llvm-project/llvm/lib/Transforms/Utils/VNCoercion.cpp

540 lines
22 KiB
C++

#include "llvm/Transforms/Utils/VNCoercion.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/Debug.h"
#define DEBUG_TYPE "vncoerce"
namespace llvm {
namespace VNCoercion {
/// Return true if coerceAvailableValueToLoadType will succeed.
bool canCoerceMustAliasedValueToLoad(Value *StoredVal, Type *LoadTy,
const DataLayout &DL) {
Type *StoredTy = StoredVal->getType();
if (StoredTy == LoadTy)
return true;
// If the loaded or stored value is an first class array or struct, don't try
// to transform them. We need to be able to bitcast to integer.
if (LoadTy->isStructTy() || LoadTy->isArrayTy() || StoredTy->isStructTy() ||
StoredTy->isArrayTy())
return false;
uint64_t StoreSize = DL.getTypeSizeInBits(StoredTy);
// The store size must be byte-aligned to support future type casts.
if (llvm::alignTo(StoreSize, 8) != StoreSize)
return false;
// The store has to be at least as big as the load.
if (StoreSize < DL.getTypeSizeInBits(LoadTy))
return false;
// Don't coerce non-integral pointers to integers or vice versa.
if (DL.isNonIntegralPointerType(StoredVal->getType()->getScalarType()) !=
DL.isNonIntegralPointerType(LoadTy->getScalarType())) {
// As a special case, allow coercion of memset used to initialize
// an array w/null. Despite non-integral pointers not generally having a
// specific bit pattern, we do assume null is zero.
if (auto *CI = dyn_cast<Constant>(StoredVal))
return CI->isNullValue();
return false;
}
return true;
}
template <class T, class HelperClass>
static T *coerceAvailableValueToLoadTypeHelper(T *StoredVal, Type *LoadedTy,
HelperClass &Helper,
const DataLayout &DL) {
assert(canCoerceMustAliasedValueToLoad(StoredVal, LoadedTy, DL) &&
"precondition violation - materialization can't fail");
if (auto *C = dyn_cast<Constant>(StoredVal))
if (auto *FoldedStoredVal = ConstantFoldConstant(C, DL))
StoredVal = FoldedStoredVal;
// If this is already the right type, just return it.
Type *StoredValTy = StoredVal->getType();
uint64_t StoredValSize = DL.getTypeSizeInBits(StoredValTy);
uint64_t LoadedValSize = DL.getTypeSizeInBits(LoadedTy);
// If the store and reload are the same size, we can always reuse it.
if (StoredValSize == LoadedValSize) {
// Pointer to Pointer -> use bitcast.
if (StoredValTy->isPtrOrPtrVectorTy() && LoadedTy->isPtrOrPtrVectorTy()) {
StoredVal = Helper.CreateBitCast(StoredVal, LoadedTy);
} else {
// Convert source pointers to integers, which can be bitcast.
if (StoredValTy->isPtrOrPtrVectorTy()) {
StoredValTy = DL.getIntPtrType(StoredValTy);
StoredVal = Helper.CreatePtrToInt(StoredVal, StoredValTy);
}
Type *TypeToCastTo = LoadedTy;
if (TypeToCastTo->isPtrOrPtrVectorTy())
TypeToCastTo = DL.getIntPtrType(TypeToCastTo);
if (StoredValTy != TypeToCastTo)
StoredVal = Helper.CreateBitCast(StoredVal, TypeToCastTo);
// Cast to pointer if the load needs a pointer type.
if (LoadedTy->isPtrOrPtrVectorTy())
StoredVal = Helper.CreateIntToPtr(StoredVal, LoadedTy);
}
if (auto *C = dyn_cast<ConstantExpr>(StoredVal))
if (auto *FoldedStoredVal = ConstantFoldConstant(C, DL))
StoredVal = FoldedStoredVal;
return StoredVal;
}
// If the loaded value is smaller than the available value, then we can
// extract out a piece from it. If the available value is too small, then we
// can't do anything.
assert(StoredValSize >= LoadedValSize &&
"canCoerceMustAliasedValueToLoad fail");
// Convert source pointers to integers, which can be manipulated.
if (StoredValTy->isPtrOrPtrVectorTy()) {
StoredValTy = DL.getIntPtrType(StoredValTy);
StoredVal = Helper.CreatePtrToInt(StoredVal, StoredValTy);
}
// Convert vectors and fp to integer, which can be manipulated.
if (!StoredValTy->isIntegerTy()) {
StoredValTy = IntegerType::get(StoredValTy->getContext(), StoredValSize);
StoredVal = Helper.CreateBitCast(StoredVal, StoredValTy);
}
// If this is a big-endian system, we need to shift the value down to the low
// bits so that a truncate will work.
if (DL.isBigEndian()) {
uint64_t ShiftAmt = DL.getTypeStoreSizeInBits(StoredValTy) -
DL.getTypeStoreSizeInBits(LoadedTy);
StoredVal = Helper.CreateLShr(
StoredVal, ConstantInt::get(StoredVal->getType(), ShiftAmt));
}
// Truncate the integer to the right size now.
Type *NewIntTy = IntegerType::get(StoredValTy->getContext(), LoadedValSize);
StoredVal = Helper.CreateTruncOrBitCast(StoredVal, NewIntTy);
if (LoadedTy != NewIntTy) {
// If the result is a pointer, inttoptr.
if (LoadedTy->isPtrOrPtrVectorTy())
StoredVal = Helper.CreateIntToPtr(StoredVal, LoadedTy);
else
// Otherwise, bitcast.
StoredVal = Helper.CreateBitCast(StoredVal, LoadedTy);
}
if (auto *C = dyn_cast<Constant>(StoredVal))
if (auto *FoldedStoredVal = ConstantFoldConstant(C, DL))
StoredVal = FoldedStoredVal;
return StoredVal;
}
/// If we saw a store of a value to memory, and
/// then a load from a must-aliased pointer of a different type, try to coerce
/// the stored value. LoadedTy is the type of the load we want to replace.
/// IRB is IRBuilder used to insert new instructions.
///
/// If we can't do it, return null.
Value *coerceAvailableValueToLoadType(Value *StoredVal, Type *LoadedTy,
IRBuilder<> &IRB, const DataLayout &DL) {
return coerceAvailableValueToLoadTypeHelper(StoredVal, LoadedTy, IRB, DL);
}
/// This function is called when we have a memdep query of a load that ends up
/// being a clobbering memory write (store, memset, memcpy, memmove). This
/// means that the write *may* provide bits used by the load but we can't be
/// sure because the pointers don't must-alias.
///
/// Check this case to see if there is anything more we can do before we give
/// up. This returns -1 if we have to give up, or a byte number in the stored
/// value of the piece that feeds the load.
static int analyzeLoadFromClobberingWrite(Type *LoadTy, Value *LoadPtr,
Value *WritePtr,
uint64_t WriteSizeInBits,
const DataLayout &DL) {
// If the loaded or stored value is a first class array or struct, don't try
// to transform them. We need to be able to bitcast to integer.
if (LoadTy->isStructTy() || LoadTy->isArrayTy())
return -1;
int64_t StoreOffset = 0, LoadOffset = 0;
Value *StoreBase =
GetPointerBaseWithConstantOffset(WritePtr, StoreOffset, DL);
Value *LoadBase = GetPointerBaseWithConstantOffset(LoadPtr, LoadOffset, DL);
if (StoreBase != LoadBase)
return -1;
// If the load and store are to the exact same address, they should have been
// a must alias. AA must have gotten confused.
// FIXME: Study to see if/when this happens. One case is forwarding a memset
// to a load from the base of the memset.
// If the load and store don't overlap at all, the store doesn't provide
// anything to the load. In this case, they really don't alias at all, AA
// must have gotten confused.
uint64_t LoadSize = DL.getTypeSizeInBits(LoadTy);
if ((WriteSizeInBits & 7) | (LoadSize & 7))
return -1;
uint64_t StoreSize = WriteSizeInBits / 8; // Convert to bytes.
LoadSize /= 8;
bool isAAFailure = false;
if (StoreOffset < LoadOffset)
isAAFailure = StoreOffset + int64_t(StoreSize) <= LoadOffset;
else
isAAFailure = LoadOffset + int64_t(LoadSize) <= StoreOffset;
if (isAAFailure)
return -1;
// If the Load isn't completely contained within the stored bits, we don't
// have all the bits to feed it. We could do something crazy in the future
// (issue a smaller load then merge the bits in) but this seems unlikely to be
// valuable.
if (StoreOffset > LoadOffset ||
StoreOffset + StoreSize < LoadOffset + LoadSize)
return -1;
// Okay, we can do this transformation. Return the number of bytes into the
// store that the load is.
return LoadOffset - StoreOffset;
}
/// This function is called when we have a
/// memdep query of a load that ends up being a clobbering store.
int analyzeLoadFromClobberingStore(Type *LoadTy, Value *LoadPtr,
StoreInst *DepSI, const DataLayout &DL) {
auto *StoredVal = DepSI->getValueOperand();
// Cannot handle reading from store of first-class aggregate yet.
if (StoredVal->getType()->isStructTy() ||
StoredVal->getType()->isArrayTy())
return -1;
// Don't coerce non-integral pointers to integers or vice versa.
if (DL.isNonIntegralPointerType(StoredVal->getType()->getScalarType()) !=
DL.isNonIntegralPointerType(LoadTy->getScalarType())) {
// Allow casts of zero values to null as a special case
auto *CI = dyn_cast<Constant>(StoredVal);
if (!CI || !CI->isNullValue())
return -1;
}
Value *StorePtr = DepSI->getPointerOperand();
uint64_t StoreSize =
DL.getTypeSizeInBits(DepSI->getValueOperand()->getType());
return analyzeLoadFromClobberingWrite(LoadTy, LoadPtr, StorePtr, StoreSize,
DL);
}
/// This function is called when we have a
/// memdep query of a load that ends up being clobbered by another load. See if
/// the other load can feed into the second load.
int analyzeLoadFromClobberingLoad(Type *LoadTy, Value *LoadPtr, LoadInst *DepLI,
const DataLayout &DL) {
// Cannot handle reading from store of first-class aggregate yet.
if (DepLI->getType()->isStructTy() || DepLI->getType()->isArrayTy())
return -1;
// Don't coerce non-integral pointers to integers or vice versa.
if (DL.isNonIntegralPointerType(DepLI->getType()->getScalarType()) !=
DL.isNonIntegralPointerType(LoadTy->getScalarType()))
return -1;
Value *DepPtr = DepLI->getPointerOperand();
uint64_t DepSize = DL.getTypeSizeInBits(DepLI->getType());
int R = analyzeLoadFromClobberingWrite(LoadTy, LoadPtr, DepPtr, DepSize, DL);
if (R != -1)
return R;
// If we have a load/load clobber an DepLI can be widened to cover this load,
// then we should widen it!
int64_t LoadOffs = 0;
const Value *LoadBase =
GetPointerBaseWithConstantOffset(LoadPtr, LoadOffs, DL);
unsigned LoadSize = DL.getTypeStoreSize(LoadTy);
unsigned Size = MemoryDependenceResults::getLoadLoadClobberFullWidthSize(
LoadBase, LoadOffs, LoadSize, DepLI);
if (Size == 0)
return -1;
// Check non-obvious conditions enforced by MDA which we rely on for being
// able to materialize this potentially available value
assert(DepLI->isSimple() && "Cannot widen volatile/atomic load!");
assert(DepLI->getType()->isIntegerTy() && "Can't widen non-integer load");
return analyzeLoadFromClobberingWrite(LoadTy, LoadPtr, DepPtr, Size * 8, DL);
}
int analyzeLoadFromClobberingMemInst(Type *LoadTy, Value *LoadPtr,
MemIntrinsic *MI, const DataLayout &DL) {
// If the mem operation is a non-constant size, we can't handle it.
ConstantInt *SizeCst = dyn_cast<ConstantInt>(MI->getLength());
if (!SizeCst)
return -1;
uint64_t MemSizeInBits = SizeCst->getZExtValue() * 8;
// If this is memset, we just need to see if the offset is valid in the size
// of the memset..
if (MI->getIntrinsicID() == Intrinsic::memset) {
if (DL.isNonIntegralPointerType(LoadTy->getScalarType())) {
auto *CI = dyn_cast<ConstantInt>(cast<MemSetInst>(MI)->getValue());
if (!CI || !CI->isZero())
return -1;
}
return analyzeLoadFromClobberingWrite(LoadTy, LoadPtr, MI->getDest(),
MemSizeInBits, DL);
}
// If we have a memcpy/memmove, the only case we can handle is if this is a
// copy from constant memory. In that case, we can read directly from the
// constant memory.
MemTransferInst *MTI = cast<MemTransferInst>(MI);
Constant *Src = dyn_cast<Constant>(MTI->getSource());
if (!Src)
return -1;
GlobalVariable *GV = dyn_cast<GlobalVariable>(GetUnderlyingObject(Src, DL));
if (!GV || !GV->isConstant() || !GV->hasDefinitiveInitializer())
return -1;
// See if the access is within the bounds of the transfer.
int Offset = analyzeLoadFromClobberingWrite(LoadTy, LoadPtr, MI->getDest(),
MemSizeInBits, DL);
if (Offset == -1)
return Offset;
// Don't coerce non-integral pointers to integers or vice versa, and the
// memtransfer is implicitly a raw byte code
if (DL.isNonIntegralPointerType(LoadTy->getScalarType()))
// TODO: Can allow nullptrs from constant zeros
return -1;
unsigned AS = Src->getType()->getPointerAddressSpace();
// Otherwise, see if we can constant fold a load from the constant with the
// offset applied as appropriate.
Src =
ConstantExpr::getBitCast(Src, Type::getInt8PtrTy(Src->getContext(), AS));
Constant *OffsetCst =
ConstantInt::get(Type::getInt64Ty(Src->getContext()), (unsigned)Offset);
Src = ConstantExpr::getGetElementPtr(Type::getInt8Ty(Src->getContext()), Src,
OffsetCst);
Src = ConstantExpr::getBitCast(Src, PointerType::get(LoadTy, AS));
if (ConstantFoldLoadFromConstPtr(Src, LoadTy, DL))
return Offset;
return -1;
}
template <class T, class HelperClass>
static T *getStoreValueForLoadHelper(T *SrcVal, unsigned Offset, Type *LoadTy,
HelperClass &Helper,
const DataLayout &DL) {
LLVMContext &Ctx = SrcVal->getType()->getContext();
// If two pointers are in the same address space, they have the same size,
// so we don't need to do any truncation, etc. This avoids introducing
// ptrtoint instructions for pointers that may be non-integral.
if (SrcVal->getType()->isPointerTy() && LoadTy->isPointerTy() &&
cast<PointerType>(SrcVal->getType())->getAddressSpace() ==
cast<PointerType>(LoadTy)->getAddressSpace()) {
return SrcVal;
}
uint64_t StoreSize = (DL.getTypeSizeInBits(SrcVal->getType()) + 7) / 8;
uint64_t LoadSize = (DL.getTypeSizeInBits(LoadTy) + 7) / 8;
// Compute which bits of the stored value are being used by the load. Convert
// to an integer type to start with.
if (SrcVal->getType()->isPtrOrPtrVectorTy())
SrcVal = Helper.CreatePtrToInt(SrcVal, DL.getIntPtrType(SrcVal->getType()));
if (!SrcVal->getType()->isIntegerTy())
SrcVal = Helper.CreateBitCast(SrcVal, IntegerType::get(Ctx, StoreSize * 8));
// Shift the bits to the least significant depending on endianness.
unsigned ShiftAmt;
if (DL.isLittleEndian())
ShiftAmt = Offset * 8;
else
ShiftAmt = (StoreSize - LoadSize - Offset) * 8;
if (ShiftAmt)
SrcVal = Helper.CreateLShr(SrcVal,
ConstantInt::get(SrcVal->getType(), ShiftAmt));
if (LoadSize != StoreSize)
SrcVal = Helper.CreateTruncOrBitCast(SrcVal,
IntegerType::get(Ctx, LoadSize * 8));
return SrcVal;
}
/// This function is called when we have a memdep query of a load that ends up
/// being a clobbering store. This means that the store provides bits used by
/// the load but the pointers don't must-alias. Check this case to see if
/// there is anything more we can do before we give up.
Value *getStoreValueForLoad(Value *SrcVal, unsigned Offset, Type *LoadTy,
Instruction *InsertPt, const DataLayout &DL) {
IRBuilder<> Builder(InsertPt);
SrcVal = getStoreValueForLoadHelper(SrcVal, Offset, LoadTy, Builder, DL);
return coerceAvailableValueToLoadTypeHelper(SrcVal, LoadTy, Builder, DL);
}
Constant *getConstantStoreValueForLoad(Constant *SrcVal, unsigned Offset,
Type *LoadTy, const DataLayout &DL) {
ConstantFolder F;
SrcVal = getStoreValueForLoadHelper(SrcVal, Offset, LoadTy, F, DL);
return coerceAvailableValueToLoadTypeHelper(SrcVal, LoadTy, F, DL);
}
/// This function is called when we have a memdep query of a load that ends up
/// being a clobbering load. This means that the load *may* provide bits used
/// by the load but we can't be sure because the pointers don't must-alias.
/// Check this case to see if there is anything more we can do before we give
/// up.
Value *getLoadValueForLoad(LoadInst *SrcVal, unsigned Offset, Type *LoadTy,
Instruction *InsertPt, const DataLayout &DL) {
// If Offset+LoadTy exceeds the size of SrcVal, then we must be wanting to
// widen SrcVal out to a larger load.
unsigned SrcValStoreSize = DL.getTypeStoreSize(SrcVal->getType());
unsigned LoadSize = DL.getTypeStoreSize(LoadTy);
if (Offset + LoadSize > SrcValStoreSize) {
assert(SrcVal->isSimple() && "Cannot widen volatile/atomic load!");
assert(SrcVal->getType()->isIntegerTy() && "Can't widen non-integer load");
// If we have a load/load clobber an DepLI can be widened to cover this
// load, then we should widen it to the next power of 2 size big enough!
unsigned NewLoadSize = Offset + LoadSize;
if (!isPowerOf2_32(NewLoadSize))
NewLoadSize = NextPowerOf2(NewLoadSize);
Value *PtrVal = SrcVal->getPointerOperand();
// Insert the new load after the old load. This ensures that subsequent
// memdep queries will find the new load. We can't easily remove the old
// load completely because it is already in the value numbering table.
IRBuilder<> Builder(SrcVal->getParent(), ++BasicBlock::iterator(SrcVal));
Type *DestTy = IntegerType::get(LoadTy->getContext(), NewLoadSize * 8);
Type *DestPTy =
PointerType::get(DestTy, PtrVal->getType()->getPointerAddressSpace());
Builder.SetCurrentDebugLocation(SrcVal->getDebugLoc());
PtrVal = Builder.CreateBitCast(PtrVal, DestPTy);
LoadInst *NewLoad = Builder.CreateLoad(DestTy, PtrVal);
NewLoad->takeName(SrcVal);
NewLoad->setAlignment(SrcVal->getAlignment());
LLVM_DEBUG(dbgs() << "GVN WIDENED LOAD: " << *SrcVal << "\n");
LLVM_DEBUG(dbgs() << "TO: " << *NewLoad << "\n");
// Replace uses of the original load with the wider load. On a big endian
// system, we need to shift down to get the relevant bits.
Value *RV = NewLoad;
if (DL.isBigEndian())
RV = Builder.CreateLShr(RV, (NewLoadSize - SrcValStoreSize) * 8);
RV = Builder.CreateTrunc(RV, SrcVal->getType());
SrcVal->replaceAllUsesWith(RV);
SrcVal = NewLoad;
}
return getStoreValueForLoad(SrcVal, Offset, LoadTy, InsertPt, DL);
}
Constant *getConstantLoadValueForLoad(Constant *SrcVal, unsigned Offset,
Type *LoadTy, const DataLayout &DL) {
unsigned SrcValStoreSize = DL.getTypeStoreSize(SrcVal->getType());
unsigned LoadSize = DL.getTypeStoreSize(LoadTy);
if (Offset + LoadSize > SrcValStoreSize)
return nullptr;
return getConstantStoreValueForLoad(SrcVal, Offset, LoadTy, DL);
}
template <class T, class HelperClass>
T *getMemInstValueForLoadHelper(MemIntrinsic *SrcInst, unsigned Offset,
Type *LoadTy, HelperClass &Helper,
const DataLayout &DL) {
LLVMContext &Ctx = LoadTy->getContext();
uint64_t LoadSize = DL.getTypeSizeInBits(LoadTy) / 8;
// We know that this method is only called when the mem transfer fully
// provides the bits for the load.
if (MemSetInst *MSI = dyn_cast<MemSetInst>(SrcInst)) {
// memset(P, 'x', 1234) -> splat('x'), even if x is a variable, and
// independently of what the offset is.
T *Val = cast<T>(MSI->getValue());
if (LoadSize != 1)
Val =
Helper.CreateZExtOrBitCast(Val, IntegerType::get(Ctx, LoadSize * 8));
T *OneElt = Val;
// Splat the value out to the right number of bits.
for (unsigned NumBytesSet = 1; NumBytesSet != LoadSize;) {
// If we can double the number of bytes set, do it.
if (NumBytesSet * 2 <= LoadSize) {
T *ShVal = Helper.CreateShl(
Val, ConstantInt::get(Val->getType(), NumBytesSet * 8));
Val = Helper.CreateOr(Val, ShVal);
NumBytesSet <<= 1;
continue;
}
// Otherwise insert one byte at a time.
T *ShVal = Helper.CreateShl(Val, ConstantInt::get(Val->getType(), 1 * 8));
Val = Helper.CreateOr(OneElt, ShVal);
++NumBytesSet;
}
return coerceAvailableValueToLoadTypeHelper(Val, LoadTy, Helper, DL);
}
// Otherwise, this is a memcpy/memmove from a constant global.
MemTransferInst *MTI = cast<MemTransferInst>(SrcInst);
Constant *Src = cast<Constant>(MTI->getSource());
unsigned AS = Src->getType()->getPointerAddressSpace();
// Otherwise, see if we can constant fold a load from the constant with the
// offset applied as appropriate.
Src =
ConstantExpr::getBitCast(Src, Type::getInt8PtrTy(Src->getContext(), AS));
Constant *OffsetCst =
ConstantInt::get(Type::getInt64Ty(Src->getContext()), (unsigned)Offset);
Src = ConstantExpr::getGetElementPtr(Type::getInt8Ty(Src->getContext()), Src,
OffsetCst);
Src = ConstantExpr::getBitCast(Src, PointerType::get(LoadTy, AS));
return ConstantFoldLoadFromConstPtr(Src, LoadTy, DL);
}
/// This function is called when we have a
/// memdep query of a load that ends up being a clobbering mem intrinsic.
Value *getMemInstValueForLoad(MemIntrinsic *SrcInst, unsigned Offset,
Type *LoadTy, Instruction *InsertPt,
const DataLayout &DL) {
IRBuilder<> Builder(InsertPt);
return getMemInstValueForLoadHelper<Value, IRBuilder<>>(SrcInst, Offset,
LoadTy, Builder, DL);
}
Constant *getConstantMemInstValueForLoad(MemIntrinsic *SrcInst, unsigned Offset,
Type *LoadTy, const DataLayout &DL) {
// The only case analyzeLoadFromClobberingMemInst cannot be converted to a
// constant is when it's a memset of a non-constant.
if (auto *MSI = dyn_cast<MemSetInst>(SrcInst))
if (!isa<Constant>(MSI->getValue()))
return nullptr;
ConstantFolder F;
return getMemInstValueForLoadHelper<Constant, ConstantFolder>(SrcInst, Offset,
LoadTy, F, DL);
}
} // namespace VNCoercion
} // namespace llvm