InstCombineLoadStoreAlloca.cpp revision b0bc6c361da9009e8414efde317d9bbff755f6c0
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->isIntegerTy() || 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->isIntegerTy() || 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 LoadInst *NewLoad = 119 IC.Builder->CreateLoad(CastOp, LI.isVolatile(), CI->getName()); 120 NewLoad->setAlignment(LI.getAlignment()); 121 // Now cast the result of the load. 122 return new BitCastInst(NewLoad, LI.getType()); 123 } 124 } 125 } 126 return 0; 127} 128 129Instruction *InstCombiner::visitLoadInst(LoadInst &LI) { 130 Value *Op = LI.getOperand(0); 131 132 // Attempt to improve the alignment. 133 if (TD) { 134 unsigned KnownAlign = 135 GetOrEnforceKnownAlignment(Op, TD->getPrefTypeAlignment(LI.getType())); 136 if (KnownAlign > 137 (LI.getAlignment() == 0 ? TD->getABITypeAlignment(LI.getType()) : 138 LI.getAlignment())) 139 LI.setAlignment(KnownAlign); 140 } 141 142 // load (cast X) --> cast (load X) iff safe. 143 if (isa<CastInst>(Op)) 144 if (Instruction *Res = InstCombineLoadCast(*this, LI, TD)) 145 return Res; 146 147 // None of the following transforms are legal for volatile loads. 148 if (LI.isVolatile()) return 0; 149 150 // Do really simple store-to-load forwarding and load CSE, to catch cases 151 // where there are several consequtive memory accesses to the same location, 152 // separated by a few arithmetic operations. 153 BasicBlock::iterator BBI = &LI; 154 if (Value *AvailableVal = FindAvailableLoadedValue(Op, LI.getParent(), BBI,6)) 155 return ReplaceInstUsesWith(LI, AvailableVal); 156 157 // load(gep null, ...) -> unreachable 158 if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) { 159 const Value *GEPI0 = GEPI->getOperand(0); 160 // TODO: Consider a target hook for valid address spaces for this xform. 161 if (isa<ConstantPointerNull>(GEPI0) && GEPI->getPointerAddressSpace() == 0){ 162 // Insert a new store to null instruction before the load to indicate 163 // that this code is not reachable. We do this instead of inserting 164 // an unreachable instruction directly because we cannot modify the 165 // CFG. 166 new StoreInst(UndefValue::get(LI.getType()), 167 Constant::getNullValue(Op->getType()), &LI); 168 return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType())); 169 } 170 } 171 172 // load null/undef -> unreachable 173 // TODO: Consider a target hook for valid address spaces for this xform. 174 if (isa<UndefValue>(Op) || 175 (isa<ConstantPointerNull>(Op) && LI.getPointerAddressSpace() == 0)) { 176 // Insert a new store to null instruction before the load to indicate that 177 // this code is not reachable. We do this instead of inserting an 178 // unreachable instruction directly because we cannot modify the CFG. 179 new StoreInst(UndefValue::get(LI.getType()), 180 Constant::getNullValue(Op->getType()), &LI); 181 return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType())); 182 } 183 184 // Instcombine load (constantexpr_cast global) -> cast (load global) 185 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Op)) 186 if (CE->isCast()) 187 if (Instruction *Res = InstCombineLoadCast(*this, LI, TD)) 188 return Res; 189 190 if (Op->hasOneUse()) { 191 // Change select and PHI nodes to select values instead of addresses: this 192 // helps alias analysis out a lot, allows many others simplifications, and 193 // exposes redundancy in the code. 194 // 195 // Note that we cannot do the transformation unless we know that the 196 // introduced loads cannot trap! Something like this is valid as long as 197 // the condition is always false: load (select bool %C, int* null, int* %G), 198 // but it would not be valid if we transformed it to load from null 199 // unconditionally. 200 // 201 if (SelectInst *SI = dyn_cast<SelectInst>(Op)) { 202 // load (select (Cond, &V1, &V2)) --> select(Cond, load &V1, load &V2). 203 unsigned Align = LI.getAlignment(); 204 if (isSafeToLoadUnconditionally(SI->getOperand(1), SI, Align, TD) && 205 isSafeToLoadUnconditionally(SI->getOperand(2), SI, Align, TD)) { 206 LoadInst *V1 = Builder->CreateLoad(SI->getOperand(1), 207 SI->getOperand(1)->getName()+".val"); 208 LoadInst *V2 = Builder->CreateLoad(SI->getOperand(2), 209 SI->getOperand(2)->getName()+".val"); 210 V1->setAlignment(Align); 211 V2->setAlignment(Align); 212 return SelectInst::Create(SI->getCondition(), V1, V2); 213 } 214 215 // load (select (cond, null, P)) -> load P 216 if (Constant *C = dyn_cast<Constant>(SI->getOperand(1))) 217 if (C->isNullValue()) { 218 LI.setOperand(0, SI->getOperand(2)); 219 return &LI; 220 } 221 222 // load (select (cond, P, null)) -> load P 223 if (Constant *C = dyn_cast<Constant>(SI->getOperand(2))) 224 if (C->isNullValue()) { 225 LI.setOperand(0, SI->getOperand(1)); 226 return &LI; 227 } 228 } 229 } 230 return 0; 231} 232 233/// InstCombineStoreToCast - Fold store V, (cast P) -> store (cast V), P 234/// when possible. This makes it generally easy to do alias analysis and/or 235/// SROA/mem2reg of the memory object. 236static Instruction *InstCombineStoreToCast(InstCombiner &IC, StoreInst &SI) { 237 User *CI = cast<User>(SI.getOperand(1)); 238 Value *CastOp = CI->getOperand(0); 239 240 const Type *DestPTy = cast<PointerType>(CI->getType())->getElementType(); 241 const PointerType *SrcTy = dyn_cast<PointerType>(CastOp->getType()); 242 if (SrcTy == 0) return 0; 243 244 const Type *SrcPTy = SrcTy->getElementType(); 245 246 if (!DestPTy->isIntegerTy() && !isa<PointerType>(DestPTy)) 247 return 0; 248 249 /// NewGEPIndices - If SrcPTy is an aggregate type, we can emit a "noop gep" 250 /// to its first element. This allows us to handle things like: 251 /// store i32 xxx, (bitcast {foo*, float}* %P to i32*) 252 /// on 32-bit hosts. 253 SmallVector<Value*, 4> NewGEPIndices; 254 255 // If the source is an array, the code below will not succeed. Check to 256 // see if a trivial 'gep P, 0, 0' will help matters. Only do this for 257 // constants. 258 if (isa<ArrayType>(SrcPTy) || isa<StructType>(SrcPTy)) { 259 // Index through pointer. 260 Constant *Zero = Constant::getNullValue(Type::getInt32Ty(SI.getContext())); 261 NewGEPIndices.push_back(Zero); 262 263 while (1) { 264 if (const StructType *STy = dyn_cast<StructType>(SrcPTy)) { 265 if (!STy->getNumElements()) /* Struct can be empty {} */ 266 break; 267 NewGEPIndices.push_back(Zero); 268 SrcPTy = STy->getElementType(0); 269 } else if (const ArrayType *ATy = dyn_cast<ArrayType>(SrcPTy)) { 270 NewGEPIndices.push_back(Zero); 271 SrcPTy = ATy->getElementType(); 272 } else { 273 break; 274 } 275 } 276 277 SrcTy = PointerType::get(SrcPTy, SrcTy->getAddressSpace()); 278 } 279 280 if (!SrcPTy->isIntegerTy() && !isa<PointerType>(SrcPTy)) 281 return 0; 282 283 // If the pointers point into different address spaces or if they point to 284 // values with different sizes, we can't do the transformation. 285 if (!IC.getTargetData() || 286 SrcTy->getAddressSpace() != 287 cast<PointerType>(CI->getType())->getAddressSpace() || 288 IC.getTargetData()->getTypeSizeInBits(SrcPTy) != 289 IC.getTargetData()->getTypeSizeInBits(DestPTy)) 290 return 0; 291 292 // Okay, we are casting from one integer or pointer type to another of 293 // the same size. Instead of casting the pointer before 294 // the store, cast the value to be stored. 295 Value *NewCast; 296 Value *SIOp0 = SI.getOperand(0); 297 Instruction::CastOps opcode = Instruction::BitCast; 298 const Type* CastSrcTy = SIOp0->getType(); 299 const Type* CastDstTy = SrcPTy; 300 if (isa<PointerType>(CastDstTy)) { 301 if (CastSrcTy->isIntegerTy()) 302 opcode = Instruction::IntToPtr; 303 } else if (isa<IntegerType>(CastDstTy)) { 304 if (isa<PointerType>(SIOp0->getType())) 305 opcode = Instruction::PtrToInt; 306 } 307 308 // SIOp0 is a pointer to aggregate and this is a store to the first field, 309 // emit a GEP to index into its first field. 310 if (!NewGEPIndices.empty()) 311 CastOp = IC.Builder->CreateInBoundsGEP(CastOp, NewGEPIndices.begin(), 312 NewGEPIndices.end()); 313 314 NewCast = IC.Builder->CreateCast(opcode, SIOp0, CastDstTy, 315 SIOp0->getName()+".c"); 316 return new StoreInst(NewCast, CastOp); 317} 318 319/// equivalentAddressValues - Test if A and B will obviously have the same 320/// value. This includes recognizing that %t0 and %t1 will have the same 321/// value in code like this: 322/// %t0 = getelementptr \@a, 0, 3 323/// store i32 0, i32* %t0 324/// %t1 = getelementptr \@a, 0, 3 325/// %t2 = load i32* %t1 326/// 327static bool equivalentAddressValues(Value *A, Value *B) { 328 // Test if the values are trivially equivalent. 329 if (A == B) return true; 330 331 // Test if the values come form identical arithmetic instructions. 332 // This uses isIdenticalToWhenDefined instead of isIdenticalTo because 333 // its only used to compare two uses within the same basic block, which 334 // means that they'll always either have the same value or one of them 335 // will have an undefined value. 336 if (isa<BinaryOperator>(A) || 337 isa<CastInst>(A) || 338 isa<PHINode>(A) || 339 isa<GetElementPtrInst>(A)) 340 if (Instruction *BI = dyn_cast<Instruction>(B)) 341 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) 342 return true; 343 344 // Otherwise they may not be equivalent. 345 return false; 346} 347 348// If this instruction has two uses, one of which is a llvm.dbg.declare, 349// return the llvm.dbg.declare. 350DbgDeclareInst *InstCombiner::hasOneUsePlusDeclare(Value *V) { 351 if (!V->hasNUses(2)) 352 return 0; 353 for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); 354 UI != E; ++UI) { 355 if (DbgDeclareInst *DI = dyn_cast<DbgDeclareInst>(UI)) 356 return DI; 357 if (isa<BitCastInst>(UI) && UI->hasOneUse()) { 358 if (DbgDeclareInst *DI = dyn_cast<DbgDeclareInst>(UI->use_begin())) 359 return DI; 360 } 361 } 362 return 0; 363} 364 365Instruction *InstCombiner::visitStoreInst(StoreInst &SI) { 366 Value *Val = SI.getOperand(0); 367 Value *Ptr = SI.getOperand(1); 368 369 // If the RHS is an alloca with a single use, zapify the store, making the 370 // alloca dead. 371 // If the RHS is an alloca with a two uses, the other one being a 372 // llvm.dbg.declare, zapify the store and the declare, making the 373 // alloca dead. We must do this to prevent declares from affecting 374 // codegen. 375 if (!SI.isVolatile()) { 376 if (Ptr->hasOneUse()) { 377 if (isa<AllocaInst>(Ptr)) 378 return EraseInstFromFunction(SI); 379 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr)) { 380 if (isa<AllocaInst>(GEP->getOperand(0))) { 381 if (GEP->getOperand(0)->hasOneUse()) 382 return EraseInstFromFunction(SI); 383 if (DbgDeclareInst *DI = hasOneUsePlusDeclare(GEP->getOperand(0))) { 384 EraseInstFromFunction(*DI); 385 return EraseInstFromFunction(SI); 386 } 387 } 388 } 389 } 390 if (DbgDeclareInst *DI = hasOneUsePlusDeclare(Ptr)) { 391 EraseInstFromFunction(*DI); 392 return EraseInstFromFunction(SI); 393 } 394 } 395 396 // Attempt to improve the alignment. 397 if (TD) { 398 unsigned KnownAlign = 399 GetOrEnforceKnownAlignment(Ptr, TD->getPrefTypeAlignment(Val->getType())); 400 if (KnownAlign > 401 (SI.getAlignment() == 0 ? TD->getABITypeAlignment(Val->getType()) : 402 SI.getAlignment())) 403 SI.setAlignment(KnownAlign); 404 } 405 406 // Do really simple DSE, to catch cases where there are several consecutive 407 // stores to the same location, separated by a few arithmetic operations. This 408 // situation often occurs with bitfield accesses. 409 BasicBlock::iterator BBI = &SI; 410 for (unsigned ScanInsts = 6; BBI != SI.getParent()->begin() && ScanInsts; 411 --ScanInsts) { 412 --BBI; 413 // Don't count debug info directives, lest they affect codegen, 414 // and we skip pointer-to-pointer bitcasts, which are NOPs. 415 if (isa<DbgInfoIntrinsic>(BBI) || 416 (isa<BitCastInst>(BBI) && isa<PointerType>(BBI->getType()))) { 417 ScanInsts++; 418 continue; 419 } 420 421 if (StoreInst *PrevSI = dyn_cast<StoreInst>(BBI)) { 422 // Prev store isn't volatile, and stores to the same location? 423 if (!PrevSI->isVolatile() &&equivalentAddressValues(PrevSI->getOperand(1), 424 SI.getOperand(1))) { 425 ++NumDeadStore; 426 ++BBI; 427 EraseInstFromFunction(*PrevSI); 428 continue; 429 } 430 break; 431 } 432 433 // If this is a load, we have to stop. However, if the loaded value is from 434 // the pointer we're loading and is producing the pointer we're storing, 435 // then *this* store is dead (X = load P; store X -> P). 436 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { 437 if (LI == Val && equivalentAddressValues(LI->getOperand(0), Ptr) && 438 !SI.isVolatile()) 439 return EraseInstFromFunction(SI); 440 441 // Otherwise, this is a load from some other location. Stores before it 442 // may not be dead. 443 break; 444 } 445 446 // Don't skip over loads or things that can modify memory. 447 if (BBI->mayWriteToMemory() || BBI->mayReadFromMemory()) 448 break; 449 } 450 451 452 if (SI.isVolatile()) return 0; // Don't hack volatile stores. 453 454 // store X, null -> turns into 'unreachable' in SimplifyCFG 455 if (isa<ConstantPointerNull>(Ptr) && SI.getPointerAddressSpace() == 0) { 456 if (!isa<UndefValue>(Val)) { 457 SI.setOperand(0, UndefValue::get(Val->getType())); 458 if (Instruction *U = dyn_cast<Instruction>(Val)) 459 Worklist.Add(U); // Dropped a use. 460 } 461 return 0; // Do not modify these! 462 } 463 464 // store undef, Ptr -> noop 465 if (isa<UndefValue>(Val)) 466 return EraseInstFromFunction(SI); 467 468 // If the pointer destination is a cast, see if we can fold the cast into the 469 // source instead. 470 if (isa<CastInst>(Ptr)) 471 if (Instruction *Res = InstCombineStoreToCast(*this, SI)) 472 return Res; 473 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr)) 474 if (CE->isCast()) 475 if (Instruction *Res = InstCombineStoreToCast(*this, SI)) 476 return Res; 477 478 479 // If this store is the last instruction in the basic block (possibly 480 // excepting debug info instructions), and if the block ends with an 481 // unconditional branch, try to move it to the successor block. 482 BBI = &SI; 483 do { 484 ++BBI; 485 } while (isa<DbgInfoIntrinsic>(BBI) || 486 (isa<BitCastInst>(BBI) && isa<PointerType>(BBI->getType()))); 487 if (BranchInst *BI = dyn_cast<BranchInst>(BBI)) 488 if (BI->isUnconditional()) 489 if (SimplifyStoreAtEndOfBlock(SI)) 490 return 0; // xform done! 491 492 return 0; 493} 494 495/// SimplifyStoreAtEndOfBlock - Turn things like: 496/// if () { *P = v1; } else { *P = v2 } 497/// into a phi node with a store in the successor. 498/// 499/// Simplify things like: 500/// *P = v1; if () { *P = v2; } 501/// into a phi node with a store in the successor. 502/// 503bool InstCombiner::SimplifyStoreAtEndOfBlock(StoreInst &SI) { 504 BasicBlock *StoreBB = SI.getParent(); 505 506 // Check to see if the successor block has exactly two incoming edges. If 507 // so, see if the other predecessor contains a store to the same location. 508 // if so, insert a PHI node (if needed) and move the stores down. 509 BasicBlock *DestBB = StoreBB->getTerminator()->getSuccessor(0); 510 511 // Determine whether Dest has exactly two predecessors and, if so, compute 512 // the other predecessor. 513 pred_iterator PI = pred_begin(DestBB); 514 BasicBlock *OtherBB = 0; 515 if (*PI != StoreBB) 516 OtherBB = *PI; 517 ++PI; 518 if (PI == pred_end(DestBB)) 519 return false; 520 521 if (*PI != StoreBB) { 522 if (OtherBB) 523 return false; 524 OtherBB = *PI; 525 } 526 if (++PI != pred_end(DestBB)) 527 return false; 528 529 // Bail out if all the relevant blocks aren't distinct (this can happen, 530 // for example, if SI is in an infinite loop) 531 if (StoreBB == DestBB || OtherBB == DestBB) 532 return false; 533 534 // Verify that the other block ends in a branch and is not otherwise empty. 535 BasicBlock::iterator BBI = OtherBB->getTerminator(); 536 BranchInst *OtherBr = dyn_cast<BranchInst>(BBI); 537 if (!OtherBr || BBI == OtherBB->begin()) 538 return false; 539 540 // If the other block ends in an unconditional branch, check for the 'if then 541 // else' case. there is an instruction before the branch. 542 StoreInst *OtherStore = 0; 543 if (OtherBr->isUnconditional()) { 544 --BBI; 545 // Skip over debugging info. 546 while (isa<DbgInfoIntrinsic>(BBI) || 547 (isa<BitCastInst>(BBI) && isa<PointerType>(BBI->getType()))) { 548 if (BBI==OtherBB->begin()) 549 return false; 550 --BBI; 551 } 552 // If this isn't a store, isn't a store to the same location, or if the 553 // alignments differ, bail out. 554 OtherStore = dyn_cast<StoreInst>(BBI); 555 if (!OtherStore || OtherStore->getOperand(1) != SI.getOperand(1) || 556 OtherStore->getAlignment() != SI.getAlignment()) 557 return false; 558 } else { 559 // Otherwise, the other block ended with a conditional branch. If one of the 560 // destinations is StoreBB, then we have the if/then case. 561 if (OtherBr->getSuccessor(0) != StoreBB && 562 OtherBr->getSuccessor(1) != StoreBB) 563 return false; 564 565 // Okay, we know that OtherBr now goes to Dest and StoreBB, so this is an 566 // if/then triangle. See if there is a store to the same ptr as SI that 567 // lives in OtherBB. 568 for (;; --BBI) { 569 // Check to see if we find the matching store. 570 if ((OtherStore = dyn_cast<StoreInst>(BBI))) { 571 if (OtherStore->getOperand(1) != SI.getOperand(1) || 572 OtherStore->getAlignment() != SI.getAlignment()) 573 return false; 574 break; 575 } 576 // If we find something that may be using or overwriting the stored 577 // value, or if we run out of instructions, we can't do the xform. 578 if (BBI->mayReadFromMemory() || BBI->mayWriteToMemory() || 579 BBI == OtherBB->begin()) 580 return false; 581 } 582 583 // In order to eliminate the store in OtherBr, we have to 584 // make sure nothing reads or overwrites the stored value in 585 // StoreBB. 586 for (BasicBlock::iterator I = StoreBB->begin(); &*I != &SI; ++I) { 587 // FIXME: This should really be AA driven. 588 if (I->mayReadFromMemory() || I->mayWriteToMemory()) 589 return false; 590 } 591 } 592 593 // Insert a PHI node now if we need it. 594 Value *MergedVal = OtherStore->getOperand(0); 595 if (MergedVal != SI.getOperand(0)) { 596 PHINode *PN = PHINode::Create(MergedVal->getType(), "storemerge"); 597 PN->reserveOperandSpace(2); 598 PN->addIncoming(SI.getOperand(0), SI.getParent()); 599 PN->addIncoming(OtherStore->getOperand(0), OtherBB); 600 MergedVal = InsertNewInstBefore(PN, DestBB->front()); 601 } 602 603 // Advance to a place where it is safe to insert the new store and 604 // insert it. 605 BBI = DestBB->getFirstNonPHI(); 606 InsertNewInstBefore(new StoreInst(MergedVal, SI.getOperand(1), 607 OtherStore->isVolatile(), 608 SI.getAlignment()), *BBI); 609 610 // Nuke the old stores. 611 EraseInstFromFunction(SI); 612 EraseInstFromFunction(*OtherStore); 613 return true; 614} 615