1 //===-- LoopIdiomRecognize.cpp - Loop idiom recognition -------------------===//
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 implements an idiom recognizer that transforms simple loops into a
11 // non-loop form. In cases that this kicks in, it can be a significant
14 //===----------------------------------------------------------------------===//
18 // Future loop memory idioms to recognize:
19 // memcmp, memmove, strlen, etc.
20 // Future floating point idioms to recognize in -ffast-math mode:
22 // Future integer operation idioms to recognize:
25 // Beware that isel's default lowering for ctpop is highly inefficient for
26 // i64 and larger types when i64 is legal and the value has few bits set. It
27 // would be good to enhance isel to emit a loop for ctpop in this case.
29 // We should enhance the memset/memcpy recognition to handle multiple stores in
30 // the loop. This would handle things like:
31 // void foo(_Complex float *P)
32 // for (i) { __real__(*P) = 0; __imag__(*P) = 0; }
33 // this is also "Example 2" from http://blog.regehr.org/archives/320
35 //===----------------------------------------------------------------------===//
37 #define DEBUG_TYPE "loop-idiom"
38 #include "llvm/Transforms/Scalar.h"
39 #include "llvm/Analysis/AliasAnalysis.h"
40 #include "llvm/Analysis/LoopPass.h"
41 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
42 #include "llvm/Analysis/ScalarEvolutionExpander.h"
43 #include "llvm/Analysis/ValueTracking.h"
44 #include "llvm/Target/TargetData.h"
45 #include "llvm/Transforms/Utils/Local.h"
46 #include "llvm/Support/Debug.h"
47 #include "llvm/Support/IRBuilder.h"
48 #include "llvm/Support/raw_ostream.h"
49 #include "llvm/ADT/Statistic.h"
52 // TODO: Recognize "N" size array multiplies: replace with call to blas or
54 STATISTIC(NumMemSet, "Number of memset's formed from loop stores");
55 STATISTIC(NumMemCpy, "Number of memcpy's formed from loop load+stores");
58 class LoopIdiomRecognize : public LoopPass {
65 explicit LoopIdiomRecognize() : LoopPass(ID) {
66 initializeLoopIdiomRecognizePass(*PassRegistry::getPassRegistry());
69 bool runOnLoop(Loop *L, LPPassManager &LPM);
70 bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
71 SmallVectorImpl<BasicBlock*> &ExitBlocks);
73 bool processLoopStore(StoreInst *SI, const SCEV *BECount);
75 bool processLoopStoreOfSplatValue(StoreInst *SI, unsigned StoreSize,
77 const SCEVAddRecExpr *Ev,
79 bool processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize,
80 const SCEVAddRecExpr *StoreEv,
81 const SCEVAddRecExpr *LoadEv,
84 /// This transformation requires natural loop information & requires that
85 /// loop preheaders be inserted into the CFG.
87 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
88 AU.addRequired<LoopInfo>();
89 AU.addPreserved<LoopInfo>();
90 AU.addRequiredID(LoopSimplifyID);
91 AU.addPreservedID(LoopSimplifyID);
92 AU.addRequiredID(LCSSAID);
93 AU.addPreservedID(LCSSAID);
94 AU.addRequired<AliasAnalysis>();
95 AU.addPreserved<AliasAnalysis>();
96 AU.addRequired<ScalarEvolution>();
97 AU.addPreserved<ScalarEvolution>();
98 AU.addPreserved<DominatorTree>();
99 AU.addRequired<DominatorTree>();
104 char LoopIdiomRecognize::ID = 0;
105 INITIALIZE_PASS_BEGIN(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms",
107 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
108 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
109 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
110 INITIALIZE_PASS_DEPENDENCY(LCSSA)
111 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
112 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
113 INITIALIZE_PASS_END(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms",
116 Pass *llvm::createLoopIdiomPass() { return new LoopIdiomRecognize(); }
118 /// DeleteDeadInstruction - Delete this instruction. Before we do, go through
119 /// and zero out all the operands of this instruction. If any of them become
120 /// dead, delete them and the computation tree that feeds them.
122 static void DeleteDeadInstruction(Instruction *I, ScalarEvolution &SE) {
123 SmallVector<Instruction*, 32> NowDeadInsts;
125 NowDeadInsts.push_back(I);
127 // Before we touch this instruction, remove it from SE!
129 Instruction *DeadInst = NowDeadInsts.pop_back_val();
131 // This instruction is dead, zap it, in stages. Start by removing it from
133 SE.forgetValue(DeadInst);
135 for (unsigned op = 0, e = DeadInst->getNumOperands(); op != e; ++op) {
136 Value *Op = DeadInst->getOperand(op);
137 DeadInst->setOperand(op, 0);
139 // If this operand just became dead, add it to the NowDeadInsts list.
140 if (!Op->use_empty()) continue;
142 if (Instruction *OpI = dyn_cast<Instruction>(Op))
143 if (isInstructionTriviallyDead(OpI))
144 NowDeadInsts.push_back(OpI);
147 DeadInst->eraseFromParent();
149 } while (!NowDeadInsts.empty());
152 bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) {
155 // The trip count of the loop must be analyzable.
156 SE = &getAnalysis<ScalarEvolution>();
157 if (!SE->hasLoopInvariantBackedgeTakenCount(L))
159 const SCEV *BECount = SE->getBackedgeTakenCount(L);
160 if (isa<SCEVCouldNotCompute>(BECount)) return false;
162 // We require target data for now.
163 TD = getAnalysisIfAvailable<TargetData>();
164 if (TD == 0) return false;
166 DT = &getAnalysis<DominatorTree>();
167 LoopInfo &LI = getAnalysis<LoopInfo>();
169 SmallVector<BasicBlock*, 8> ExitBlocks;
170 CurLoop->getUniqueExitBlocks(ExitBlocks);
172 bool MadeChange = false;
173 // Scan all the blocks in the loop that are not in subloops.
174 for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E;
176 // Ignore blocks in subloops.
177 if (LI.getLoopFor(*BI) != CurLoop)
180 MadeChange |= runOnLoopBlock(*BI, BECount, ExitBlocks);
185 /// runOnLoopBlock - Process the specified block, which lives in a counted loop
186 /// with the specified backedge count. This block is known to be in the current
187 /// loop and not in any subloops.
188 bool LoopIdiomRecognize::runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
189 SmallVectorImpl<BasicBlock*> &ExitBlocks) {
190 // We can only promote stores in this block if they are unconditionally
191 // executed in the loop. For a block to be unconditionally executed, it has
192 // to dominate all the exit blocks of the loop. Verify this now.
193 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
194 if (!DT->dominates(BB, ExitBlocks[i]))
197 DEBUG(dbgs() << "loop-idiom Scanning: F[" << BB->getParent()->getName()
198 << "] Loop %" << BB->getName() << "\n");
200 bool MadeChange = false;
201 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
202 // Look for store instructions, which may be memsets.
203 StoreInst *SI = dyn_cast<StoreInst>(I++);
204 if (SI == 0 || SI->isVolatile()) continue;
207 if (!processLoopStore(SI, BECount)) continue;
211 // If processing the store invalidated our iterator, start over from the
221 /// scanBlock - Look over a block to see if we can promote anything out of it.
222 bool LoopIdiomRecognize::processLoopStore(StoreInst *SI, const SCEV *BECount) {
223 Value *StoredVal = SI->getValueOperand();
224 Value *StorePtr = SI->getPointerOperand();
226 // Reject stores that are so large that they overflow an unsigned.
227 uint64_t SizeInBits = TD->getTypeSizeInBits(StoredVal->getType());
228 if ((SizeInBits & 7) || (SizeInBits >> 32) != 0)
231 // See if the pointer expression is an AddRec like {base,+,1} on the current
232 // loop, which indicates a strided store. If we have something else, it's a
233 // random store we can't handle.
234 const SCEVAddRecExpr *StoreEv =
235 dyn_cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
236 if (StoreEv == 0 || StoreEv->getLoop() != CurLoop || !StoreEv->isAffine())
239 // Check to see if the stride matches the size of the store. If so, then we
240 // know that every byte is touched in the loop.
241 unsigned StoreSize = (unsigned)SizeInBits >> 3;
242 const SCEVConstant *Stride = dyn_cast<SCEVConstant>(StoreEv->getOperand(1));
244 // TODO: Could also handle negative stride here someday, that will require the
245 // validity check in mayLoopModRefLocation to be updated though.
246 if (Stride == 0 || StoreSize != Stride->getValue()->getValue())
249 // If the stored value is a byte-wise value (like i32 -1), then it may be
250 // turned into a memset of i8 -1, assuming that all the consequtive bytes
251 // are stored. A store of i32 0x01020304 can never be turned into a memset.
252 if (Value *SplatValue = isBytewiseValue(StoredVal))
253 if (processLoopStoreOfSplatValue(SI, StoreSize, SplatValue, StoreEv,
257 // If the stored value is a strided load in the same loop with the same stride
258 // this this may be transformable into a memcpy. This kicks in for stuff like
259 // for (i) A[i] = B[i];
260 if (LoadInst *LI = dyn_cast<LoadInst>(StoredVal)) {
261 const SCEVAddRecExpr *LoadEv =
262 dyn_cast<SCEVAddRecExpr>(SE->getSCEV(LI->getOperand(0)));
263 if (LoadEv && LoadEv->getLoop() == CurLoop && LoadEv->isAffine() &&
264 StoreEv->getOperand(1) == LoadEv->getOperand(1) && !LI->isVolatile())
265 if (processLoopStoreOfLoopLoad(SI, StoreSize, StoreEv, LoadEv, BECount))
268 //errs() << "UNHANDLED strided store: " << *StoreEv << " - " << *SI << "\n";
273 /// mayLoopModRefLocation - Return true if the specified loop might do a load or
274 /// store to the same location that the specified store could store to, which is
275 /// a loop-strided access.
276 static bool mayLoopModRefLocation(Value *Ptr, Loop *L, const SCEV *BECount,
277 unsigned StoreSize, AliasAnalysis &AA,
278 StoreInst *IgnoredStore) {
279 // Get the location that may be stored across the loop. Since the access is
280 // strided positively through memory, we say that the modified location starts
281 // at the pointer and has infinite size.
282 uint64_t AccessSize = AliasAnalysis::UnknownSize;
284 // If the loop iterates a fixed number of times, we can refine the access size
285 // to be exactly the size of the memset, which is (BECount+1)*StoreSize
286 if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
287 AccessSize = (BECst->getValue()->getZExtValue()+1)*StoreSize;
289 // TODO: For this to be really effective, we have to dive into the pointer
290 // operand in the store. Store to &A[i] of 100 will always return may alias
291 // with store of &A[100], we need to StoreLoc to be "A" with size of 100,
292 // which will then no-alias a store to &A[100].
293 AliasAnalysis::Location StoreLoc(Ptr, AccessSize);
295 for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E;
297 for (BasicBlock::iterator I = (*BI)->begin(), E = (*BI)->end(); I != E; ++I)
298 if (&*I != IgnoredStore &&
299 AA.getModRefInfo(I, StoreLoc) != AliasAnalysis::NoModRef)
305 /// processLoopStoreOfSplatValue - We see a strided store of a memsetable value.
306 /// If we can transform this into a memset in the loop preheader, do so.
307 bool LoopIdiomRecognize::
308 processLoopStoreOfSplatValue(StoreInst *SI, unsigned StoreSize,
310 const SCEVAddRecExpr *Ev, const SCEV *BECount) {
311 // Verify that the stored value is loop invariant. If not, we can't promote
313 if (!CurLoop->isLoopInvariant(SplatValue))
316 // Okay, we have a strided store "p[i]" of a splattable value. We can turn
317 // this into a memset in the loop preheader now if we want. However, this
318 // would be unsafe to do if there is anything else in the loop that may read
319 // or write to the aliased location. Check for an alias.
320 if (mayLoopModRefLocation(SI->getPointerOperand(), CurLoop, BECount,
321 StoreSize, getAnalysis<AliasAnalysis>(), SI))
324 // Okay, everything looks good, insert the memset.
325 BasicBlock *Preheader = CurLoop->getLoopPreheader();
327 IRBuilder<> Builder(Preheader->getTerminator());
329 // The trip count of the loop and the base pointer of the addrec SCEV is
330 // guaranteed to be loop invariant, which means that it should dominate the
331 // header. Just insert code for it in the preheader.
332 SCEVExpander Expander(*SE);
334 unsigned AddrSpace = SI->getPointerAddressSpace();
336 Expander.expandCodeFor(Ev->getStart(), Builder.getInt8PtrTy(AddrSpace),
337 Preheader->getTerminator());
339 // The # stored bytes is (BECount+1)*Size. Expand the trip count out to
340 // pointer size if it isn't already.
341 const Type *IntPtr = TD->getIntPtrType(SI->getContext());
342 unsigned BESize = SE->getTypeSizeInBits(BECount->getType());
343 if (BESize < TD->getPointerSizeInBits())
344 BECount = SE->getZeroExtendExpr(BECount, IntPtr);
345 else if (BESize > TD->getPointerSizeInBits())
346 BECount = SE->getTruncateExpr(BECount, IntPtr);
348 const SCEV *NumBytesS = SE->getAddExpr(BECount, SE->getConstant(IntPtr, 1),
349 true, true /*nooverflow*/);
351 NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtr, StoreSize),
352 true, true /*nooverflow*/);
355 Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator());
358 Builder.CreateMemSet(BasePtr, SplatValue, NumBytes, SI->getAlignment());
360 DEBUG(dbgs() << " Formed memset: " << *NewCall << "\n"
361 << " from store to: " << *Ev << " at: " << *SI << "\n");
364 // Okay, the memset has been formed. Zap the original store and anything that
366 DeleteDeadInstruction(SI, *SE);
371 /// processLoopStoreOfLoopLoad - We see a strided store whose value is a
372 /// same-strided load.
373 bool LoopIdiomRecognize::
374 processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize,
375 const SCEVAddRecExpr *StoreEv,
376 const SCEVAddRecExpr *LoadEv,
377 const SCEV *BECount) {
378 LoadInst *LI = cast<LoadInst>(SI->getValueOperand());
380 // Okay, we have a strided store "p[i]" of a loaded value. We can turn
381 // this into a memcpy in the loop preheader now if we want. However, this
382 // would be unsafe to do if there is anything else in the loop that may read
383 // or write to the aliased location (including the load feeding the stores).
384 // Check for an alias.
385 if (mayLoopModRefLocation(SI->getPointerOperand(), CurLoop, BECount,
386 StoreSize, getAnalysis<AliasAnalysis>(), SI))
389 // Okay, everything looks good, insert the memcpy.
390 BasicBlock *Preheader = CurLoop->getLoopPreheader();
392 IRBuilder<> Builder(Preheader->getTerminator());
394 // The trip count of the loop and the base pointer of the addrec SCEV is
395 // guaranteed to be loop invariant, which means that it should dominate the
396 // header. Just insert code for it in the preheader.
397 SCEVExpander Expander(*SE);
400 Expander.expandCodeFor(LoadEv->getStart(),
401 Builder.getInt8PtrTy(LI->getPointerAddressSpace()),
402 Preheader->getTerminator());
403 Value *StoreBasePtr =
404 Expander.expandCodeFor(StoreEv->getStart(),
405 Builder.getInt8PtrTy(SI->getPointerAddressSpace()),
406 Preheader->getTerminator());
408 // The # stored bytes is (BECount+1)*Size. Expand the trip count out to
409 // pointer size if it isn't already.
410 const Type *IntPtr = TD->getIntPtrType(SI->getContext());
411 unsigned BESize = SE->getTypeSizeInBits(BECount->getType());
412 if (BESize < TD->getPointerSizeInBits())
413 BECount = SE->getZeroExtendExpr(BECount, IntPtr);
414 else if (BESize > TD->getPointerSizeInBits())
415 BECount = SE->getTruncateExpr(BECount, IntPtr);
417 const SCEV *NumBytesS = SE->getAddExpr(BECount, SE->getConstant(IntPtr, 1),
418 true, true /*nooverflow*/);
420 NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtr, StoreSize),
421 true, true /*nooverflow*/);
424 Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator());
427 Builder.CreateMemCpy(StoreBasePtr, LoadBasePtr, NumBytes,
428 std::min(SI->getAlignment(), LI->getAlignment()));
430 DEBUG(dbgs() << " Formed memcpy: " << *NewCall << "\n"
431 << " from load ptr=" << *LoadEv << " at: " << *LI << "\n"
432 << " from store ptr=" << *StoreEv << " at: " << *SI << "\n");
435 // Okay, the memset has been formed. Zap the original store and anything that
437 DeleteDeadInstruction(SI, *SE);