llvm-6502/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp

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//===-- SeparateConstOffsetFromGEP.cpp - ------------------------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Loop unrolling may create many similar GEPs for array accesses.
// e.g., a 2-level loop
//
// float a[32][32]; // global variable
//
// for (int i = 0; i < 2; ++i) {
// for (int j = 0; j < 2; ++j) {
// ...
// ... = a[x + i][y + j];
// ...
// }
// }
//
// will probably be unrolled to:
//
// gep %a, 0, %x, %y; load
// gep %a, 0, %x, %y + 1; load
// gep %a, 0, %x + 1, %y; load
// gep %a, 0, %x + 1, %y + 1; load
//
// LLVM's GVN does not use partial redundancy elimination yet, and is thus
// unable to reuse (gep %a, 0, %x, %y). As a result, this misoptimization incurs
// significant slowdown in targets with limited addressing modes. For instance,
// because the PTX target does not support the reg+reg addressing mode, the
// NVPTX backend emits PTX code that literally computes the pointer address of
// each GEP, wasting tons of registers. It emits the following PTX for the
// first load and similar PTX for other loads.
//
// mov.u32 %r1, %x;
// mov.u32 %r2, %y;
// mul.wide.u32 %rl2, %r1, 128;
// mov.u64 %rl3, a;
// add.s64 %rl4, %rl3, %rl2;
// mul.wide.u32 %rl5, %r2, 4;
// add.s64 %rl6, %rl4, %rl5;
// ld.global.f32 %f1, [%rl6];
//
// To reduce the register pressure, the optimization implemented in this file
// merges the common part of a group of GEPs, so we can compute each pointer
// address by adding a simple offset to the common part, saving many registers.
//
// It works by splitting each GEP into a variadic base and a constant offset.
// The variadic base can be computed once and reused by multiple GEPs, and the
// constant offsets can be nicely folded into the reg+immediate addressing mode
// (supported by most targets) without using any extra register.
//
// For instance, we transform the four GEPs and four loads in the above example
// into:
//
// base = gep a, 0, x, y
// load base
// laod base + 1 * sizeof(float)
// load base + 32 * sizeof(float)
// load base + 33 * sizeof(float)
//
// Given the transformed IR, a backend that supports the reg+immediate
// addressing mode can easily fold the pointer arithmetics into the loads. For
// example, the NVPTX backend can easily fold the pointer arithmetics into the
// ld.global.f32 instructions, and the resultant PTX uses much fewer registers.
//
// mov.u32 %r1, %tid.x;
// mov.u32 %r2, %tid.y;
// mul.wide.u32 %rl2, %r1, 128;
// mov.u64 %rl3, a;
// add.s64 %rl4, %rl3, %rl2;
// mul.wide.u32 %rl5, %r2, 4;
// add.s64 %rl6, %rl4, %rl5;
// ld.global.f32 %f1, [%rl6]; // so far the same as unoptimized PTX
// ld.global.f32 %f2, [%rl6+4]; // much better
// ld.global.f32 %f3, [%rl6+128]; // much better
// ld.global.f32 %f4, [%rl6+132]; // much better
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
using namespace llvm;
static cl::opt<bool> DisableSeparateConstOffsetFromGEP(
"disable-separate-const-offset-from-gep", cl::init(false),
cl::desc("Do not separate the constant offset from a GEP instruction"),
cl::Hidden);
namespace {
/// \brief A helper class for separating a constant offset from a GEP index.
///
/// In real programs, a GEP index may be more complicated than a simple addition
/// of something and a constant integer which can be trivially splitted. For
/// example, to split ((a << 3) | 5) + b, we need to search deeper for the
/// constant offset, so that we can separate the index to (a << 3) + b and 5.
///
/// Therefore, this class looks into the expression that computes a given GEP
/// index, and tries to find a constant integer that can be hoisted to the
/// outermost level of the expression as an addition. Not every constant in an
/// expression can jump out. e.g., we cannot transform (b * (a + 5)) to (b * a +
/// 5); nor can we transform (3 * (a + 5)) to (3 * a + 5), however in this case,
/// -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
/// new index representing the remainder (equal to the original index minus
/// the constant offset).
/// \p Idx The given GEP index
/// \p NewIdx The new index to replace
/// \p DL The datalayout of the module
/// \p IP Calculating the new index requires new instructions. IP indicates
/// where to insert them (typically right before the GEP).
static int64_t Extract(Value *Idx, Value *&NewIdx, const DataLayout *DL,
Instruction *IP);
/// Looks for a constant offset without extracting it. The meaning of the
/// arguments and the return value are the same as Extract.
static int64_t Find(Value *Idx, const DataLayout *DL);
private:
ConstantOffsetExtractor(const DataLayout *Layout, Instruction *InsertionPt)
: DL(Layout), IP(InsertionPt) {}
/// Searches the expression that computes V for a constant offset. If the
/// searching is successful, update UserChain as a path from V to the constant
/// offset.
int64_t find(Value *V);
/// A helper function to look into both operands of a binary operator U.
/// \p IsSub Whether U is a sub operator. If so, we need to negate the
/// constant offset at some point.
int64_t findInEitherOperand(User *U, bool IsSub);
/// After finding the constant offset and how it is reached from the GEP
/// index, we build a new index which is a clone of the old one except the
/// constant offset is removed. For example, given (a + (b + 5)) and knowning
/// the constant offset is 5, this function returns (a + b).
///
/// We cannot simply change the constant to zero because the expression that
/// computes the index or its intermediate result may be used by others.
Value *rebuildWithoutConstantOffset();
// A helper function for rebuildWithoutConstantOffset that rebuilds the direct
// user (U) of the constant offset (C).
Value *rebuildLeafWithoutConstantOffset(User *U, Value *C);
/// Returns a clone of U except the first occurrence of From with To.
Value *cloneAndReplace(User *U, Value *From, Value *To);
/// Returns true if LHS and RHS have no bits in common, i.e., LHS | RHS == 0.
bool NoCommonBits(Value *LHS, Value *RHS) const;
/// Computes which bits are known to be one or zero.
/// \p KnownOne Mask of all bits that are known to be one.
/// \p KnownZero Mask of all bits that are known to be zero.
void ComputeKnownBits(Value *V, APInt &KnownOne, APInt &KnownZero) const;
/// Finds the first use of Used in U. Returns -1 if not found.
static unsigned FindFirstUse(User *U, Value *Used);
/// The path from the constant offset to the old GEP index. e.g., if the GEP
/// index is "a * b + (c + 5)". After running function find, UserChain[0] will
/// be the constant 5, UserChain[1] will be the subexpression "c + 5", and
/// UserChain[2] will be the entire expression "a * b + (c + 5)".
///
/// This path helps rebuildWithoutConstantOffset rebuild the new GEP index.
SmallVector<User *, 8> UserChain;
/// The data layout of the module. Used in ComputeKnownBits.
const DataLayout *DL;
Instruction *IP; /// Insertion position of cloned instructions.
};
/// \brief A pass that tries to split every GEP in the function into a variadic
/// base and a constant offset. It is a FunctionPass because searching for the
/// constant offset may inspect other basic blocks.
class SeparateConstOffsetFromGEP : public FunctionPass {
public:
static char ID;
SeparateConstOffsetFromGEP() : FunctionPass(ID) {
initializeSeparateConstOffsetFromGEPPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<DataLayoutPass>();
AU.addRequired<TargetTransformInfo>();
}
bool runOnFunction(Function &F) override;
private:
/// 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.
int64_t accumulateByteOffset(GetElementPtrInst *GEP, const DataLayout *DL,
bool &NeedsExtraction);
};
} // anonymous namespace
char SeparateConstOffsetFromGEP::ID = 0;
INITIALIZE_PASS_BEGIN(
SeparateConstOffsetFromGEP, "separate-const-offset-from-gep",
"Split GEPs to a variadic base and a constant offset for better CSE", false,
false)
INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
INITIALIZE_PASS_DEPENDENCY(DataLayoutPass)
INITIALIZE_PASS_END(
SeparateConstOffsetFromGEP, "separate-const-offset-from-gep",
"Split GEPs to a variadic base and a constant offset for better CSE", false,
false)
FunctionPass *llvm::createSeparateConstOffsetFromGEPPass() {
return new SeparateConstOffsetFromGEP();
}
int64_t ConstantOffsetExtractor::findInEitherOperand(User *U, bool IsSub) {
assert(U->getNumOperands() == 2);
int64_t ConstantOffset = find(U->getOperand(0));
// If we found a constant offset in the left operand, stop and return that.
// This shortcut might cause us to miss opportunities of combining the
// constant offsets in both operands, e.g., (a + 4) + (b + 5) => (a + b) + 9.
// However, such cases are probably already handled by -instcombine,
// given this pass runs after the standard optimizations.
if (ConstantOffset != 0) return ConstantOffset;
ConstantOffset = find(U->getOperand(1));
// If U is a sub operator, negate the constant offset found in the right
// operand.
return IsSub ? -ConstantOffset : ConstantOffset;
}
int64_t ConstantOffsetExtractor::find(Value *V) {
// TODO(jingyue): We can even trace into integer/pointer casts, such as
// inttoptr, ptrtoint, bitcast, and addrspacecast. We choose to handle only
// integers because it gives good enough results for our benchmarks.
assert(V->getType()->isIntegerTy());
User *U = dyn_cast<User>(V);
// We cannot do much with Values that are not a User, such as BasicBlock and
// MDNode.
if (U == nullptr) return 0;
int64_t ConstantOffset = 0;
if (ConstantInt *CI = dyn_cast<ConstantInt>(U)) {
// Hooray, we found it!
ConstantOffset = CI->getSExtValue();
} else if (Operator *O = dyn_cast<Operator>(U)) {
// The GEP index may be more complicated than a simple addition of a
// varaible and a constant. Therefore, we trace into subexpressions for more
// hoisting opportunities.
switch (O->getOpcode()) {
case Instruction::Add: {
ConstantOffset = findInEitherOperand(U, false);
break;
}
case Instruction::Sub: {
ConstantOffset = findInEitherOperand(U, true);
break;
}
case Instruction::Or: {
// If LHS and RHS don't have common bits, (LHS | RHS) is equivalent to
// (LHS + RHS).
if (NoCommonBits(U->getOperand(0), U->getOperand(1)))
ConstantOffset = findInEitherOperand(U, false);
break;
}
case Instruction::SExt: {
// For safety, we trace into sext only when its operand is marked
// "nsw" because xxx.nsw guarantees no signed wrap. e.g., we can safely
// transform "sext (add nsw a, 5)" into "add nsw (sext a), 5".
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(U->getOperand(0))) {
if (BO->hasNoSignedWrap())
ConstantOffset = find(U->getOperand(0));
}
break;
}
case Instruction::ZExt: {
// Similarly, we trace into zext only when its operand is marked with
// "nuw" because zext (add nuw a, b) == add nuw (zext a), (zext b).
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(U->getOperand(0))) {
if (BO->hasNoUnsignedWrap())
ConstantOffset = find(U->getOperand(0));
}
break;
}
}
}
// If we found a non-zero constant offset, adds it to the path for future
// transformation (rebuildWithoutConstantOffset). Zero is a valid constant
// offset, but doesn't help this optimization.
if (ConstantOffset != 0)
UserChain.push_back(U);
return ConstantOffset;
}
unsigned ConstantOffsetExtractor::FindFirstUse(User *U, Value *Used) {
for (unsigned I = 0, E = U->getNumOperands(); I < E; ++I) {
if (U->getOperand(I) == Used)
return I;
}
return -1;
}
Value *ConstantOffsetExtractor::cloneAndReplace(User *U, Value *From,
Value *To) {
// Finds in U the first use of From. It is safe to ignore future occurrences
// of From, because findInEitherOperand similarly stops searching the right
// operand when the first operand has a non-zero constant offset.
unsigned OpNo = FindFirstUse(U, From);
assert(OpNo != (unsigned)-1 && "UserChain wasn't built correctly");
// ConstantOffsetExtractor::find only follows Operators (i.e., Instructions
// and ConstantExprs). Therefore, U is either an Instruction or a
// ConstantExpr.
if (Instruction *I = dyn_cast<Instruction>(U)) {
Instruction *Clone = I->clone();
Clone->setOperand(OpNo, To);
Clone->insertBefore(IP);
return Clone;
}
// cast<Constant>(To) is safe because a ConstantExpr only uses Constants.
return cast<ConstantExpr>(U)
->getWithOperandReplaced(OpNo, cast<Constant>(To));
}
Value *ConstantOffsetExtractor::rebuildLeafWithoutConstantOffset(User *U,
Value *C) {
assert(U->getNumOperands() <= 2 &&
"We didn't trace into any operator with more than 2 operands");
// If U has only one operand which is the constant offset, removing the
// constant offset leaves U as a null value.
if (U->getNumOperands() == 1)
return Constant::getNullValue(U->getType());
// U->getNumOperands() == 2
unsigned OpNo = FindFirstUse(U, C); // U->getOperand(OpNo) == C
assert(OpNo < 2 && "UserChain wasn't built correctly");
Value *TheOther = U->getOperand(1 - OpNo); // The other operand of U
// If U = C - X, removing C makes U = -X; otherwise U will simply be X.
if (!isa<SubOperator>(U) || OpNo == 1)
return TheOther;
if (isa<ConstantExpr>(U))
return ConstantExpr::getNeg(cast<Constant>(TheOther));
return BinaryOperator::CreateNeg(TheOther, "", IP);
}
Value *ConstantOffsetExtractor::rebuildWithoutConstantOffset() {
assert(UserChain.size() > 0 && "you at least found a constant, right?");
// Start with the constant and go up through UserChain, each time building a
// clone of the subexpression but with the constant removed.
// e.g., to build a clone of (a + (b + (c + 5)) but with the 5 removed, we
// first c, then (b + c), and finally (a + (b + c)).
//
// Fast path: if the GEP index is a constant, simply returns 0.
if (UserChain.size() == 1)
return ConstantInt::get(UserChain[0]->getType(), 0);
Value *Remainder =
rebuildLeafWithoutConstantOffset(UserChain[1], UserChain[0]);
for (size_t I = 2; I < UserChain.size(); ++I)
Remainder = cloneAndReplace(UserChain[I], UserChain[I - 1], Remainder);
return Remainder;
}
int64_t ConstantOffsetExtractor::Extract(Value *Idx, Value *&NewIdx,
const DataLayout *DL,
Instruction *IP) {
ConstantOffsetExtractor Extractor(DL, IP);
// Find a non-zero constant offset first.
int64_t ConstantOffset = Extractor.find(Idx);
if (ConstantOffset == 0)
return 0;
// Then rebuild a new index with the constant removed.
NewIdx = Extractor.rebuildWithoutConstantOffset();
return ConstantOffset;
}
int64_t ConstantOffsetExtractor::Find(Value *Idx, const DataLayout *DL) {
return ConstantOffsetExtractor(DL, nullptr).find(Idx);
}
void ConstantOffsetExtractor::ComputeKnownBits(Value *V, APInt &KnownOne,
APInt &KnownZero) const {
IntegerType *IT = cast<IntegerType>(V->getType());
KnownOne = APInt(IT->getBitWidth(), 0);
KnownZero = APInt(IT->getBitWidth(), 0);
llvm::computeKnownBits(V, KnownZero, KnownOne, DL, 0);
}
bool ConstantOffsetExtractor::NoCommonBits(Value *LHS, Value *RHS) const {
assert(LHS->getType() == RHS->getType() &&
"LHS and RHS should have the same type");
APInt LHSKnownOne, LHSKnownZero, RHSKnownOne, RHSKnownZero;
ComputeKnownBits(LHS, LHSKnownOne, LHSKnownZero);
ComputeKnownBits(RHS, RHSKnownOne, RHSKnownZero);
return (LHSKnownZero | RHSKnownZero).isAllOnesValue();
}
int64_t SeparateConstOffsetFromGEP::accumulateByteOffset(
GetElementPtrInst *GEP, const DataLayout *DL, bool &NeedsExtraction) {
NeedsExtraction = false;
int64_t AccumulativeByteOffset = 0;
gep_type_iterator GTI = gep_type_begin(*GEP);
for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
if (isa<SequentialType>(*GTI)) {
// Tries to extract a constant offset from this GEP index.
int64_t ConstantOffset =
ConstantOffsetExtractor::Find(GEP->getOperand(I), DL);
if (ConstantOffset != 0) {
NeedsExtraction = true;
// A GEP may have multiple indices. We accumulate the extracted
// constant offset to a byte offset, and later offset the remainder of
// the original GEP with this byte offset.
AccumulativeByteOffset +=
ConstantOffset * DL->getTypeAllocSize(GTI.getIndexedType());
}
}
}
return AccumulativeByteOffset;
}
bool SeparateConstOffsetFromGEP::splitGEP(GetElementPtrInst *GEP) {
// Skip vector GEPs.
if (GEP->getType()->isVectorTy())
return false;
// The backend can already nicely handle the case where all indices are
// constant.
if (GEP->hasAllConstantIndices())
return false;
bool Changed = false;
// Shortcuts integer casts. Eliminating these explicit casts can make
// subsequent optimizations more obvious: ConstantOffsetExtractor needn't
// trace into these casts.
if (GEP->isInBounds()) {
// Doing this to inbounds GEPs is safe because their indices are guaranteed
// to be non-negative and in bounds.
gep_type_iterator GTI = gep_type_begin(*GEP);
for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
if (isa<SequentialType>(*GTI)) {
if (Operator *O = dyn_cast<Operator>(GEP->getOperand(I))) {
if (O->getOpcode() == Instruction::SExt ||
O->getOpcode() == Instruction::ZExt) {
GEP->setOperand(I, O->getOperand(0));
Changed = true;
}
}
}
}
}
const DataLayout *DL = &getAnalysis<DataLayoutPass>().getDataLayout();
bool NeedsExtraction;
int64_t AccumulativeByteOffset =
accumulateByteOffset(GEP, DL, NeedsExtraction);
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;
}
// Remove the constant offset in each GEP index. The resultant GEP computes
// the variadic base.
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 && "ConstantOffset != 0 implies NewIdx is set");
GEP->setOperand(I, NewIdx);
// Clear the inbounds attribute because the new index may be off-bound.
// e.g.,
//
// b = add i64 a, 5
// addr = gep inbounds float* p, i64 b
//
// is transformed to:
//
// addr2 = gep float* p, i64 a
// addr = gep float* addr2, i64 5
//
// If a is -4, although the old index b is in bounds, the new index a is
// off-bound. http://llvm.org/docs/LangRef.html#id181 says "if the
// inbounds keyword is not present, the offsets are added to the base
// address with silently-wrapping two's complement arithmetic".
// Therefore, the final code will be a semantically equivalent.
//
// TODO(jingyue): do some range analysis to keep as many inbounds as
// possible. GEPs with inbounds are more friendly to alias analysis.
GEP->setIsInBounds(false);
Changed = true;
}
}
}
// Offsets the base with the accumulative byte offset.
//
// %gep ; the base
// ... %gep ...
//
// => add the offset
//
// %gep2 ; clone of %gep
// %0 = ptrtoint %gep2
// %1 = add %0, <offset>
// %new.gep = inttoptr %1
// %gep ; will be removed
// ... %gep ...
//
// => replace all uses of %gep with %new.gep and remove %gep
//
// %gep2 ; clone of %gep
// %0 = ptrtoint %gep2
// %1 = add %0, <offset>
// %new.gep = inttoptr %1
// ... %new.gep ...
//
// TODO(jingyue): Emit a GEP instead of an "uglygep"
// (http://llvm.org/docs/GetElementPtr.html#what-s-an-uglygep) to make the IR
// prettier and more alias analysis friendly. One caveat: if the original GEP
// ends with a StructType, we need to split the GEP at the last
// SequentialType. For instance, consider the following IR:
//
// %struct.S = type { float, double }
// @array = global [1024 x %struct.S]
// %p = getelementptr %array, 0, %i + 5, 1
//
// To separate the constant 5 from %p, we would need to split %p at the last
// array index so that we have:
//
// %addr = gep %array, 0, %i
// %p = gep %addr, 5, 1
Instruction *NewGEP = GEP->clone();
NewGEP->insertBefore(GEP);
Type *IntPtrTy = DL->getIntPtrType(GEP->getType());
Value *Addr = new PtrToIntInst(NewGEP, IntPtrTy, "", GEP);
Addr = BinaryOperator::CreateAdd(
Addr, ConstantInt::get(IntPtrTy, AccumulativeByteOffset, true), "", GEP);
Addr = new IntToPtrInst(Addr, GEP->getType(), "", GEP);
GEP->replaceAllUsesWith(Addr);
GEP->eraseFromParent();
return true;
}
bool SeparateConstOffsetFromGEP::runOnFunction(Function &F) {
if (DisableSeparateConstOffsetFromGEP)
return false;
bool Changed = false;
for (Function::iterator B = F.begin(), BE = F.end(); B != BE; ++B) {
for (BasicBlock::iterator I = B->begin(), IE = B->end(); I != IE; ) {
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I++)) {
Changed |= splitGEP(GEP);
}
// No need to split GEP ConstantExprs because all its indices are constant
// already.
}
}
return Changed;
}