Loads.cpp revision ebe69fe11e48d322045d5949c83283927a0d790b
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/Analysis/ValueTracking.h" 17#include "llvm/IR/DataLayout.h" 18#include "llvm/IR/GlobalAlias.h" 19#include "llvm/IR/GlobalVariable.h" 20#include "llvm/IR/IntrinsicInst.h" 21#include "llvm/IR/LLVMContext.h" 22#include "llvm/IR/Operator.h" 23using namespace llvm; 24 25/// \brief Test if A and B will obviously have the same value. 26/// 27/// This includes recognizing that %t0 and %t1 will have the same 28/// value in code like this: 29/// \code 30/// %t0 = getelementptr \@a, 0, 3 31/// store i32 0, i32* %t0 32/// %t1 = getelementptr \@a, 0, 3 33/// %t2 = load i32* %t1 34/// \endcode 35/// 36static bool AreEquivalentAddressValues(const Value *A, const Value *B) { 37 // Test if the values are trivially equivalent. 38 if (A == B) 39 return true; 40 41 // Test if the values come from identical arithmetic instructions. 42 // Use isIdenticalToWhenDefined instead of isIdenticalTo because 43 // this function is only used when one address use dominates the 44 // other, which means that they'll always either have the same 45 // value or one of them will have an undefined value. 46 if (isa<BinaryOperator>(A) || isa<CastInst>(A) || isa<PHINode>(A) || 47 isa<GetElementPtrInst>(A)) 48 if (const Instruction *BI = dyn_cast<Instruction>(B)) 49 if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) 50 return true; 51 52 // Otherwise they may not be equivalent. 53 return false; 54} 55 56/// \brief Check if executing a load of this pointer value cannot trap. 57/// 58/// If it is not obviously safe to load from the specified pointer, we do 59/// a quick local scan of the basic block containing \c ScanFrom, to determine 60/// if the address is already accessed. 61/// 62/// This uses the pointee type to determine how many bytes need to be safe to 63/// load from the pointer. 64bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom, 65 unsigned Align, const DataLayout *DL) { 66 int64_t ByteOffset = 0; 67 Value *Base = V; 68 Base = GetPointerBaseWithConstantOffset(V, ByteOffset, DL); 69 70 if (ByteOffset < 0) // out of bounds 71 return false; 72 73 Type *BaseType = nullptr; 74 unsigned BaseAlign = 0; 75 if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) { 76 // An alloca is safe to load from as load as it is suitably aligned. 77 BaseType = AI->getAllocatedType(); 78 BaseAlign = AI->getAlignment(); 79 } else if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) { 80 // Global variables are not necessarily safe to load from if they are 81 // overridden. Their size may change or they may be weak and require a test 82 // to determine if they were in fact provided. 83 if (!GV->mayBeOverridden()) { 84 BaseType = GV->getType()->getElementType(); 85 BaseAlign = GV->getAlignment(); 86 } 87 } 88 89 PointerType *AddrTy = cast<PointerType>(V->getType()); 90 uint64_t LoadSize = DL ? DL->getTypeStoreSize(AddrTy->getElementType()) : 0; 91 92 // If we found a base allocated type from either an alloca or global variable, 93 // try to see if we are definitively within the allocated region. We need to 94 // know the size of the base type and the loaded type to do anything in this 95 // case, so only try this when we have the DataLayout available. 96 if (BaseType && BaseType->isSized() && DL) { 97 if (BaseAlign == 0) 98 BaseAlign = DL->getPrefTypeAlignment(BaseType); 99 100 if (Align <= BaseAlign) { 101 // Check if the load is within the bounds of the underlying object. 102 if (ByteOffset + LoadSize <= DL->getTypeAllocSize(BaseType) && 103 (Align == 0 || (ByteOffset % Align) == 0)) 104 return true; 105 } 106 } 107 108 // Otherwise, be a little bit aggressive by scanning the local block where we 109 // want to check to see if the pointer is already being loaded or stored 110 // from/to. If so, the previous load or store would have already trapped, 111 // so there is no harm doing an extra load (also, CSE will later eliminate 112 // the load entirely). 113 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin(); 114 115 // We can at least always strip pointer casts even though we can't use the 116 // base here. 117 V = V->stripPointerCasts(); 118 119 while (BBI != E) { 120 --BBI; 121 122 // If we see a free or a call which may write to memory (i.e. which might do 123 // a free) the pointer could be marked invalid. 124 if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() && 125 !isa<DbgInfoIntrinsic>(BBI)) 126 return false; 127 128 Value *AccessedPtr; 129 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) 130 AccessedPtr = LI->getPointerOperand(); 131 else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) 132 AccessedPtr = SI->getPointerOperand(); 133 else 134 continue; 135 136 // Handle trivial cases even w/o DataLayout or other work. 137 if (AccessedPtr == V) 138 return true; 139 140 if (!DL) 141 continue; 142 143 auto *AccessedTy = cast<PointerType>(AccessedPtr->getType()); 144 if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) && 145 LoadSize <= DL->getTypeStoreSize(AccessedTy->getElementType())) 146 return true; 147 } 148 return false; 149} 150 151/// \brief Scan the ScanBB block backwards to see if we have the value at the 152/// memory address *Ptr locally available within a small number of instructions. 153/// 154/// The scan starts from \c ScanFrom. \c MaxInstsToScan specifies the maximum 155/// instructions to scan in the block. If it is set to \c 0, it will scan the whole 156/// block. 157/// 158/// If the value is available, this function returns it. If not, it returns the 159/// iterator for the last validated instruction that the value would be live 160/// through. If we scanned the entire block and didn't find something that 161/// invalidates \c *Ptr or provides it, \c ScanFrom is left at the last 162/// instruction processed and this returns null. 163/// 164/// You can also optionally specify an alias analysis implementation, which 165/// makes this more precise. 166/// 167/// If \c AATags is non-null and a load or store is found, the AA tags from the 168/// load or store are recorded there. If there are no AA tags or if no access is 169/// found, it is left unmodified. 170Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 171 BasicBlock::iterator &ScanFrom, 172 unsigned MaxInstsToScan, 173 AliasAnalysis *AA, AAMDNodes *AATags) { 174 if (MaxInstsToScan == 0) 175 MaxInstsToScan = ~0U; 176 177 Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 178 179 // Try to get the DataLayout for this module. This may be null, in which case 180 // the optimizations will be limited. 181 const DataLayout *DL = ScanBB->getDataLayout(); 182 183 // Try to get the store size for the type. 184 uint64_t AccessSize = DL ? DL->getTypeStoreSize(AccessTy) 185 : AA ? AA->getTypeStoreSize(AccessTy) : 0; 186 187 Value *StrippedPtr = Ptr->stripPointerCasts(); 188 189 while (ScanFrom != ScanBB->begin()) { 190 // We must ignore debug info directives when counting (otherwise they 191 // would affect codegen). 192 Instruction *Inst = --ScanFrom; 193 if (isa<DbgInfoIntrinsic>(Inst)) 194 continue; 195 196 // Restore ScanFrom to expected value in case next test succeeds 197 ScanFrom++; 198 199 // Don't scan huge blocks. 200 if (MaxInstsToScan-- == 0) 201 return nullptr; 202 203 --ScanFrom; 204 // If this is a load of Ptr, the loaded value is available. 205 // (This is true even if the load is volatile or atomic, although 206 // those cases are unlikely.) 207 if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 208 if (AreEquivalentAddressValues( 209 LI->getPointerOperand()->stripPointerCasts(), StrippedPtr) && 210 CastInst::isBitOrNoopPointerCastable(LI->getType(), AccessTy, DL)) { 211 if (AATags) 212 LI->getAAMetadata(*AATags); 213 return LI; 214 } 215 216 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 217 Value *StorePtr = SI->getPointerOperand()->stripPointerCasts(); 218 // If this is a store through Ptr, the value is available! 219 // (This is true even if the store is volatile or atomic, although 220 // those cases are unlikely.) 221 if (AreEquivalentAddressValues(StorePtr, StrippedPtr) && 222 CastInst::isBitOrNoopPointerCastable(SI->getValueOperand()->getType(), 223 AccessTy, DL)) { 224 if (AATags) 225 SI->getAAMetadata(*AATags); 226 return SI->getOperand(0); 227 } 228 229 // If both StrippedPtr and StorePtr reach all the way to an alloca or 230 // global and they are different, ignore the store. This is a trivial form 231 // of alias analysis that is important for reg2mem'd code. 232 if ((isa<AllocaInst>(StrippedPtr) || isa<GlobalVariable>(StrippedPtr)) && 233 (isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)) && 234 StrippedPtr != StorePtr) 235 continue; 236 237 // If we have alias analysis and it says the store won't modify the loaded 238 // value, ignore the store. 239 if (AA && 240 (AA->getModRefInfo(SI, StrippedPtr, AccessSize) & 241 AliasAnalysis::Mod) == 0) 242 continue; 243 244 // Otherwise the store that may or may not alias the pointer, bail out. 245 ++ScanFrom; 246 return nullptr; 247 } 248 249 // If this is some other instruction that may clobber Ptr, bail out. 250 if (Inst->mayWriteToMemory()) { 251 // If alias analysis claims that it really won't modify the load, 252 // ignore it. 253 if (AA && 254 (AA->getModRefInfo(Inst, StrippedPtr, AccessSize) & 255 AliasAnalysis::Mod) == 0) 256 continue; 257 258 // May modify the pointer, bail out. 259 ++ScanFrom; 260 return nullptr; 261 } 262 } 263 264 // Got to the start of the block, we didn't find it, but are done for this 265 // block. 266 return nullptr; 267} 268