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" 20 #include "llvm/Operator.h" 21 using namespace llvm; 22 23 /// AreEquivalentAddressValues - Test if A and B will obviously have the same 24 /// value. This includes recognizing that %t0 and %t1 will have the same 25 /// value in code like this: 26 /// %t0 = getelementptr \@a, 0, 3 27 /// store i32 0, i32* %t0 28 /// %t1 = getelementptr \@a, 0, 3 29 /// %t2 = load i32* %t1 30 /// 31 static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 32 // Test if the values are trivially equivalent. 33 if (A == B) return true; 34 35 // Test if the values come from identical arithmetic instructions. 36 // Use isIdenticalToWhenDefined instead of isIdenticalTo because 37 // this function is only used when one address use dominates the 38 // other, which means that they'll always either have the same 39 // value or one of them will have an undefined value. 40 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 41 isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 42 if (const Instruction *BI = dyn_cast<Instruction>(B)) 43 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) 44 return true; 45 46 // Otherwise they may not be equivalent. 47 return false; 48 } 49 50 /// getUnderlyingObjectWithOffset - Strip off up to MaxLookup GEPs and 51 /// bitcasts to get back to the underlying object being addressed, keeping 52 /// track of the offset in bytes from the GEPs relative to the result. 53 /// This is closely related to GetUnderlyingObject but is located 54 /// here to avoid making VMCore depend on TargetData. 55 static Value *getUnderlyingObjectWithOffset(Value *V, const TargetData *TD, 56 uint64_t &ByteOffset, 57 unsigned MaxLookup = 6) { 58 if (!V->getType()->isPointerTy()) 59 return V; 60 for (unsigned Count = 0; MaxLookup == 0 || Count < MaxLookup; ++Count) { 61 if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) { 62 if (!GEP->hasAllConstantIndices()) 63 return V; 64 SmallVector<Value*, 8> Indices(GEP->op_begin() + 1, GEP->op_end()); 65 ByteOffset += TD->getIndexedOffset(GEP->getPointerOperandType(), 66 Indices); 67 V = GEP->getPointerOperand(); 68 } else if (Operator::getOpcode(V) == Instruction::BitCast) { 69 V = cast<Operator>(V)->getOperand(0); 70 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) { 71 if (GA->mayBeOverridden()) 72 return V; 73 V = GA->getAliasee(); 74 } else { 75 return V; 76 } 77 assert(V->getType()->isPointerTy() && "Unexpected operand type!"); 78 } 79 return V; 80 } 81 82 /// isSafeToLoadUnconditionally - Return true if we know that executing a load 83 /// from this value cannot trap. If it is not obviously safe to load from the 84 /// specified pointer, we do a quick local scan of the basic block containing 85 /// ScanFrom, to determine if the address is already accessed. 86 bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom, 87 unsigned Align, const TargetData *TD) { 88 uint64_t ByteOffset = 0; 89 Value *Base = V; 90 if (TD) 91 Base = getUnderlyingObjectWithOffset(V, TD, ByteOffset); 92 93 Type *BaseType = 0; 94 unsigned BaseAlign = 0; 95 if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) { 96 // An alloca is safe to load from as load as it is suitably aligned. 97 BaseType = AI->getAllocatedType(); 98 BaseAlign = AI->getAlignment(); 99 } else if (const GlobalValue *GV = dyn_cast<GlobalValue>(Base)) { 100 // Global variables are safe to load from but their size cannot be 101 // guaranteed if they are overridden. 102 if (!isa<GlobalAlias>(GV) && !GV->mayBeOverridden()) { 103 BaseType = GV->getType()->getElementType(); 104 BaseAlign = GV->getAlignment(); 105 } 106 } 107 108 if (BaseType && BaseType->isSized()) { 109 if (TD && BaseAlign == 0) 110 BaseAlign = TD->getPrefTypeAlignment(BaseType); 111 112 if (Align <= BaseAlign) { 113 if (!TD) 114 return true; // Loading directly from an alloca or global is OK. 115 116 // Check if the load is within the bounds of the underlying object. 117 PointerType *AddrTy = cast<PointerType>(V->getType()); 118 uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType()); 119 if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) && 120 (Align == 0 || (ByteOffset % Align) == 0)) 121 return true; 122 } 123 } 124 125 // Otherwise, be a little bit aggressive by scanning the local block where we 126 // want to check to see if the pointer is already being loaded or stored 127 // from/to. If so, the previous load or store would have already trapped, 128 // so there is no harm doing an extra load (also, CSE will later eliminate 129 // the load entirely). 130 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin(); 131 132 while (BBI != E) { 133 --BBI; 134 135 // If we see a free or a call which may write to memory (i.e. which might do 136 // a free) the pointer could be marked invalid. 137 if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() && 138 !isa<DbgInfoIntrinsic>(BBI)) 139 return false; 140 141 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { 142 if (AreEquivalentAddressValues(LI->getOperand(0), V)) return true; 143 } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) { 144 if (AreEquivalentAddressValues(SI->getOperand(1), V)) return true; 145 } 146 } 147 return false; 148 } 149 150 /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 151 /// instruction before ScanFrom) checking to see if we have the value at the 152 /// memory address *Ptr locally available within a small number of instructions. 153 /// If the value is available, return it. 154 /// 155 /// If not, return the iterator for the last validated instruction that the 156 /// value would be live through. If we scanned the entire block and didn't find 157 /// something that invalidates *Ptr or provides it, ScanFrom would be left at 158 /// begin() and this returns null. ScanFrom could also be left 159 /// 160 /// MaxInstsToScan specifies the maximum instructions to scan in the block. If 161 /// it is set to 0, it will scan the whole block. You can also optionally 162 /// specify an alias analysis implementation, which makes this more precise. 163 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 164 BasicBlock::iterator &ScanFrom, 165 unsigned MaxInstsToScan, 166 AliasAnalysis *AA) { 167 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 168 169 // If we're using alias analysis to disambiguate get the size of *Ptr. 170 uint64_t AccessSize = 0; 171 if (AA) { 172 Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 173 AccessSize = AA->getTypeStoreSize(AccessTy); 174 } 175 176 while (ScanFrom != ScanBB->begin()) { 177 // We must ignore debug info directives when counting (otherwise they 178 // would affect codegen). 179 Instruction *Inst = --ScanFrom; 180 if (isa<DbgInfoIntrinsic>(Inst)) 181 continue; 182 183 // Restore ScanFrom to expected value in case next test succeeds 184 ScanFrom++; 185 186 // Don't scan huge blocks. 187 if (MaxInstsToScan-- == 0) return 0; 188 189 --ScanFrom; 190 // If this is a load of Ptr, the loaded value is available. 191 // (This is true even if the load is volatile or atomic, although 192 // those cases are unlikely.) 193 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 194 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) 195 return LI; 196 197 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 198 // If this is a store through Ptr, the value is available! 199 // (This is true even if the store is volatile or atomic, although 200 // those cases are unlikely.) 201 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) 202 return SI->getOperand(0); 203 204 // If Ptr is an alloca and this is a store to a different alloca, ignore 205 // the store. This is a trivial form of alias analysis that is important 206 // for reg2mem'd code. 207 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 208 (isa<AllocaInst>(SI->getOperand(1)) || 209 isa<GlobalVariable>(SI->getOperand(1)))) 210 continue; 211 212 // If we have alias analysis and it says the store won't modify the loaded 213 // value, ignore the store. 214 if (AA && 215 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 216 continue; 217 218 // Otherwise the store that may or may not alias the pointer, bail out. 219 ++ScanFrom; 220 return 0; 221 } 222 223 // If this is some other instruction that may clobber Ptr, bail out. 224 if (Inst->mayWriteToMemory()) { 225 // If alias analysis claims that it really won't modify the load, 226 // ignore it. 227 if (AA && 228 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 229 continue; 230 231 // May modify the pointer, bail out. 232 ++ScanFrom; 233 return 0; 234 } 235 } 236 237 // Got to the start of the block, we didn't find it, but are done for this 238 // block. 239 return 0; 240 } 241
