1 //===-- LoopIdiomRecognize.cpp - Loop idiom recognition -------------------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 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 12 // performance win. 13 // 14 //===----------------------------------------------------------------------===// 15 // 16 // TODO List: 17 // 18 // Future loop memory idioms to recognize: 19 // memcmp, memmove, strlen, etc. 20 // Future floating point idioms to recognize in -ffast-math mode: 21 // fpowi 22 // Future integer operation idioms to recognize: 23 // ctpop, ctlz, cttz 24 // 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. 28 // 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 // 34 // This could recognize common matrix multiplies and dot product idioms and 35 // replace them with calls to BLAS (if linked in??). 36 // 37 //===----------------------------------------------------------------------===// 38 39 #include "llvm/Transforms/Scalar.h" 40 #include "llvm/ADT/Statistic.h" 41 #include "llvm/Analysis/AliasAnalysis.h" 42 #include "llvm/Analysis/BasicAliasAnalysis.h" 43 #include "llvm/Analysis/GlobalsModRef.h" 44 #include "llvm/Analysis/LoopPass.h" 45 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h" 46 #include "llvm/Analysis/ScalarEvolutionExpander.h" 47 #include "llvm/Analysis/ScalarEvolutionExpressions.h" 48 #include "llvm/Analysis/TargetLibraryInfo.h" 49 #include "llvm/Analysis/TargetTransformInfo.h" 50 #include "llvm/Analysis/ValueTracking.h" 51 #include "llvm/IR/DataLayout.h" 52 #include "llvm/IR/Dominators.h" 53 #include "llvm/IR/IRBuilder.h" 54 #include "llvm/IR/IntrinsicInst.h" 55 #include "llvm/IR/Module.h" 56 #include "llvm/Support/Debug.h" 57 #include "llvm/Support/raw_ostream.h" 58 #include "llvm/Transforms/Utils/Local.h" 59 using namespace llvm; 60 61 #define DEBUG_TYPE "loop-idiom" 62 63 STATISTIC(NumMemSet, "Number of memset's formed from loop stores"); 64 STATISTIC(NumMemCpy, "Number of memcpy's formed from loop load+stores"); 65 66 namespace { 67 68 class LoopIdiomRecognize : public LoopPass { 69 Loop *CurLoop; 70 AliasAnalysis *AA; 71 DominatorTree *DT; 72 LoopInfo *LI; 73 ScalarEvolution *SE; 74 TargetLibraryInfo *TLI; 75 const TargetTransformInfo *TTI; 76 const DataLayout *DL; 77 78 public: 79 static char ID; 80 explicit LoopIdiomRecognize() : LoopPass(ID) { 81 initializeLoopIdiomRecognizePass(*PassRegistry::getPassRegistry()); 82 } 83 84 bool runOnLoop(Loop *L, LPPassManager &LPM) override; 85 86 /// This transformation requires natural loop information & requires that 87 /// loop preheaders be inserted into the CFG. 88 /// 89 void getAnalysisUsage(AnalysisUsage &AU) const override { 90 AU.addRequired<LoopInfoWrapperPass>(); 91 AU.addPreserved<LoopInfoWrapperPass>(); 92 AU.addRequiredID(LoopSimplifyID); 93 AU.addPreservedID(LoopSimplifyID); 94 AU.addRequiredID(LCSSAID); 95 AU.addPreservedID(LCSSAID); 96 AU.addRequired<AAResultsWrapperPass>(); 97 AU.addPreserved<AAResultsWrapperPass>(); 98 AU.addRequired<ScalarEvolutionWrapperPass>(); 99 AU.addPreserved<ScalarEvolutionWrapperPass>(); 100 AU.addPreserved<SCEVAAWrapperPass>(); 101 AU.addRequired<DominatorTreeWrapperPass>(); 102 AU.addPreserved<DominatorTreeWrapperPass>(); 103 AU.addRequired<TargetLibraryInfoWrapperPass>(); 104 AU.addRequired<TargetTransformInfoWrapperPass>(); 105 AU.addPreserved<BasicAAWrapperPass>(); 106 AU.addPreserved<GlobalsAAWrapperPass>(); 107 } 108 109 private: 110 typedef SmallVector<StoreInst *, 8> StoreList; 111 StoreList StoreRefs; 112 113 /// \name Countable Loop Idiom Handling 114 /// @{ 115 116 bool runOnCountableLoop(); 117 bool runOnLoopBlock(BasicBlock *BB, const SCEV *BECount, 118 SmallVectorImpl<BasicBlock *> &ExitBlocks); 119 120 void collectStores(BasicBlock *BB); 121 bool isLegalStore(StoreInst *SI); 122 bool processLoopStore(StoreInst *SI, const SCEV *BECount); 123 bool processLoopMemSet(MemSetInst *MSI, const SCEV *BECount); 124 125 bool processLoopStridedStore(Value *DestPtr, unsigned StoreSize, 126 unsigned StoreAlignment, Value *SplatValue, 127 Instruction *TheStore, const SCEVAddRecExpr *Ev, 128 const SCEV *BECount, bool NegStride); 129 bool processLoopStoreOfLoopLoad(StoreInst *SI, unsigned StoreSize, 130 const SCEVAddRecExpr *StoreEv, 131 const SCEV *BECount, bool NegStride); 132 133 /// @} 134 /// \name Noncountable Loop Idiom Handling 135 /// @{ 136 137 bool runOnNoncountableLoop(); 138 139 bool recognizePopcount(); 140 void transformLoopToPopcount(BasicBlock *PreCondBB, Instruction *CntInst, 141 PHINode *CntPhi, Value *Var); 142 143 /// @} 144 }; 145 146 } // End anonymous namespace. 147 148 char LoopIdiomRecognize::ID = 0; 149 INITIALIZE_PASS_BEGIN(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms", 150 false, false) 151 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass) 152 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 153 INITIALIZE_PASS_DEPENDENCY(LoopSimplify) 154 INITIALIZE_PASS_DEPENDENCY(LCSSA) 155 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass) 156 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass) 157 INITIALIZE_PASS_DEPENDENCY(BasicAAWrapperPass) 158 INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass) 159 INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass) 160 INITIALIZE_PASS_DEPENDENCY(SCEVAAWrapperPass) 161 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) 162 INITIALIZE_PASS_END(LoopIdiomRecognize, "loop-idiom", "Recognize loop idioms", 163 false, false) 164 165 Pass *llvm::createLoopIdiomPass() { return new LoopIdiomRecognize(); } 166 167 /// deleteDeadInstruction - Delete this instruction. Before we do, go through 168 /// and zero out all the operands of this instruction. If any of them become 169 /// dead, delete them and the computation tree that feeds them. 170 /// 171 static void deleteDeadInstruction(Instruction *I, 172 const TargetLibraryInfo *TLI) { 173 SmallVector<Value *, 16> Operands(I->value_op_begin(), I->value_op_end()); 174 I->replaceAllUsesWith(UndefValue::get(I->getType())); 175 I->eraseFromParent(); 176 for (Value *Op : Operands) 177 RecursivelyDeleteTriviallyDeadInstructions(Op, TLI); 178 } 179 180 //===----------------------------------------------------------------------===// 181 // 182 // Implementation of LoopIdiomRecognize 183 // 184 //===----------------------------------------------------------------------===// 185 186 bool LoopIdiomRecognize::runOnLoop(Loop *L, LPPassManager &LPM) { 187 if (skipOptnoneFunction(L)) 188 return false; 189 190 CurLoop = L; 191 // If the loop could not be converted to canonical form, it must have an 192 // indirectbr in it, just give up. 193 if (!L->getLoopPreheader()) 194 return false; 195 196 // Disable loop idiom recognition if the function's name is a common idiom. 197 StringRef Name = L->getHeader()->getParent()->getName(); 198 if (Name == "memset" || Name == "memcpy") 199 return false; 200 201 AA = &getAnalysis<AAResultsWrapperPass>().getAAResults(); 202 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 203 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo(); 204 SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE(); 205 TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(); 206 TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI( 207 *CurLoop->getHeader()->getParent()); 208 DL = &CurLoop->getHeader()->getModule()->getDataLayout(); 209 210 if (SE->hasLoopInvariantBackedgeTakenCount(L)) 211 return runOnCountableLoop(); 212 213 return runOnNoncountableLoop(); 214 } 215 216 bool LoopIdiomRecognize::runOnCountableLoop() { 217 const SCEV *BECount = SE->getBackedgeTakenCount(CurLoop); 218 assert(!isa<SCEVCouldNotCompute>(BECount) && 219 "runOnCountableLoop() called on a loop without a predictable" 220 "backedge-taken count"); 221 222 // If this loop executes exactly one time, then it should be peeled, not 223 // optimized by this pass. 224 if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount)) 225 if (BECst->getAPInt() == 0) 226 return false; 227 228 SmallVector<BasicBlock *, 8> ExitBlocks; 229 CurLoop->getUniqueExitBlocks(ExitBlocks); 230 231 DEBUG(dbgs() << "loop-idiom Scanning: F[" 232 << CurLoop->getHeader()->getParent()->getName() << "] Loop %" 233 << CurLoop->getHeader()->getName() << "\n"); 234 235 bool MadeChange = false; 236 // Scan all the blocks in the loop that are not in subloops. 237 for (auto *BB : CurLoop->getBlocks()) { 238 // Ignore blocks in subloops. 239 if (LI->getLoopFor(BB) != CurLoop) 240 continue; 241 242 MadeChange |= runOnLoopBlock(BB, BECount, ExitBlocks); 243 } 244 return MadeChange; 245 } 246 247 static unsigned getStoreSizeInBytes(StoreInst *SI, const DataLayout *DL) { 248 uint64_t SizeInBits = DL->getTypeSizeInBits(SI->getValueOperand()->getType()); 249 assert(((SizeInBits & 7) || (SizeInBits >> 32) == 0) && 250 "Don't overflow unsigned."); 251 return (unsigned)SizeInBits >> 3; 252 } 253 254 static unsigned getStoreStride(const SCEVAddRecExpr *StoreEv) { 255 const SCEVConstant *ConstStride = cast<SCEVConstant>(StoreEv->getOperand(1)); 256 return ConstStride->getAPInt().getZExtValue(); 257 } 258 259 /// getMemSetPatternValue - If a strided store of the specified value is safe to 260 /// turn into a memset_pattern16, return a ConstantArray of 16 bytes that should 261 /// be passed in. Otherwise, return null. 262 /// 263 /// Note that we don't ever attempt to use memset_pattern8 or 4, because these 264 /// just replicate their input array and then pass on to memset_pattern16. 265 static Constant *getMemSetPatternValue(Value *V, const DataLayout *DL) { 266 // If the value isn't a constant, we can't promote it to being in a constant 267 // array. We could theoretically do a store to an alloca or something, but 268 // that doesn't seem worthwhile. 269 Constant *C = dyn_cast<Constant>(V); 270 if (!C) 271 return nullptr; 272 273 // Only handle simple values that are a power of two bytes in size. 274 uint64_t Size = DL->getTypeSizeInBits(V->getType()); 275 if (Size == 0 || (Size & 7) || (Size & (Size - 1))) 276 return nullptr; 277 278 // Don't care enough about darwin/ppc to implement this. 279 if (DL->isBigEndian()) 280 return nullptr; 281 282 // Convert to size in bytes. 283 Size /= 8; 284 285 // TODO: If CI is larger than 16-bytes, we can try slicing it in half to see 286 // if the top and bottom are the same (e.g. for vectors and large integers). 287 if (Size > 16) 288 return nullptr; 289 290 // If the constant is exactly 16 bytes, just use it. 291 if (Size == 16) 292 return C; 293 294 // Otherwise, we'll use an array of the constants. 295 unsigned ArraySize = 16 / Size; 296 ArrayType *AT = ArrayType::get(V->getType(), ArraySize); 297 return ConstantArray::get(AT, std::vector<Constant *>(ArraySize, C)); 298 } 299 300 bool LoopIdiomRecognize::isLegalStore(StoreInst *SI) { 301 // Don't touch volatile stores. 302 if (!SI->isSimple()) 303 return false; 304 305 Value *StoredVal = SI->getValueOperand(); 306 Value *StorePtr = SI->getPointerOperand(); 307 308 // Reject stores that are so large that they overflow an unsigned. 309 uint64_t SizeInBits = DL->getTypeSizeInBits(StoredVal->getType()); 310 if ((SizeInBits & 7) || (SizeInBits >> 32) != 0) 311 return false; 312 313 // See if the pointer expression is an AddRec like {base,+,1} on the current 314 // loop, which indicates a strided store. If we have something else, it's a 315 // random store we can't handle. 316 const SCEVAddRecExpr *StoreEv = 317 dyn_cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr)); 318 if (!StoreEv || StoreEv->getLoop() != CurLoop || !StoreEv->isAffine()) 319 return false; 320 321 // Check to see if we have a constant stride. 322 if (!isa<SCEVConstant>(StoreEv->getOperand(1))) 323 return false; 324 325 return true; 326 } 327 328 void LoopIdiomRecognize::collectStores(BasicBlock *BB) { 329 StoreRefs.clear(); 330 for (Instruction &I : *BB) { 331 StoreInst *SI = dyn_cast<StoreInst>(&I); 332 if (!SI) 333 continue; 334 335 // Make sure this is a strided store with a constant stride. 336 if (!isLegalStore(SI)) 337 continue; 338 339 // Save the store locations. 340 StoreRefs.push_back(SI); 341 } 342 } 343 344 /// runOnLoopBlock - Process the specified block, which lives in a counted loop 345 /// with the specified backedge count. This block is known to be in the current 346 /// loop and not in any subloops. 347 bool LoopIdiomRecognize::runOnLoopBlock( 348 BasicBlock *BB, const SCEV *BECount, 349 SmallVectorImpl<BasicBlock *> &ExitBlocks) { 350 // We can only promote stores in this block if they are unconditionally 351 // executed in the loop. For a block to be unconditionally executed, it has 352 // to dominate all the exit blocks of the loop. Verify this now. 353 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) 354 if (!DT->dominates(BB, ExitBlocks[i])) 355 return false; 356 357 bool MadeChange = false; 358 // Look for store instructions, which may be optimized to memset/memcpy. 359 collectStores(BB); 360 for (auto &SI : StoreRefs) 361 MadeChange |= processLoopStore(SI, BECount); 362 363 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) { 364 Instruction *Inst = &*I++; 365 // Look for memset instructions, which may be optimized to a larger memset. 366 if (MemSetInst *MSI = dyn_cast<MemSetInst>(Inst)) { 367 WeakVH InstPtr(&*I); 368 if (!processLoopMemSet(MSI, BECount)) 369 continue; 370 MadeChange = true; 371 372 // If processing the memset invalidated our iterator, start over from the 373 // top of the block. 374 if (!InstPtr) 375 I = BB->begin(); 376 continue; 377 } 378 } 379 380 return MadeChange; 381 } 382 383 /// processLoopStore - See if this store can be promoted to a memset or memcpy. 384 bool LoopIdiomRecognize::processLoopStore(StoreInst *SI, const SCEV *BECount) { 385 assert(SI->isSimple() && "Expected only non-volatile stores."); 386 387 Value *StoredVal = SI->getValueOperand(); 388 Value *StorePtr = SI->getPointerOperand(); 389 390 // Check to see if the stride matches the size of the store. If so, then we 391 // know that every byte is touched in the loop. 392 const SCEVAddRecExpr *StoreEv = cast<SCEVAddRecExpr>(SE->getSCEV(StorePtr)); 393 unsigned Stride = getStoreStride(StoreEv); 394 unsigned StoreSize = getStoreSizeInBytes(SI, DL); 395 if (StoreSize != Stride && StoreSize != -Stride) 396 return false; 397 398 bool NegStride = StoreSize == -Stride; 399 400 // See if we can optimize just this store in isolation. 401 if (processLoopStridedStore(StorePtr, StoreSize, SI->getAlignment(), 402 StoredVal, SI, StoreEv, BECount, NegStride)) 403 return true; 404 405 // Optimize the store into a memcpy, if it feeds an similarly strided load. 406 return processLoopStoreOfLoopLoad(SI, StoreSize, StoreEv, BECount, NegStride); 407 } 408 409 /// processLoopMemSet - See if this memset can be promoted to a large memset. 410 bool LoopIdiomRecognize::processLoopMemSet(MemSetInst *MSI, 411 const SCEV *BECount) { 412 // We can only handle non-volatile memsets with a constant size. 413 if (MSI->isVolatile() || !isa<ConstantInt>(MSI->getLength())) 414 return false; 415 416 // If we're not allowed to hack on memset, we fail. 417 if (!TLI->has(LibFunc::memset)) 418 return false; 419 420 Value *Pointer = MSI->getDest(); 421 422 // See if the pointer expression is an AddRec like {base,+,1} on the current 423 // loop, which indicates a strided store. If we have something else, it's a 424 // random store we can't handle. 425 const SCEVAddRecExpr *Ev = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(Pointer)); 426 if (!Ev || Ev->getLoop() != CurLoop || !Ev->isAffine()) 427 return false; 428 429 // Reject memsets that are so large that they overflow an unsigned. 430 uint64_t SizeInBytes = cast<ConstantInt>(MSI->getLength())->getZExtValue(); 431 if ((SizeInBytes >> 32) != 0) 432 return false; 433 434 // Check to see if the stride matches the size of the memset. If so, then we 435 // know that every byte is touched in the loop. 436 const SCEVConstant *Stride = dyn_cast<SCEVConstant>(Ev->getOperand(1)); 437 438 // TODO: Could also handle negative stride here someday, that will require the 439 // validity check in mayLoopAccessLocation to be updated though. 440 if (!Stride || MSI->getLength() != Stride->getValue()) 441 return false; 442 443 return processLoopStridedStore(Pointer, (unsigned)SizeInBytes, 444 MSI->getAlignment(), MSI->getValue(), MSI, Ev, 445 BECount, /*NegStride=*/false); 446 } 447 448 /// mayLoopAccessLocation - Return true if the specified loop might access the 449 /// specified pointer location, which is a loop-strided access. The 'Access' 450 /// argument specifies what the verboten forms of access are (read or write). 451 static bool mayLoopAccessLocation(Value *Ptr, ModRefInfo Access, Loop *L, 452 const SCEV *BECount, unsigned StoreSize, 453 AliasAnalysis &AA, 454 Instruction *IgnoredStore) { 455 // Get the location that may be stored across the loop. Since the access is 456 // strided positively through memory, we say that the modified location starts 457 // at the pointer and has infinite size. 458 uint64_t AccessSize = MemoryLocation::UnknownSize; 459 460 // If the loop iterates a fixed number of times, we can refine the access size 461 // to be exactly the size of the memset, which is (BECount+1)*StoreSize 462 if (const SCEVConstant *BECst = dyn_cast<SCEVConstant>(BECount)) 463 AccessSize = (BECst->getValue()->getZExtValue() + 1) * StoreSize; 464 465 // TODO: For this to be really effective, we have to dive into the pointer 466 // operand in the store. Store to &A[i] of 100 will always return may alias 467 // with store of &A[100], we need to StoreLoc to be "A" with size of 100, 468 // which will then no-alias a store to &A[100]. 469 MemoryLocation StoreLoc(Ptr, AccessSize); 470 471 for (Loop::block_iterator BI = L->block_begin(), E = L->block_end(); BI != E; 472 ++BI) 473 for (BasicBlock::iterator I = (*BI)->begin(), E = (*BI)->end(); I != E; ++I) 474 if (&*I != IgnoredStore && (AA.getModRefInfo(&*I, StoreLoc) & Access)) 475 return true; 476 477 return false; 478 } 479 480 // If we have a negative stride, Start refers to the end of the memory location 481 // we're trying to memset. Therefore, we need to recompute the base pointer, 482 // which is just Start - BECount*Size. 483 static const SCEV *getStartForNegStride(const SCEV *Start, const SCEV *BECount, 484 Type *IntPtr, unsigned StoreSize, 485 ScalarEvolution *SE) { 486 const SCEV *Index = SE->getTruncateOrZeroExtend(BECount, IntPtr); 487 if (StoreSize != 1) 488 Index = SE->getMulExpr(Index, SE->getConstant(IntPtr, StoreSize), 489 SCEV::FlagNUW); 490 return SE->getMinusSCEV(Start, Index); 491 } 492 493 /// processLoopStridedStore - We see a strided store of some value. If we can 494 /// transform this into a memset or memset_pattern in the loop preheader, do so. 495 bool LoopIdiomRecognize::processLoopStridedStore( 496 Value *DestPtr, unsigned StoreSize, unsigned StoreAlignment, 497 Value *StoredVal, Instruction *TheStore, const SCEVAddRecExpr *Ev, 498 const SCEV *BECount, bool NegStride) { 499 500 // If the stored value is a byte-wise value (like i32 -1), then it may be 501 // turned into a memset of i8 -1, assuming that all the consecutive bytes 502 // are stored. A store of i32 0x01020304 can never be turned into a memset, 503 // but it can be turned into memset_pattern if the target supports it. 504 Value *SplatValue = isBytewiseValue(StoredVal); 505 Constant *PatternValue = nullptr; 506 unsigned DestAS = DestPtr->getType()->getPointerAddressSpace(); 507 508 // If we're allowed to form a memset, and the stored value would be acceptable 509 // for memset, use it. 510 if (SplatValue && TLI->has(LibFunc::memset) && 511 // Verify that the stored value is loop invariant. If not, we can't 512 // promote the memset. 513 CurLoop->isLoopInvariant(SplatValue)) { 514 // Keep and use SplatValue. 515 PatternValue = nullptr; 516 } else if (DestAS == 0 && TLI->has(LibFunc::memset_pattern16) && 517 (PatternValue = getMemSetPatternValue(StoredVal, DL))) { 518 // Don't create memset_pattern16s with address spaces. 519 // It looks like we can use PatternValue! 520 SplatValue = nullptr; 521 } else { 522 // Otherwise, this isn't an idiom we can transform. For example, we can't 523 // do anything with a 3-byte store. 524 return false; 525 } 526 527 // The trip count of the loop and the base pointer of the addrec SCEV is 528 // guaranteed to be loop invariant, which means that it should dominate the 529 // header. This allows us to insert code for it in the preheader. 530 BasicBlock *Preheader = CurLoop->getLoopPreheader(); 531 IRBuilder<> Builder(Preheader->getTerminator()); 532 SCEVExpander Expander(*SE, *DL, "loop-idiom"); 533 534 Type *DestInt8PtrTy = Builder.getInt8PtrTy(DestAS); 535 Type *IntPtr = Builder.getIntPtrTy(*DL, DestAS); 536 537 const SCEV *Start = Ev->getStart(); 538 // Handle negative strided loops. 539 if (NegStride) 540 Start = getStartForNegStride(Start, BECount, IntPtr, StoreSize, SE); 541 542 // Okay, we have a strided store "p[i]" of a splattable value. We can turn 543 // this into a memset in the loop preheader now if we want. However, this 544 // would be unsafe to do if there is anything else in the loop that may read 545 // or write to the aliased location. Check for any overlap by generating the 546 // base pointer and checking the region. 547 Value *BasePtr = 548 Expander.expandCodeFor(Start, DestInt8PtrTy, Preheader->getTerminator()); 549 if (mayLoopAccessLocation(BasePtr, MRI_ModRef, CurLoop, BECount, StoreSize, 550 *AA, TheStore)) { 551 Expander.clear(); 552 // If we generated new code for the base pointer, clean up. 553 RecursivelyDeleteTriviallyDeadInstructions(BasePtr, TLI); 554 return false; 555 } 556 557 // Okay, everything looks good, insert the memset. 558 559 // The # stored bytes is (BECount+1)*Size. Expand the trip count out to 560 // pointer size if it isn't already. 561 BECount = SE->getTruncateOrZeroExtend(BECount, IntPtr); 562 563 const SCEV *NumBytesS = 564 SE->getAddExpr(BECount, SE->getOne(IntPtr), SCEV::FlagNUW); 565 if (StoreSize != 1) { 566 NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtr, StoreSize), 567 SCEV::FlagNUW); 568 } 569 570 Value *NumBytes = 571 Expander.expandCodeFor(NumBytesS, IntPtr, Preheader->getTerminator()); 572 573 CallInst *NewCall; 574 if (SplatValue) { 575 NewCall = 576 Builder.CreateMemSet(BasePtr, SplatValue, NumBytes, StoreAlignment); 577 } else { 578 // Everything is emitted in default address space 579 Type *Int8PtrTy = DestInt8PtrTy; 580 581 Module *M = TheStore->getModule(); 582 Value *MSP = 583 M->getOrInsertFunction("memset_pattern16", Builder.getVoidTy(), 584 Int8PtrTy, Int8PtrTy, IntPtr, (void *)nullptr); 585 586 // Otherwise we should form a memset_pattern16. PatternValue is known to be 587 // an constant array of 16-bytes. Plop the value into a mergable global. 588 GlobalVariable *GV = new GlobalVariable(*M, PatternValue->getType(), true, 589 GlobalValue::PrivateLinkage, 590 PatternValue, ".memset_pattern"); 591 GV->setUnnamedAddr(true); // Ok to merge these. 592 GV->setAlignment(16); 593 Value *PatternPtr = ConstantExpr::getBitCast(GV, Int8PtrTy); 594 NewCall = Builder.CreateCall(MSP, {BasePtr, PatternPtr, NumBytes}); 595 } 596 597 DEBUG(dbgs() << " Formed memset: " << *NewCall << "\n" 598 << " from store to: " << *Ev << " at: " << *TheStore << "\n"); 599 NewCall->setDebugLoc(TheStore->getDebugLoc()); 600 601 // Okay, the memset has been formed. Zap the original store and anything that 602 // feeds into it. 603 deleteDeadInstruction(TheStore, TLI); 604 ++NumMemSet; 605 return true; 606 } 607 608 /// If the stored value is a strided load in the same loop with the same stride 609 /// this may be transformable into a memcpy. This kicks in for stuff like 610 /// for (i) A[i] = B[i]; 611 bool LoopIdiomRecognize::processLoopStoreOfLoopLoad( 612 StoreInst *SI, unsigned StoreSize, const SCEVAddRecExpr *StoreEv, 613 const SCEV *BECount, bool NegStride) { 614 // If we're not allowed to form memcpy, we fail. 615 if (!TLI->has(LibFunc::memcpy)) 616 return false; 617 618 // The store must be feeding a non-volatile load. 619 LoadInst *LI = dyn_cast<LoadInst>(SI->getValueOperand()); 620 if (!LI || !LI->isSimple()) 621 return false; 622 623 // See if the pointer expression is an AddRec like {base,+,1} on the current 624 // loop, which indicates a strided load. If we have something else, it's a 625 // random load we can't handle. 626 const SCEVAddRecExpr *LoadEv = 627 dyn_cast<SCEVAddRecExpr>(SE->getSCEV(LI->getPointerOperand())); 628 if (!LoadEv || LoadEv->getLoop() != CurLoop || !LoadEv->isAffine()) 629 return false; 630 631 // The store and load must share the same stride. 632 if (StoreEv->getOperand(1) != LoadEv->getOperand(1)) 633 return false; 634 635 // The trip count of the loop and the base pointer of the addrec SCEV is 636 // guaranteed to be loop invariant, which means that it should dominate the 637 // header. This allows us to insert code for it in the preheader. 638 BasicBlock *Preheader = CurLoop->getLoopPreheader(); 639 IRBuilder<> Builder(Preheader->getTerminator()); 640 SCEVExpander Expander(*SE, *DL, "loop-idiom"); 641 642 const SCEV *StrStart = StoreEv->getStart(); 643 unsigned StrAS = SI->getPointerAddressSpace(); 644 Type *IntPtrTy = Builder.getIntPtrTy(*DL, StrAS); 645 646 // Handle negative strided loops. 647 if (NegStride) 648 StrStart = getStartForNegStride(StrStart, BECount, IntPtrTy, StoreSize, SE); 649 650 // Okay, we have a strided store "p[i]" of a loaded value. We can turn 651 // this into a memcpy in the loop preheader now if we want. However, this 652 // would be unsafe to do if there is anything else in the loop that may read 653 // or write the memory region we're storing to. This includes the load that 654 // feeds the stores. Check for an alias by generating the base address and 655 // checking everything. 656 Value *StoreBasePtr = Expander.expandCodeFor( 657 StrStart, Builder.getInt8PtrTy(StrAS), Preheader->getTerminator()); 658 659 if (mayLoopAccessLocation(StoreBasePtr, MRI_ModRef, CurLoop, BECount, 660 StoreSize, *AA, SI)) { 661 Expander.clear(); 662 // If we generated new code for the base pointer, clean up. 663 RecursivelyDeleteTriviallyDeadInstructions(StoreBasePtr, TLI); 664 return false; 665 } 666 667 const SCEV *LdStart = LoadEv->getStart(); 668 unsigned LdAS = LI->getPointerAddressSpace(); 669 670 // Handle negative strided loops. 671 if (NegStride) 672 LdStart = getStartForNegStride(LdStart, BECount, IntPtrTy, StoreSize, SE); 673 674 // For a memcpy, we have to make sure that the input array is not being 675 // mutated by the loop. 676 Value *LoadBasePtr = Expander.expandCodeFor( 677 LdStart, Builder.getInt8PtrTy(LdAS), Preheader->getTerminator()); 678 679 if (mayLoopAccessLocation(LoadBasePtr, MRI_Mod, CurLoop, BECount, StoreSize, 680 *AA, SI)) { 681 Expander.clear(); 682 // If we generated new code for the base pointer, clean up. 683 RecursivelyDeleteTriviallyDeadInstructions(LoadBasePtr, TLI); 684 RecursivelyDeleteTriviallyDeadInstructions(StoreBasePtr, TLI); 685 return false; 686 } 687 688 // Okay, everything is safe, we can transform this! 689 690 // The # stored bytes is (BECount+1)*Size. Expand the trip count out to 691 // pointer size if it isn't already. 692 BECount = SE->getTruncateOrZeroExtend(BECount, IntPtrTy); 693 694 const SCEV *NumBytesS = 695 SE->getAddExpr(BECount, SE->getOne(IntPtrTy), SCEV::FlagNUW); 696 if (StoreSize != 1) 697 NumBytesS = SE->getMulExpr(NumBytesS, SE->getConstant(IntPtrTy, StoreSize), 698 SCEV::FlagNUW); 699 700 Value *NumBytes = 701 Expander.expandCodeFor(NumBytesS, IntPtrTy, Preheader->getTerminator()); 702 703 CallInst *NewCall = 704 Builder.CreateMemCpy(StoreBasePtr, LoadBasePtr, NumBytes, 705 std::min(SI->getAlignment(), LI->getAlignment())); 706 NewCall->setDebugLoc(SI->getDebugLoc()); 707 708 DEBUG(dbgs() << " Formed memcpy: " << *NewCall << "\n" 709 << " from load ptr=" << *LoadEv << " at: " << *LI << "\n" 710 << " from store ptr=" << *StoreEv << " at: " << *SI << "\n"); 711 712 // Okay, the memcpy has been formed. Zap the original store and anything that 713 // feeds into it. 714 deleteDeadInstruction(SI, TLI); 715 ++NumMemCpy; 716 return true; 717 } 718 719 bool LoopIdiomRecognize::runOnNoncountableLoop() { 720 return recognizePopcount(); 721 } 722 723 /// Check if the given conditional branch is based on the comparison between 724 /// a variable and zero, and if the variable is non-zero, the control yields to 725 /// the loop entry. If the branch matches the behavior, the variable involved 726 /// in the comparion is returned. This function will be called to see if the 727 /// precondition and postcondition of the loop are in desirable form. 728 static Value *matchCondition(BranchInst *BI, BasicBlock *LoopEntry) { 729 if (!BI || !BI->isConditional()) 730 return nullptr; 731 732 ICmpInst *Cond = dyn_cast<ICmpInst>(BI->getCondition()); 733 if (!Cond) 734 return nullptr; 735 736 ConstantInt *CmpZero = dyn_cast<ConstantInt>(Cond->getOperand(1)); 737 if (!CmpZero || !CmpZero->isZero()) 738 return nullptr; 739 740 ICmpInst::Predicate Pred = Cond->getPredicate(); 741 if ((Pred == ICmpInst::ICMP_NE && BI->getSuccessor(0) == LoopEntry) || 742 (Pred == ICmpInst::ICMP_EQ && BI->getSuccessor(1) == LoopEntry)) 743 return Cond->getOperand(0); 744 745 return nullptr; 746 } 747 748 /// Return true iff the idiom is detected in the loop. 749 /// 750 /// Additionally: 751 /// 1) \p CntInst is set to the instruction counting the population bit. 752 /// 2) \p CntPhi is set to the corresponding phi node. 753 /// 3) \p Var is set to the value whose population bits are being counted. 754 /// 755 /// The core idiom we are trying to detect is: 756 /// \code 757 /// if (x0 != 0) 758 /// goto loop-exit // the precondition of the loop 759 /// cnt0 = init-val; 760 /// do { 761 /// x1 = phi (x0, x2); 762 /// cnt1 = phi(cnt0, cnt2); 763 /// 764 /// cnt2 = cnt1 + 1; 765 /// ... 766 /// x2 = x1 & (x1 - 1); 767 /// ... 768 /// } while(x != 0); 769 /// 770 /// loop-exit: 771 /// \endcode 772 static bool detectPopcountIdiom(Loop *CurLoop, BasicBlock *PreCondBB, 773 Instruction *&CntInst, PHINode *&CntPhi, 774 Value *&Var) { 775 // step 1: Check to see if the look-back branch match this pattern: 776 // "if (a!=0) goto loop-entry". 777 BasicBlock *LoopEntry; 778 Instruction *DefX2, *CountInst; 779 Value *VarX1, *VarX0; 780 PHINode *PhiX, *CountPhi; 781 782 DefX2 = CountInst = nullptr; 783 VarX1 = VarX0 = nullptr; 784 PhiX = CountPhi = nullptr; 785 LoopEntry = *(CurLoop->block_begin()); 786 787 // step 1: Check if the loop-back branch is in desirable form. 788 { 789 if (Value *T = matchCondition( 790 dyn_cast<BranchInst>(LoopEntry->getTerminator()), LoopEntry)) 791 DefX2 = dyn_cast<Instruction>(T); 792 else 793 return false; 794 } 795 796 // step 2: detect instructions corresponding to "x2 = x1 & (x1 - 1)" 797 { 798 if (!DefX2 || DefX2->getOpcode() != Instruction::And) 799 return false; 800 801 BinaryOperator *SubOneOp; 802 803 if ((SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(0)))) 804 VarX1 = DefX2->getOperand(1); 805 else { 806 VarX1 = DefX2->getOperand(0); 807 SubOneOp = dyn_cast<BinaryOperator>(DefX2->getOperand(1)); 808 } 809 if (!SubOneOp) 810 return false; 811 812 Instruction *SubInst = cast<Instruction>(SubOneOp); 813 ConstantInt *Dec = dyn_cast<ConstantInt>(SubInst->getOperand(1)); 814 if (!Dec || 815 !((SubInst->getOpcode() == Instruction::Sub && Dec->isOne()) || 816 (SubInst->getOpcode() == Instruction::Add && 817 Dec->isAllOnesValue()))) { 818 return false; 819 } 820 } 821 822 // step 3: Check the recurrence of variable X 823 { 824 PhiX = dyn_cast<PHINode>(VarX1); 825 if (!PhiX || 826 (PhiX->getOperand(0) != DefX2 && PhiX->getOperand(1) != DefX2)) { 827 return false; 828 } 829 } 830 831 // step 4: Find the instruction which count the population: cnt2 = cnt1 + 1 832 { 833 CountInst = nullptr; 834 for (BasicBlock::iterator Iter = LoopEntry->getFirstNonPHI()->getIterator(), 835 IterE = LoopEntry->end(); 836 Iter != IterE; Iter++) { 837 Instruction *Inst = &*Iter; 838 if (Inst->getOpcode() != Instruction::Add) 839 continue; 840 841 ConstantInt *Inc = dyn_cast<ConstantInt>(Inst->getOperand(1)); 842 if (!Inc || !Inc->isOne()) 843 continue; 844 845 PHINode *Phi = dyn_cast<PHINode>(Inst->getOperand(0)); 846 if (!Phi || Phi->getParent() != LoopEntry) 847 continue; 848 849 // Check if the result of the instruction is live of the loop. 850 bool LiveOutLoop = false; 851 for (User *U : Inst->users()) { 852 if ((cast<Instruction>(U))->getParent() != LoopEntry) { 853 LiveOutLoop = true; 854 break; 855 } 856 } 857 858 if (LiveOutLoop) { 859 CountInst = Inst; 860 CountPhi = Phi; 861 break; 862 } 863 } 864 865 if (!CountInst) 866 return false; 867 } 868 869 // step 5: check if the precondition is in this form: 870 // "if (x != 0) goto loop-head ; else goto somewhere-we-don't-care;" 871 { 872 auto *PreCondBr = dyn_cast<BranchInst>(PreCondBB->getTerminator()); 873 Value *T = matchCondition(PreCondBr, CurLoop->getLoopPreheader()); 874 if (T != PhiX->getOperand(0) && T != PhiX->getOperand(1)) 875 return false; 876 877 CntInst = CountInst; 878 CntPhi = CountPhi; 879 Var = T; 880 } 881 882 return true; 883 } 884 885 /// Recognizes a population count idiom in a non-countable loop. 886 /// 887 /// If detected, transforms the relevant code to issue the popcount intrinsic 888 /// function call, and returns true; otherwise, returns false. 889 bool LoopIdiomRecognize::recognizePopcount() { 890 if (TTI->getPopcntSupport(32) != TargetTransformInfo::PSK_FastHardware) 891 return false; 892 893 // Counting population are usually conducted by few arithmetic instructions. 894 // Such instructions can be easily "absorbed" by vacant slots in a 895 // non-compact loop. Therefore, recognizing popcount idiom only makes sense 896 // in a compact loop. 897 898 // Give up if the loop has multiple blocks or multiple backedges. 899 if (CurLoop->getNumBackEdges() != 1 || CurLoop->getNumBlocks() != 1) 900 return false; 901 902 BasicBlock *LoopBody = *(CurLoop->block_begin()); 903 if (LoopBody->size() >= 20) { 904 // The loop is too big, bail out. 905 return false; 906 } 907 908 // It should have a preheader containing nothing but an unconditional branch. 909 BasicBlock *PH = CurLoop->getLoopPreheader(); 910 if (!PH) 911 return false; 912 if (&PH->front() != PH->getTerminator()) 913 return false; 914 auto *EntryBI = dyn_cast<BranchInst>(PH->getTerminator()); 915 if (!EntryBI || EntryBI->isConditional()) 916 return false; 917 918 // It should have a precondition block where the generated popcount instrinsic 919 // function can be inserted. 920 auto *PreCondBB = PH->getSinglePredecessor(); 921 if (!PreCondBB) 922 return false; 923 auto *PreCondBI = dyn_cast<BranchInst>(PreCondBB->getTerminator()); 924 if (!PreCondBI || PreCondBI->isUnconditional()) 925 return false; 926 927 Instruction *CntInst; 928 PHINode *CntPhi; 929 Value *Val; 930 if (!detectPopcountIdiom(CurLoop, PreCondBB, CntInst, CntPhi, Val)) 931 return false; 932 933 transformLoopToPopcount(PreCondBB, CntInst, CntPhi, Val); 934 return true; 935 } 936 937 static CallInst *createPopcntIntrinsic(IRBuilder<> &IRBuilder, Value *Val, 938 DebugLoc DL) { 939 Value *Ops[] = {Val}; 940 Type *Tys[] = {Val->getType()}; 941 942 Module *M = IRBuilder.GetInsertBlock()->getParent()->getParent(); 943 Value *Func = Intrinsic::getDeclaration(M, Intrinsic::ctpop, Tys); 944 CallInst *CI = IRBuilder.CreateCall(Func, Ops); 945 CI->setDebugLoc(DL); 946 947 return CI; 948 } 949 950 void LoopIdiomRecognize::transformLoopToPopcount(BasicBlock *PreCondBB, 951 Instruction *CntInst, 952 PHINode *CntPhi, Value *Var) { 953 BasicBlock *PreHead = CurLoop->getLoopPreheader(); 954 auto *PreCondBr = dyn_cast<BranchInst>(PreCondBB->getTerminator()); 955 const DebugLoc DL = CntInst->getDebugLoc(); 956 957 // Assuming before transformation, the loop is following: 958 // if (x) // the precondition 959 // do { cnt++; x &= x - 1; } while(x); 960 961 // Step 1: Insert the ctpop instruction at the end of the precondition block 962 IRBuilder<> Builder(PreCondBr); 963 Value *PopCnt, *PopCntZext, *NewCount, *TripCnt; 964 { 965 PopCnt = createPopcntIntrinsic(Builder, Var, DL); 966 NewCount = PopCntZext = 967 Builder.CreateZExtOrTrunc(PopCnt, cast<IntegerType>(CntPhi->getType())); 968 969 if (NewCount != PopCnt) 970 (cast<Instruction>(NewCount))->setDebugLoc(DL); 971 972 // TripCnt is exactly the number of iterations the loop has 973 TripCnt = NewCount; 974 975 // If the population counter's initial value is not zero, insert Add Inst. 976 Value *CntInitVal = CntPhi->getIncomingValueForBlock(PreHead); 977 ConstantInt *InitConst = dyn_cast<ConstantInt>(CntInitVal); 978 if (!InitConst || !InitConst->isZero()) { 979 NewCount = Builder.CreateAdd(NewCount, CntInitVal); 980 (cast<Instruction>(NewCount))->setDebugLoc(DL); 981 } 982 } 983 984 // Step 2: Replace the precondition from "if (x == 0) goto loop-exit" to 985 // "if (NewCount == 0) loop-exit". Without this change, the intrinsic 986 // function would be partial dead code, and downstream passes will drag 987 // it back from the precondition block to the preheader. 988 { 989 ICmpInst *PreCond = cast<ICmpInst>(PreCondBr->getCondition()); 990 991 Value *Opnd0 = PopCntZext; 992 Value *Opnd1 = ConstantInt::get(PopCntZext->getType(), 0); 993 if (PreCond->getOperand(0) != Var) 994 std::swap(Opnd0, Opnd1); 995 996 ICmpInst *NewPreCond = cast<ICmpInst>( 997 Builder.CreateICmp(PreCond->getPredicate(), Opnd0, Opnd1)); 998 PreCondBr->setCondition(NewPreCond); 999 1000 RecursivelyDeleteTriviallyDeadInstructions(PreCond, TLI); 1001 } 1002 1003 // Step 3: Note that the population count is exactly the trip count of the 1004 // loop in question, which enable us to to convert the loop from noncountable 1005 // loop into a countable one. The benefit is twofold: 1006 // 1007 // - If the loop only counts population, the entire loop becomes dead after 1008 // the transformation. It is a lot easier to prove a countable loop dead 1009 // than to prove a noncountable one. (In some C dialects, an infinite loop 1010 // isn't dead even if it computes nothing useful. In general, DCE needs 1011 // to prove a noncountable loop finite before safely delete it.) 1012 // 1013 // - If the loop also performs something else, it remains alive. 1014 // Since it is transformed to countable form, it can be aggressively 1015 // optimized by some optimizations which are in general not applicable 1016 // to a noncountable loop. 1017 // 1018 // After this step, this loop (conceptually) would look like following: 1019 // newcnt = __builtin_ctpop(x); 1020 // t = newcnt; 1021 // if (x) 1022 // do { cnt++; x &= x-1; t--) } while (t > 0); 1023 BasicBlock *Body = *(CurLoop->block_begin()); 1024 { 1025 auto *LbBr = dyn_cast<BranchInst>(Body->getTerminator()); 1026 ICmpInst *LbCond = cast<ICmpInst>(LbBr->getCondition()); 1027 Type *Ty = TripCnt->getType(); 1028 1029 PHINode *TcPhi = PHINode::Create(Ty, 2, "tcphi", &Body->front()); 1030 1031 Builder.SetInsertPoint(LbCond); 1032 Instruction *TcDec = cast<Instruction>( 1033 Builder.CreateSub(TcPhi, ConstantInt::get(Ty, 1), 1034 "tcdec", false, true)); 1035 1036 TcPhi->addIncoming(TripCnt, PreHead); 1037 TcPhi->addIncoming(TcDec, Body); 1038 1039 CmpInst::Predicate Pred = 1040 (LbBr->getSuccessor(0) == Body) ? CmpInst::ICMP_UGT : CmpInst::ICMP_SLE; 1041 LbCond->setPredicate(Pred); 1042 LbCond->setOperand(0, TcDec); 1043 LbCond->setOperand(1, ConstantInt::get(Ty, 0)); 1044 } 1045 1046 // Step 4: All the references to the original population counter outside 1047 // the loop are replaced with the NewCount -- the value returned from 1048 // __builtin_ctpop(). 1049 CntInst->replaceUsesOutsideBlock(NewCount, Body); 1050 1051 // step 5: Forget the "non-computable" trip-count SCEV associated with the 1052 // loop. The loop would otherwise not be deleted even if it becomes empty. 1053 SE->forgetLoop(CurLoop); 1054 } 1055