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/LLVMContext.h" 21 #include "llvm/Operator.h" 22 using namespace llvm; 23 24 /// AreEquivalentAddressValues - Test if A and B will obviously have the same 25 /// value. This includes recognizing that %t0 and %t1 will have the same 26 /// value in code like this: 27 /// %t0 = getelementptr \@a, 0, 3 28 /// store i32 0, i32* %t0 29 /// %t1 = getelementptr \@a, 0, 3 30 /// %t2 = load i32* %t1 31 /// 32 static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 33 // Test if the values are trivially equivalent. 34 if (A == B) return true; 35 36 // Test if the values come from identical arithmetic instructions. 37 // Use isIdenticalToWhenDefined instead of isIdenticalTo because 38 // this function is only used when one address use dominates the 39 // other, which means that they'll always either have the same 40 // value or one of them will have an undefined value. 41 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 42 isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 43 if (const Instruction *BI = dyn_cast<Instruction>(B)) 44 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) 45 return true; 46 47 // Otherwise they may not be equivalent. 48 return false; 49 } 50 51 /// getUnderlyingObjectWithOffset - Strip off up to MaxLookup GEPs and 52 /// bitcasts to get back to the underlying object being addressed, keeping 53 /// track of the offset in bytes from the GEPs relative to the result. 54 /// This is closely related to GetUnderlyingObject but is located 55 /// here to avoid making VMCore depend on TargetData. 56 static Value *getUnderlyingObjectWithOffset(Value *V, const TargetData *TD, 57 uint64_t &ByteOffset, 58 unsigned MaxLookup = 6) { 59 if (!V->getType()->isPointerTy()) 60 return V; 61 for (unsigned Count = 0; MaxLookup == 0 || Count < MaxLookup; ++Count) { 62 if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) { 63 if (!GEP->hasAllConstantIndices()) 64 return V; 65 SmallVector<Value*, 8> Indices(GEP->op_begin() + 1, GEP->op_end()); 66 ByteOffset += TD->getIndexedOffset(GEP->getPointerOperandType(), 67 Indices); 68 V = GEP->getPointerOperand(); 69 } else if (Operator::getOpcode(V) == Instruction::BitCast) { 70 V = cast<Operator>(V)->getOperand(0); 71 } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) { 72 if (GA->mayBeOverridden()) 73 return V; 74 V = GA->getAliasee(); 75 } else { 76 return V; 77 } 78 assert(V->getType()->isPointerTy() && "Unexpected operand type!"); 79 } 80 return V; 81 } 82 83 /// isSafeToLoadUnconditionally - Return true if we know that executing a load 84 /// from this value cannot trap. If it is not obviously safe to load from the 85 /// specified pointer, we do a quick local scan of the basic block containing 86 /// ScanFrom, to determine if the address is already accessed. 87 bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom, 88 unsigned Align, const TargetData *TD) { 89 uint64_t ByteOffset = 0; 90 Value *Base = V; 91 if (TD) 92 Base = getUnderlyingObjectWithOffset(V, TD, ByteOffset); 93 94 Type *BaseType = 0; 95 unsigned BaseAlign = 0; 96 if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) { 97 // An alloca is safe to load from as load as it is suitably aligned. 98 BaseType = AI->getAllocatedType(); 99 BaseAlign = AI->getAlignment(); 100 } else if (const GlobalValue *GV = dyn_cast<GlobalValue>(Base)) { 101 // Global variables are safe to load from but their size cannot be 102 // guaranteed if they are overridden. 103 if (!isa<GlobalAlias>(GV) && !GV->mayBeOverridden()) { 104 BaseType = GV->getType()->getElementType(); 105 BaseAlign = GV->getAlignment(); 106 } 107 } 108 109 if (BaseType && BaseType->isSized()) { 110 if (TD && BaseAlign == 0) 111 BaseAlign = TD->getPrefTypeAlignment(BaseType); 112 113 if (Align <= BaseAlign) { 114 if (!TD) 115 return true; // Loading directly from an alloca or global is OK. 116 117 // Check if the load is within the bounds of the underlying object. 118 PointerType *AddrTy = cast<PointerType>(V->getType()); 119 uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType()); 120 if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) && 121 (Align == 0 || (ByteOffset % Align) == 0)) 122 return true; 123 } 124 } 125 126 // Otherwise, be a little bit aggressive by scanning the local block where we 127 // want to check to see if the pointer is already being loaded or stored 128 // from/to. If so, the previous load or store would have already trapped, 129 // so there is no harm doing an extra load (also, CSE will later eliminate 130 // the load entirely). 131 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin(); 132 133 while (BBI != E) { 134 --BBI; 135 136 // If we see a free or a call which may write to memory (i.e. which might do 137 // a free) the pointer could be marked invalid. 138 if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() && 139 !isa<DbgInfoIntrinsic>(BBI)) 140 return false; 141 142 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { 143 if (AreEquivalentAddressValues(LI->getOperand(0), V)) return true; 144 } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) { 145 if (AreEquivalentAddressValues(SI->getOperand(1), V)) return true; 146 } 147 } 148 return false; 149 } 150 151 /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 152 /// instruction before ScanFrom) checking to see if we have the value at the 153 /// memory address *Ptr locally available within a small number of instructions. 154 /// If the value is available, return it. 155 /// 156 /// If not, return the iterator for the last validated instruction that the 157 /// value would be live through. If we scanned the entire block and didn't find 158 /// something that invalidates *Ptr or provides it, ScanFrom would be left at 159 /// begin() and this returns null. ScanFrom could also be left 160 /// 161 /// MaxInstsToScan specifies the maximum instructions to scan in the block. If 162 /// it is set to 0, it will scan the whole block. You can also optionally 163 /// specify an alias analysis implementation, which makes this more precise. 164 /// 165 /// If TBAATag is non-null and a load or store is found, the TBAA tag from the 166 /// load or store is recorded there. If there is no TBAA tag or if no access 167 /// is found, it is left unmodified. 168 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 169 BasicBlock::iterator &ScanFrom, 170 unsigned MaxInstsToScan, 171 AliasAnalysis *AA, 172 MDNode **TBAATag) { 173 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 174 175 // If we're using alias analysis to disambiguate get the size of *Ptr. 176 uint64_t AccessSize = 0; 177 if (AA) { 178 Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 179 AccessSize = AA->getTypeStoreSize(AccessTy); 180 } 181 182 while (ScanFrom != ScanBB->begin()) { 183 // We must ignore debug info directives when counting (otherwise they 184 // would affect codegen). 185 Instruction *Inst = --ScanFrom; 186 if (isa<DbgInfoIntrinsic>(Inst)) 187 continue; 188 189 // Restore ScanFrom to expected value in case next test succeeds 190 ScanFrom++; 191 192 // Don't scan huge blocks. 193 if (MaxInstsToScan-- == 0) return 0; 194 195 --ScanFrom; 196 // If this is a load of Ptr, the loaded value is available. 197 // (This is true even if the load is volatile or atomic, although 198 // those cases are unlikely.) 199 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 200 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) { 201 if (TBAATag) *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa); 202 return LI; 203 } 204 205 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 206 // If this is a store through Ptr, the value is available! 207 // (This is true even if the store is volatile or atomic, although 208 // those cases are unlikely.) 209 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) { 210 if (TBAATag) *TBAATag = SI->getMetadata(LLVMContext::MD_tbaa); 211 return SI->getOperand(0); 212 } 213 214 // If Ptr is an alloca and this is a store to a different alloca, ignore 215 // the store. This is a trivial form of alias analysis that is important 216 // for reg2mem'd code. 217 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 218 (isa<AllocaInst>(SI->getOperand(1)) || 219 isa<GlobalVariable>(SI->getOperand(1)))) 220 continue; 221 222 // If we have alias analysis and it says the store won't modify the loaded 223 // value, ignore the store. 224 if (AA && 225 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 226 continue; 227 228 // Otherwise the store that may or may not alias the pointer, bail out. 229 ++ScanFrom; 230 return 0; 231 } 232 233 // If this is some other instruction that may clobber Ptr, bail out. 234 if (Inst->mayWriteToMemory()) { 235 // If alias analysis claims that it really won't modify the load, 236 // ignore it. 237 if (AA && 238 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 239 continue; 240 241 // May modify the pointer, bail out. 242 ++ScanFrom; 243 return 0; 244 } 245 } 246 247 // Got to the start of the block, we didn't find it, but are done for this 248 // block. 249 return 0; 250 } 251
