InstCombineLoadStoreAlloca.cpp revision 29fa5e98863332eb808a6f5c1e29138ca6fd7f5c
1//===- InstCombineLoadStoreAlloca.cpp -------------------------------------===// 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 file implements the visit functions for load, store and alloca. 11// 12//===----------------------------------------------------------------------===// 13 14#include "InstCombine.h" 15#include "llvm/IntrinsicInst.h" 16#include "llvm/Target/TargetData.h" 17#include "llvm/Transforms/Utils/BasicBlockUtils.h" 18#include "llvm/Transforms/Utils/Local.h" 19#include "llvm/ADT/Statistic.h" 20using namespace llvm; 21 22STATISTIC(NumDeadStore, "Number of dead stores eliminated"); 23 24Instruction *InstCombiner::visitAllocaInst(AllocaInst &AI) { 25 // Convert: alloca Ty, C - where C is a constant != 1 into: alloca [C x Ty], 1 26 if (AI.isArrayAllocation()) { // Check C != 1 27 if (const ConstantInt *C = dyn_cast<ConstantInt>(AI.getArraySize())) { 28 const Type *NewTy = 29 ArrayType::get(AI.getAllocatedType(), C->getZExtValue()); 30 assert(isa<AllocaInst>(AI) && "Unknown type of allocation inst!"); 31 AllocaInst *New = Builder->CreateAlloca(NewTy, 0, AI.getName()); 32 New->setAlignment(AI.getAlignment()); 33 34 // Scan to the end of the allocation instructions, to skip over a block of 35 // allocas if possible...also skip interleaved debug info 36 // 37 BasicBlock::iterator It = New; 38 while (isa<AllocaInst>(*It) || isa<DbgInfoIntrinsic>(*It)) ++It; 39 40 // Now that I is pointing to the first non-allocation-inst in the block, 41 // insert our getelementptr instruction... 42 // 43 Value *NullIdx =Constant::getNullValue(Type::getInt32Ty(AI.getContext())); 44 Value *Idx[2]; 45 Idx[0] = NullIdx; 46 Idx[1] = NullIdx; 47 Value *V = GetElementPtrInst::CreateInBounds(New, Idx, Idx + 2, 48 New->getName()+".sub", It); 49 50 // Now make everything use the getelementptr instead of the original 51 // allocation. 52 return ReplaceInstUsesWith(AI, V); 53 } else if (isa<UndefValue>(AI.getArraySize())) { 54 return ReplaceInstUsesWith(AI, Constant::getNullValue(AI.getType())); 55 } 56 } 57 58 if (TD && isa<AllocaInst>(AI) && AI.getAllocatedType()->isSized()) { 59 // If alloca'ing a zero byte object, replace the alloca with a null pointer. 60 // Note that we only do this for alloca's, because malloc should allocate 61 // and return a unique pointer, even for a zero byte allocation. 62 if (TD->getTypeAllocSize(AI.getAllocatedType()) == 0) 63 return ReplaceInstUsesWith(AI, Constant::getNullValue(AI.getType())); 64 65 // If the alignment is 0 (unspecified), assign it the preferred alignment. 66 if (AI.getAlignment() == 0) 67 AI.setAlignment(TD->getPrefTypeAlignment(AI.getAllocatedType())); 68 } 69 70 return 0; 71} 72 73 74/// InstCombineLoadCast - Fold 'load (cast P)' -> cast (load P)' when possible. 75static Instruction *InstCombineLoadCast(InstCombiner &IC, LoadInst &LI, 76 const TargetData *TD) { 77 User *CI = cast<User>(LI.getOperand(0)); 78 Value *CastOp = CI->getOperand(0); 79 80 const PointerType *DestTy = cast<PointerType>(CI->getType()); 81 const Type *DestPTy = DestTy->getElementType(); 82 if (const PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType())) { 83 84 // If the address spaces don't match, don't eliminate the cast. 85 if (DestTy->getAddressSpace() != SrcTy->getAddressSpace()) 86 return 0; 87 88 const Type *SrcPTy = SrcTy->getElementType(); 89 90 if (DestPTy->isInteger() || isa<PointerType>(DestPTy) || 91 isa<VectorType>(DestPTy)) { 92 // If the source is an array, the code below will not succeed. Check to 93 // see if a trivial 'gep P, 0, 0' will help matters. Only do this for 94 // constants. 95 if (const ArrayType *ASrcTy = dyn_cast<ArrayType>(SrcPTy)) 96 if (Constant *CSrc = dyn_cast<Constant>(CastOp)) 97 if (ASrcTy->getNumElements() != 0) { 98 Value *Idxs[2]; 99 Idxs[0] = Constant::getNullValue(Type::getInt32Ty(LI.getContext())); 100 Idxs[1] = Idxs[0]; 101 CastOp = ConstantExpr::getGetElementPtr(CSrc, Idxs, 2); 102 SrcTy = cast<PointerType>(CastOp->getType()); 103 SrcPTy = SrcTy->getElementType(); 104 } 105 106 if (IC.getTargetData() && 107 (SrcPTy->isInteger() || isa<PointerType>(SrcPTy) || 108 isa<VectorType>(SrcPTy)) && 109 // Do not allow turning this into a load of an integer, which is then 110 // casted to a pointer, this pessimizes pointer analysis a lot. 111 (isa<PointerType>(SrcPTy) == isa<PointerType>(LI.getType())) && 112 IC.getTargetData()->getTypeSizeInBits(SrcPTy) == 113 IC.getTargetData()->getTypeSizeInBits(DestPTy)) { 114 115 // Okay, we are casting from one integer or pointer type to another of 116 // the same size. Instead of casting the pointer before the load, cast 117 // the result of the loaded value. 118 Value *NewLoad = 119 IC.Builder->CreateLoad(CastOp, LI.isVolatile(), CI->getName()); 120 // Now cast the result of the load. 121 return new BitCastInst(NewLoad, LI.getType()); 122 } 123 } 124 } 125 return 0; 126} 127 128Instruction *InstCombiner::visitLoadInst(LoadInst &LI) { 129 Value *Op = LI.getOperand(0); 130 131 // Attempt to improve the alignment. 132 if (TD) { 133 unsigned KnownAlign = 134 GetOrEnforceKnownAlignment(Op, TD->getPrefTypeAlignment(LI.getType())); 135 if (KnownAlign > 136 (LI.getAlignment() == 0 ? TD->getABITypeAlignment(LI.getType()) : 137 LI.getAlignment())) 138 LI.setAlignment(KnownAlign); 139 } 140 141 // load (cast X) --> cast (load X) iff safe. 142 if (isa<CastInst>(Op)) 143 if (Instruction *Res = InstCombineLoadCast(*this, LI, TD)) 144 return Res; 145 146 // None of the following transforms are legal for volatile loads. 147 if (LI.isVolatile()) return 0; 148 149 // Do really simple store-to-load forwarding and load CSE, to catch cases 150 // where there are several consequtive memory accesses to the same location, 151 // separated by a few arithmetic operations. 152 BasicBlock::iterator BBI = &LI; 153 if (Value *AvailableVal = FindAvailableLoadedValue(Op, LI.getParent(), BBI,6)) 154 return ReplaceInstUsesWith(LI, AvailableVal); 155 156 // load(gep null, ...) -> unreachable 157 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) { 158 const Value *GEPI0 = GEPI->getOperand(0); 159 // TODO: Consider a target hook for valid address spaces for this xform. 160 if (isa<ConstantPointerNull>(GEPI0) && GEPI->getPointerAddressSpace() == 0){ 161 // Insert a new store to null instruction before the load to indicate 162 // that this code is not reachable. We do this instead of inserting 163 // an unreachable instruction directly because we cannot modify the 164 // CFG. 165 new StoreInst(UndefValue::get(LI.getType()), 166 Constant::getNullValue(Op->getType()), &LI); 167 return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType())); 168 } 169 } 170 171 // load null/undef -> unreachable 172 // TODO: Consider a target hook for valid address spaces for this xform. 173 if (isa<UndefValue>(Op) || 174 (isa<ConstantPointerNull>(Op) && LI.getPointerAddressSpace() == 0)) { 175 // Insert a new store to null instruction before the load to indicate that 176 // this code is not reachable. We do this instead of inserting an 177 // unreachable instruction directly because we cannot modify the CFG. 178 new StoreInst(UndefValue::get(LI.getType()), 179 Constant::getNullValue(Op->getType()), &LI); 180 return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType())); 181 } 182 183 // Instcombine load (constantexpr_cast global) -> cast (load global) 184 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Op)) 185 if (CE->isCast()) 186 if (Instruction *Res = InstCombineLoadCast(*this, LI, TD)) 187 return Res; 188 189 if (Op->hasOneUse()) { 190 // Change select and PHI nodes to select values instead of addresses: this 191 // helps alias analysis out a lot, allows many others simplifications, and 192 // exposes redundancy in the code. 193 // 194 // Note that we cannot do the transformation unless we know that the 195 // introduced loads cannot trap! Something like this is valid as long as 196 // the condition is always false: load (select bool %C, int* null, int* %G), 197 // but it would not be valid if we transformed it to load from null 198 // unconditionally. 199 // 200 if (SelectInst *SI = dyn_cast<SelectInst>(Op)) { 201 // load (select (Cond, &V1, &V2)) --> select(Cond, load &V1, load &V2). 202 if (isSafeToLoadUnconditionally(SI->getOperand(1), SI) && 203 isSafeToLoadUnconditionally(SI->getOperand(2), SI)) { 204 Value *V1 = Builder->CreateLoad(SI->getOperand(1), 205 SI->getOperand(1)->getName()+".val"); 206 Value *V2 = Builder->CreateLoad(SI->getOperand(2), 207 SI->getOperand(2)->getName()+".val"); 208 return SelectInst::Create(SI->getCondition(), V1, V2); 209 } 210 211 // load (select (cond, null, P)) -> load P 212 if (Constant *C = dyn_cast<Constant>(SI->getOperand(1))) 213 if (C->isNullValue()) { 214 LI.setOperand(0, SI->getOperand(2)); 215 return &LI; 216 } 217 218 // load (select (cond, P, null)) -> load P 219 if (Constant *C = dyn_cast<Constant>(SI->getOperand(2))) 220 if (C->isNullValue()) { 221 LI.setOperand(0, SI->getOperand(1)); 222 return &LI; 223 } 224 } 225 } 226 return 0; 227} 228 229/// InstCombineStoreToCast - Fold store V, (cast P) -> store (cast V), P 230/// when possible. This makes it generally easy to do alias analysis and/or 231/// SROA/mem2reg of the memory object. 232static Instruction *InstCombineStoreToCast(InstCombiner &IC, StoreInst &SI) { 233 User *CI = cast<User>(SI.getOperand(1)); 234 Value *CastOp = CI->getOperand(0); 235 236 const Type *DestPTy = cast<PointerType>(CI->getType())->getElementType(); 237 const PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType()); 238 if (SrcTy == 0) return 0; 239 240 const Type *SrcPTy = SrcTy->getElementType(); 241 242 if (!DestPTy->isInteger() && !isa<PointerType>(DestPTy)) 243 return 0; 244 245 /// NewGEPIndices - If SrcPTy is an aggregate type, we can emit a "noop gep" 246 /// to its first element. This allows us to handle things like: 247 /// store i32 xxx, (bitcast {foo*, float}* %P to i32*) 248 /// on 32-bit hosts. 249 SmallVector<Value*, 4> NewGEPIndices; 250 251 // If the source is an array, the code below will not succeed. Check to 252 // see if a trivial 'gep P, 0, 0' will help matters. Only do this for 253 // constants. 254 if (isa<ArrayType>(SrcPTy) || isa<StructType>(SrcPTy)) { 255 // Index through pointer. 256 Constant *Zero = Constant::getNullValue(Type::getInt32Ty(SI.getContext())); 257 NewGEPIndices.push_back(Zero); 258 259 while (1) { 260 if (const StructType *STy = dyn_cast<StructType>(SrcPTy)) { 261 if (!STy->getNumElements()) /* Struct can be empty {} */ 262 break; 263 NewGEPIndices.push_back(Zero); 264 SrcPTy = STy->getElementType(0); 265 } else if (const ArrayType *ATy = dyn_cast<ArrayType>(SrcPTy)) { 266 NewGEPIndices.push_back(Zero); 267 SrcPTy = ATy->getElementType(); 268 } else { 269 break; 270 } 271 } 272 273 SrcTy = PointerType::get(SrcPTy, SrcTy->getAddressSpace()); 274 } 275 276 if (!SrcPTy->isInteger() && !isa<PointerType>(SrcPTy)) 277 return 0; 278 279 // If the pointers point into different address spaces or if they point to 280 // values with different sizes, we can't do the transformation. 281 if (!IC.getTargetData() || 282 SrcTy->getAddressSpace() != 283 cast<PointerType>(CI->getType())->getAddressSpace() || 284 IC.getTargetData()->getTypeSizeInBits(SrcPTy) != 285 IC.getTargetData()->getTypeSizeInBits(DestPTy)) 286 return 0; 287 288 // Okay, we are casting from one integer or pointer type to another of 289 // the same size. Instead of casting the pointer before 290 // the store, cast the value to be stored. 291 Value *NewCast; 292 Value *SIOp0 = SI.getOperand(0); 293 Instruction::CastOps opcode = Instruction::BitCast; 294 const Type* CastSrcTy = SIOp0->getType(); 295 const Type* CastDstTy = SrcPTy; 296 if (isa<PointerType>(CastDstTy)) { 297 if (CastSrcTy->isInteger()) 298 opcode = Instruction::IntToPtr; 299 } else if (isa<IntegerType>(CastDstTy)) { 300 if (isa<PointerType>(SIOp0->getType())) 301 opcode = Instruction::PtrToInt; 302 } 303 304 // SIOp0 is a pointer to aggregate and this is a store to the first field, 305 // emit a GEP to index into its first field. 306 if (!NewGEPIndices.empty()) 307 CastOp = IC.Builder->CreateInBoundsGEP(CastOp, NewGEPIndices.begin(), 308 NewGEPIndices.end()); 309 310 NewCast = IC.Builder->CreateCast(opcode, SIOp0, CastDstTy, 311 SIOp0->getName()+".c"); 312 return new StoreInst(NewCast, CastOp); 313} 314 315/// equivalentAddressValues - Test if A and B will obviously have the same 316/// value. This includes recognizing that %t0 and %t1 will have the same 317/// value in code like this: 318/// %t0 = getelementptr \@a, 0, 3 319/// store i32 0, i32* %t0 320/// %t1 = getelementptr \@a, 0, 3 321/// %t2 = load i32* %t1 322/// 323static bool equivalentAddressValues(Value *A, Value *B) { 324 // Test if the values are trivially equivalent. 325 if (A == B) return true; 326 327 // Test if the values come form identical arithmetic instructions. 328 // This uses isIdenticalToWhenDefined instead of isIdenticalTo because 329 // its only used to compare two uses within the same basic block, which 330 // means that they'll always either have the same value or one of them 331 // will have an undefined value. 332 if (isa<BinaryOperator>(A) || 333 isa<CastInst>(A) || 334 isa<PHINode>(A) || 335 isa<GetElementPtrInst>(A)) 336 if (Instruction *BI = dyn_cast<Instruction>(B)) 337 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) 338 return true; 339 340 // Otherwise they may not be equivalent. 341 return false; 342} 343 344// If this instruction has two uses, one of which is a llvm.dbg.declare, 345// return the llvm.dbg.declare. 346DbgDeclareInst *InstCombiner::hasOneUsePlusDeclare(Value *V) { 347 if (!V->hasNUses(2)) 348 return 0; 349 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); 350 UI != E; ++UI) { 351 if (DbgDeclareInst *DI = dyn_cast<DbgDeclareInst>(UI)) 352 return DI; 353 if (isa<BitCastInst>(UI) && UI->hasOneUse()) { 354 if (DbgDeclareInst *DI = dyn_cast<DbgDeclareInst>(UI->use_begin())) 355 return DI; 356 } 357 } 358 return 0; 359} 360 361Instruction *InstCombiner::visitStoreInst(StoreInst &SI) { 362 Value *Val = SI.getOperand(0); 363 Value *Ptr = SI.getOperand(1); 364 365 // If the RHS is an alloca with a single use, zapify the store, making the 366 // alloca dead. 367 // If the RHS is an alloca with a two uses, the other one being a 368 // llvm.dbg.declare, zapify the store and the declare, making the 369 // alloca dead. We must do this to prevent declares from affecting 370 // codegen. 371 if (!SI.isVolatile()) { 372 if (Ptr->hasOneUse()) { 373 if (isa<AllocaInst>(Ptr)) 374 return EraseInstFromFunction(SI); 375 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr)) { 376 if (isa<AllocaInst>(GEP->getOperand(0))) { 377 if (GEP->getOperand(0)->hasOneUse()) 378 return EraseInstFromFunction(SI); 379 if (DbgDeclareInst *DI = hasOneUsePlusDeclare(GEP->getOperand(0))) { 380 EraseInstFromFunction(*DI); 381 return EraseInstFromFunction(SI); 382 } 383 } 384 } 385 } 386 if (DbgDeclareInst *DI = hasOneUsePlusDeclare(Ptr)) { 387 EraseInstFromFunction(*DI); 388 return EraseInstFromFunction(SI); 389 } 390 } 391 392 // Attempt to improve the alignment. 393 if (TD) { 394 unsigned KnownAlign = 395 GetOrEnforceKnownAlignment(Ptr, TD->getPrefTypeAlignment(Val->getType())); 396 if (KnownAlign > 397 (SI.getAlignment() == 0 ? TD->getABITypeAlignment(Val->getType()) : 398 SI.getAlignment())) 399 SI.setAlignment(KnownAlign); 400 } 401 402 // Do really simple DSE, to catch cases where there are several consecutive 403 // stores to the same location, separated by a few arithmetic operations. This 404 // situation often occurs with bitfield accesses. 405 BasicBlock::iterator BBI = &SI; 406 for (unsigned ScanInsts = 6; BBI != SI.getParent()->begin() && ScanInsts; 407 --ScanInsts) { 408 --BBI; 409 // Don't count debug info directives, lest they affect codegen, 410 // and we skip pointer-to-pointer bitcasts, which are NOPs. 411 if (isa<DbgInfoIntrinsic>(BBI) || 412 (isa<BitCastInst>(BBI) && isa<PointerType>(BBI->getType()))) { 413 ScanInsts++; 414 continue; 415 } 416 417 if (StoreInst *PrevSI = dyn_cast<StoreInst>(BBI)) { 418 // Prev store isn't volatile, and stores to the same location? 419 if (!PrevSI->isVolatile() &&equivalentAddressValues(PrevSI->getOperand(1), 420 SI.getOperand(1))) { 421 ++NumDeadStore; 422 ++BBI; 423 EraseInstFromFunction(*PrevSI); 424 continue; 425 } 426 break; 427 } 428 429 // If this is a load, we have to stop. However, if the loaded value is from 430 // the pointer we're loading and is producing the pointer we're storing, 431 // then *this* store is dead (X = load P; store X -> P). 432 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { 433 if (LI == Val && equivalentAddressValues(LI->getOperand(0), Ptr) && 434 !SI.isVolatile()) 435 return EraseInstFromFunction(SI); 436 437 // Otherwise, this is a load from some other location. Stores before it 438 // may not be dead. 439 break; 440 } 441 442 // Don't skip over loads or things that can modify memory. 443 if (BBI->mayWriteToMemory() || BBI->mayReadFromMemory()) 444 break; 445 } 446 447 448 if (SI.isVolatile()) return 0; // Don't hack volatile stores. 449 450 // store X, null -> turns into 'unreachable' in SimplifyCFG 451 if (isa<ConstantPointerNull>(Ptr) && SI.getPointerAddressSpace() == 0) { 452 if (!isa<UndefValue>(Val)) { 453 SI.setOperand(0, UndefValue::get(Val->getType())); 454 if (Instruction *U = dyn_cast<Instruction>(Val)) 455 Worklist.Add(U); // Dropped a use. 456 } 457 return 0; // Do not modify these! 458 } 459 460 // store undef, Ptr -> noop 461 if (isa<UndefValue>(Val)) 462 return EraseInstFromFunction(SI); 463 464 // If the pointer destination is a cast, see if we can fold the cast into the 465 // source instead. 466 if (isa<CastInst>(Ptr)) 467 if (Instruction *Res = InstCombineStoreToCast(*this, SI)) 468 return Res; 469 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr)) 470 if (CE->isCast()) 471 if (Instruction *Res = InstCombineStoreToCast(*this, SI)) 472 return Res; 473 474 475 // If this store is the last instruction in the basic block (possibly 476 // excepting debug info instructions), and if the block ends with an 477 // unconditional branch, try to move it to the successor block. 478 BBI = &SI; 479 do { 480 ++BBI; 481 } while (isa<DbgInfoIntrinsic>(BBI) || 482 (isa<BitCastInst>(BBI) && isa<PointerType>(BBI->getType()))); 483 if (BranchInst *BI = dyn_cast<BranchInst>(BBI)) 484 if (BI->isUnconditional()) 485 if (SimplifyStoreAtEndOfBlock(SI)) 486 return 0; // xform done! 487 488 return 0; 489} 490 491/// SimplifyStoreAtEndOfBlock - Turn things like: 492/// if () { *P = v1; } else { *P = v2 } 493/// into a phi node with a store in the successor. 494/// 495/// Simplify things like: 496/// *P = v1; if () { *P = v2; } 497/// into a phi node with a store in the successor. 498/// 499bool InstCombiner::SimplifyStoreAtEndOfBlock(StoreInst &SI) { 500 BasicBlock *StoreBB = SI.getParent(); 501 502 // Check to see if the successor block has exactly two incoming edges. If 503 // so, see if the other predecessor contains a store to the same location. 504 // if so, insert a PHI node (if needed) and move the stores down. 505 BasicBlock *DestBB = StoreBB->getTerminator()->getSuccessor(0); 506 507 // Determine whether Dest has exactly two predecessors and, if so, compute 508 // the other predecessor. 509 pred_iterator PI = pred_begin(DestBB); 510 BasicBlock *OtherBB = 0; 511 if (*PI != StoreBB) 512 OtherBB = *PI; 513 ++PI; 514 if (PI == pred_end(DestBB)) 515 return false; 516 517 if (*PI != StoreBB) { 518 if (OtherBB) 519 return false; 520 OtherBB = *PI; 521 } 522 if (++PI != pred_end(DestBB)) 523 return false; 524 525 // Bail out if all the relevant blocks aren't distinct (this can happen, 526 // for example, if SI is in an infinite loop) 527 if (StoreBB == DestBB || OtherBB == DestBB) 528 return false; 529 530 // Verify that the other block ends in a branch and is not otherwise empty. 531 BasicBlock::iterator BBI = OtherBB->getTerminator(); 532 BranchInst *OtherBr = dyn_cast<BranchInst>(BBI); 533 if (!OtherBr || BBI == OtherBB->begin()) 534 return false; 535 536 // If the other block ends in an unconditional branch, check for the 'if then 537 // else' case. there is an instruction before the branch. 538 StoreInst *OtherStore = 0; 539 if (OtherBr->isUnconditional()) { 540 --BBI; 541 // Skip over debugging info. 542 while (isa<DbgInfoIntrinsic>(BBI) || 543 (isa<BitCastInst>(BBI) && isa<PointerType>(BBI->getType()))) { 544 if (BBI==OtherBB->begin()) 545 return false; 546 --BBI; 547 } 548 // If this isn't a store, isn't a store to the same location, or if the 549 // alignments differ, bail out. 550 OtherStore = dyn_cast<StoreInst>(BBI); 551 if (!OtherStore || OtherStore->getOperand(1) != SI.getOperand(1) || 552 OtherStore->getAlignment() != SI.getAlignment()) 553 return false; 554 } else { 555 // Otherwise, the other block ended with a conditional branch. If one of the 556 // destinations is StoreBB, then we have the if/then case. 557 if (OtherBr->getSuccessor(0) != StoreBB && 558 OtherBr->getSuccessor(1) != StoreBB) 559 return false; 560 561 // Okay, we know that OtherBr now goes to Dest and StoreBB, so this is an 562 // if/then triangle. See if there is a store to the same ptr as SI that 563 // lives in OtherBB. 564 for (;; --BBI) { 565 // Check to see if we find the matching store. 566 if ((OtherStore = dyn_cast<StoreInst>(BBI))) { 567 if (OtherStore->getOperand(1) != SI.getOperand(1) || 568 OtherStore->getAlignment() != SI.getAlignment()) 569 return false; 570 break; 571 } 572 // If we find something that may be using or overwriting the stored 573 // value, or if we run out of instructions, we can't do the xform. 574 if (BBI->mayReadFromMemory() || BBI->mayWriteToMemory() || 575 BBI == OtherBB->begin()) 576 return false; 577 } 578 579 // In order to eliminate the store in OtherBr, we have to 580 // make sure nothing reads or overwrites the stored value in 581 // StoreBB. 582 for (BasicBlock::iterator I = StoreBB->begin(); &*I != &SI; ++I) { 583 // FIXME: This should really be AA driven. 584 if (I->mayReadFromMemory() || I->mayWriteToMemory()) 585 return false; 586 } 587 } 588 589 // Insert a PHI node now if we need it. 590 Value *MergedVal = OtherStore->getOperand(0); 591 if (MergedVal != SI.getOperand(0)) { 592 PHINode *PN = PHINode::Create(MergedVal->getType(), "storemerge"); 593 PN->reserveOperandSpace(2); 594 PN->addIncoming(SI.getOperand(0), SI.getParent()); 595 PN->addIncoming(OtherStore->getOperand(0), OtherBB); 596 MergedVal = InsertNewInstBefore(PN, DestBB->front()); 597 } 598 599 // Advance to a place where it is safe to insert the new store and 600 // insert it. 601 BBI = DestBB->getFirstNonPHI(); 602 InsertNewInstBefore(new StoreInst(MergedVal, SI.getOperand(1), 603 OtherStore->isVolatile(), 604 SI.getAlignment()), *BBI); 605 606 // Nuke the old stores. 607 EraseInstFromFunction(SI); 608 EraseInstFromFunction(*OtherStore); 609 return true; 610} 611