rewritten, delete the old instructions from the body
856 // of the loop. They should all be dead now.
857 for (unsigned i = 0, e = LoopUses.size(); i != e; ++i) {
858 Instruction *User = LoopUses[i];
860 // If this is a load that still has uses, then the load must have been added
861 // as a live value in the SSAUpdate data structure for a block (e.g. because
862 // the loaded value was stored later). In this case, we need to recursively
863 // propagate the updates until we get to the real value.
864 if (!User->use_empty()) {
865 Value *NewVal = ReplacedLoads[User];
866 assert(NewVal && "not a replaced load?");
868 // Propagate down to the ultimate replacee. The intermediately loads
869 // could theoretically already have been deleted, so we don't want to
870 // dereference the Value*'s.
871 DenseMap<Value*, Value*>::iterator RLI = ReplacedLoads.find(NewVal);
872 while (RLI != ReplacedLoads.end()) {
873 NewVal = RLI->second;
874 RLI = ReplacedLoads.find(NewVal);
877 User->replaceAllUsesWith(NewVal);
878 CurAST->copyValue(User, NewVal);
881 CurAST->deleteValue(User);
882 User->eraseFromParent();
885 // If the preheader load is itself a pointer, we need to tell alias analysis
886 // about the new pointer we created in the preheader block and about any PHI
887 // nodes that just got inserted.
888 if (PreheaderLoad->getType()->isPointerTy()) {
889 // Copy any value stored to or loaded from a must-alias of the pointer.
890 CurAST->copyValue(SomeValue, PreheaderLoad);
892 for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
893 CurAST->copyValue(SomeValue, NewPHIs[i]);
900 /// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
901 void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
902 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);
906 AST->copyValue(From, To);
909 /// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
911 void LICM::deleteAnalysisValue(Value *V, Loop *L) {
912 AliasSetTracker *AST = LoopToAliasSetMap.lookup(L);