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//===- InstCombineNegator.cpp -----------------------------------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements sinking of negation into expression trees,
// as long as that can be done without increasing instruction count.
#include "InstCombineInternal.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/None.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Twine.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Analysis/TargetFolder.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/DebugCounter.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/InstCombine/InstCombiner.h"
#include <cassert>
#include <cstdint>
#include <functional>
#include <tuple>
#include <type_traits>
#include <utility>
namespace llvm {
class AssumptionCache;
class DataLayout;
class DominatorTree;
class LLVMContext;
} // namespace llvm
using namespace llvm;
#define DEBUG_TYPE "instcombine"
"Negator: Number of negations attempted to be sinked");
"Negator: Number of negations successfully sinked");
STATISTIC(NegatorMaxDepthVisited, "Negator: Maximal traversal depth ever "
"reached while attempting to sink negation");
"Negator: How many times did the traversal depth limit was reached "
"during sinking");
"Negator: Total number of values visited during attempts to sink negation");
"Negator: How many negations did we retrieve/reuse from cache");
"Negator: Maximal number of values ever visited while attempting to "
"sink negation");
"Negator: Number of new negated instructions created, total");
"Negator: Maximal number of new instructions created during negation "
"Negator: Number of new negated instructions created in successful "
"negation sinking attempts");
DEBUG_COUNTER(NegatorCounter, "instcombine-negator",
"Controls Negator transformations in InstCombine pass");
static cl::opt<bool>
NegatorEnabled("instcombine-negator-enabled", cl::init(true),
cl::desc("Should we attempt to sink negations?"));
static cl::opt<unsigned>
cl::desc("What is the maximal lookup depth when trying to "
"check for viability of negation sinking."));
Negator::Negator(LLVMContext &C, const DataLayout &DL_, AssumptionCache &AC_,
const DominatorTree &DT_, bool IsTrulyNegation_)
: Builder(C, TargetFolder(DL_),
IRBuilderCallbackInserter([&](Instruction *I) {
DL(DL_), AC(AC_), DT(DT_), IsTrulyNegation(IsTrulyNegation_) {}
Negator::~Negator() {
// Due to the InstCombine's worklist management, there are no guarantees that
// each instruction we'll encounter has been visited by InstCombine already.
// In particular, most importantly for us, that means we have to canonicalize
// constants to RHS ourselves, since that is helpful sometimes.
std::array<Value *, 2> Negator::getSortedOperandsOfBinOp(Instruction *I) {
assert(I->getNumOperands() == 2 && "Only for binops!");
std::array<Value *, 2> Ops{I->getOperand(0), I->getOperand(1)};
if (I->isCommutative() && InstCombiner::getComplexity(I->getOperand(0)) <
std::swap(Ops[0], Ops[1]);
return Ops;
// FIXME: can this be reworked into a worklist-based algorithm while preserving
// the depth-first, early bailout traversal?
LLVM_NODISCARD Value *Negator::visitImpl(Value *V, unsigned Depth) {
// -(undef) -> undef.
if (match(V, m_Undef()))
return V;
// In i1, negation can simply be ignored.
if (V->getType()->isIntOrIntVectorTy(1))
return V;
Value *X;
// -(-(X)) -> X.
if (match(V, m_Neg(m_Value(X))))
return X;
// Integral constants can be freely negated.
if (match(V, m_AnyIntegralConstant()))
return ConstantExpr::getNeg(cast<Constant>(V), /*HasNUW=*/false,
// If we have a non-instruction, then give up.
if (!isa<Instruction>(V))
return nullptr;
// If we have started with a true negation (i.e. `sub 0, %y`), then if we've
// got instruction that does not require recursive reasoning, we can still
// negate it even if it has other uses, without increasing instruction count.
if (!V->hasOneUse() && !IsTrulyNegation)
return nullptr;
auto *I = cast<Instruction>(V);
unsigned BitWidth = I->getType()->getScalarSizeInBits();
// We must preserve the insertion point and debug info that is set in the
// builder at the time this function is called.
InstCombiner::BuilderTy::InsertPointGuard Guard(Builder);
// And since we are trying to negate instruction I, that tells us about the
// insertion point and the debug info that we need to keep.
// In some cases we can give the answer without further recursion.
switch (I->getOpcode()) {
case Instruction::Add: {
std::array<Value *, 2> Ops = getSortedOperandsOfBinOp(I);
// `inc` is always negatible.
if (match(Ops[1], m_One()))
return Builder.CreateNot(Ops[0], I->getName() + ".neg");
case Instruction::Xor:
// `not` is always negatible.
if (match(I, m_Not(m_Value(X))))
return Builder.CreateAdd(X, ConstantInt::get(X->getType(), 1),
I->getName() + ".neg");
case Instruction::AShr:
case Instruction::LShr: {
// Right-shift sign bit smear is negatible.
const APInt *Op1Val;
if (match(I->getOperand(1), m_APInt(Op1Val)) && *Op1Val == BitWidth - 1) {
Value *BO = I->getOpcode() == Instruction::AShr
? Builder.CreateLShr(I->getOperand(0), I->getOperand(1))
: Builder.CreateAShr(I->getOperand(0), I->getOperand(1));
if (auto *NewInstr = dyn_cast<Instruction>(BO)) {
NewInstr->setName(I->getName() + ".neg");
return BO;
// While we could negate exact arithmetic shift:
// ashr exact %x, C --> sdiv exact i8 %x, -1<<C
// iff C != 0 and C u< bitwidth(%x), we don't want to,
// because division is *THAT* much worse than a shift.
case Instruction::SExt:
case Instruction::ZExt:
// `*ext` of i1 is always negatible
if (I->getOperand(0)->getType()->isIntOrIntVectorTy(1))
return I->getOpcode() == Instruction::SExt
? Builder.CreateZExt(I->getOperand(0), I->getType(),
I->getName() + ".neg")
: Builder.CreateSExt(I->getOperand(0), I->getType(),
I->getName() + ".neg");
break; // Other instructions require recursive reasoning.
if (I->getOpcode() == Instruction::Sub &&
(I->hasOneUse() || match(I->getOperand(0), m_ImmConstant()))) {
// `sub` is always negatible.
// However, only do this either if the old `sub` doesn't stick around, or
// it was subtracting from a constant. Otherwise, this isn't profitable.
return Builder.CreateSub(I->getOperand(1), I->getOperand(0),
I->getName() + ".neg");
// Some other cases, while still don't require recursion,
// are restricted to the one-use case.
if (!V->hasOneUse())
return nullptr;
switch (I->getOpcode()) {
case Instruction::SDiv:
// `sdiv` is negatible if divisor is not undef/INT_MIN/1.
// While this is normally not behind a use-check,
// let's consider division to be special since it's costly.
if (auto *Op1C = dyn_cast<Constant>(I->getOperand(1))) {
if (!Op1C->containsUndefOrPoisonElement() &&
Op1C->isNotMinSignedValue() && Op1C->isNotOneValue()) {
Value *BO =
Builder.CreateSDiv(I->getOperand(0), ConstantExpr::getNeg(Op1C),
I->getName() + ".neg");
if (auto *NewInstr = dyn_cast<Instruction>(BO))
return BO;
// Rest of the logic is recursive, so if it's time to give up then it's time.
if (Depth > NegatorMaxDepth) {
LLVM_DEBUG(dbgs() << "Negator: reached maximal allowed traversal depth in "
<< *V << ". Giving up.\n");
return nullptr;
switch (I->getOpcode()) {
case Instruction::Freeze: {
// `freeze` is negatible if its operand is negatible.
Value *NegOp = negate(I->getOperand(0), Depth + 1);
if (!NegOp) // Early return.
return nullptr;
return Builder.CreateFreeze(NegOp, I->getName() + ".neg");
case Instruction::PHI: {
// `phi` is negatible if all the incoming values are negatible.
auto *PHI = cast<PHINode>(I);
SmallVector<Value *, 4> NegatedIncomingValues(PHI->getNumOperands());
for (auto I : zip(PHI->incoming_values(), NegatedIncomingValues)) {
if (!(std::get<1>(I) =
negate(std::get<0>(I), Depth + 1))) // Early return.
return nullptr;
// All incoming values are indeed negatible. Create negated PHI node.
PHINode *NegatedPHI = Builder.CreatePHI(
PHI->getType(), PHI->getNumOperands(), PHI->getName() + ".neg");
for (auto I : zip(NegatedIncomingValues, PHI->blocks()))
NegatedPHI->addIncoming(std::get<0>(I), std::get<1>(I));
return NegatedPHI;
case Instruction::Select: {
if (isKnownNegation(I->getOperand(1), I->getOperand(2))) {
// Of one hand of select is known to be negation of another hand,
// just swap the hands around.
auto *NewSelect = cast<SelectInst>(I->clone());
// Just swap the operands of the select.
// Don't swap prof metadata, we didn't change the branch behavior.
NewSelect->setName(I->getName() + ".neg");
return NewSelect;
// `select` is negatible if both hands of `select` are negatible.
Value *NegOp1 = negate(I->getOperand(1), Depth + 1);
if (!NegOp1) // Early return.
return nullptr;
Value *NegOp2 = negate(I->getOperand(2), Depth + 1);
if (!NegOp2)
return nullptr;
// Do preserve the metadata!
return Builder.CreateSelect(I->getOperand(0), NegOp1, NegOp2,
I->getName() + ".neg", /*MDFrom=*/I);
case Instruction::ShuffleVector: {
// `shufflevector` is negatible if both operands are negatible.
auto *Shuf = cast<ShuffleVectorInst>(I);
Value *NegOp0 = negate(I->getOperand(0), Depth + 1);
if (!NegOp0) // Early return.
return nullptr;
Value *NegOp1 = negate(I->getOperand(1), Depth + 1);
if (!NegOp1)
return nullptr;
return Builder.CreateShuffleVector(NegOp0, NegOp1, Shuf->getShuffleMask(),
I->getName() + ".neg");
case Instruction::ExtractElement: {
// `extractelement` is negatible if source operand is negatible.
auto *EEI = cast<ExtractElementInst>(I);
Value *NegVector = negate(EEI->getVectorOperand(), Depth + 1);
if (!NegVector) // Early return.
return nullptr;
return Builder.CreateExtractElement(NegVector, EEI->getIndexOperand(),
I->getName() + ".neg");
case Instruction::InsertElement: {
// `insertelement` is negatible if both the source vector and
// element-to-be-inserted are negatible.
auto *IEI = cast<InsertElementInst>(I);
Value *NegVector = negate(IEI->getOperand(0), Depth + 1);
if (!NegVector) // Early return.
return nullptr;
Value *NegNewElt = negate(IEI->getOperand(1), Depth + 1);
if (!NegNewElt) // Early return.
return nullptr;
return Builder.CreateInsertElement(NegVector, NegNewElt, IEI->getOperand(2),
I->getName() + ".neg");
case Instruction::Trunc: {
// `trunc` is negatible if its operand is negatible.
Value *NegOp = negate(I->getOperand(0), Depth + 1);
if (!NegOp) // Early return.
return nullptr;
return Builder.CreateTrunc(NegOp, I->getType(), I->getName() + ".neg");
case Instruction::Shl: {
// `shl` is negatible if the first operand is negatible.
if (Value *NegOp0 = negate(I->getOperand(0), Depth + 1))
return Builder.CreateShl(NegOp0, I->getOperand(1), I->getName() + ".neg");
// Otherwise, `shl %x, C` can be interpreted as `mul %x, 1<<C`.
auto *Op1C = dyn_cast<Constant>(I->getOperand(1));
if (!Op1C) // Early return.
return nullptr;
return Builder.CreateMul(
ConstantExpr::getShl(Constant::getAllOnesValue(Op1C->getType()), Op1C),
I->getName() + ".neg");
case Instruction::Or: {
if (!haveNoCommonBitsSet(I->getOperand(0), I->getOperand(1), DL, &AC, I,
return nullptr; // Don't know how to handle `or` in general.
std::array<Value *, 2> Ops = getSortedOperandsOfBinOp(I);
// `or`/`add` are interchangeable when operands have no common bits set.
// `inc` is always negatible.
if (match(Ops[1], m_One()))
return Builder.CreateNot(Ops[0], I->getName() + ".neg");
// Else, just defer to Instruction::Add handling.
case Instruction::Add: {
// `add` is negatible if both of its operands are negatible.
SmallVector<Value *, 2> NegatedOps, NonNegatedOps;
for (Value *Op : I->operands()) {
// Can we sink the negation into this operand?
if (Value *NegOp = negate(Op, Depth + 1)) {
NegatedOps.emplace_back(NegOp); // Successfully negated operand!
// Failed to sink negation into this operand. IFF we started from negation
// and we manage to sink negation into one operand, we can still do this.
if (!IsTrulyNegation)
return nullptr;
NonNegatedOps.emplace_back(Op); // Just record which operand that was.
assert((NegatedOps.size() + NonNegatedOps.size()) == 2 &&
"Internal consistency sanity check.");
// Did we manage to sink negation into both of the operands?
if (NegatedOps.size() == 2) // Then we get to keep the `add`!
return Builder.CreateAdd(NegatedOps[0], NegatedOps[1],
I->getName() + ".neg");
assert(IsTrulyNegation && "We should have early-exited then.");
// Completely failed to sink negation?
if (NonNegatedOps.size() == 2)
return nullptr;
// 0-(a+b) --> (-a)-b
return Builder.CreateSub(NegatedOps[0], NonNegatedOps[0],
I->getName() + ".neg");
case Instruction::Xor: {
std::array<Value *, 2> Ops = getSortedOperandsOfBinOp(I);
// `xor` is negatible if one of its operands is invertible.
// FIXME: InstCombineInverter? But how to connect Inverter and Negator?
if (auto *C = dyn_cast<Constant>(Ops[1])) {
Value *Xor = Builder.CreateXor(Ops[0], ConstantExpr::getNot(C));
return Builder.CreateAdd(Xor, ConstantInt::get(Xor->getType(), 1),
I->getName() + ".neg");
return nullptr;
case Instruction::Mul: {
std::array<Value *, 2> Ops = getSortedOperandsOfBinOp(I);
// `mul` is negatible if one of its operands is negatible.
Value *NegatedOp, *OtherOp;
// First try the second operand, in case it's a constant it will be best to
// just invert it instead of sinking the `neg` deeper.
if (Value *NegOp1 = negate(Ops[1], Depth + 1)) {
NegatedOp = NegOp1;
OtherOp = Ops[0];
} else if (Value *NegOp0 = negate(Ops[0], Depth + 1)) {
NegatedOp = NegOp0;
OtherOp = Ops[1];
} else
// Can't negate either of them.
return nullptr;
return Builder.CreateMul(NegatedOp, OtherOp, I->getName() + ".neg");
return nullptr; // Don't know, likely not negatible for free.
llvm_unreachable("Can't get here. We always return from switch.");
LLVM_NODISCARD Value *Negator::negate(Value *V, unsigned Depth) {
#ifndef NDEBUG
// We can't ever have a Value with such an address.
Value *Placeholder = reinterpret_cast<Value *>(static_cast<uintptr_t>(-1));
// Did we already try to negate this value?
auto NegationsCacheIterator = NegationsCache.find(V);
if (NegationsCacheIterator != NegationsCache.end()) {
Value *NegatedV = NegationsCacheIterator->second;
assert(NegatedV != Placeholder && "Encountered a cycle during negation.");
return NegatedV;
#ifndef NDEBUG
// We did not find a cached result for negation of V. While there,
// let's temporairly cache a placeholder value, with the idea that if later
// during negation we fetch it from cache, we'll know we're in a cycle.
NegationsCache[V] = Placeholder;
// No luck. Try negating it for real.
Value *NegatedV = visitImpl(V, Depth);
// And cache the (real) result for the future.
NegationsCache[V] = NegatedV;
return NegatedV;
LLVM_NODISCARD Optional<Negator::Result> Negator::run(Value *Root) {
Value *Negated = negate(Root, /*Depth=*/0);
if (!Negated) {
// We must cleanup newly-inserted instructions, to avoid any potential
// endless combine looping.
for (Instruction *I : llvm::reverse(NewInstructions))
return llvm::None;
return std::make_pair(ArrayRef<Instruction *>(NewInstructions), Negated);
LLVM_NODISCARD Value *Negator::Negate(bool LHSIsZero, Value *Root,
InstCombinerImpl &IC) {
LLVM_DEBUG(dbgs() << "Negator: attempting to sink negation into " << *Root
<< "\n");
if (!NegatorEnabled || !DebugCounter::shouldExecute(NegatorCounter))
return nullptr;
Negator N(Root->getContext(), IC.getDataLayout(), IC.getAssumptionCache(),
IC.getDominatorTree(), LHSIsZero);
Optional<Result> Res =;
if (!Res) { // Negation failed.
LLVM_DEBUG(dbgs() << "Negator: failed to sink negation into " << *Root
<< "\n");
return nullptr;
LLVM_DEBUG(dbgs() << "Negator: successfully sunk negation into " << *Root
<< "\n NEW: " << *Res->second << "\n");
// We must temporarily unset the 'current' insertion point and DebugLoc of the
// InstCombine's IRBuilder so that it won't interfere with the ones we have
// already specified when producing negated instructions.
InstCombiner::BuilderTy::InsertPointGuard Guard(IC.Builder);
// And finally, we must add newly-created instructions into the InstCombine's
// worklist (in a proper order!) so it can attempt to combine them.
LLVM_DEBUG(dbgs() << "Negator: Propagating " << Res->first.size()
<< " instrs to InstCombine\n");
NegatorNumInstructionsNegatedSuccess += Res->first.size();
// They are in def-use order, so nothing fancy, just insert them in order.
for (Instruction *I : Res->first)
IC.Builder.Insert(I, I->getName());
// And return the new root.
return Res->second;