InstCombineLoadStoreAlloca.cpp revision d3dc3cc98f0366a9860a68eaa1f823dfc7fa310b
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 // It is necessary for correctness to skip those that feed into a 412 // llvm.dbg.declare, as these are not present when debugging is off. 413 if (isa<DbgInfoIntrinsic>(BBI) || 414 (isa<BitCastInst>(BBI) && isa<PointerType>(BBI->getType()))) { 415 ScanInsts++; 416 continue; 417 } 418 419 if (StoreInst *PrevSI = dyn_cast<StoreInst>(BBI)) { 420 // Prev store isn't volatile, and stores to the same location? 421 if (!PrevSI->isVolatile() &&equivalentAddressValues(PrevSI->getOperand(1), 422 SI.getOperand(1))) { 423 ++NumDeadStore; 424 ++BBI; 425 EraseInstFromFunction(*PrevSI); 426 continue; 427 } 428 break; 429 } 430 431 // If this is a load, we have to stop. However, if the loaded value is from 432 // the pointer we're loading and is producing the pointer we're storing, 433 // then *this* store is dead (X = load P; store X -> P). 434 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { 435 if (LI == Val && equivalentAddressValues(LI->getOperand(0), Ptr) && 436 !SI.isVolatile()) 437 return EraseInstFromFunction(SI); 438 439 // Otherwise, this is a load from some other location. Stores before it 440 // may not be dead. 441 break; 442 } 443 444 // Don't skip over loads or things that can modify memory. 445 if (BBI->mayWriteToMemory() || BBI->mayReadFromMemory()) 446 break; 447 } 448 449 450 if (SI.isVolatile()) return 0; // Don't hack volatile stores. 451 452 // store X, null -> turns into 'unreachable' in SimplifyCFG 453 if (isa<ConstantPointerNull>(Ptr) && SI.getPointerAddressSpace() == 0) { 454 if (!isa<UndefValue>(Val)) { 455 SI.setOperand(0, UndefValue::get(Val->getType())); 456 if (Instruction *U = dyn_cast<Instruction>(Val)) 457 Worklist.Add(U); // Dropped a use. 458 } 459 return 0; // Do not modify these! 460 } 461 462 // store undef, Ptr -> noop 463 if (isa<UndefValue>(Val)) 464 return EraseInstFromFunction(SI); 465 466 // If the pointer destination is a cast, see if we can fold the cast into the 467 // source instead. 468 if (isa<CastInst>(Ptr)) 469 if (Instruction *Res = InstCombineStoreToCast(*this, SI)) 470 return Res; 471 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr)) 472 if (CE->isCast()) 473 if (Instruction *Res = InstCombineStoreToCast(*this, SI)) 474 return Res; 475 476 477 // If this store is the last instruction in the basic block (possibly 478 // excepting debug info instructions and the pointer bitcasts that feed 479 // into them), and if the block ends with an unconditional branch, try 480 // to move it to the successor block. 481 BBI = &SI; 482 do { 483 ++BBI; 484 } while (isa<DbgInfoIntrinsic>(BBI) || 485 (isa<BitCastInst>(BBI) && isa<PointerType>(BBI->getType()))); 486 if (BranchInst *BI = dyn_cast<BranchInst>(BBI)) 487 if (BI->isUnconditional()) 488 if (SimplifyStoreAtEndOfBlock(SI)) 489 return 0; // xform done! 490 491 return 0; 492} 493 494/// SimplifyStoreAtEndOfBlock - Turn things like: 495/// if () { *P = v1; } else { *P = v2 } 496/// into a phi node with a store in the successor. 497/// 498/// Simplify things like: 499/// *P = v1; if () { *P = v2; } 500/// into a phi node with a store in the successor. 501/// 502bool InstCombiner::SimplifyStoreAtEndOfBlock(StoreInst &SI) { 503 BasicBlock *StoreBB = SI.getParent(); 504 505 // Check to see if the successor block has exactly two incoming edges. If 506 // so, see if the other predecessor contains a store to the same location. 507 // if so, insert a PHI node (if needed) and move the stores down. 508 BasicBlock *DestBB = StoreBB->getTerminator()->getSuccessor(0); 509 510 // Determine whether Dest has exactly two predecessors and, if so, compute 511 // the other predecessor. 512 pred_iterator PI = pred_begin(DestBB); 513 BasicBlock *OtherBB = 0; 514 if (*PI != StoreBB) 515 OtherBB = *PI; 516 ++PI; 517 if (PI == pred_end(DestBB)) 518 return false; 519 520 if (*PI != StoreBB) { 521 if (OtherBB) 522 return false; 523 OtherBB = *PI; 524 } 525 if (++PI != pred_end(DestBB)) 526 return false; 527 528 // Bail out if all the relevant blocks aren't distinct (this can happen, 529 // for example, if SI is in an infinite loop) 530 if (StoreBB == DestBB || OtherBB == DestBB) 531 return false; 532 533 // Verify that the other block ends in a branch and is not otherwise empty. 534 BasicBlock::iterator BBI = OtherBB->getTerminator(); 535 BranchInst *OtherBr = dyn_cast<BranchInst>(BBI); 536 if (!OtherBr || BBI == OtherBB->begin()) 537 return false; 538 539 // If the other block ends in an unconditional branch, check for the 'if then 540 // else' case. there is an instruction before the branch. 541 StoreInst *OtherStore = 0; 542 if (OtherBr->isUnconditional()) { 543 --BBI; 544 // Skip over debugging info. 545 while (isa<DbgInfoIntrinsic>(BBI) || 546 (isa<BitCastInst>(BBI) && isa<PointerType>(BBI->getType()))) { 547 if (BBI==OtherBB->begin()) 548 return false; 549 --BBI; 550 } 551 // If this isn't a store, isn't a store to the same location, or if the 552 // alignments differ, bail out. 553 OtherStore = dyn_cast<StoreInst>(BBI); 554 if (!OtherStore || OtherStore->getOperand(1) != SI.getOperand(1) || 555 OtherStore->getAlignment() != SI.getAlignment()) 556 return false; 557 } else { 558 // Otherwise, the other block ended with a conditional branch. If one of the 559 // destinations is StoreBB, then we have the if/then case. 560 if (OtherBr->getSuccessor(0) != StoreBB && 561 OtherBr->getSuccessor(1) != StoreBB) 562 return false; 563 564 // Okay, we know that OtherBr now goes to Dest and StoreBB, so this is an 565 // if/then triangle. See if there is a store to the same ptr as SI that 566 // lives in OtherBB. 567 for (;; --BBI) { 568 // Check to see if we find the matching store. 569 if ((OtherStore = dyn_cast<StoreInst>(BBI))) { 570 if (OtherStore->getOperand(1) != SI.getOperand(1) || 571 OtherStore->getAlignment() != SI.getAlignment()) 572 return false; 573 break; 574 } 575 // If we find something that may be using or overwriting the stored 576 // value, or if we run out of instructions, we can't do the xform. 577 if (BBI->mayReadFromMemory() || BBI->mayWriteToMemory() || 578 BBI == OtherBB->begin()) 579 return false; 580 } 581 582 // In order to eliminate the store in OtherBr, we have to 583 // make sure nothing reads or overwrites the stored value in 584 // StoreBB. 585 for (BasicBlock::iterator I = StoreBB->begin(); &*I != &SI; ++I) { 586 // FIXME: This should really be AA driven. 587 if (I->mayReadFromMemory() || I->mayWriteToMemory()) 588 return false; 589 } 590 } 591 592 // Insert a PHI node now if we need it. 593 Value *MergedVal = OtherStore->getOperand(0); 594 if (MergedVal != SI.getOperand(0)) { 595 PHINode *PN = PHINode::Create(MergedVal->getType(), "storemerge"); 596 PN->reserveOperandSpace(2); 597 PN->addIncoming(SI.getOperand(0), SI.getParent()); 598 PN->addIncoming(OtherStore->getOperand(0), OtherBB); 599 MergedVal = InsertNewInstBefore(PN, DestBB->front()); 600 } 601 602 // Advance to a place where it is safe to insert the new store and 603 // insert it. 604 BBI = DestBB->getFirstNonPHI(); 605 InsertNewInstBefore(new StoreInst(MergedVal, SI.getOperand(1), 606 OtherStore->isVolatile(), 607 SI.getAlignment()), *BBI); 608 609 // Nuke the old stores. 610 EraseInstFromFunction(SI); 611 EraseInstFromFunction(*OtherStore); 612 return true; 613} 614