421 lines
16 KiB
C++
421 lines
16 KiB
C++
//===- 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"
|
|
#include "llvm/IR/Statepoint.h"
|
|
|
|
using namespace llvm;
|
|
|
|
static bool isAligned(const Value *Base, const APInt &Offset, unsigned Align,
|
|
const DataLayout &DL) {
|
|
APInt BaseAlign(Offset.getBitWidth(), Base->getPointerAlignment(DL));
|
|
|
|
if (!BaseAlign) {
|
|
Type *Ty = Base->getType()->getPointerElementType();
|
|
if (!Ty->isSized())
|
|
return false;
|
|
BaseAlign = DL.getABITypeAlignment(Ty);
|
|
}
|
|
|
|
APInt Alignment(Offset.getBitWidth(), Align);
|
|
|
|
assert(Alignment.isPowerOf2() && "must be a power of 2!");
|
|
return BaseAlign.uge(Alignment) && !(Offset & (Alignment-1));
|
|
}
|
|
|
|
static bool isAligned(const Value *Base, unsigned Align, const DataLayout &DL) {
|
|
Type *Ty = Base->getType();
|
|
assert(Ty->isSized() && "must be sized");
|
|
APInt Offset(DL.getTypeStoreSizeInBits(Ty), 0);
|
|
return isAligned(Base, Offset, Align, DL);
|
|
}
|
|
|
|
/// Test if V is always a pointer to allocated and suitably aligned memory for
|
|
/// a simple load or store.
|
|
static bool isDereferenceableAndAlignedPointer(
|
|
const Value *V, unsigned Align, const APInt &Size, const DataLayout &DL,
|
|
const Instruction *CtxI, const DominatorTree *DT,
|
|
SmallPtrSetImpl<const Value *> &Visited) {
|
|
// Note that it is not safe to speculate into a malloc'd region because
|
|
// malloc may return null.
|
|
|
|
// bitcast instructions are no-ops as far as dereferenceability is concerned.
|
|
if (const BitCastOperator *BC = dyn_cast<BitCastOperator>(V))
|
|
return isDereferenceableAndAlignedPointer(BC->getOperand(0), Align, Size,
|
|
DL, CtxI, DT, Visited);
|
|
|
|
bool CheckForNonNull = false;
|
|
APInt KnownDerefBytes(Size.getBitWidth(),
|
|
V->getPointerDereferenceableBytes(DL, CheckForNonNull));
|
|
if (KnownDerefBytes.getBoolValue()) {
|
|
if (KnownDerefBytes.uge(Size))
|
|
if (!CheckForNonNull || isKnownNonNullAt(V, CtxI, DT))
|
|
return isAligned(V, Align, DL);
|
|
}
|
|
|
|
// For GEPs, determine if the indexing lands within the allocated object.
|
|
if (const GEPOperator *GEP = dyn_cast<GEPOperator>(V)) {
|
|
const Value *Base = GEP->getPointerOperand();
|
|
|
|
APInt Offset(DL.getPointerTypeSizeInBits(GEP->getType()), 0);
|
|
if (!GEP->accumulateConstantOffset(DL, Offset) || Offset.isNegative() ||
|
|
!Offset.urem(APInt(Offset.getBitWidth(), Align)).isMinValue())
|
|
return false;
|
|
|
|
// If the base pointer is dereferenceable for Offset+Size bytes, then the
|
|
// GEP (== Base + Offset) is dereferenceable for Size bytes. If the base
|
|
// pointer is aligned to Align bytes, and the Offset is divisible by Align
|
|
// then the GEP (== Base + Offset == k_0 * Align + k_1 * Align) is also
|
|
// aligned to Align bytes.
|
|
|
|
return Visited.insert(Base).second &&
|
|
isDereferenceableAndAlignedPointer(Base, Align, Offset + Size, DL,
|
|
CtxI, DT, Visited);
|
|
}
|
|
|
|
// For gc.relocate, look through relocations
|
|
if (const GCRelocateInst *RelocateInst = dyn_cast<GCRelocateInst>(V))
|
|
return isDereferenceableAndAlignedPointer(
|
|
RelocateInst->getDerivedPtr(), Align, Size, DL, CtxI, DT, Visited);
|
|
|
|
if (const AddrSpaceCastInst *ASC = dyn_cast<AddrSpaceCastInst>(V))
|
|
return isDereferenceableAndAlignedPointer(ASC->getOperand(0), Align, Size,
|
|
DL, CtxI, DT, Visited);
|
|
|
|
if (auto CS = ImmutableCallSite(V))
|
|
if (const Value *RV = CS.getReturnedArgOperand())
|
|
return isDereferenceableAndAlignedPointer(RV, Align, Size, DL, CtxI, DT,
|
|
Visited);
|
|
|
|
// If we don't know, assume the worst.
|
|
return false;
|
|
}
|
|
|
|
bool llvm::isDereferenceableAndAlignedPointer(const Value *V, unsigned Align,
|
|
const DataLayout &DL,
|
|
const Instruction *CtxI,
|
|
const DominatorTree *DT) {
|
|
// When dereferenceability information is provided by a dereferenceable
|
|
// attribute, we know exactly how many bytes are dereferenceable. If we can
|
|
// determine the exact offset to the attributed variable, we can use that
|
|
// information here.
|
|
Type *VTy = V->getType();
|
|
Type *Ty = VTy->getPointerElementType();
|
|
|
|
// Require ABI alignment for loads without alignment specification
|
|
if (Align == 0)
|
|
Align = DL.getABITypeAlignment(Ty);
|
|
|
|
if (!Ty->isSized())
|
|
return false;
|
|
|
|
SmallPtrSet<const Value *, 32> Visited;
|
|
return ::isDereferenceableAndAlignedPointer(
|
|
V, Align, APInt(DL.getTypeSizeInBits(VTy), DL.getTypeStoreSize(Ty)), DL,
|
|
CtxI, DT, Visited);
|
|
}
|
|
|
|
bool llvm::isDereferenceablePointer(const Value *V, const DataLayout &DL,
|
|
const Instruction *CtxI,
|
|
const DominatorTree *DT) {
|
|
return isDereferenceableAndAlignedPointer(V, 1, DL, CtxI, DT);
|
|
}
|
|
|
|
/// \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 DT and ScanFrom are specified this method performs context-sensitive
|
|
/// analysis and returns true if it is safe to load immediately before ScanFrom.
|
|
///
|
|
/// 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, unsigned Align,
|
|
const DataLayout &DL,
|
|
Instruction *ScanFrom,
|
|
const DominatorTree *DT) {
|
|
// 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));
|
|
|
|
// If DT is not specified we can't make context-sensitive query
|
|
const Instruction* CtxI = DT ? ScanFrom : nullptr;
|
|
if (isDereferenceableAndAlignedPointer(V, Align, DL, CtxI, DT))
|
|
return true;
|
|
|
|
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
|
|
// interposed arbitrarily. Their size may change or they may be weak and
|
|
// require a test to determine if they were in fact provided.
|
|
if (!GV->isInterposable()) {
|
|
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;
|
|
}
|
|
}
|
|
|
|
if (!ScanFrom)
|
|
return false;
|
|
|
|
// 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"));
|
|
|
|
Value *llvm::FindAvailableLoadedValue(LoadInst *Load, BasicBlock *ScanBB,
|
|
BasicBlock::iterator &ScanFrom,
|
|
unsigned MaxInstsToScan,
|
|
AliasAnalysis *AA, AAMDNodes *AATags,
|
|
bool *IsLoadCSE) {
|
|
if (MaxInstsToScan == 0)
|
|
MaxInstsToScan = ~0U;
|
|
|
|
Value *Ptr = Load->getPointerOperand();
|
|
Type *AccessTy = Load->getType();
|
|
|
|
// We can never remove a volatile load
|
|
if (Load->isVolatile())
|
|
return nullptr;
|
|
|
|
// Anything stronger than unordered is currently unimplemented.
|
|
if (!Load->isUnordered())
|
|
return nullptr;
|
|
|
|
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)) {
|
|
|
|
// We can value forward from an atomic to a non-atomic, but not the
|
|
// other way around.
|
|
if (LI->isAtomic() < Load->isAtomic())
|
|
return nullptr;
|
|
|
|
if (AATags)
|
|
LI->getAAMetadata(*AATags);
|
|
if (IsLoadCSE)
|
|
*IsLoadCSE = true;
|
|
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)) {
|
|
|
|
// We can value forward from an atomic to a non-atomic, but not the
|
|
// other way around.
|
|
if (SI->isAtomic() < Load->isAtomic())
|
|
return nullptr;
|
|
|
|
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;
|
|
}
|