| //===- 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/Analysis/ValueTracking.h" |
| #include "llvm/IR/DataLayout.h" |
| #include "llvm/IR/GlobalAlias.h" |
| #include "llvm/IR/GlobalVariable.h" |
| #include "llvm/IR/IntrinsicInst.h" |
| #include "llvm/IR/LLVMContext.h" |
| #include "llvm/IR/Module.h" |
| #include "llvm/IR/Operator.h" |
| using namespace llvm; |
| |
| /// \brief 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: |
| /// \code |
| /// %t0 = getelementptr \@a, 0, 3 |
| /// store i32 0, i32* %t0 |
| /// %t1 = getelementptr \@a, 0, 3 |
| /// %t2 = load i32* %t1 |
| /// \endcode |
| /// |
| 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; |
| } |
| |
| /// \brief Check if executing a load of this pointer 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 \c ScanFrom, to determine |
| /// if the address is already accessed. |
| /// |
| /// This uses the pointee type to determine how many bytes need to be safe to |
| /// load from the pointer. |
| bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom, |
| unsigned Align) { |
| const DataLayout &DL = ScanFrom->getModule()->getDataLayout(); |
| |
| // Zero alignment means that the load has the ABI alignment for the target |
| if (Align == 0) |
| Align = DL.getABITypeAlignment(V->getType()->getPointerElementType()); |
| assert(isPowerOf2_32(Align)); |
| |
| int64_t ByteOffset = 0; |
| Value *Base = V; |
| Base = GetPointerBaseWithConstantOffset(V, ByteOffset, DL); |
| |
| if (ByteOffset < 0) // out of bounds |
| return false; |
| |
| Type *BaseType = nullptr; |
| 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 GlobalVariable *GV = dyn_cast<GlobalVariable>(Base)) { |
| // Global variables are not necessarily safe to load from if they are |
| // overridden. Their size may change or they may be weak and require a test |
| // to determine if they were in fact provided. |
| if (!GV->mayBeOverridden()) { |
| BaseType = GV->getType()->getElementType(); |
| BaseAlign = GV->getAlignment(); |
| } |
| } |
| |
| PointerType *AddrTy = cast<PointerType>(V->getType()); |
| uint64_t LoadSize = DL.getTypeStoreSize(AddrTy->getElementType()); |
| |
| // If we found a base allocated type from either an alloca or global variable, |
| // try to see if we are definitively within the allocated region. We need to |
| // know the size of the base type and the loaded type to do anything in this |
| // case. |
| if (BaseType && BaseType->isSized()) { |
| if (BaseAlign == 0) |
| BaseAlign = DL.getPrefTypeAlignment(BaseType); |
| |
| if (Align <= BaseAlign) { |
| // Check if the load is within the bounds of the underlying object. |
| if (ByteOffset + LoadSize <= DL.getTypeAllocSize(BaseType) && |
| ((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->getIterator(), |
| E = ScanFrom->getParent()->begin(); |
| |
| // We can at least always strip pointer casts even though we can't use the |
| // base here. |
| V = V->stripPointerCasts(); |
| |
| 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; |
| |
| Value *AccessedPtr; |
| unsigned AccessedAlign; |
| if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { |
| AccessedPtr = LI->getPointerOperand(); |
| AccessedAlign = LI->getAlignment(); |
| } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) { |
| AccessedPtr = SI->getPointerOperand(); |
| AccessedAlign = SI->getAlignment(); |
| } else |
| continue; |
| |
| Type *AccessedTy = AccessedPtr->getType()->getPointerElementType(); |
| if (AccessedAlign == 0) |
| AccessedAlign = DL.getABITypeAlignment(AccessedTy); |
| if (AccessedAlign < Align) |
| continue; |
| |
| // Handle trivial cases. |
| if (AccessedPtr == V) |
| return true; |
| |
| if (AreEquivalentAddressValues(AccessedPtr->stripPointerCasts(), V) && |
| LoadSize <= DL.getTypeStoreSize(AccessedTy)) |
| return true; |
| } |
| return false; |
| } |
| |
| /// DefMaxInstsToScan - the default number of maximum instructions |
| /// to scan in the block, used by FindAvailableLoadedValue(). |
| /// FindAvailableLoadedValue() was introduced in r60148, to improve jump |
| /// threading in part by eliminating partially redundant loads. |
| /// At that point, the value of MaxInstsToScan was already set to '6' |
| /// without documented explanation. |
| cl::opt<unsigned> |
| llvm::DefMaxInstsToScan("available-load-scan-limit", cl::init(6), cl::Hidden, |
| cl::desc("Use this to specify the default maximum number of instructions " |
| "to scan backward from a given instruction, when searching for " |
| "available loaded value")); |
| |
| /// \brief Scan the ScanBB block backwards to see if we have the value at the |
| /// memory address *Ptr locally available within a small number of instructions. |
| /// |
| /// The scan starts from \c ScanFrom. \c MaxInstsToScan specifies the maximum |
| /// instructions to scan in the block. If it is set to \c 0, it will scan the whole |
| /// block. |
| /// |
| /// If the value is available, this function returns it. If not, it returns 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 \c *Ptr or provides it, \c ScanFrom is left at the last |
| /// instruction processed and this returns null. |
| /// |
| /// You can also optionally specify an alias analysis implementation, which |
| /// makes this more precise. |
| /// |
| /// If \c AATags is non-null and a load or store is found, the AA tags from the |
| /// load or store are recorded there. If there are no AA tags or if no access is |
| /// found, it is left unmodified. |
| Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB, |
| BasicBlock::iterator &ScanFrom, |
| unsigned MaxInstsToScan, |
| AliasAnalysis *AA, AAMDNodes *AATags) { |
| if (MaxInstsToScan == 0) |
| MaxInstsToScan = ~0U; |
| |
| Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType(); |
| |
| const DataLayout &DL = ScanBB->getModule()->getDataLayout(); |
| |
| // Try to get the store size for the type. |
| uint64_t AccessSize = DL.getTypeStoreSize(AccessTy); |
| |
| Value *StrippedPtr = Ptr->stripPointerCasts(); |
| |
| 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 nullptr; |
| |
| --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->getPointerOperand()->stripPointerCasts(), StrippedPtr) && |
| CastInst::isBitOrNoopPointerCastable(LI->getType(), AccessTy, DL)) { |
| if (AATags) |
| LI->getAAMetadata(*AATags); |
| return LI; |
| } |
| |
| if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) { |
| Value *StorePtr = SI->getPointerOperand()->stripPointerCasts(); |
| // 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(StorePtr, StrippedPtr) && |
| CastInst::isBitOrNoopPointerCastable(SI->getValueOperand()->getType(), |
| AccessTy, DL)) { |
| if (AATags) |
| SI->getAAMetadata(*AATags); |
| return SI->getOperand(0); |
| } |
| |
| // If both StrippedPtr and StorePtr reach all the way to an alloca or |
| // global and they are different, ignore the store. This is a trivial form |
| // of alias analysis that is important for reg2mem'd code. |
| if ((isa<AllocaInst>(StrippedPtr) || isa<GlobalVariable>(StrippedPtr)) && |
| (isa<AllocaInst>(StorePtr) || isa<GlobalVariable>(StorePtr)) && |
| StrippedPtr != StorePtr) |
| 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, StrippedPtr, AccessSize) & MRI_Mod) == 0) |
| continue; |
| |
| // Otherwise the store that may or may not alias the pointer, bail out. |
| ++ScanFrom; |
| return nullptr; |
| } |
| |
| // 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, StrippedPtr, AccessSize) & MRI_Mod) == 0) |
| continue; |
| |
| // May modify the pointer, bail out. |
| ++ScanFrom; |
| return nullptr; |
| } |
| } |
| |
| // Got to the start of the block, we didn't find it, but are done for this |
| // block. |
| return nullptr; |
| } |