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llvm-project/llvm/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
//
// 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/ScalarEvolution.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Operator.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetSubtargetInfo.h"
#include "llvm/IR/IRBuilder.h"
using namespace llvm;
using namespace llvm::PatternMatch;
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);
// Setting this flag may emit false positives when the input module already
// contains dead instructions. Therefore, we set it only in unit tests that are
// free of dead code.
static cl::opt<bool>
VerifyNoDeadCode("reassociate-geps-verify-no-dead-code", cl::init(false),
cl::desc("Verify this pass produces no dead code"),
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 returns the
/// new index representing the remainder (equal to the original index minus
/// the constant offset), or nullptr if we cannot extract a constant offset.
/// \p Idx The given GEP index
/// \p GEP The given GEP
/// \p UserChainTail Outputs the tail of UserChain so that we can
/// garbage-collect unused instructions in UserChain.
static Value *Extract(Value *Idx, GetElementPtrInst *GEP,
User *&UserChainTail, const DominatorTree *DT);
/// 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, GetElementPtrInst *GEP,
const DominatorTree *DT);
private:
ConstantOffsetExtractor(Instruction *InsertionPt, const DominatorTree *DT)
: IP(InsertionPt), DL(InsertionPt->getModule()->getDataLayout()), DT(DT) {
}
/// 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);
/// 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;
Instruction *IP; /// Insertion position of cloned instructions.
const DataLayout &DL;
const DominatorTree *DT;
};
/// \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(const TargetMachine *TM = nullptr,
bool LowerGEP = false)
: FunctionPass(ID), DL(nullptr), DT(nullptr), TM(TM), LowerGEP(LowerGEP) {
initializeSeparateConstOffsetFromGEPPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<ScalarEvolutionWrapperPass>();
AU.addRequired<TargetTransformInfoWrapperPass>();
AU.setPreservesCFG();
}
bool doInitialization(Module &M) override {
DL = &M.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);
/// 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
/// 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);
/// Optimize sext(a)+sext(b) to sext(a+b) when a+b can't sign overflow.
/// SeparateConstOffsetFromGEP distributes a sext to leaves before extracting
/// the constant offset. After extraction, it becomes desirable to reunion the
/// distributed sexts. For example,
///
/// &a[sext(i +nsw (j +nsw 5)]
/// => distribute &a[sext(i) +nsw (sext(j) +nsw 5)]
/// => constant extraction &a[sext(i) + sext(j)] + 5
/// => reunion &a[sext(i +nsw j)] + 5
bool reuniteExts(Function &F);
/// A helper that reunites sexts in an instruction.
bool reuniteExts(Instruction *I);
/// Find the closest dominator of <Dominatee> that is equivalent to <Key>.
Instruction *findClosestMatchingDominator(const SCEV *Key,
Instruction *Dominatee);
/// Verify F is free of dead code.
void verifyNoDeadCode(Function &F);
const DataLayout *DL;
DominatorTree *DT;
ScalarEvolution *SE;
const TargetMachine *TM;
/// Whether to lower a GEP with multiple indices into arithmetic operations or
/// multiple GEPs with a single index.
bool LowerGEP;
DenseMap<const SCEV *, SmallVector<Instruction *, 2>> DominatingExprs;
};
} // 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_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
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(const TargetMachine *TM,
bool LowerGEP) {
return new SeparateConstOffsetFromGEP(TM, LowerGEP);
}
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 &&
!haveNoCommonBitsSet(LHS, RHS, DL, nullptr, BO, DT))
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]);
assert(BO->getNumUses() <= 1 &&
"distributeExtsAndCloneChain clones each BinaryOperator in "
"UserChain, so no one should be used more than "
"once");
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;
}
BinaryOperator::BinaryOps NewOp = BO->getOpcode();
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
NewOp = Instruction::Add;
}
BinaryOperator *NewBO;
if (OpNo == 0) {
NewBO = BinaryOperator::Create(NewOp, NextInChain, TheOther, "", IP);
} else {
NewBO = BinaryOperator::Create(NewOp, TheOther, NextInChain, "", IP);
}
NewBO->takeName(BO);
return NewBO;
}
Value *ConstantOffsetExtractor::Extract(Value *Idx, GetElementPtrInst *GEP,
User *&UserChainTail,
const DominatorTree *DT) {
ConstantOffsetExtractor Extractor(GEP, DT);
// Find a non-zero constant offset first.
APInt ConstantOffset =
Extractor.find(Idx, /* SignExtended */ false, /* ZeroExtended */ false,
GEP->isInBounds());
if (ConstantOffset == 0) {
UserChainTail = nullptr;
return nullptr;
}
// Separates the constant offset from the GEP index.
Value *IdxWithoutConstOffset = Extractor.rebuildWithoutConstOffset();
UserChainTail = Extractor.UserChain.back();
return IdxWithoutConstOffset;
}
int64_t ConstantOffsetExtractor::Find(Value *Idx, GetElementPtrInst *GEP,
const DominatorTree *DT) {
// If Idx is an index of an inbound GEP, Idx is guaranteed to be non-negative.
return ConstantOffsetExtractor(GEP, DT)
.find(Idx, /* SignExtended */ false, /* ZeroExtended */ false,
GEP->isInBounds())
.getSExtValue();
}
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), GEP, DT);
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());
}
} 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(Builder.getInt8Ty(), ResultPtr, Idx, "uglygep");
}
}
// Create a GEP with the constant offset index.
if (AccumulativeByteOffset != 0) {
Value *Offset = ConstantInt::get(IntPtrTy, AccumulativeByteOffset);
ResultPtr =
Builder.CreateGEP(Builder.getInt8Ty(), 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())
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;
// 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<TargetTransformInfoWrapperPass>().getTTI(
*GEP->getParent()->getParent());
unsigned AddrSpace = GEP->getPointerAddressSpace();
if (!TTI.isLegalAddressingMode(GEP->getType()->getElementType(),
/*BaseGV=*/nullptr, AccumulativeByteOffset,
/*HasBaseReg=*/true, /*Scale=*/0,
AddrSpace)) {
return Changed;
}
}
// 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)) {
// Splits this GEP index into a variadic part and a constant offset, and
// uses the variadic part as the new index.
Value *OldIdx = GEP->getOperand(I);
User *UserChainTail;
Value *NewIdx =
ConstantOffsetExtractor::Extract(OldIdx, GEP, UserChainTail, DT);
if (NewIdx != nullptr) {
// Switches to the index with the constant offset removed.
GEP->setOperand(I, NewIdx);
// After switching to the new index, we can garbage-collect UserChain
// and the old index if they are not used.
RecursivelyDeleteTriviallyDeadInstructions(UserChainTail);
RecursivelyDeleteTriviallyDeadInstructions(OldIdx);
}
}
}
// Clear the inbounds attribute because the new index may be off-bound.
// e.g.,
//
// b = add i64 a, 5
// addr = gep inbounds float, float* p, i64 b
//
// is transformed to:
//
// addr2 = gep float, float* p, i64 a ; inbounds removed
// addr = gep inbounds float, 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.
bool GEPWasInBounds = GEP->isInBounds();
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->getSubtargetImpl(*GEP->getParent()->getParent())->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
// ... %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);
// Per ANSI C standard, signed / unsigned = unsigned and signed % unsigned =
// unsigned.. Therefore, we cast ElementTypeSizeOfGEP to signed because it is
// used with unsigned integers later.
int64_t ElementTypeSizeOfGEP = static_cast<int64_t>(
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).
int64_t Index = AccumulativeByteOffset / ElementTypeSizeOfGEP;
NewGEP = GetElementPtrInst::Create(GEP->getResultElementType(), NewGEP,
ConstantInt::get(IntPtrTy, Index, true),
GEP->getName(), GEP);
// Inherit the inbounds attribute of the original GEP.
cast<GetElementPtrInst>(NewGEP)->setIsInBounds(GEPWasInBounds);
} 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(
Type::getInt8Ty(GEP->getContext()), NewGEP,
ConstantInt::get(IntPtrTy, AccumulativeByteOffset, true), "uglygep",
GEP);
// Inherit the inbounds attribute of the original GEP.
cast<GetElementPtrInst>(NewGEP)->setIsInBounds(GEPWasInBounds);
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 (skipOptnoneFunction(F))
return false;
if (DisableSeparateConstOffsetFromGEP)
return false;
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
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.
}
}
Changed |= reuniteExts(F);
if (VerifyNoDeadCode)
verifyNoDeadCode(F);
return Changed;
}
Instruction *SeparateConstOffsetFromGEP::findClosestMatchingDominator(
const SCEV *Key, Instruction *Dominatee) {
auto Pos = DominatingExprs.find(Key);
if (Pos == DominatingExprs.end())
return nullptr;
auto &Candidates = Pos->second;
// Because we process the basic blocks in pre-order of the dominator tree, a
// candidate that doesn't dominate the current instruction won't dominate any
// future instruction either. Therefore, we pop it out of the stack. This
// optimization makes the algorithm O(n).
while (!Candidates.empty()) {
Instruction *Candidate = Candidates.back();
if (DT->dominates(Candidate, Dominatee))
return Candidate;
Candidates.pop_back();
}
return nullptr;
}
bool SeparateConstOffsetFromGEP::reuniteExts(Instruction *I) {
if (!SE->isSCEVable(I->getType()))
return false;
// Dom: LHS+RHS
// I: sext(LHS)+sext(RHS)
// If Dom can't sign overflow and Dom dominates I, optimize I to sext(Dom).
// TODO: handle zext
Value *LHS = nullptr, *RHS = nullptr;
if (match(I, m_Add(m_SExt(m_Value(LHS)), m_SExt(m_Value(RHS)))) ||
match(I, m_Sub(m_SExt(m_Value(LHS)), m_SExt(m_Value(RHS))))) {
if (LHS->getType() == RHS->getType()) {
const SCEV *Key =
SE->getAddExpr(SE->getUnknown(LHS), SE->getUnknown(RHS));
if (auto *Dom = findClosestMatchingDominator(Key, I)) {
Instruction *NewSExt = new SExtInst(Dom, I->getType(), "", I);
NewSExt->takeName(I);
I->replaceAllUsesWith(NewSExt);
RecursivelyDeleteTriviallyDeadInstructions(I);
return true;
}
}
}
// Add I to DominatingExprs if it's an add/sub that can't sign overflow.
if (match(I, m_NSWAdd(m_Value(LHS), m_Value(RHS))) ||
match(I, m_NSWSub(m_Value(LHS), m_Value(RHS)))) {
if (isKnownNotFullPoison(I)) {
const SCEV *Key =
SE->getAddExpr(SE->getUnknown(LHS), SE->getUnknown(RHS));
DominatingExprs[Key].push_back(I);
}
}
return false;
}
bool SeparateConstOffsetFromGEP::reuniteExts(Function &F) {
bool Changed = false;
DominatingExprs.clear();
for (auto Node = GraphTraits<DominatorTree *>::nodes_begin(DT);
Node != GraphTraits<DominatorTree *>::nodes_end(DT); ++Node) {
BasicBlock *BB = Node->getBlock();
for (auto I = BB->begin(); I != BB->end(); ) {
Instruction *Cur = I++;
Changed |= reuniteExts(Cur);
}
}
return Changed;
}
void SeparateConstOffsetFromGEP::verifyNoDeadCode(Function &F) {
for (auto &B : F) {
for (auto &I : B) {
if (isInstructionTriviallyDead(&I)) {
std::string ErrMessage;
raw_string_ostream RSO(ErrMessage);
RSO << "Dead instruction detected!\n" << I << "\n";
llvm_unreachable(RSO.str().c_str());
}
}
}
}
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