Loads.cpp revision 218893
1//===- Loads.cpp - Local load analysis ------------------------------------===// 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 defines simple local analyses for load instructions. 11// 12//===----------------------------------------------------------------------===// 13 14#include "llvm/Analysis/Loads.h" 15#include "llvm/Analysis/AliasAnalysis.h" 16#include "llvm/Target/TargetData.h" 17#include "llvm/GlobalAlias.h" 18#include "llvm/GlobalVariable.h" 19#include "llvm/IntrinsicInst.h" 20using namespace llvm; 21 22/// AreEquivalentAddressValues - Test if A and B will obviously have the same 23/// value. This includes recognizing that %t0 and %t1 will have the same 24/// value in code like this: 25/// %t0 = getelementptr \@a, 0, 3 26/// store i32 0, i32* %t0 27/// %t1 = getelementptr \@a, 0, 3 28/// %t2 = load i32* %t1 29/// 30static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 31 // Test if the values are trivially equivalent. 32 if (A == B) return true; 33 34 // Test if the values come from identical arithmetic instructions. 35 // Use isIdenticalToWhenDefined instead of isIdenticalTo because 36 // this function is only used when one address use dominates the 37 // other, which means that they'll always either have the same 38 // value or one of them will have an undefined value. 39 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 40 isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 41 if (const Instruction *BI = dyn_cast<Instruction>(B)) 42 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) 43 return true; 44 45 // Otherwise they may not be equivalent. 46 return false; 47} 48 49/// getUnderlyingObjectWithOffset - Strip off up to MaxLookup GEPs and 50/// bitcasts to get back to the underlying object being addressed, keeping 51/// track of the offset in bytes from the GEPs relative to the result. 52/// This is closely related to GetUnderlyingObject but is located 53/// here to avoid making VMCore depend on TargetData. 54static Value *getUnderlyingObjectWithOffset(Value *V, const TargetData *TD, 55 uint64_t &ByteOffset, 56 unsigned MaxLookup = 6) { 57 if (!V->getType()->isPointerTy()) 58 return V; 59 for (unsigned Count = 0; MaxLookup == 0 || Count < MaxLookup; ++Count) { 60 if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) { 61 if (!GEP->hasAllConstantIndices()) 62 return V; 63 SmallVector<Value*, 8> Indices(GEP->op_begin() + 1, GEP->op_end()); 64 ByteOffset += TD->getIndexedOffset(GEP->getPointerOperandType(), 65 &Indices[0], Indices.size()); 66 V = GEP->getPointerOperand(); 67 } else if (Operator::getOpcode(V) == Instruction::BitCast) { 68 V = cast<Operator>(V)->getOperand(0); 69 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) { 70 if (GA->mayBeOverridden()) 71 return V; 72 V = GA->getAliasee(); 73 } else { 74 return V; 75 } 76 assert(V->getType()->isPointerTy() && "Unexpected operand type!"); 77 } 78 return V; 79} 80 81/// isSafeToLoadUnconditionally - Return true if we know that executing a load 82/// from this value cannot trap. If it is not obviously safe to load from the 83/// specified pointer, we do a quick local scan of the basic block containing 84/// ScanFrom, to determine if the address is already accessed. 85bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom, 86 unsigned Align, const TargetData *TD) { 87 uint64_t ByteOffset = 0; 88 Value *Base = V; 89 if (TD) 90 Base = getUnderlyingObjectWithOffset(V, TD, ByteOffset); 91 92 const Type *BaseType = 0; 93 unsigned BaseAlign = 0; 94 if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) { 95 // An alloca is safe to load from as load as it is suitably aligned. 96 BaseType = AI->getAllocatedType(); 97 BaseAlign = AI->getAlignment(); 98 } else if (const GlobalValue *GV = dyn_cast<GlobalValue>(Base)) { 99 // Global variables are safe to load from but their size cannot be 100 // guaranteed if they are overridden. 101 if (!isa<GlobalAlias>(GV) && !GV->mayBeOverridden()) { 102 BaseType = GV->getType()->getElementType(); 103 BaseAlign = GV->getAlignment(); 104 } 105 } 106 107 if (BaseType && BaseType->isSized()) { 108 if (TD && BaseAlign == 0) 109 BaseAlign = TD->getPrefTypeAlignment(BaseType); 110 111 if (Align <= BaseAlign) { 112 if (!TD) 113 return true; // Loading directly from an alloca or global is OK. 114 115 // Check if the load is within the bounds of the underlying object. 116 const PointerType *AddrTy = cast<PointerType>(V->getType()); 117 uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType()); 118 if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) && 119 (Align == 0 || (ByteOffset % Align) == 0)) 120 return true; 121 } 122 } 123 124 // Otherwise, be a little bit aggressive by scanning the local block where we 125 // want to check to see if the pointer is already being loaded or stored 126 // from/to. If so, the previous load or store would have already trapped, 127 // so there is no harm doing an extra load (also, CSE will later eliminate 128 // the load entirely). 129 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin(); 130 131 while (BBI != E) { 132 --BBI; 133 134 // If we see a free or a call which may write to memory (i.e. which might do 135 // a free) the pointer could be marked invalid. 136 if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() && 137 !isa<DbgInfoIntrinsic>(BBI)) 138 return false; 139 140 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { 141 if (AreEquivalentAddressValues(LI->getOperand(0), V)) return true; 142 } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) { 143 if (AreEquivalentAddressValues(SI->getOperand(1), V)) return true; 144 } 145 } 146 return false; 147} 148 149/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 150/// instruction before ScanFrom) checking to see if we have the value at the 151/// memory address *Ptr locally available within a small number of instructions. 152/// If the value is available, return it. 153/// 154/// If not, return the iterator for the last validated instruction that the 155/// value would be live through. If we scanned the entire block and didn't find 156/// something that invalidates *Ptr or provides it, ScanFrom would be left at 157/// begin() and this returns null. ScanFrom could also be left 158/// 159/// MaxInstsToScan specifies the maximum instructions to scan in the block. If 160/// it is set to 0, it will scan the whole block. You can also optionally 161/// specify an alias analysis implementation, which makes this more precise. 162Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 163 BasicBlock::iterator &ScanFrom, 164 unsigned MaxInstsToScan, 165 AliasAnalysis *AA) { 166 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 167 168 // If we're using alias analysis to disambiguate get the size of *Ptr. 169 uint64_t AccessSize = 0; 170 if (AA) { 171 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 172 AccessSize = AA->getTypeStoreSize(AccessTy); 173 } 174 175 while (ScanFrom != ScanBB->begin()) { 176 // We must ignore debug info directives when counting (otherwise they 177 // would affect codegen). 178 Instruction *Inst = --ScanFrom; 179 if (isa<DbgInfoIntrinsic>(Inst)) 180 continue; 181 182 // Restore ScanFrom to expected value in case next test succeeds 183 ScanFrom++; 184 185 // Don't scan huge blocks. 186 if (MaxInstsToScan-- == 0) return 0; 187 188 --ScanFrom; 189 // If this is a load of Ptr, the loaded value is available. 190 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 191 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) 192 return LI; 193 194 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 195 // If this is a store through Ptr, the value is available! 196 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) 197 return SI->getOperand(0); 198 199 // If Ptr is an alloca and this is a store to a different alloca, ignore 200 // the store. This is a trivial form of alias analysis that is important 201 // for reg2mem'd code. 202 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 203 (isa<AllocaInst>(SI->getOperand(1)) || 204 isa<GlobalVariable>(SI->getOperand(1)))) 205 continue; 206 207 // If we have alias analysis and it says the store won't modify the loaded 208 // value, ignore the store. 209 if (AA && 210 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 211 continue; 212 213 // Otherwise the store that may or may not alias the pointer, bail out. 214 ++ScanFrom; 215 return 0; 216 } 217 218 // If this is some other instruction that may clobber Ptr, bail out. 219 if (Inst->mayWriteToMemory()) { 220 // If alias analysis claims that it really won't modify the load, 221 // ignore it. 222 if (AA && 223 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 224 continue; 225 226 // May modify the pointer, bail out. 227 ++ScanFrom; 228 return 0; 229 } 230 } 231 232 // Got to the start of the block, we didn't find it, but are done for this 233 // block. 234 return 0; 235} 236