| //===- Loads.cpp - Local load analysis ------------------------------------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file defines simple local analyses for load instructions. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Analysis/Loads.h" |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Target/TargetData.h" |
| #include "llvm/GlobalAlias.h" |
| #include "llvm/GlobalVariable.h" |
| #include "llvm/IntrinsicInst.h" |
| #include "llvm/Operator.h" |
| using namespace llvm; |
| |
| /// AreEquivalentAddressValues - Test if A and B will obviously have the same |
| /// value. This includes recognizing that %t0 and %t1 will have the same |
| /// value in code like this: |
| /// %t0 = getelementptr \@a, 0, 3 |
| /// store i32 0, i32* %t0 |
| /// %t1 = getelementptr \@a, 0, 3 |
| /// %t2 = load i32* %t1 |
| /// |
| static bool AreEquivalentAddressValues(const Value *A, const Value *B) { |
| // Test if the values are trivially equivalent. |
| if (A == B) return true; |
| |
| // Test if the values come from identical arithmetic instructions. |
| // Use isIdenticalToWhenDefined instead of isIdenticalTo because |
| // this function is only used when one address use dominates the |
| // other, which means that they'll always either have the same |
| // value or one of them will have an undefined value. |
| if (isa<BinaryOperator>(A) || isa<CastInst>(A) || |
| isa<PHINode>(A) || isa<GetElementPtrInst>(A)) |
| if (const Instruction *BI = dyn_cast<Instruction>(B)) |
| if (cast<Instruction>(A)->isIdenticalToWhenDefined(BI)) |
| return true; |
| |
| // Otherwise they may not be equivalent. |
| return false; |
| } |
| |
| /// getUnderlyingObjectWithOffset - Strip off up to MaxLookup GEPs and |
| /// bitcasts to get back to the underlying object being addressed, keeping |
| /// track of the offset in bytes from the GEPs relative to the result. |
| /// This is closely related to GetUnderlyingObject but is located |
| /// here to avoid making VMCore depend on TargetData. |
| static Value *getUnderlyingObjectWithOffset(Value *V, const TargetData *TD, |
| uint64_t &ByteOffset, |
| unsigned MaxLookup = 6) { |
| if (!V->getType()->isPointerTy()) |
| return V; |
| for (unsigned Count = 0; MaxLookup == 0 || Count < MaxLookup; ++Count) { |
| if (GEPOperator *GEP = dyn_cast<GEPOperator>(V)) { |
| if (!GEP->hasAllConstantIndices()) |
| return V; |
| SmallVector<Value*, 8> Indices(GEP->op_begin() + 1, GEP->op_end()); |
| ByteOffset += TD->getIndexedOffset(GEP->getPointerOperandType(), |
| Indices); |
| V = GEP->getPointerOperand(); |
| } else if (Operator::getOpcode(V) == Instruction::BitCast) { |
| V = cast<Operator>(V)->getOperand(0); |
| } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) { |
| if (GA->mayBeOverridden()) |
| return V; |
| V = GA->getAliasee(); |
| } else { |
| return V; |
| } |
| assert(V->getType()->isPointerTy() && "Unexpected operand type!"); |
| } |
| return V; |
| } |
| |
| /// isSafeToLoadUnconditionally - Return true if we know that executing a load |
| /// from this value cannot trap. If it is not obviously safe to load from the |
| /// specified pointer, we do a quick local scan of the basic block containing |
| /// ScanFrom, to determine if the address is already accessed. |
| bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom, |
| unsigned Align, const TargetData *TD) { |
| uint64_t ByteOffset = 0; |
| Value *Base = V; |
| if (TD) |
| Base = getUnderlyingObjectWithOffset(V, TD, ByteOffset); |
| |
| Type *BaseType = 0; |
| unsigned BaseAlign = 0; |
| if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) { |
| // An alloca is safe to load from as load as it is suitably aligned. |
| BaseType = AI->getAllocatedType(); |
| BaseAlign = AI->getAlignment(); |
| } else if (const GlobalValue *GV = dyn_cast<GlobalValue>(Base)) { |
| // Global variables are safe to load from but their size cannot be |
| // guaranteed if they are overridden. |
| if (!isa<GlobalAlias>(GV) && !GV->mayBeOverridden()) { |
| BaseType = GV->getType()->getElementType(); |
| BaseAlign = GV->getAlignment(); |
| } |
| } |
| |
| if (BaseType && BaseType->isSized()) { |
| if (TD && BaseAlign == 0) |
| BaseAlign = TD->getPrefTypeAlignment(BaseType); |
| |
| if (Align <= BaseAlign) { |
| if (!TD) |
| return true; // Loading directly from an alloca or global is OK. |
| |
| // Check if the load is within the bounds of the underlying object. |
| PointerType *AddrTy = cast<PointerType>(V->getType()); |
| uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType()); |
| if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) && |
| (Align == 0 || (ByteOffset % Align) == 0)) |
| return true; |
| } |
| } |
| |
| // Otherwise, be a little bit aggressive by scanning the local block where we |
| // want to check to see if the pointer is already being loaded or stored |
| // from/to. If so, the previous load or store would have already trapped, |
| // so there is no harm doing an extra load (also, CSE will later eliminate |
| // the load entirely). |
| BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin(); |
| |
| while (BBI != E) { |
| --BBI; |
| |
| // If we see a free or a call which may write to memory (i.e. which might do |
| // a free) the pointer could be marked invalid. |
| if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() && |
| !isa<DbgInfoIntrinsic>(BBI)) |
| return false; |
| |
| if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { |
| if (AreEquivalentAddressValues(LI->getOperand(0), V)) return true; |
| } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) { |
| if (AreEquivalentAddressValues(SI->getOperand(1), V)) return true; |
| } |
| } |
| return false; |
| } |
| |
| /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the |
| /// instruction before ScanFrom) checking to see if we have the value at the |
| /// memory address *Ptr locally available within a small number of instructions. |
| /// If the value is available, return it. |
| /// |
| /// If not, return the iterator for the last validated instruction that the |
| /// value would be live through. If we scanned the entire block and didn't find |
| /// something that invalidates *Ptr or provides it, ScanFrom would be left at |
| /// begin() and this returns null. ScanFrom could also be left |
| /// |
| /// MaxInstsToScan specifies the maximum instructions to scan in the block. If |
| /// it is set to 0, it will scan the whole block. You can also optionally |
| /// specify an alias analysis implementation, which makes this more precise. |
| Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, |
| BasicBlock::iterator &ScanFrom, |
| unsigned MaxInstsToScan, |
| AliasAnalysis *AA) { |
| if (MaxInstsToScan == 0) MaxInstsToScan = ~0U; |
| |
| // If we're using alias analysis to disambiguate get the size of *Ptr. |
| uint64_t AccessSize = 0; |
| if (AA) { |
| Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); |
| AccessSize = AA->getTypeStoreSize(AccessTy); |
| } |
| |
| while (ScanFrom != ScanBB->begin()) { |
| // We must ignore debug info directives when counting (otherwise they |
| // would affect codegen). |
| Instruction *Inst = --ScanFrom; |
| if (isa<DbgInfoIntrinsic>(Inst)) |
| continue; |
| |
| // Restore ScanFrom to expected value in case next test succeeds |
| ScanFrom++; |
| |
| // Don't scan huge blocks. |
| if (MaxInstsToScan-- == 0) return 0; |
| |
| --ScanFrom; |
| // If this is a load of Ptr, the loaded value is available. |
| // (This is true even if the load is volatile or atomic, although |
| // those cases are unlikely.) |
| if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) |
| if (AreEquivalentAddressValues(LI->getOperand(0), Ptr)) |
| return LI; |
| |
| if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { |
| // If this is a store through Ptr, the value is available! |
| // (This is true even if the store is volatile or atomic, although |
| // those cases are unlikely.) |
| if (AreEquivalentAddressValues(SI->getOperand(1), Ptr)) |
| return SI->getOperand(0); |
| |
| // If Ptr is an alloca and this is a store to a different alloca, ignore |
| // the store. This is a trivial form of alias analysis that is important |
| // for reg2mem'd code. |
| if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) && |
| (isa<AllocaInst>(SI->getOperand(1)) || |
| isa<GlobalVariable>(SI->getOperand(1)))) |
| continue; |
| |
| // If we have alias analysis and it says the store won't modify the loaded |
| // value, ignore the store. |
| if (AA && |
| (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) |
| continue; |
| |
| // Otherwise the store that may or may not alias the pointer, bail out. |
| ++ScanFrom; |
| return 0; |
| } |
| |
| // If this is some other instruction that may clobber Ptr, bail out. |
| if (Inst->mayWriteToMemory()) { |
| // If alias analysis claims that it really won't modify the load, |
| // ignore it. |
| if (AA && |
| (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0) |
| continue; |
| |
| // May modify the pointer, bail out. |
| ++ScanFrom; |
| return 0; |
| } |
| } |
| |
| // Got to the start of the block, we didn't find it, but are done for this |
| // block. |
| return 0; |
| } |
