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[SeparateConstOffsetFromGEP] Allow SeparateConstOffsetFromGEP pass to lower GEPs.

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If LowerGEP is enabled, it can lower a GEP with multiple indices into GEPs with a single index
or arithmetic operations. Lowering GEPs can always extract structure indices. Lowering GEPs can
also give use more optimization opportunities. It can benefit passes like CSE, LICM and CGP.

Reviewed in http://reviews.llvm.org/D5864


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222328 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hao Liu 2014-11-19 06:24:44 +00:00
parent 1d4f28c6bc
commit eb52f383c2
2 changed files with 275 additions and 41 deletions
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4
include/llvm/Transforms/Scalar.h
View File

@ -395,7 +395,9 @@ FunctionPass *createAddDiscriminatorsPass();
//
// SeparateConstOffsetFromGEP - Split GEPs for better CSE
//
FunctionPass *createSeparateConstOffsetFromGEPPass();
FunctionPass *
createSeparateConstOffsetFromGEPPass(const TargetMachine *TM = nullptr,
bool LowerGEP = false);
//===----------------------------------------------------------------------===//
//

312
lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
View File

@ -79,6 +79,81 @@
// ld.global.f32 %f3, [%rl6+128]; // much better
// ld.global.f32 %f4, [%rl6+132]; // much better
//
// Another improvement enabled by the LowerGEP flag is to lower a GEP with
// multiple indices to either multiple GEPs with a single index or arithmetic
// operations (depending on whether the target uses alias analysis in codegen).
// Such transformation can have following benefits:
// (1) It can always extract constants in the indices of structure type.
// (2) After such Lowering, there are more optimization opportunities such as
// CSE, LICM and CGP.
//
// E.g. The following GEPs have multiple indices:
// BB1:
// %p = getelementptr [10 x %struct]* %ptr, i64 %i, i64 %j1, i32 3
// load %p
// ...
// BB2:
// %p2 = getelementptr [10 x %struct]* %ptr, i64 %i, i64 %j1, i32 2
// load %p2
// ...
//
// We can not do CSE for to the common part related to index "i64 %i". Lowering
// GEPs can achieve such goals.
// If the target does not use alias analysis in codegen, this pass will
// lower a GEP with multiple indices into arithmetic operations:
// BB1:
// %1 = ptrtoint [10 x %struct]* %ptr to i64 ; CSE opportunity
// %2 = mul i64 %i, length_of_10xstruct ; CSE opportunity
// %3 = add i64 %1, %2 ; CSE opportunity
// %4 = mul i64 %j1, length_of_struct
// %5 = add i64 %3, %4
// %6 = add i64 %3, struct_field_3 ; Constant offset
// %p = inttoptr i64 %6 to i32*
// load %p
// ...
// BB2:
// %7 = ptrtoint [10 x %struct]* %ptr to i64 ; CSE opportunity
// %8 = mul i64 %i, length_of_10xstruct ; CSE opportunity
// %9 = add i64 %7, %8 ; CSE opportunity
// %10 = mul i64 %j2, length_of_struct
// %11 = add i64 %9, %10
// %12 = add i64 %11, struct_field_2 ; Constant offset
// %p = inttoptr i64 %12 to i32*
// load %p2
// ...
//
// If the target uses alias analysis in codegen, this pass will lower a GEP
// with multiple indices into multiple GEPs with a single index:
// BB1:
// %1 = bitcast [10 x %struct]* %ptr to i8* ; CSE opportunity
// %2 = mul i64 %i, length_of_10xstruct ; CSE opportunity
// %3 = getelementptr i8* %1, i64 %2 ; CSE opportunity
// %4 = mul i64 %j1, length_of_struct
// %5 = getelementptr i8* %3, i64 %4
// %6 = getelementptr i8* %5, struct_field_3 ; Constant offset
// %p = bitcast i8* %6 to i32*
// load %p
// ...
// BB2:
// %7 = bitcast [10 x %struct]* %ptr to i8* ; CSE opportunity
// %8 = mul i64 %i, length_of_10xstruct ; CSE opportunity
// %9 = getelementptr i8* %7, i64 %8 ; CSE opportunity
// %10 = mul i64 %j2, length_of_struct
// %11 = getelementptr i8* %9, i64 %10
// %12 = getelementptr i8* %11, struct_field_2 ; Constant offset
// %p2 = bitcast i8* %12 to i32*
// load %p2
// ...
//
// Lowering GEPs can also benefit other passes such as LICM and CGP.
// LICM (Loop Invariant Code Motion) can not hoist/sink a GEP of multiple
// indices if one of the index is variant. If we lower such GEP into invariant
// parts and variant parts, LICM can hoist/sink those invariant parts.
// CGP (CodeGen Prepare) tries to sink address calculations that match the
// target's addressing modes. A GEP with multiple indices may not match and will
// not be sunk. If we lower such GEP into smaller parts, CGP may sink some of
// them. So we end up with a better addressing mode.
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/TargetTransformInfo.h"
@ -92,6 +167,9 @@
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include "llvm/IR/IRBuilder.h"
using namespace llvm;
@ -117,18 +195,17 @@ namespace {
/// -instcombine probably already optimized (3 * (a + 5)) to (3 * a + 15).
class ConstantOffsetExtractor {
public:
/// Extracts a constant offset from the given GEP index. It outputs the
/// numeric value of the extracted constant offset (0 if failed), and a
/// Extracts a constant offset from the given GEP index. It returns the
/// new index representing the remainder (equal to the original index minus
/// the constant offset).
/// the constant offset), or nullptr if we cannot extract a constant offset.
/// \p Idx The given GEP index
/// \p NewIdx The new index to replace (output)
/// \p DL The datalayout of the module
/// \p GEP The given GEP
static int64_t Extract(Value *Idx, Value *&NewIdx, const DataLayout *DL,
GetElementPtrInst *GEP);
/// Looks for a constant offset without extracting it. The meaning of the
/// arguments and the return value are the same as Extract.
static Value *Extract(Value *Idx, const DataLayout *DL,
GetElementPtrInst *GEP);
/// Looks for a constant offset from the given GEP index without extracting
/// it. It returns the numeric value of the extracted constant offset (0 if
/// failed). The meaning of the arguments are the same as Extract.
static int64_t Find(Value *Idx, const DataLayout *DL, GetElementPtrInst *GEP);
private:
@ -228,7 +305,9 @@ class ConstantOffsetExtractor {
class SeparateConstOffsetFromGEP : public FunctionPass {
public:
static char ID;
SeparateConstOffsetFromGEP() : FunctionPass(ID) {
SeparateConstOffsetFromGEP(const TargetMachine *TM = nullptr,
bool LowerGEP = false)
: FunctionPass(ID), TM(TM), LowerGEP(LowerGEP) {
initializeSeparateConstOffsetFromGEPPass(*PassRegistry::getPassRegistry());
}
@ -251,10 +330,29 @@ class SeparateConstOffsetFromGEP : public FunctionPass {
/// Tries to split the given GEP into a variadic base and a constant offset,
/// and returns true if the splitting succeeds.
bool splitGEP(GetElementPtrInst *GEP);
/// Finds the constant offset within each index, and accumulates them. This
/// function only inspects the GEP without changing it. The output
/// NeedsExtraction indicates whether we can extract a non-zero constant
/// offset from any index.
/// Lower a GEP with multiple indices into multiple GEPs with a single index.
/// Function splitGEP already split the original GEP into a variadic part and
/// a constant offset (i.e., AccumulativeByteOffset). This function lowers the
/// variadic part into a set of GEPs with a single index and applies
/// AccumulativeByteOffset to it.
/// \p Variadic The variadic part of the original GEP.
/// \p AccumulativeByteOffset The constant offset.
void lowerToSingleIndexGEPs(GetElementPtrInst *Variadic,
int64_t AccumulativeByteOffset);
/// Lower a GEP with multiple indices into ptrtoint+arithmetics+inttoptr form.
/// Function splitGEP already split the original GEP into a variadic part and
/// a constant offset (i.e., AccumulativeByteOffset). This function lowers the
/// variadic part into a set of arithmetic operations and applies
/// AccumulativeByteOffset to it.
/// \p Variadic The variadic part of the original GEP.
/// \p AccumulativeByteOffset The constant offset.
void lowerToArithmetics(GetElementPtrInst *Variadic,
int64_t AccumulativeByteOffset);
/// Finds the constant offset within each index and accumulates them. If
/// LowerGEP is true, it finds in indices of both sequential and structure
/// types, otherwise it only finds in sequential indices. The output
/// NeedsExtraction indicates whether we successfully find a non-zero constant
/// offset.
int64_t accumulateByteOffset(GetElementPtrInst *GEP, bool &NeedsExtraction);
/// Canonicalize array indices to pointer-size integers. This helps to
/// simplify the logic of splitting a GEP. For example, if a + b is a
@ -274,6 +372,10 @@ class SeparateConstOffsetFromGEP : public FunctionPass {
bool canonicalizeArrayIndicesToPointerSize(GetElementPtrInst *GEP);
const DataLayout *DL;
const TargetMachine *TM;
/// Whether to lower a GEP with multiple indices into arithmetic operations or
/// multiple GEPs with a single index.
bool LowerGEP;
};
} // anonymous namespace
@ -289,8 +391,10 @@ INITIALIZE_PASS_END(
"Split GEPs to a variadic base and a constant offset for better CSE", false,
false)
FunctionPass *llvm::createSeparateConstOffsetFromGEPPass() {
return new SeparateConstOffsetFromGEP();
FunctionPass *
llvm::createSeparateConstOffsetFromGEPPass(const TargetMachine *TM,
bool LowerGEP) {
return new SeparateConstOffsetFromGEP(TM, LowerGEP);
}
bool ConstantOffsetExtractor::CanTraceInto(bool SignExtended,
@ -542,19 +646,17 @@ Value *ConstantOffsetExtractor::removeConstOffset(unsigned ChainIndex) {
return BO;
}
int64_t ConstantOffsetExtractor::Extract(Value *Idx, Value *&NewIdx,
const DataLayout *DL,
GetElementPtrInst *GEP) {
Value *ConstantOffsetExtractor::Extract(Value *Idx, const DataLayout *DL,
GetElementPtrInst *GEP) {
ConstantOffsetExtractor Extractor(DL, GEP);
// Find a non-zero constant offset first.
APInt ConstantOffset =
Extractor.find(Idx, /* SignExtended */ false, /* ZeroExtended */ false,
GEP->isInBounds());
if (ConstantOffset != 0) {
// Separates the constant offset from the GEP index.
NewIdx = Extractor.rebuildWithoutConstOffset();
}
return ConstantOffset.getSExtValue();
if (ConstantOffset == 0)
return nullptr;
// Separates the constant offset from the GEP index.
return Extractor.rebuildWithoutConstOffset();
}
int64_t ConstantOffsetExtractor::Find(Value *Idx, const DataLayout *DL,
@ -620,11 +722,116 @@ SeparateConstOffsetFromGEP::accumulateByteOffset(GetElementPtrInst *GEP,
AccumulativeByteOffset +=
ConstantOffset * DL->getTypeAllocSize(GTI.getIndexedType());
}
} else if (LowerGEP) {
StructType *StTy = cast<StructType>(*GTI);
uint64_t Field = cast<ConstantInt>(GEP->getOperand(I))->getZExtValue();
// Skip field 0 as the offset is always 0.
if (Field != 0) {
NeedsExtraction = true;
AccumulativeByteOffset +=
DL->getStructLayout(StTy)->getElementOffset(Field);
}
}
}
return AccumulativeByteOffset;
}
void SeparateConstOffsetFromGEP::lowerToSingleIndexGEPs(
GetElementPtrInst *Variadic, int64_t AccumulativeByteOffset) {
IRBuilder<> Builder(Variadic);
Type *IntPtrTy = DL->getIntPtrType(Variadic->getType());
Type *I8PtrTy =
Builder.getInt8PtrTy(Variadic->getType()->getPointerAddressSpace());
Value *ResultPtr = Variadic->getOperand(0);
if (ResultPtr->getType() != I8PtrTy)
ResultPtr = Builder.CreateBitCast(ResultPtr, I8PtrTy);
gep_type_iterator GTI = gep_type_begin(*Variadic);
// Create an ugly GEP for each sequential index. We don't create GEPs for
// structure indices, as they are accumulated in the constant offset index.
for (unsigned I = 1, E = Variadic->getNumOperands(); I != E; ++I, ++GTI) {
if (isa<SequentialType>(*GTI)) {
Value *Idx = Variadic->getOperand(I);
// Skip zero indices.
if (ConstantInt *CI = dyn_cast<ConstantInt>(Idx))
if (CI->isZero())
continue;
APInt ElementSize = APInt(IntPtrTy->getIntegerBitWidth(),
DL->getTypeAllocSize(GTI.getIndexedType()));
// Scale the index by element size.
if (ElementSize != 1) {
if (ElementSize.isPowerOf2()) {
Idx = Builder.CreateShl(
Idx, ConstantInt::get(IntPtrTy, ElementSize.logBase2()));
} else {
Idx = Builder.CreateMul(Idx, ConstantInt::get(IntPtrTy, ElementSize));
}
}
// Create an ugly GEP with a single index for each index.
ResultPtr = Builder.CreateGEP(ResultPtr, Idx, "uglygep");
}
}
// Create a GEP with the constant offset index.
if (AccumulativeByteOffset != 0) {
Value *Offset = ConstantInt::get(IntPtrTy, AccumulativeByteOffset);
ResultPtr = Builder.CreateGEP(ResultPtr, Offset, "uglygep");
}
if (ResultPtr->getType() != Variadic->getType())
ResultPtr = Builder.CreateBitCast(ResultPtr, Variadic->getType());
Variadic->replaceAllUsesWith(ResultPtr);
Variadic->eraseFromParent();
}
void
SeparateConstOffsetFromGEP::lowerToArithmetics(GetElementPtrInst *Variadic,
int64_t AccumulativeByteOffset) {
IRBuilder<> Builder(Variadic);
Type *IntPtrTy = DL->getIntPtrType(Variadic->getType());
Value *ResultPtr = Builder.CreatePtrToInt(Variadic->getOperand(0), IntPtrTy);
gep_type_iterator GTI = gep_type_begin(*Variadic);
// Create ADD/SHL/MUL arithmetic operations for each sequential indices. We
// don't create arithmetics for structure indices, as they are accumulated
// in the constant offset index.
for (unsigned I = 1, E = Variadic->getNumOperands(); I != E; ++I, ++GTI) {
if (isa<SequentialType>(*GTI)) {
Value *Idx = Variadic->getOperand(I);
// Skip zero indices.
if (ConstantInt *CI = dyn_cast<ConstantInt>(Idx))
if (CI->isZero())
continue;
APInt ElementSize = APInt(IntPtrTy->getIntegerBitWidth(),
DL->getTypeAllocSize(GTI.getIndexedType()));
// Scale the index by element size.
if (ElementSize != 1) {
if (ElementSize.isPowerOf2()) {
Idx = Builder.CreateShl(
Idx, ConstantInt::get(IntPtrTy, ElementSize.logBase2()));
} else {
Idx = Builder.CreateMul(Idx, ConstantInt::get(IntPtrTy, ElementSize));
}
}
// Create an ADD for each index.
ResultPtr = Builder.CreateAdd(ResultPtr, Idx);
}
}
// Create an ADD for the constant offset index.
if (AccumulativeByteOffset != 0) {
ResultPtr = Builder.CreateAdd(
ResultPtr, ConstantInt::get(IntPtrTy, AccumulativeByteOffset));
}
ResultPtr = Builder.CreateIntToPtr(ResultPtr, Variadic->getType());
Variadic->replaceAllUsesWith(ResultPtr);
Variadic->eraseFromParent();
}
bool SeparateConstOffsetFromGEP::splitGEP(GetElementPtrInst *GEP) {
// Skip vector GEPs.
if (GEP->getType()->isVectorTy())
@ -642,32 +849,42 @@ bool SeparateConstOffsetFromGEP::splitGEP(GetElementPtrInst *GEP) {
if (!NeedsExtraction)
return Changed;
// Before really splitting the GEP, check whether the backend supports the
// addressing mode we are about to produce. If no, this splitting probably
// won't be beneficial.
TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
if (!TTI.isLegalAddressingMode(GEP->getType()->getElementType(),
/*BaseGV=*/nullptr, AccumulativeByteOffset,
/*HasBaseReg=*/true, /*Scale=*/0)) {
return Changed;
// If LowerGEP is disabled, before really splitting the GEP, check whether the
// backend supports the addressing mode we are about to produce. If no, this
// splitting probably won't be beneficial.
// If LowerGEP is enabled, even the extracted constant offset can not match
// the addressing mode, we can still do optimizations to other lowered parts
// of variable indices. Therefore, we don't check for addressing modes in that
// case.
if (!LowerGEP) {
TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
if (!TTI.isLegalAddressingMode(GEP->getType()->getElementType(),
/*BaseGV=*/nullptr, AccumulativeByteOffset,
/*HasBaseReg=*/true, /*Scale=*/0)) {
return Changed;
}
}
// Remove the constant offset in each GEP index. The resultant GEP computes
// the variadic base.
// Remove the constant offset in each sequential index. The resultant GEP
// computes the variadic base.
// Notice that we don't remove struct field indices here. If LowerGEP is
// disabled, a structure index is not accumulated and we still use the old
// one. If LowerGEP is enabled, a structure index is accumulated in the
// constant offset. LowerToSingleIndexGEPs or lowerToArithmetics will later
// handle the constant offset and won't need a new structure index.
gep_type_iterator GTI = gep_type_begin(*GEP);
for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
if (isa<SequentialType>(*GTI)) {
Value *NewIdx = nullptr;
// Tries to extract a constant offset from this GEP index.
int64_t ConstantOffset =
ConstantOffsetExtractor::Extract(GEP->getOperand(I), NewIdx, DL, GEP);
if (ConstantOffset != 0) {
assert(NewIdx != nullptr &&
"ConstantOffset != 0 implies NewIdx is set");
// Splits this GEP index into a variadic part and a constant offset, and
// uses the variadic part as the new index.
Value *NewIdx =
ConstantOffsetExtractor::Extract(GEP->getOperand(I), DL, GEP);
if (NewIdx != nullptr) {
GEP->setOperand(I, NewIdx);
}
}
}
// Clear the inbounds attribute because the new index may be off-bound.
// e.g.,
//
@ -689,6 +906,21 @@ bool SeparateConstOffsetFromGEP::splitGEP(GetElementPtrInst *GEP) {
// possible. GEPs with inbounds are more friendly to alias analysis.
GEP->setIsInBounds(false);
// Lowers a GEP to either GEPs with a single index or arithmetic operations.
if (LowerGEP) {
// As currently BasicAA does not analyze ptrtoint/inttoptr, do not lower to
// arithmetic operations if the target uses alias analysis in codegen.
if (TM && TM->getSubtarget<TargetSubtargetInfo>().useAA())
lowerToSingleIndexGEPs(GEP, AccumulativeByteOffset);
else
lowerToArithmetics(GEP, AccumulativeByteOffset);
return true;
}
// No need to create another GEP if the accumulative byte offset is 0.
if (AccumulativeByteOffset == 0)
return true;
// Offsets the base with the accumulative byte offset.
//
// %gep ; the base