maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity
llvm-project/llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp

777 lines
31 KiB
C++
Raw Blame History

//===-- 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 (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 int64_t Find(Value *Idx, const DataLayout *DL, GetElementPtrInst *GEP);
private:
ConstantOffsetExtractor(const DataLayout *Layout, Instruction *InsertionPt)
: DL(Layout), IP(InsertionPt) {}
/// Searches the expression that computes V for a non-zero constant C s.t.
/// V can be reassociated into the form V' + C. If the searching is
/// successful, returns C and update UserChain as a def-use chain from C to V;
/// otherwise, UserChain is empty.
///
/// \p V The given expression
/// \p SignExtended Whether V will be sign-extended in the computation of the
/// GEP index
/// \p ZeroExtended Whether V will be zero-extended in the computation of the
/// GEP index
/// \p NonNegative Whether V is guaranteed to be non-negative. For example,
/// an index of an inbounds GEP is guaranteed to be
/// non-negative. Levaraging this, we can better split
/// inbounds GEPs.
APInt find(Value *V, bool SignExtended, bool ZeroExtended, bool NonNegative);
/// A helper function to look into both operands of a binary operator.
APInt findInEitherOperand(BinaryOperator *BO, bool SignExtended,
bool ZeroExtended);
/// After finding the constant offset C from the GEP index I, we build a new
/// index I' s.t. I' + C = I. This function builds and returns the new
/// index I' according to UserChain produced by function "find".
///
/// The building conceptually takes two steps:
/// 1) iteratively distribute s/zext towards the leaves of the expression tree
/// that computes I
/// 2) reassociate the expression tree to the form I' + C.
///
/// For example, to extract the 5 from sext(a + (b + 5)), we first distribute
/// sext to a, b and 5 so that we have
/// sext(a) + (sext(b) + 5).
/// Then, we reassociate it to
/// (sext(a) + sext(b)) + 5.
/// Given this form, we know I' is sext(a) + sext(b).
Value *rebuildWithoutConstOffset();
/// After the first step of rebuilding the GEP index without the constant
/// offset, distribute s/zext to the operands of all operators in UserChain.
/// e.g., zext(sext(a + (b + 5)) (assuming no overflow) =>
/// zext(sext(a)) + (zext(sext(b)) + zext(sext(5))).
///
/// The function also updates UserChain to point to new subexpressions after
/// distributing s/zext. e.g., the old UserChain of the above example is
/// 5 -> b + 5 -> a + (b + 5) -> sext(...) -> zext(sext(...)),
/// and the new UserChain is
/// zext(sext(5)) -> zext(sext(b)) + zext(sext(5)) ->
/// zext(sext(a)) + (zext(sext(b)) + zext(sext(5))
///
/// \p ChainIndex The index to UserChain. ChainIndex is initially
/// UserChain.size() - 1, and is decremented during
/// the recursion.
Value *distributeExtsAndCloneChain(unsigned ChainIndex);
/// Reassociates the GEP index to the form I' + C and returns I'.
Value *removeConstOffset(unsigned ChainIndex);
/// A helper function to apply ExtInsts, a list of s/zext, to value V.
/// e.g., if ExtInsts = [sext i32 to i64, zext i16 to i32], this function
/// returns "sext i32 (zext i16 V to i32) to i64".
Value *applyExts(Value *V);
/// 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;
/// A helper function that returns whether we can trace into the operands
/// of binary operator BO for a constant offset.
///
/// \p SignExtended Whether BO is surrounded by sext
/// \p ZeroExtended Whether BO is surrounded by zext
/// \p NonNegative Whether BO is known to be non-negative, e.g., an in-bound
/// array index.
bool CanTraceInto(bool SignExtended, bool ZeroExtended, BinaryOperator *BO,
bool NonNegative);
/// 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 to rebuild the new GEP index.
SmallVector<User *, 8> UserChain;
/// A data structure used in rebuildWithoutConstOffset. Contains all
/// sext/zext instructions along UserChain.
SmallVector<CastInst *, 16> ExtInsts;
/// 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 doInitialization(Module &M) override {
DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
if (DLP == nullptr)
report_fatal_error("data layout missing");
DL = &DLP->getDataLayout();
return false;
}
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, 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
/// pointer-size integer, we have
/// gep base, a + b = gep (gep base, a), b
/// However, this equality may not hold if the size of a + b is smaller than
/// the pointer size, because LLVM conceptually sign-extends GEP indices to
/// pointer size before computing the address
/// (http://llvm.org/docs/LangRef.html#id181).
///
/// This canonicalization is very likely already done in clang and
/// instcombine. Therefore, the program will probably remain the same.
///
/// Returns true if the module changes.
///
/// Verified in @i32_add in split-gep.ll
bool canonicalizeArrayIndicesToPointerSize(GetElementPtrInst *GEP);
const DataLayout *DL;
};
} // 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();
}
bool ConstantOffsetExtractor::CanTraceInto(bool SignExtended,
bool ZeroExtended,
BinaryOperator *BO,
bool NonNegative) {
// We only consider ADD, SUB and OR, because a non-zero constant found in
// expressions composed of these operations can be easily hoisted as a
// constant offset by reassociation.
if (BO->getOpcode() != Instruction::Add &&
BO->getOpcode() != Instruction::Sub &&
BO->getOpcode() != Instruction::Or) {
return false;
}
Value *LHS = BO->getOperand(0), *RHS = BO->getOperand(1);
// Do not trace into "or" unless it is equivalent to "add". If LHS and RHS
// don't have common bits, (LHS | RHS) is equivalent to (LHS + RHS).
if (BO->getOpcode() == Instruction::Or && !NoCommonBits(LHS, RHS))
return false;
// In addition, tracing into BO requires that its surrounding s/zext (if
// any) is distributable to both operands.
//
// Suppose BO = A op B.
// SignExtended | ZeroExtended | Distributable?
// --------------+--------------+----------------------------------
// 0 | 0 | true because no s/zext exists
// 0 | 1 | zext(BO) == zext(A) op zext(B)
// 1 | 0 | sext(BO) == sext(A) op sext(B)
// 1 | 1 | zext(sext(BO)) ==
// | | zext(sext(A)) op zext(sext(B))
if (BO->getOpcode() == Instruction::Add && !ZeroExtended && NonNegative) {
// If a + b >= 0 and (a >= 0 or b >= 0), then
// sext(a + b) = sext(a) + sext(b)
// even if the addition is not marked nsw.
//
// Leveraging this invarient, we can trace into an sext'ed inbound GEP
// index if the constant offset is non-negative.
//
// Verified in @sext_add in split-gep.ll.
if (ConstantInt *ConstLHS = dyn_cast<ConstantInt>(LHS)) {
if (!ConstLHS->isNegative())
return true;
}
if (ConstantInt *ConstRHS = dyn_cast<ConstantInt>(RHS)) {
if (!ConstRHS->isNegative())
return true;
}
}
// sext (add/sub nsw A, B) == add/sub nsw (sext A), (sext B)
// zext (add/sub nuw A, B) == add/sub nuw (zext A), (zext B)
if (BO->getOpcode() == Instruction::Add ||
BO->getOpcode() == Instruction::Sub) {
if (SignExtended && !BO->hasNoSignedWrap())
return false;
if (ZeroExtended && !BO->hasNoUnsignedWrap())
return false;
}
return true;
}
APInt ConstantOffsetExtractor::findInEitherOperand(BinaryOperator *BO,
bool SignExtended,
bool ZeroExtended) {
// BO being non-negative does not shed light on whether its operands are
// non-negative. Clear the NonNegative flag here.
APInt ConstantOffset = find(BO->getOperand(0), SignExtended, ZeroExtended,
/* NonNegative */ false);
// 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(BO->getOperand(1), SignExtended, ZeroExtended,
/* NonNegative */ false);
// If U is a sub operator, negate the constant offset found in the right
// operand.
if (BO->getOpcode() == Instruction::Sub)
ConstantOffset = -ConstantOffset;
return ConstantOffset;
}
APInt ConstantOffsetExtractor::find(Value *V, bool SignExtended,
bool ZeroExtended, bool NonNegative) {
// TODO(jingyue): We could 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.
unsigned BitWidth = cast<IntegerType>(V->getType())->getBitWidth();
// We cannot do much with Values that are not a User, such as an Argument.
User *U = dyn_cast<User>(V);
if (U == nullptr) return APInt(BitWidth, 0);
APInt ConstantOffset(BitWidth, 0);
if (ConstantInt *CI = dyn_cast<ConstantInt>(V)) {
// Hooray, we found it!
ConstantOffset = CI->getValue();
} else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V)) {
// Trace into subexpressions for more hoisting opportunities.
if (CanTraceInto(SignExtended, ZeroExtended, BO, NonNegative)) {
ConstantOffset = findInEitherOperand(BO, SignExtended, ZeroExtended);
}
} else if (isa<SExtInst>(V)) {
ConstantOffset = find(U->getOperand(0), /* SignExtended */ true,
ZeroExtended, NonNegative).sext(BitWidth);
} else if (isa<ZExtInst>(V)) {
// As an optimization, we can clear the SignExtended flag because
// sext(zext(a)) = zext(a). Verified in @sext_zext in split-gep.ll.
//
// Clear the NonNegative flag, because zext(a) >= 0 does not imply a >= 0.
ConstantOffset =
find(U->getOperand(0), /* SignExtended */ false,
/* ZeroExtended */ true, /* NonNegative */ false).zext(BitWidth);
}
// If we found a non-zero constant offset, add it to the path for
// rebuildWithoutConstOffset. Zero is a valid constant offset, but doesn't
// help this optimization.
if (ConstantOffset != 0)
UserChain.push_back(U);
return ConstantOffset;
}
Value *ConstantOffsetExtractor::applyExts(Value *V) {
Value *Current = V;
// ExtInsts is built in the use-def order. Therefore, we apply them to V
// in the reversed order.
for (auto I = ExtInsts.rbegin(), E = ExtInsts.rend(); I != E; ++I) {
if (Constant *C = dyn_cast<Constant>(Current)) {
// If Current is a constant, apply s/zext using ConstantExpr::getCast.
// ConstantExpr::getCast emits a ConstantInt if C is a ConstantInt.
Current = ConstantExpr::getCast((*I)->getOpcode(), C, (*I)->getType());
} else {
Instruction *Ext = (*I)->clone();
Ext->setOperand(0, Current);
Ext->insertBefore(IP);
Current = Ext;
}
}
return Current;
}
Value *ConstantOffsetExtractor::rebuildWithoutConstOffset() {
distributeExtsAndCloneChain(UserChain.size() - 1);
// Remove all nullptrs (used to be s/zext) from UserChain.
unsigned NewSize = 0;
for (auto I = UserChain.begin(), E = UserChain.end(); I != E; ++I) {
if (*I != nullptr) {
UserChain[NewSize] = *I;
NewSize++;
}
}
UserChain.resize(NewSize);
return removeConstOffset(UserChain.size() - 1);
}
Value *
ConstantOffsetExtractor::distributeExtsAndCloneChain(unsigned ChainIndex) {
User *U = UserChain[ChainIndex];
if (ChainIndex == 0) {
assert(isa<ConstantInt>(U));
// If U is a ConstantInt, applyExts will return a ConstantInt as well.
return UserChain[ChainIndex] = cast<ConstantInt>(applyExts(U));
}
if (CastInst *Cast = dyn_cast<CastInst>(U)) {
assert((isa<SExtInst>(Cast) || isa<ZExtInst>(Cast)) &&
"We only traced into two types of CastInst: sext and zext");
ExtInsts.push_back(Cast);
UserChain[ChainIndex] = nullptr;
return distributeExtsAndCloneChain(ChainIndex - 1);
}
// Function find only trace into BinaryOperator and CastInst.
BinaryOperator *BO = cast<BinaryOperator>(U);
// OpNo = which operand of BO is UserChain[ChainIndex - 1]
unsigned OpNo = (BO->getOperand(0) == UserChain[ChainIndex - 1] ? 0 : 1);
Value *TheOther = applyExts(BO->getOperand(1 - OpNo));
Value *NextInChain = distributeExtsAndCloneChain(ChainIndex - 1);
BinaryOperator *NewBO = nullptr;
if (OpNo == 0) {
NewBO = BinaryOperator::Create(BO->getOpcode(), NextInChain, TheOther,
BO->getName(), IP);
} else {
NewBO = BinaryOperator::Create(BO->getOpcode(), TheOther, NextInChain,
BO->getName(), IP);
}
return UserChain[ChainIndex] = NewBO;
}
Value *ConstantOffsetExtractor::removeConstOffset(unsigned ChainIndex) {
if (ChainIndex == 0) {
assert(isa<ConstantInt>(UserChain[ChainIndex]));
return ConstantInt::getNullValue(UserChain[ChainIndex]->getType());
}
BinaryOperator *BO = cast<BinaryOperator>(UserChain[ChainIndex]);
unsigned OpNo = (BO->getOperand(0) == UserChain[ChainIndex - 1] ? 0 : 1);
assert(BO->getOperand(OpNo) == UserChain[ChainIndex - 1]);
Value *NextInChain = removeConstOffset(ChainIndex - 1);
Value *TheOther = BO->getOperand(1 - OpNo);
// If NextInChain is 0 and not the LHS of a sub, we can simplify the
// sub-expression to be just TheOther.
if (ConstantInt *CI = dyn_cast<ConstantInt>(NextInChain)) {
if (CI->isZero() && !(BO->getOpcode() == Instruction::Sub && OpNo == 0))
return TheOther;
}
if (BO->getOpcode() == Instruction::Or) {
// Rebuild "or" as "add", because "or" may be invalid for the new
// epxression.
//
// For instance, given
// a | (b + 5) where a and b + 5 have no common bits,
// we can extract 5 as the constant offset.
//
// However, reusing the "or" in the new index would give us
// (a | b) + 5
// which does not equal a | (b + 5).
//
// Replacing the "or" with "add" is fine, because
// a | (b + 5) = a + (b + 5) = (a + b) + 5
return BinaryOperator::CreateAdd(BO->getOperand(0), BO->getOperand(1),
BO->getName(), IP);
}
// We can reuse BO in this case, because the new expression shares the same
// instruction type and BO is used at most once.
assert(BO->getNumUses() <= 1 &&
"distributeExtsAndCloneChain clones each BinaryOperator in "
"UserChain, so no one should be used more than "
"once");
BO->setOperand(OpNo, NextInChain);
BO->setHasNoSignedWrap(false);
BO->setHasNoUnsignedWrap(false);
// Make sure it appears after all instructions we've inserted so far.
BO->moveBefore(IP);
return BO;
}
int64_t ConstantOffsetExtractor::Extract(Value *Idx, Value *&NewIdx,
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();
}
int64_t ConstantOffsetExtractor::Find(Value *Idx, const DataLayout *DL,
GetElementPtrInst *GEP) {
// If Idx is an index of an inbound GEP, Idx is guaranteed to be non-negative.
return ConstantOffsetExtractor(DL, GEP)
.find(Idx, /* SignExtended */ false, /* ZeroExtended */ false,
GEP->isInBounds())
.getSExtValue();
}
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();
}
bool SeparateConstOffsetFromGEP::canonicalizeArrayIndicesToPointerSize(
GetElementPtrInst *GEP) {
bool Changed = false;
Type *IntPtrTy = DL->getIntPtrType(GEP->getType());
gep_type_iterator GTI = gep_type_begin(*GEP);
for (User::op_iterator I = GEP->op_begin() + 1, E = GEP->op_end();
I != E; ++I, ++GTI) {
// Skip struct member indices which must be i32.
if (isa<SequentialType>(*GTI)) {
if ((*I)->getType() != IntPtrTy) {
*I = CastInst::CreateIntegerCast(*I, IntPtrTy, true, "idxprom", GEP);
Changed = true;
}
}
}
return Changed;
}
int64_t
SeparateConstOffsetFromGEP::accumulateByteOffset(GetElementPtrInst *GEP,
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, GEP);
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 = canonicalizeArrayIndicesToPointerSize(GEP);
bool NeedsExtraction;
int64_t AccumulativeByteOffset = accumulateByteOffset(GEP, 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 != nullptr &&
"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);
// Offsets the base with the accumulative byte offset.
//
// %gep ; the base
// ... %gep ...
//
// => add the offset
//
// %gep2 ; clone of %gep
// %new.gep = gep %gep2, <offset / sizeof(*%gep)>
// %gep ; will be removed
// ... %gep ...
//
// => replace all uses of %gep with %new.gep and remove %gep
//
// %gep2 ; clone of %gep
// %new.gep = gep %gep2, <offset / sizeof(*%gep)>
// ... %new.gep ...
//
// If AccumulativeByteOffset is not a multiple of sizeof(*%gep), we emit an
// uglygep (http://llvm.org/docs/GetElementPtr.html#what-s-an-uglygep):
// bitcast %gep2 to i8*, add the offset, and bitcast the result back to the
// type of %gep.
//
// %gep2 ; clone of %gep
// %0 = bitcast %gep2 to i8*
// %uglygep = gep %0, <offset>
// %new.gep = bitcast %uglygep to <type of %gep>
// ... %new.gep ...
Instruction *NewGEP = GEP->clone();
NewGEP->insertBefore(GEP);
uint64_t ElementTypeSizeOfGEP =
DL->getTypeAllocSize(GEP->getType()->getElementType());
Type *IntPtrTy = DL->getIntPtrType(GEP->getType());
if (AccumulativeByteOffset % ElementTypeSizeOfGEP == 0) {
// Very likely. As long as %gep is natually aligned, the byte offset we
// extracted should be a multiple of sizeof(*%gep).
// Per ANSI C standard, signed / unsigned = unsigned. Therefore, we
// cast ElementTypeSizeOfGEP to signed.
int64_t Index =
AccumulativeByteOffset / static_cast<int64_t>(ElementTypeSizeOfGEP);
NewGEP = GetElementPtrInst::Create(
NewGEP, ConstantInt::get(IntPtrTy, Index, true), GEP->getName(), GEP);
} else {
// Unlikely but possible. For example,
// #pragma pack(1)
// struct S {
// int a[3];
// int64 b[8];
// };
// #pragma pack()
//
// Suppose the gep before extraction is &s[i + 1].b[j + 3]. After
// extraction, it becomes &s[i].b[j] and AccumulativeByteOffset is
// sizeof(S) + 3 * sizeof(int64) = 100, which is not a multiple of
// sizeof(int64).
//
// Emit an uglygep in this case.
Type *I8PtrTy = Type::getInt8PtrTy(GEP->getContext(),
GEP->getPointerAddressSpace());
NewGEP = new BitCastInst(NewGEP, I8PtrTy, "", GEP);
NewGEP = GetElementPtrInst::Create(
NewGEP, ConstantInt::get(IntPtrTy, AccumulativeByteOffset, true),
"uglygep", GEP);
if (GEP->getType() != I8PtrTy)
NewGEP = new BitCastInst(NewGEP, GEP->getType(), GEP->getName(), GEP);
}
GEP->replaceAllUsesWith(NewGEP);
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;
}
Reference in New Issue View Git Blame Copy Permalink