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; }
34 // We should enhance this to handle negative strides through memory.
35 // Alternatively (and perhaps better) we could rely on an earlier pass to force
36 // forward iteration through memory, which is generally better for cache
37 // behavior. Negative strides *do* happen for memset/memcpy loops.
39 // This could recognize common matrix multiplies and dot product idioms and
40 // replace them with calls to BLAS (if linked in??).
42 //===----------------------------------------------------------------------===//
44 #include "llvm/Transforms/Scalar.h"
45 #include "llvm/ADT/Statistic.h"
46 #include "llvm/Analysis/AliasAnalysis.h"
47 #include "llvm/Analysis/LoopPass.h"
48 #include "llvm/Analysis/ScalarEvolutionExpander.h"
49 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
50 #include "llvm/Analysis/TargetLibraryInfo.h"
51 #include "llvm/Analysis/TargetTransformInfo.h"
52 #include "llvm/Analysis/ValueTracking.h"
53 #include "llvm/IR/DataLayout.h"
54 #include "llvm/IR/Dominators.h"
55 #include "llvm/IR/IRBuilder.h"
56 #include "llvm/IR/IntrinsicInst.h"
57 #include "llvm/IR/Module.h"
58 #include "llvm/Support/Debug.h"
59 #include "llvm/Support/raw_ostream.h"
60 #include "llvm/Transforms/Utils/Local.h"
63 #define DEBUG_TYPE "loop-idiom"
65 STATISTIC(NumMemSet, "Number of memset's formed from loop stores");
66 STATISTIC(NumMemCpy, "Number of memcpy's formed from loop load+stores");
70 class LoopIdiomRecognize : public LoopPass {
76 TargetLibraryInfo *TLI;
77 const TargetTransformInfo *TTI;
81 explicit LoopIdiomRecognize() : LoopPass(ID) {
82 initializeLoopIdiomRecognizePass(*PassRegistry::getPassRegistry());
85 bool runOnLoop(Loop *L, LPPassManager &LPM) override;
87 /// This transformation requires natural loop information & requires that
88 /// loop preheaders be inserted into the CFG.
90 void getAnalysisUsage(AnalysisUsage &AU) const override {
91 AU.addRequired<LoopInfoWrapperPass>();
92 AU.addPreserved<LoopInfoWrapperPass>();
93 AU.addRequiredID(LoopSimplifyID);
94 AU.addPreservedID(LoopSimplifyID);
95 AU.addRequiredID(LCSSAID);
96 AU.addPreservedID(LCSSAID);
97 AU.addRequired<AliasAnalysis>();
98 AU.addPreserved<AliasAnalysis>();
99 AU.addRequired<ScalarEvolutionWrapperPass>();
100 AU.addPreserved<ScalarEvolutionWrapperPass>();
101 AU.addPreserved<DominatorTreeWrapperPass>();
102 AU.addRequired<DominatorTreeWrapperPass>();
103 AU.addRequired<TargetLibraryInfoWrapperPass>();
104 AU.addRequired<TargetTransformInfoWrapperPass>();
108 /// \name Countable Loop Idiom Handling
111 bool runOnCountableLoop();
112 bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount,
113 SmallVectorImpl<BasicBlock *> &ExitBlocks);
115 bool processLoopStore(StoreInst *SI, const SCEV *BECount);
116 bool processLoopMemSet(MemSetInst *MSI, const SCEV *BECount);
118 bool processLoopStridedStore(Value *DestPtr, unsigned StoreSize,
119 unsigned StoreAlignment, Value *SplatValue,
120 Instruction *TheStore, const SCEVAddRecExpr *Ev,
121 const SCEV *BECount);
122 bool processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize,
123 const SCEVAddRecExpr *StoreEv,
124 const SCEVAddRecExpr *LoadEv,
125 const SCEV *BECount);
128 /// \name Noncountable Loop Idiom Handling
131 bool runOnNoncountableLoop();
133 bool recognizePopcount();
134 void transformLoopToPopcount(BasicBlock *PreCondBB, Instruction *CntInst,
135 PHINode *CntPhi, Value *Var);
140 } // End anonymous namespace.
142 char LoopIdiomRecognize::ID = 0;
143 INITIALIZE_PASS_BEGIN(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms",
145 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
146 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
147 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
148 INITIALIZE_PASS_DEPENDENCY(LCSSA)
149 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
150 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
151 INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
152 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
153 INITIALIZE_PASS_END(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms",
156 Pass *llvm::createLoopIdiomPass() { return new LoopIdiomRecognize(); }
158 /// deleteDeadInstruction - Delete this instruction. Before we do, go through
159 /// and zero out all the operands of this instruction. If any of them become
160 /// dead, delete them and the computation tree that feeds them.
162 static void deleteDeadInstruction(Instruction *I,
163 const TargetLibraryInfo *TLI) {
164 SmallVector<Value *, 16> Operands(I->value_op_begin(), I->value_op_end());
165 I->replaceAllUsesWith(UndefValue::get(I->getType()));
166 I->eraseFromParent();
167 for (Value *Op : Operands)
168 RecursivelyDeleteTriviallyDeadInstructions(Op, TLI);
171 //===----------------------------------------------------------------------===//
173 // Implementation of LoopIdiomRecognize
175 //===----------------------------------------------------------------------===//
177 bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) {
178 if (skipOptnoneFunction(L))
182 // If the loop could not be converted to canonical form, it must have an
183 // indirectbr in it, just give up.
184 if (!L->getLoopPreheader())
187 // Disable loop idiom recognition if the function's name is a common idiom.
188 StringRef Name = L->getHeader()->getParent()->getName();
189 if (Name == "memset" || Name == "memcpy")
192 AA = &getAnalysis<AliasAnalysis>();
193 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
194 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
195 SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
196 TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
197 TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(
198 *CurLoop->getHeader()->getParent());
200 if (SE->hasLoopInvariantBackedgeTakenCount(L))
201 return runOnCountableLoop();
203 return runOnNoncountableLoop();
206 bool LoopIdiomRecognize::runOnCountableLoop() {
207 const SCEV *BECount = SE->getBackedgeTakenCount(CurLoop);
208 assert(!isa<SCEVCouldNotCompute>(BECount) &&
209 "runOnCountableLoop() called on a loop without a predictable"
210 "backedge-taken count");
212 // If this loop executes exactly one time, then it should be peeled, not
213 // optimized by this pass.
214 if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
215 if (BECst->getValue()->getValue() == 0)
218 SmallVector<BasicBlock *, 8> ExitBlocks;
219 CurLoop->getUniqueExitBlocks(ExitBlocks);
221 DEBUG(dbgs() << "loop-idiom Scanning: F["
222 << CurLoop->getHeader()->getParent()->getName() << "] Loop %"
223 << CurLoop->getHeader()->getName() << "\n");
225 bool MadeChange = false;
226 // Scan all the blocks in the loop that are not in subloops.
227 for (auto *BB : CurLoop->getBlocks()) {
228 // Ignore blocks in subloops.
229 if (LI->getLoopFor(BB) != CurLoop)
232 MadeChange |= runOnLoopBlock(BB, BECount, ExitBlocks);
237 /// runOnLoopBlock - Process the specified block, which lives in a counted loop
238 /// with the specified backedge count. This block is known to be in the current
239 /// loop and not in any subloops.
240 bool LoopIdiomRecognize::runOnLoopBlock(
241 BasicBlock *BB, const SCEV *BECount,
242 SmallVectorImpl<BasicBlock *> &ExitBlocks) {
243 // We can only promote stores in this block if they are unconditionally
244 // executed in the loop. For a block to be unconditionally executed, it has
245 // to dominate all the exit blocks of the loop. Verify this now.
246 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
247 if (!DT->dominates(BB, ExitBlocks[i]))
250 bool MadeChange = false;
251 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
252 Instruction *Inst = I++;
253 // Look for store instructions, which may be optimized to memset/memcpy.
254 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
256 if (!processLoopStore(SI, BECount))
260 // If processing the store invalidated our iterator, start over from the
267 // Look for memset instructions, which may be optimized to a larger memset.
268 if (MemSetInst *MSI = dyn_cast<MemSetInst>(Inst)) {
270 if (!processLoopMemSet(MSI, BECount))
274 // If processing the memset invalidated our iterator, start over from the
285 /// processLoopStore - See if this store can be promoted to a memset or memcpy.
286 bool LoopIdiomRecognize::processLoopStore(StoreInst *SI, const SCEV *BECount) {
290 Value *StoredVal = SI->getValueOperand();
291 Value *StorePtr = SI->getPointerOperand();
293 // Reject stores that are so large that they overflow an unsigned.
294 auto &DL = CurLoop->getHeader()->getModule()->getDataLayout();
295 uint64_t SizeInBits = DL.getTypeSizeInBits(StoredVal->getType());
296 if ((SizeInBits & 7) || (SizeInBits >> 32) != 0)
299 // See if the pointer expression is an AddRec like {base,+,1} on the current
300 // loop, which indicates a strided store. If we have something else, it's a
301 // random store we can't handle.
302 const SCEVAddRecExpr *StoreEv =
303 dyn_cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr));
304 if (!StoreEv || StoreEv->getLoop() != CurLoop || !StoreEv->isAffine())
307 // Check to see if the stride matches the size of the store. If so, then we
308 // know that every byte is touched in the loop.
309 unsigned StoreSize = (unsigned)SizeInBits >> 3;
310 const SCEVConstant *Stride = dyn_cast<SCEVConstant>(StoreEv->getOperand(1));
312 if (!Stride || StoreSize != Stride->getValue()->getValue()) {
313 // TODO: Could also handle negative stride here someday, that will require
314 // the validity check in mayLoopAccessLocation to be updated though.
315 // Enable this to print exact negative strides.
316 if (0 && Stride && StoreSize == -Stride->getValue()->getValue()) {
317 dbgs() << "NEGATIVE STRIDE: " << *SI << "\n";
318 dbgs() << "BB: " << *SI->getParent();
324 // See if we can optimize just this store in isolation.
325 if (processLoopStridedStore(StorePtr, StoreSize, SI->getAlignment(),
326 StoredVal, SI, StoreEv, BECount))
329 // If the stored value is a strided load in the same loop with the same stride
330 // this this may be transformable into a memcpy. This kicks in for stuff like
331 // for (i) A[i] = B[i];
332 if (LoadInst *LI = dyn_cast<LoadInst>(StoredVal)) {
333 const SCEVAddRecExpr *LoadEv =
334 dyn_cast<SCEVAddRecExpr>(SE->getSCEV(LI->getOperand(0)));
335 if (LoadEv && LoadEv->getLoop() == CurLoop && LoadEv->isAffine() &&
336 StoreEv->getOperand(1) == LoadEv->getOperand(1) && LI->isSimple())
337 if (processLoopStoreOfLoopLoad(SI, StoreSize, StoreEv, LoadEv, BECount))
340 // errs() << "UNHANDLED strided store: " << *StoreEv << " - " << *SI << "\n";
345 /// processLoopMemSet - See if this memset can be promoted to a large memset.
346 bool LoopIdiomRecognize::processLoopMemSet(MemSetInst *MSI,
347 const SCEV *BECount) {
348 // We can only handle non-volatile memsets with a constant size.
349 if (MSI->isVolatile() || !isa<ConstantInt>(MSI->getLength()))
352 // If we're not allowed to hack on memset, we fail.
353 if (!TLI->has(LibFunc::memset))
356 Value *Pointer = MSI->getDest();
358 // See if the pointer expression is an AddRec like {base,+,1} on the current
359 // loop, which indicates a strided store. If we have something else, it's a
360 // random store we can't handle.
361 const SCEVAddRecExpr *Ev = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(Pointer));
362 if (!Ev || Ev->getLoop() != CurLoop || !Ev->isAffine())
365 // Reject memsets that are so large that they overflow an unsigned.
366 uint64_t SizeInBytes = cast<ConstantInt>(MSI->getLength())->getZExtValue();
367 if ((SizeInBytes >> 32) != 0)
370 // Check to see if the stride matches the size of the memset. If so, then we
371 // know that every byte is touched in the loop.
372 const SCEVConstant *Stride = dyn_cast<SCEVConstant>(Ev->getOperand(1));
374 // TODO: Could also handle negative stride here someday, that will require the
375 // validity check in mayLoopAccessLocation to be updated though.
376 if (!Stride || MSI->getLength() != Stride->getValue())
379 return processLoopStridedStore(Pointer, (unsigned)SizeInBytes,
380 MSI->getAlignment(), MSI->getValue(), MSI, Ev,
384 /// mayLoopAccessLocation - Return true if the specified loop might access the
385 /// specified pointer location, which is a loop-strided access. The 'Access'
386 /// argument specifies what the verboten forms of access are (read or write).
387 static bool mayLoopAccessLocation(Value *Ptr, ModRefInfo Access, Loop *L,
388 const SCEV *BECount, unsigned StoreSize,
390 Instruction *IgnoredStore) {
391 // Get the location that may be stored across the loop. Since the access is
392 // strided positively through memory, we say that the modified location starts
393 // at the pointer and has infinite size.
394 uint64_t AccessSize = MemoryLocation::UnknownSize;
396 // If the loop iterates a fixed number of times, we can refine the access size
397 // to be exactly the size of the memset, which is (BECount+1)*StoreSize
398 if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount))
399 AccessSize = (BECst->getValue()->getZExtValue() + 1) * StoreSize;
401 // TODO: For this to be really effective, we have to dive into the pointer
402 // operand in the store. Store to &A[i] of 100 will always return may alias
403 // with store of &A[100], we need to StoreLoc to be "A" with size of 100,
404 // which will then no-alias a store to &A[100].
405 MemoryLocation StoreLoc(Ptr, AccessSize);
407 for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E;
409 for (BasicBlock::iterator I = (*BI)->begin(), E = (*BI)->end(); I != E; ++I)
410 if (&*I != IgnoredStore && (AA.getModRefInfo(I, StoreLoc) & Access))
416 /// getMemSetPatternValue - If a strided store of the specified value is safe to
417 /// turn into a memset_pattern16, return a ConstantArray of 16 bytes that should
418 /// be passed in. Otherwise, return null.
420 /// Note that we don't ever attempt to use memset_pattern8 or 4, because these
421 /// just replicate their input array and then pass on to memset_pattern16.
422 static Constant *getMemSetPatternValue(Value *V, const DataLayout &DL) {
423 // If the value isn't a constant, we can't promote it to being in a constant
424 // array. We could theoretically do a store to an alloca or something, but
425 // that doesn't seem worthwhile.
426 Constant *C = dyn_cast<Constant>(V);
430 // Only handle simple values that are a power of two bytes in size.
431 uint64_t Size = DL.getTypeSizeInBits(V->getType());
432 if (Size == 0 || (Size & 7) || (Size & (Size - 1)))
435 // Don't care enough about darwin/ppc to implement this.
436 if (DL.isBigEndian())
439 // Convert to size in bytes.
442 // TODO: If CI is larger than 16-bytes, we can try slicing it in half to see
443 // if the top and bottom are the same (e.g. for vectors and large integers).
447 // If the constant is exactly 16 bytes, just use it.
451 // Otherwise, we'll use an array of the constants.
452 unsigned ArraySize = 16 / Size;
453 ArrayType *AT = ArrayType::get(V->getType(), ArraySize);
454 return ConstantArray::get(AT, std::vector<Constant *>(ArraySize, C));
457 /// processLoopStridedStore - We see a strided store of some value. If we can
458 /// transform this into a memset or memset_pattern in the loop preheader, do so.
459 bool LoopIdiomRecognize::processLoopStridedStore(
460 Value *DestPtr, unsigned StoreSize, unsigned StoreAlignment,
461 Value *StoredVal, Instruction *TheStore, const SCEVAddRecExpr *Ev,
462 const SCEV *BECount) {
464 // If the stored value is a byte-wise value (like i32 -1), then it may be
465 // turned into a memset of i8 -1, assuming that all the consecutive bytes
466 // are stored. A store of i32 0x01020304 can never be turned into a memset,
467 // but it can be turned into memset_pattern if the target supports it.
468 Value *SplatValue = isBytewiseValue(StoredVal);
469 Constant *PatternValue = nullptr;
470 auto &DL = CurLoop->getHeader()->getModule()->getDataLayout();
471 unsigned DestAS = DestPtr->getType()->getPointerAddressSpace();
473 // If we're allowed to form a memset, and the stored value would be acceptable
474 // for memset, use it.
475 if (SplatValue && TLI->has(LibFunc::memset) &&
476 // Verify that the stored value is loop invariant. If not, we can't
477 // promote the memset.
478 CurLoop->isLoopInvariant(SplatValue)) {
479 // Keep and use SplatValue.
480 PatternValue = nullptr;
481 } else if (DestAS == 0 && TLI->has(LibFunc::memset_pattern16) &&
482 (PatternValue = getMemSetPatternValue(StoredVal, DL))) {
483 // Don't create memset_pattern16s with address spaces.
484 // It looks like we can use PatternValue!
485 SplatValue = nullptr;
487 // Otherwise, this isn't an idiom we can transform. For example, we can't
488 // do anything with a 3-byte store.
492 // The trip count of the loop and the base pointer of the addrec SCEV is
493 // guaranteed to be loop invariant, which means that it should dominate the
494 // header. This allows us to insert code for it in the preheader.
495 BasicBlock *Preheader = CurLoop->getLoopPreheader();
496 IRBuilder<> Builder(Preheader->getTerminator());
497 SCEVExpander Expander(*SE, DL, "loop-idiom");
499 Type *DestInt8PtrTy = Builder.getInt8PtrTy(DestAS);
501 // Okay, we have a strided store "p[i]" of a splattable value. We can turn
502 // this into a memset in the loop preheader now if we want. However, this
503 // would be unsafe to do if there is anything else in the loop that may read
504 // or write to the aliased location. Check for any overlap by generating the
505 // base pointer and checking the region.
506 Value *BasePtr = Expander.expandCodeFor(Ev->getStart(), DestInt8PtrTy,
507 Preheader->getTerminator());
509 if (mayLoopAccessLocation(BasePtr, MRI_ModRef, CurLoop, BECount, StoreSize,
512 // If we generated new code for the base pointer, clean up.
513 RecursivelyDeleteTriviallyDeadInstructions(BasePtr, TLI);
517 // Okay, everything looks good, insert the memset.
519 // The # stored bytes is (BECount+1)*Size. Expand the trip count out to
520 // pointer size if it isn't already.
521 Type *IntPtr = Builder.getIntPtrTy(DL, DestAS);
522 BECount = SE->getTruncateOrZeroExtend(BECount, IntPtr);
524 const SCEV *NumBytesS =
525 SE->getAddExpr(BECount, SE->getConstant(IntPtr, 1), SCEV::FlagNUW);
526 if (StoreSize != 1) {
527 NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtr, StoreSize),
532 Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator());
537 Builder.CreateMemSet(BasePtr, SplatValue, NumBytes, StoreAlignment);
539 // Everything is emitted in default address space
540 Type *Int8PtrTy = DestInt8PtrTy;
542 Module *M = TheStore->getParent()->getParent()->getParent();
544 M->getOrInsertFunction("memset_pattern16", Builder.getVoidTy(),
545 Int8PtrTy, Int8PtrTy, IntPtr, (void *)nullptr);
547 // Otherwise we should form a memset_pattern16. PatternValue is known to be
548 // an constant array of 16-bytes. Plop the value into a mergable global.
549 GlobalVariable *GV = new GlobalVariable(*M, PatternValue->getType(), true,
550 GlobalValue::PrivateLinkage,
551 PatternValue, ".memset_pattern");
552 GV->setUnnamedAddr(true); // Ok to merge these.
553 GV->setAlignment(16);
554 Value *PatternPtr = ConstantExpr::getBitCast(GV, Int8PtrTy);
555 NewCall = Builder.CreateCall(MSP, {BasePtr, PatternPtr, NumBytes});
558 DEBUG(dbgs() << " Formed memset: " << *NewCall << "\n"
559 << " from store to: " << *Ev << " at: " << *TheStore << "\n");
560 NewCall->setDebugLoc(TheStore->getDebugLoc());
562 // Okay, the memset has been formed. Zap the original store and anything that
564 deleteDeadInstruction(TheStore, TLI);
569 /// processLoopStoreOfLoopLoad - We see a strided store whose value is a
570 /// same-strided load.
571 bool LoopIdiomRecognize::processLoopStoreOfLoopLoad(
572 StoreInst *SI, unsigned StoreSize, const SCEVAddRecExpr *StoreEv,
573 const SCEVAddRecExpr *LoadEv, const SCEV *BECount) {
574 // If we're not allowed to form memcpy, we fail.
575 if (!TLI->has(LibFunc::memcpy))
578 LoadInst *LI = cast<LoadInst>(SI->getValueOperand());
580 // The trip count of the loop and the base pointer of the addrec SCEV is
581 // guaranteed to be loop invariant, which means that it should dominate the
582 // header. This allows us to insert code for it in the preheader.
583 BasicBlock *Preheader = CurLoop->getLoopPreheader();
584 IRBuilder<> Builder(Preheader->getTerminator());
585 const DataLayout &DL = Preheader->getModule()->getDataLayout();
586 SCEVExpander Expander(*SE, DL, "loop-idiom");
588 // Okay, we have a strided store "p[i]" of a loaded value. We can turn
589 // this into a memcpy in the loop preheader now if we want. However, this
590 // would be unsafe to do if there is anything else in the loop that may read
591 // or write the memory region we're storing to. This includes the load that
592 // feeds the stores. Check for an alias by generating the base address and
593 // checking everything.
594 Value *StoreBasePtr = Expander.expandCodeFor(
595 StoreEv->getStart(), Builder.getInt8PtrTy(SI->getPointerAddressSpace()),
596 Preheader->getTerminator());
598 if (mayLoopAccessLocation(StoreBasePtr, MRI_ModRef, CurLoop, BECount,
599 StoreSize, *AA, SI)) {
601 // If we generated new code for the base pointer, clean up.
602 RecursivelyDeleteTriviallyDeadInstructions(StoreBasePtr, TLI);
606 // For a memcpy, we have to make sure that the input array is not being
607 // mutated by the loop.
608 Value *LoadBasePtr = Expander.expandCodeFor(
609 LoadEv->getStart(), Builder.getInt8PtrTy(LI->getPointerAddressSpace()),
610 Preheader->getTerminator());
612 if (mayLoopAccessLocation(LoadBasePtr, MRI_Mod, CurLoop, BECount, StoreSize,
615 // If we generated new code for the base pointer, clean up.
616 RecursivelyDeleteTriviallyDeadInstructions(LoadBasePtr, TLI);
617 RecursivelyDeleteTriviallyDeadInstructions(StoreBasePtr, TLI);
621 // Okay, everything is safe, we can transform this!
623 // The # stored bytes is (BECount+1)*Size. Expand the trip count out to
624 // pointer size if it isn't already.
625 Type *IntPtrTy = Builder.getIntPtrTy(DL, SI->getPointerAddressSpace());
626 BECount = SE->getTruncateOrZeroExtend(BECount, IntPtrTy);
628 const SCEV *NumBytesS =
629 SE->getAddExpr(BECount, SE->getConstant(IntPtrTy, 1), SCEV::FlagNUW);
631 NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtrTy, StoreSize),
635 Expander.expandCodeFor(NumBytesS, IntPtrTy, Preheader->getTerminator());
638 Builder.CreateMemCpy(StoreBasePtr, LoadBasePtr, NumBytes,
639 std::min(SI->getAlignment(), LI->getAlignment()));
640 NewCall->setDebugLoc(SI->getDebugLoc());
642 DEBUG(dbgs() << " Formed memcpy: " << *NewCall << "\n"
643 << " from load ptr=" << *LoadEv << " at: " << *LI << "\n"
644 << " from store ptr=" << *StoreEv << " at: " << *SI << "\n");
646 // Okay, the memset has been formed. Zap the original store and anything that
648 deleteDeadInstruction(SI, TLI);
653 bool LoopIdiomRecognize::runOnNoncountableLoop() {
654 if (recognizePopcount())
660 /// Check if the given conditional branch is based on the comparison between
661 /// a variable and zero, and if the variable is non-zero, the control yields to
662 /// the loop entry. If the branch matches the behavior, the variable involved
663 /// in the comparion is returned. This function will be called to see if the
664 /// precondition and postcondition of the loop are in desirable form.
665 static Value *matchCondition(BranchInst *BI, BasicBlock *LoopEntry) {
666 if (!BI || !BI->isConditional())
669 ICmpInst *Cond = dyn_cast<ICmpInst>(BI->getCondition());
673 ConstantInt *CmpZero = dyn_cast<ConstantInt>(Cond->getOperand(1));
674 if (!CmpZero || !CmpZero->isZero())
677 ICmpInst::Predicate Pred = Cond->getPredicate();
678 if ((Pred == ICmpInst::ICMP_NE && BI->getSuccessor(0) == LoopEntry) ||
679 (Pred == ICmpInst::ICMP_EQ && BI->getSuccessor(1) == LoopEntry))
680 return Cond->getOperand(0);
685 /// Return true iff the idiom is detected in the loop.
688 /// 1) \p CntInst is set to the instruction counting the population bit.
689 /// 2) \p CntPhi is set to the corresponding phi node.
690 /// 3) \p Var is set to the value whose population bits are being counted.
692 /// The core idiom we are trying to detect is:
695 /// goto loop-exit // the precondition of the loop
698 /// x1 = phi (x0, x2);
699 /// cnt1 = phi(cnt0, cnt2);
703 /// x2 = x1 & (x1 - 1);
709 static bool detectPopcountIdiom(Loop *CurLoop, BasicBlock *PreCondBB,
710 Instruction *&CntInst, PHINode *&CntPhi,
712 // step 1: Check to see if the look-back branch match this pattern:
713 // "if (a!=0) goto loop-entry".
714 BasicBlock *LoopEntry;
715 Instruction *DefX2, *CountInst;
716 Value *VarX1, *VarX0;
717 PHINode *PhiX, *CountPhi;
719 DefX2 = CountInst = nullptr;
720 VarX1 = VarX0 = nullptr;
721 PhiX = CountPhi = nullptr;
722 LoopEntry = *(CurLoop->block_begin());
724 // step 1: Check if the loop-back branch is in desirable form.
726 if (Value *T = matchCondition(
727 dyn_cast<BranchInst>(LoopEntry->getTerminator()), LoopEntry))
728 DefX2 = dyn_cast<Instruction>(T);
733 // step 2: detect instructions corresponding to "x2 = x1 & (x1 - 1)"
735 if (!DefX2 || DefX2->getOpcode() != Instruction::And)
738 BinaryOperator *SubOneOp;
740 if ((SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(0))))
741 VarX1 = DefX2->getOperand(1);
743 VarX1 = DefX2->getOperand(0);
744 SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(1));
749 Instruction *SubInst = cast<Instruction>(SubOneOp);
750 ConstantInt *Dec = dyn_cast<ConstantInt>(SubInst->getOperand(1));
752 !((SubInst->getOpcode() == Instruction::Sub && Dec->isOne()) ||
753 (SubInst->getOpcode() == Instruction::Add &&
754 Dec->isAllOnesValue()))) {
759 // step 3: Check the recurrence of variable X
761 PhiX = dyn_cast<PHINode>(VarX1);
763 (PhiX->getOperand(0) != DefX2 && PhiX->getOperand(1) != DefX2)) {
768 // step 4: Find the instruction which count the population: cnt2 = cnt1 + 1
771 for (BasicBlock::iterator Iter = LoopEntry->getFirstNonPHI(),
772 IterE = LoopEntry->end();
773 Iter != IterE; Iter++) {
774 Instruction *Inst = Iter;
775 if (Inst->getOpcode() != Instruction::Add)
778 ConstantInt *Inc = dyn_cast<ConstantInt>(Inst->getOperand(1));
779 if (!Inc || !Inc->isOne())
782 PHINode *Phi = dyn_cast<PHINode>(Inst->getOperand(0));
783 if (!Phi || Phi->getParent() != LoopEntry)
786 // Check if the result of the instruction is live of the loop.
787 bool LiveOutLoop = false;
788 for (User *U : Inst->users()) {
789 if ((cast<Instruction>(U))->getParent() != LoopEntry) {
806 // step 5: check if the precondition is in this form:
807 // "if (x != 0) goto loop-head ; else goto somewhere-we-don't-care;"
809 auto *PreCondBr = dyn_cast<BranchInst>(PreCondBB->getTerminator());
810 Value *T = matchCondition(PreCondBr, CurLoop->getLoopPreheader());
811 if (T != PhiX->getOperand(0) && T != PhiX->getOperand(1))
822 /// Recognizes a population count idiom in a non-countable loop.
824 /// If detected, transforms the relevant code to issue the popcount intrinsic
825 /// function call, and returns true; otherwise, returns false.
826 bool LoopIdiomRecognize::recognizePopcount() {
827 if (TTI->getPopcntSupport(32) != TargetTransformInfo::PSK_FastHardware)
830 // Counting population are usually conducted by few arithmetic instructions.
831 // Such instructions can be easilly "absorbed" by vacant slots in a
832 // non-compact loop. Therefore, recognizing popcount idiom only makes sense
833 // in a compact loop.
835 // Give up if the loop has multiple blocks or multiple backedges.
836 if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1)
839 BasicBlock *LoopBody = *(CurLoop->block_begin());
840 if (LoopBody->size() >= 20) {
841 // The loop is too big, bail out.
845 // It should have a preheader containing nothing but an unconditional branch.
846 BasicBlock *PH = CurLoop->getLoopPreheader();
849 if (&PH->front() != PH->getTerminator())
851 auto *EntryBI = dyn_cast<BranchInst>(PH->getTerminator());
852 if (!EntryBI || EntryBI->isConditional())
855 // It should have a precondition block where the generated popcount instrinsic
856 // function can be inserted.
857 auto *PreCondBB = PH->getSinglePredecessor();
860 auto *PreCondBI = dyn_cast<BranchInst>(PreCondBB->getTerminator());
861 if (!PreCondBI || PreCondBI->isUnconditional())
864 Instruction *CntInst;
867 if (!detectPopcountIdiom(CurLoop, PreCondBB, CntInst, CntPhi, Val))
870 transformLoopToPopcount(PreCondBB, CntInst, CntPhi, Val);
874 static CallInst *createPopcntIntrinsic(IRBuilder<> &IRBuilder, Value *Val,
876 Value *Ops[] = {Val};
877 Type *Tys[] = {Val->getType()};
879 Module *M = IRBuilder.GetInsertBlock()->getParent()->getParent();
880 Value *Func = Intrinsic::getDeclaration(M, Intrinsic::ctpop, Tys);
881 CallInst *CI = IRBuilder.CreateCall(Func, Ops);
887 void LoopIdiomRecognize::transformLoopToPopcount(BasicBlock *PreCondBB,
888 Instruction *CntInst,
889 PHINode *CntPhi, Value *Var) {
890 BasicBlock *PreHead = CurLoop->getLoopPreheader();
891 auto *PreCondBr = dyn_cast<BranchInst>(PreCondBB->getTerminator());
892 const DebugLoc DL = CntInst->getDebugLoc();
894 // Assuming before transformation, the loop is following:
895 // if (x) // the precondition
896 // do { cnt++; x &= x - 1; } while(x);
898 // Step 1: Insert the ctpop instruction at the end of the precondition block
899 IRBuilder<> Builder(PreCondBr);
900 Value *PopCnt, *PopCntZext, *NewCount, *TripCnt;
902 PopCnt = createPopcntIntrinsic(Builder, Var, DL);
903 NewCount = PopCntZext =
904 Builder.CreateZExtOrTrunc(PopCnt, cast<IntegerType>(CntPhi->getType()));
906 if (NewCount != PopCnt)
907 (cast<Instruction>(NewCount))->setDebugLoc(DL);
909 // TripCnt is exactly the number of iterations the loop has
912 // If the population counter's initial value is not zero, insert Add Inst.
913 Value *CntInitVal = CntPhi->getIncomingValueForBlock(PreHead);
914 ConstantInt *InitConst = dyn_cast<ConstantInt>(CntInitVal);
915 if (!InitConst || !InitConst->isZero()) {
916 NewCount = Builder.CreateAdd(NewCount, CntInitVal);
917 (cast<Instruction>(NewCount))->setDebugLoc(DL);
921 // Step 2: Replace the precondition from "if(x == 0) goto loop-exit" to
922 // "if(NewCount == 0) loop-exit". Withtout this change, the intrinsic
923 // function would be partial dead code, and downstream passes will drag
924 // it back from the precondition block to the preheader.
926 ICmpInst *PreCond = cast<ICmpInst>(PreCondBr->getCondition());
928 Value *Opnd0 = PopCntZext;
929 Value *Opnd1 = ConstantInt::get(PopCntZext->getType(), 0);
930 if (PreCond->getOperand(0) != Var)
931 std::swap(Opnd0, Opnd1);
933 ICmpInst *NewPreCond = cast<ICmpInst>(
934 Builder.CreateICmp(PreCond->getPredicate(), Opnd0, Opnd1));
935 PreCondBr->setCondition(NewPreCond);
937 RecursivelyDeleteTriviallyDeadInstructions(PreCond, TLI);
940 // Step 3: Note that the population count is exactly the trip count of the
941 // loop in question, which enble us to to convert the loop from noncountable
942 // loop into a countable one. The benefit is twofold:
944 // - If the loop only counts population, the entire loop become dead after
945 // the transformation. It is lots easier to prove a countable loop dead
946 // than to prove a noncountable one. (In some C dialects, a infite loop
947 // isn't dead even if it computes nothing useful. In general, DCE needs
948 // to prove a noncountable loop finite before safely delete it.)
950 // - If the loop also performs something else, it remains alive.
951 // Since it is transformed to countable form, it can be aggressively
952 // optimized by some optimizations which are in general not applicable
953 // to a noncountable loop.
955 // After this step, this loop (conceptually) would look like following:
956 // newcnt = __builtin_ctpop(x);
959 // do { cnt++; x &= x-1; t--) } while (t > 0);
960 BasicBlock *Body = *(CurLoop->block_begin());
962 auto *LbBr = dyn_cast<BranchInst>(Body->getTerminator());
963 ICmpInst *LbCond = cast<ICmpInst>(LbBr->getCondition());
964 Type *Ty = TripCnt->getType();
966 PHINode *TcPhi = PHINode::Create(Ty, 2, "tcphi", Body->begin());
968 Builder.SetInsertPoint(LbCond);
969 Value *Opnd1 = cast<Value>(TcPhi);
970 Value *Opnd2 = cast<Value>(ConstantInt::get(Ty, 1));
971 Instruction *TcDec = cast<Instruction>(
972 Builder.CreateSub(Opnd1, Opnd2, "tcdec", false, true));
974 TcPhi->addIncoming(TripCnt, PreHead);
975 TcPhi->addIncoming(TcDec, Body);
977 CmpInst::Predicate Pred =
978 (LbBr->getSuccessor(0) == Body) ? CmpInst::ICMP_UGT : CmpInst::ICMP_SLE;
979 LbCond->setPredicate(Pred);
980 LbCond->setOperand(0, TcDec);
981 LbCond->setOperand(1, cast<Value>(ConstantInt::get(Ty, 0)));
984 // Step 4: All the references to the original population counter outside
985 // the loop are replaced with the NewCount -- the value returned from
986 // __builtin_ctpop().
987 CntInst->replaceUsesOutsideBlock(NewCount, Body);
989 // step 5: Forget the "non-computable" trip-count SCEV associated with the
990 // loop. The loop would otherwise not be deleted even if it becomes empty.
991 SE->forgetLoop(CurLoop);