1210006Srdivacky//===- Loads.cpp - Local load analysis ------------------------------------===// 2210006Srdivacky// 3210006Srdivacky// The LLVM Compiler Infrastructure 4210006Srdivacky// 5210006Srdivacky// This file is distributed under the University of Illinois Open Source 6210006Srdivacky// License. See LICENSE.TXT for details. 7210006Srdivacky// 8210006Srdivacky//===----------------------------------------------------------------------===// 9210006Srdivacky// 10210006Srdivacky// This file defines simple local analyses for load instructions. 11210006Srdivacky// 12210006Srdivacky//===----------------------------------------------------------------------===// 13210006Srdivacky 14210006Srdivacky#include "llvm/Analysis/Loads.h" 15210006Srdivacky#include "llvm/Analysis/AliasAnalysis.h" 16249423Sdim#include "llvm/Analysis/ValueTracking.h" 17249423Sdim#include "llvm/IR/DataLayout.h" 18249423Sdim#include "llvm/IR/GlobalAlias.h" 19249423Sdim#include "llvm/IR/GlobalVariable.h" 20249423Sdim#include "llvm/IR/IntrinsicInst.h" 21249423Sdim#include "llvm/IR/LLVMContext.h" 22249423Sdim#include "llvm/IR/Operator.h" 23210006Srdivackyusing namespace llvm; 24210006Srdivacky 25210006Srdivacky/// AreEquivalentAddressValues - Test if A and B will obviously have the same 26210006Srdivacky/// value. This includes recognizing that %t0 and %t1 will have the same 27210006Srdivacky/// value in code like this: 28210006Srdivacky/// %t0 = getelementptr \@a, 0, 3 29210006Srdivacky/// store i32 0, i32* %t0 30210006Srdivacky/// %t1 = getelementptr \@a, 0, 3 31210006Srdivacky/// %t2 = load i32* %t1 32210006Srdivacky/// 33210006Srdivackystatic bool AreEquivalentAddressValues(const Value *A, const Value *B) { 34210006Srdivacky // Test if the values are trivially equivalent. 35210006Srdivacky if (A == B) return true; 36223017Sdim 37210006Srdivacky // Test if the values come from identical arithmetic instructions. 38210006Srdivacky // Use isIdenticalToWhenDefined instead of isIdenticalTo because 39210006Srdivacky // this function is only used when one address use dominates the 40210006Srdivacky // other, which means that they'll always either have the same 41210006Srdivacky // value or one of them will have an undefined value. 42210006Srdivacky if (isa<BinaryOperator>(A) || isa<CastInst>(A) || 43210006Srdivacky isa<PHINode>(A) || isa<GetElementPtrInst>(A)) 44210006Srdivacky if (const Instruction *BI = dyn_cast<Instruction>(B)) 45210006Srdivacky if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) 46210006Srdivacky return true; 47223017Sdim 48210006Srdivacky // Otherwise they may not be equivalent. 49210006Srdivacky return false; 50210006Srdivacky} 51210006Srdivacky 52210006Srdivacky/// isSafeToLoadUnconditionally - Return true if we know that executing a load 53210006Srdivacky/// from this value cannot trap. If it is not obviously safe to load from the 54210006Srdivacky/// specified pointer, we do a quick local scan of the basic block containing 55210006Srdivacky/// ScanFrom, to determine if the address is already accessed. 56210006Srdivackybool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom, 57243830Sdim unsigned Align, const DataLayout *TD) { 58249423Sdim int64_t ByteOffset = 0; 59210006Srdivacky Value *Base = V; 60249423Sdim Base = GetPointerBaseWithConstantOffset(V, ByteOffset, TD); 61210006Srdivacky 62249423Sdim if (ByteOffset < 0) // out of bounds 63249423Sdim return false; 64249423Sdim 65226633Sdim Type *BaseType = 0; 66210006Srdivacky unsigned BaseAlign = 0; 67210006Srdivacky if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) { 68210006Srdivacky // An alloca is safe to load from as load as it is suitably aligned. 69210006Srdivacky BaseType = AI->getAllocatedType(); 70210006Srdivacky BaseAlign = AI->getAlignment(); 71249423Sdim } else if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) { 72210006Srdivacky // Global variables are safe to load from but their size cannot be 73210006Srdivacky // guaranteed if they are overridden. 74249423Sdim if (!GV->mayBeOverridden()) { 75210006Srdivacky BaseType = GV->getType()->getElementType(); 76210006Srdivacky BaseAlign = GV->getAlignment(); 77210006Srdivacky } 78210006Srdivacky } 79210006Srdivacky 80210006Srdivacky if (BaseType && BaseType->isSized()) { 81210006Srdivacky if (TD && BaseAlign == 0) 82210006Srdivacky BaseAlign = TD->getPrefTypeAlignment(BaseType); 83210006Srdivacky 84210006Srdivacky if (Align <= BaseAlign) { 85210006Srdivacky if (!TD) 86210006Srdivacky return true; // Loading directly from an alloca or global is OK. 87210006Srdivacky 88210006Srdivacky // Check if the load is within the bounds of the underlying object. 89226633Sdim PointerType *AddrTy = cast<PointerType>(V->getType()); 90210006Srdivacky uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType()); 91210006Srdivacky if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) && 92210006Srdivacky (Align == 0 || (ByteOffset % Align) == 0)) 93210006Srdivacky return true; 94210006Srdivacky } 95210006Srdivacky } 96210006Srdivacky 97210006Srdivacky // Otherwise, be a little bit aggressive by scanning the local block where we 98210006Srdivacky // want to check to see if the pointer is already being loaded or stored 99210006Srdivacky // from/to. If so, the previous load or store would have already trapped, 100210006Srdivacky // so there is no harm doing an extra load (also, CSE will later eliminate 101210006Srdivacky // the load entirely). 102210006Srdivacky BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin(); 103210006Srdivacky 104210006Srdivacky while (BBI != E) { 105210006Srdivacky --BBI; 106210006Srdivacky 107210006Srdivacky // If we see a free or a call which may write to memory (i.e. which might do 108210006Srdivacky // a free) the pointer could be marked invalid. 109210006Srdivacky if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() && 110210006Srdivacky !isa<DbgInfoIntrinsic>(BBI)) 111210006Srdivacky return false; 112210006Srdivacky 113210006Srdivacky if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { 114210006Srdivacky if (AreEquivalentAddressValues(LI->getOperand(0), V)) return true; 115210006Srdivacky } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) { 116210006Srdivacky if (AreEquivalentAddressValues(SI->getOperand(1), V)) return true; 117210006Srdivacky } 118210006Srdivacky } 119210006Srdivacky return false; 120210006Srdivacky} 121210006Srdivacky 122210006Srdivacky/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the 123210006Srdivacky/// instruction before ScanFrom) checking to see if we have the value at the 124210006Srdivacky/// memory address *Ptr locally available within a small number of instructions. 125210006Srdivacky/// If the value is available, return it. 126210006Srdivacky/// 127210006Srdivacky/// If not, return the iterator for the last validated instruction that the 128210006Srdivacky/// value would be live through. If we scanned the entire block and didn't find 129210006Srdivacky/// something that invalidates *Ptr or provides it, ScanFrom would be left at 130210006Srdivacky/// begin() and this returns null. ScanFrom could also be left 131210006Srdivacky/// 132210006Srdivacky/// MaxInstsToScan specifies the maximum instructions to scan in the block. If 133210006Srdivacky/// it is set to 0, it will scan the whole block. You can also optionally 134210006Srdivacky/// specify an alias analysis implementation, which makes this more precise. 135234353Sdim/// 136234353Sdim/// If TBAATag is non-null and a load or store is found, the TBAA tag from the 137234353Sdim/// load or store is recorded there. If there is no TBAA tag or if no access 138234353Sdim/// is found, it is left unmodified. 139210006SrdivackyValue *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, 140210006Srdivacky BasicBlock::iterator &ScanFrom, 141210006Srdivacky unsigned MaxInstsToScan, 142234353Sdim AliasAnalysis *AA, 143234353Sdim MDNode **TBAATag) { 144210006Srdivacky if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; 145210006Srdivacky 146210006Srdivacky // If we're using alias analysis to disambiguate get the size of *Ptr. 147218893Sdim uint64_t AccessSize = 0; 148210006Srdivacky if (AA) { 149226633Sdim Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); 150210006Srdivacky AccessSize = AA->getTypeStoreSize(AccessTy); 151210006Srdivacky } 152210006Srdivacky 153210006Srdivacky while (ScanFrom != ScanBB->begin()) { 154210006Srdivacky // We must ignore debug info directives when counting (otherwise they 155210006Srdivacky // would affect codegen). 156210006Srdivacky Instruction *Inst = --ScanFrom; 157210006Srdivacky if (isa<DbgInfoIntrinsic>(Inst)) 158210006Srdivacky continue; 159210006Srdivacky 160210006Srdivacky // Restore ScanFrom to expected value in case next test succeeds 161210006Srdivacky ScanFrom++; 162210006Srdivacky 163210006Srdivacky // Don't scan huge blocks. 164210006Srdivacky if (MaxInstsToScan-- == 0) return 0; 165210006Srdivacky 166210006Srdivacky --ScanFrom; 167210006Srdivacky // If this is a load of Ptr, the loaded value is available. 168226633Sdim // (This is true even if the load is volatile or atomic, although 169226633Sdim // those cases are unlikely.) 170210006Srdivacky if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) 171234353Sdim if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) { 172234353Sdim if (TBAATag) *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa); 173210006Srdivacky return LI; 174234353Sdim } 175210006Srdivacky 176210006Srdivacky if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { 177210006Srdivacky // If this is a store through Ptr, the value is available! 178226633Sdim // (This is true even if the store is volatile or atomic, although 179226633Sdim // those cases are unlikely.) 180234353Sdim if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) { 181234353Sdim if (TBAATag) *TBAATag = SI->getMetadata(LLVMContext::MD_tbaa); 182210006Srdivacky return SI->getOperand(0); 183234353Sdim } 184210006Srdivacky 185210006Srdivacky // If Ptr is an alloca and this is a store to a different alloca, ignore 186210006Srdivacky // the store. This is a trivial form of alias analysis that is important 187210006Srdivacky // for reg2mem'd code. 188210006Srdivacky if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && 189210006Srdivacky (isa<AllocaInst>(SI->getOperand(1)) || 190210006Srdivacky isa<GlobalVariable>(SI->getOperand(1)))) 191210006Srdivacky continue; 192210006Srdivacky 193210006Srdivacky // If we have alias analysis and it says the store won't modify the loaded 194210006Srdivacky // value, ignore the store. 195210006Srdivacky if (AA && 196210006Srdivacky (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 197210006Srdivacky continue; 198210006Srdivacky 199210006Srdivacky // Otherwise the store that may or may not alias the pointer, bail out. 200210006Srdivacky ++ScanFrom; 201210006Srdivacky return 0; 202210006Srdivacky } 203210006Srdivacky 204210006Srdivacky // If this is some other instruction that may clobber Ptr, bail out. 205210006Srdivacky if (Inst->mayWriteToMemory()) { 206210006Srdivacky // If alias analysis claims that it really won't modify the load, 207210006Srdivacky // ignore it. 208210006Srdivacky if (AA && 209210006Srdivacky (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) 210210006Srdivacky continue; 211210006Srdivacky 212210006Srdivacky // May modify the pointer, bail out. 213210006Srdivacky ++ScanFrom; 214210006Srdivacky return 0; 215210006Srdivacky } 216210006Srdivacky } 217210006Srdivacky 218210006Srdivacky // Got to the start of the block, we didn't find it, but are done for this 219210006Srdivacky // block. 220210006Srdivacky return 0; 221210006Srdivacky} 222