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 // If this loop executes exactly one time, then it should be peeled, not
163 // optimized by this pass.
164 if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
165 if (BECst->getValue()->getValue() == 0)
168 // We require target data for now.
169 TD = getAnalysisIfAvailable<TargetData>();
170 if (TD == 0) return false;
172 DT = &getAnalysis<DominatorTree>();
173 LoopInfo &LI = getAnalysis<LoopInfo>();
175 SmallVector<BasicBlock*, 8> ExitBlocks;
176 CurLoop->getUniqueExitBlocks(ExitBlocks);
178 bool MadeChange = false;
179 // Scan all the blocks in the loop that are not in subloops.
180 for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E;
182 // Ignore blocks in subloops.
183 if (LI.getLoopFor(*BI) != CurLoop)
186 MadeChange |= runOnLoopBlock(*BI, BECount, ExitBlocks);
191 /// runOnLoopBlock - Process the specified block, which lives in a counted loop
192 /// with the specified backedge count. This block is known to be in the current
193 /// loop and not in any subloops.
194 bool LoopIdiomRecognize::runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
195 SmallVectorImpl<BasicBlock*> &ExitBlocks) {
196 // We can only promote stores in this block if they are unconditionally
197 // executed in the loop. For a block to be unconditionally executed, it has
198 // to dominate all the exit blocks of the loop. Verify this now.
199 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
200 if (!DT->dominates(BB, ExitBlocks[i]))
203 DEBUG(dbgs() << "loop-idiom Scanning: F[" << BB->getParent()->getName()
204 << "] Loop %" << BB->getName() << "\n");
206 bool MadeChange = false;
207 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ) {
208 // Look for store instructions, which may be memsets.
209 StoreInst *SI = dyn_cast<StoreInst>(I++);
210 if (SI == 0 || SI->isVolatile()) continue;
213 if (!processLoopStore(SI, BECount)) continue;
217 // If processing the store invalidated our iterator, start over from the
227 /// scanBlock - Look over a block to see if we can promote anything out of it.
228 bool LoopIdiomRecognize::processLoopStore(StoreInst *SI, const SCEV *BECount) {
229 Value *StoredVal = SI->getValueOperand();
230 Value *StorePtr = SI->getPointerOperand();
232 // Reject stores that are so large that they overflow an unsigned.
233 uint64_t SizeInBits = TD->getTypeSizeInBits(StoredVal->getType());
234 if ((SizeInBits & 7) || (SizeInBits >> 32) != 0)
237 // See if the pointer expression is an AddRec like {base,+,1} on the current
238 // loop, which indicates a strided store. If we have something else, it's a
239 // random store we can't handle.
240 const SCEVAddRecExpr *StoreEv =
241 dyn_cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
242 if (StoreEv == 0 || StoreEv->getLoop() != CurLoop || !StoreEv->isAffine())
245 // Check to see if the stride matches the size of the store. If so, then we
246 // know that every byte is touched in the loop.
247 unsigned StoreSize = (unsigned)SizeInBits >> 3;
248 const SCEVConstant *Stride = dyn_cast<SCEVConstant>(StoreEv->getOperand(1));
250 // TODO: Could also handle negative stride here someday, that will require the
251 // validity check in mayLoopModRefLocation to be updated though.
252 if (Stride == 0 || StoreSize != Stride->getValue()->getValue())
255 // If the stored value is a byte-wise value (like i32 -1), then it may be
256 // turned into a memset of i8 -1, assuming that all the consequtive bytes
257 // are stored. A store of i32 0x01020304 can never be turned into a memset.
258 if (Value *SplatValue = isBytewiseValue(StoredVal))
259 if (processLoopStoreOfSplatValue(SI, StoreSize, SplatValue, StoreEv,
263 // If the stored value is a strided load in the same loop with the same stride
264 // this this may be transformable into a memcpy. This kicks in for stuff like
265 // for (i) A[i] = B[i];
266 if (LoadInst *LI = dyn_cast<LoadInst>(StoredVal)) {
267 const SCEVAddRecExpr *LoadEv =
268 dyn_cast<SCEVAddRecExpr>(SE->getSCEV(LI->getOperand(0)));
269 if (LoadEv && LoadEv->getLoop() == CurLoop && LoadEv->isAffine() &&
270 StoreEv->getOperand(1) == LoadEv->getOperand(1) && !LI->isVolatile())
271 if (processLoopStoreOfLoopLoad(SI, StoreSize, StoreEv, LoadEv, BECount))
274 //errs() << "UNHANDLED strided store: " << *StoreEv << " - " << *SI << "\n";
279 /// mayLoopModRefLocation - Return true if the specified loop might do a load or
280 /// store to the same location that the specified store could store to, which is
281 /// a loop-strided access.
282 static bool mayLoopModRefLocation(Value *Ptr, Loop *L, const SCEV *BECount,
283 unsigned StoreSize, AliasAnalysis &AA,
284 StoreInst *IgnoredStore) {
285 // Get the location that may be stored across the loop. Since the access is
286 // strided positively through memory, we say that the modified location starts
287 // at the pointer and has infinite size.
288 uint64_t AccessSize = AliasAnalysis::UnknownSize;
290 // If the loop iterates a fixed number of times, we can refine the access size
291 // to be exactly the size of the memset, which is (BECount+1)*StoreSize
292 if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
293 AccessSize = (BECst->getValue()->getZExtValue()+1)*StoreSize;
295 // TODO: For this to be really effective, we have to dive into the pointer
296 // operand in the store. Store to &A[i] of 100 will always return may alias
297 // with store of &A[100], we need to StoreLoc to be "A" with size of 100,
298 // which will then no-alias a store to &A[100].
299 AliasAnalysis::Location StoreLoc(Ptr, AccessSize);
301 for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E;
303 for (BasicBlock::iterator I = (*BI)->begin(), E = (*BI)->end(); I != E; ++I)
304 if (&*I != IgnoredStore &&
305 AA.getModRefInfo(I, StoreLoc) != AliasAnalysis::NoModRef)
311 /// processLoopStoreOfSplatValue - We see a strided store of a memsetable value.
312 /// If we can transform this into a memset in the loop preheader, do so.
313 bool LoopIdiomRecognize::
314 processLoopStoreOfSplatValue(StoreInst *SI, unsigned StoreSize,
316 const SCEVAddRecExpr *Ev, const SCEV *BECount) {
317 // Verify that the stored value is loop invariant. If not, we can't promote
319 if (!CurLoop->isLoopInvariant(SplatValue))
322 // Okay, we have a strided store "p[i]" of a splattable value. We can turn
323 // this into a memset in the loop preheader now if we want. However, this
324 // would be unsafe to do if there is anything else in the loop that may read
325 // or write to the aliased location. Check for an alias.
326 if (mayLoopModRefLocation(SI->getPointerOperand(), CurLoop, BECount,
327 StoreSize, getAnalysis<AliasAnalysis>(), SI))
330 // Okay, everything looks good, insert the memset.
331 BasicBlock *Preheader = CurLoop->getLoopPreheader();
333 IRBuilder<> Builder(Preheader->getTerminator());
335 // The trip count of the loop and the base pointer of the addrec SCEV is
336 // guaranteed to be loop invariant, which means that it should dominate the
337 // header. Just insert code for it in the preheader.
338 SCEVExpander Expander(*SE);
340 unsigned AddrSpace = SI->getPointerAddressSpace();
342 Expander.expandCodeFor(Ev->getStart(), Builder.getInt8PtrTy(AddrSpace),
343 Preheader->getTerminator());
345 // The # stored bytes is (BECount+1)*Size. Expand the trip count out to
346 // pointer size if it isn't already.
347 const Type *IntPtr = TD->getIntPtrType(SI->getContext());
348 unsigned BESize = SE->getTypeSizeInBits(BECount->getType());
349 if (BESize < TD->getPointerSizeInBits())
350 BECount = SE->getZeroExtendExpr(BECount, IntPtr);
351 else if (BESize > TD->getPointerSizeInBits())
352 BECount = SE->getTruncateExpr(BECount, IntPtr);
354 const SCEV *NumBytesS = SE->getAddExpr(BECount, SE->getConstant(IntPtr, 1),
355 true, true /*nooverflow*/);
357 NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtr, StoreSize),
358 true, true /*nooverflow*/);
361 Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator());
364 Builder.CreateMemSet(BasePtr, SplatValue, NumBytes, SI->getAlignment());
366 DEBUG(dbgs() << " Formed memset: " << *NewCall << "\n"
367 << " from store to: " << *Ev << " at: " << *SI << "\n");
370 // Okay, the memset has been formed. Zap the original store and anything that
372 DeleteDeadInstruction(SI, *SE);
377 /// processLoopStoreOfLoopLoad - We see a strided store whose value is a
378 /// same-strided load.
379 bool LoopIdiomRecognize::
380 processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize,
381 const SCEVAddRecExpr *StoreEv,
382 const SCEVAddRecExpr *LoadEv,
383 const SCEV *BECount) {
384 LoadInst *LI = cast<LoadInst>(SI->getValueOperand());
386 // Okay, we have a strided store "p[i]" of a loaded value. We can turn
387 // this into a memcpy in the loop preheader now if we want. However, this
388 // would be unsafe to do if there is anything else in the loop that may read
389 // or write to the aliased location (including the load feeding the stores).
390 // Check for an alias.
391 if (mayLoopModRefLocation(SI->getPointerOperand(), CurLoop, BECount,
392 StoreSize, getAnalysis<AliasAnalysis>(), SI))
395 // Okay, everything looks good, insert the memcpy.
396 BasicBlock *Preheader = CurLoop->getLoopPreheader();
398 IRBuilder<> Builder(Preheader->getTerminator());
400 // The trip count of the loop and the base pointer of the addrec SCEV is
401 // guaranteed to be loop invariant, which means that it should dominate the
402 // header. Just insert code for it in the preheader.
403 SCEVExpander Expander(*SE);
406 Expander.expandCodeFor(LoadEv->getStart(),
407 Builder.getInt8PtrTy(LI->getPointerAddressSpace()),
408 Preheader->getTerminator());
409 Value *StoreBasePtr =
410 Expander.expandCodeFor(StoreEv->getStart(),
411 Builder.getInt8PtrTy(SI->getPointerAddressSpace()),
412 Preheader->getTerminator());
414 // The # stored bytes is (BECount+1)*Size. Expand the trip count out to
415 // pointer size if it isn't already.
416 const Type *IntPtr = TD->getIntPtrType(SI->getContext());
417 unsigned BESize = SE->getTypeSizeInBits(BECount->getType());
418 if (BESize < TD->getPointerSizeInBits())
419 BECount = SE->getZeroExtendExpr(BECount, IntPtr);
420 else if (BESize > TD->getPointerSizeInBits())
421 BECount = SE->getTruncateExpr(BECount, IntPtr);
423 const SCEV *NumBytesS = SE->getAddExpr(BECount, SE->getConstant(IntPtr, 1),
424 true, true /*nooverflow*/);
426 NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtr, StoreSize),
427 true, true /*nooverflow*/);
430 Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator());
433 Builder.CreateMemCpy(StoreBasePtr, LoadBasePtr, NumBytes,
434 std::min(SI->getAlignment(), LI->getAlignment()));
436 DEBUG(dbgs() << " Formed memcpy: " << *NewCall << "\n"
437 << " from load ptr=" << *LoadEv << " at: " << *LI << "\n"
438 << " from store ptr=" << *StoreEv << " at: " << *SI << "\n");
441 // Okay, the memset has been formed. Zap the original store and anything that
443 DeleteDeadInstruction(SI, *SE);