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[InstCombine] Make takeLog2 usable throughout InstCombine; NFC #122498

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[InstCombine] Make takeLog2 usable throughout InstCombine; NFC #122498

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b21edc4
[InstCombine] Move `takeLog2` to InstructionCombiner; NFC
goldsteinn Jan 10, 2025
fea0bde
[InstCombine] Add convenience helper `tryGetLog2`; NFC
goldsteinn Jan 10, 2025
305c6c8
Fixup
goldsteinn Jan 10, 2025
fd6a4f9
Leave in orig spot
goldsteinn Jan 10, 2025
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12 changes: 12 additions & 0 deletions llvm/include/llvm/Transforms/InstCombine/InstCombiner.h
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Original file line number Diff line number Diff line change
Expand Up @@ -195,6 +195,18 @@ class LLVM_LIBRARY_VISIBILITY InstCombiner {
PatternMatch::m_Value()));
}

// Take the exact integer log2 of the value. If DoFold is true, create the
// actual instructions, otherwise return a non-null dummy value. Return
// nullptr on failure. Note, if DoFold is true the caller must ensure that
// takeLog2 will succeed, otherwise it may create stray instructions.
Value *takeLog2(Value *Op, unsigned Depth, bool AssumeNonZero, bool DoFold);

Value *tryGetLog2(Value *Op, bool AssumeNonZero) {
if (takeLog2(Op, /*Depth=*/0, AssumeNonZero, /*DoFold=*/false))
return takeLog2(Op, /*Depth=*/0, AssumeNonZero, /*DoFold=*/true);
return nullptr;
}

/// Return nonnull value if V is free to invert under the condition of
/// WillInvertAllUses.
/// If Builder is nonnull, it will return a simplified ~V.
Expand Down
124 changes: 3 additions & 121 deletions llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -185,9 +185,6 @@ static Value *foldMulShl1(BinaryOperator &Mul, bool CommuteOperands,
return nullptr;
}

static Value *takeLog2(IRBuilderBase &Builder, Value *Op, unsigned Depth,
bool AssumeNonZero, bool DoFold);

Instruction *InstCombinerImpl::visitMul(BinaryOperator &I) {
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
if (Value *V =
Expand Down Expand Up @@ -531,19 +528,13 @@ Instruction *InstCombinerImpl::visitMul(BinaryOperator &I) {
// (shl Op1, Log2(Op0))
// if Log2(Op1) folds away ->
// (shl Op0, Log2(Op1))
if (takeLog2(Builder, Op0, /*Depth*/ 0, /*AssumeNonZero*/ false,
/*DoFold*/ false)) {
Value *Res = takeLog2(Builder, Op0, /*Depth*/ 0, /*AssumeNonZero*/ false,
/*DoFold*/ true);
if (Value *Res = tryGetLog2(Op0, /*AssumeNonZero=*/false)) {
BinaryOperator *Shl = BinaryOperator::CreateShl(Op1, Res);
// We can only propegate nuw flag.
Shl->setHasNoUnsignedWrap(HasNUW);
return Shl;
}
if (takeLog2(Builder, Op1, /*Depth*/ 0, /*AssumeNonZero*/ false,
/*DoFold*/ false)) {
Value *Res = takeLog2(Builder, Op1, /*Depth*/ 0, /*AssumeNonZero*/ false,
/*DoFold*/ true);
if (Value *Res = tryGetLog2(Op0, /*AssumeNonZero=*/false)) {
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Contributor Author

@goldsteinn goldsteinn Jan 10, 2025

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Mistake here.

BinaryOperator *Shl = BinaryOperator::CreateShl(Op0, Res);
// We can only propegate nuw flag.
Shl->setHasNoUnsignedWrap(HasNUW);
Expand Down Expand Up @@ -1407,111 +1398,6 @@ Instruction *InstCombinerImpl::commonIDivTransforms(BinaryOperator &I) {
return nullptr;
}

static const unsigned MaxDepth = 6;

// Take the exact integer log2 of the value. If DoFold is true, create the
// actual instructions, otherwise return a non-null dummy value. Return nullptr
// on failure.
static Value *takeLog2(IRBuilderBase &Builder, Value *Op, unsigned Depth,
bool AssumeNonZero, bool DoFold) {
auto IfFold = [DoFold](function_ref<Value *()> Fn) {
if (!DoFold)
return reinterpret_cast<Value *>(-1);
return Fn();
};

// FIXME: assert that Op1 isn't/doesn't contain undef.

// log2(2^C) -> C
if (match(Op, m_Power2()))
return IfFold([&]() {
Constant *C = ConstantExpr::getExactLogBase2(cast<Constant>(Op));
if (!C)
llvm_unreachable("Failed to constant fold udiv -> logbase2");
return C;
});

// The remaining tests are all recursive, so bail out if we hit the limit.
if (Depth++ == MaxDepth)
return nullptr;

// log2(zext X) -> zext log2(X)
// FIXME: Require one use?
Value *X, *Y;
if (match(Op, m_ZExt(m_Value(X))))
if (Value *LogX = takeLog2(Builder, X, Depth, AssumeNonZero, DoFold))
return IfFold([&]() { return Builder.CreateZExt(LogX, Op->getType()); });

// log2(trunc x) -> trunc log2(X)
// FIXME: Require one use?
if (match(Op, m_Trunc(m_Value(X)))) {
auto *TI = cast<TruncInst>(Op);
if (AssumeNonZero || TI->hasNoUnsignedWrap())
if (Value *LogX = takeLog2(Builder, X, Depth, AssumeNonZero, DoFold))
return IfFold([&]() {
return Builder.CreateTrunc(LogX, Op->getType(), "",
/*IsNUW=*/TI->hasNoUnsignedWrap());
});
}

// log2(X << Y) -> log2(X) + Y
// FIXME: Require one use unless X is 1?
if (match(Op, m_Shl(m_Value(X), m_Value(Y)))) {
auto *BO = cast<OverflowingBinaryOperator>(Op);
// nuw will be set if the `shl` is trivially non-zero.
if (AssumeNonZero || BO->hasNoUnsignedWrap() || BO->hasNoSignedWrap())
if (Value *LogX = takeLog2(Builder, X, Depth, AssumeNonZero, DoFold))
return IfFold([&]() { return Builder.CreateAdd(LogX, Y); });
}

// log2(X >>u Y) -> log2(X) - Y
// FIXME: Require one use?
if (match(Op, m_LShr(m_Value(X), m_Value(Y)))) {
auto *PEO = cast<PossiblyExactOperator>(Op);
if (AssumeNonZero || PEO->isExact())
if (Value *LogX = takeLog2(Builder, X, Depth, AssumeNonZero, DoFold))
return IfFold([&]() { return Builder.CreateSub(LogX, Y); });
}

// log2(X & Y) -> either log2(X) or log2(Y)
// This requires `AssumeNonZero` as `X & Y` may be zero when X != Y.
if (AssumeNonZero && match(Op, m_And(m_Value(X), m_Value(Y)))) {
if (Value *LogX = takeLog2(Builder, X, Depth, AssumeNonZero, DoFold))
return IfFold([&]() { return LogX; });
if (Value *LogY = takeLog2(Builder, Y, Depth, AssumeNonZero, DoFold))
return IfFold([&]() { return LogY; });
}

// log2(Cond ? X : Y) -> Cond ? log2(X) : log2(Y)
// FIXME: Require one use?
if (SelectInst *SI = dyn_cast<SelectInst>(Op))
if (Value *LogX = takeLog2(Builder, SI->getOperand(1), Depth,
AssumeNonZero, DoFold))
if (Value *LogY = takeLog2(Builder, SI->getOperand(2), Depth,
AssumeNonZero, DoFold))
return IfFold([&]() {
return Builder.CreateSelect(SI->getOperand(0), LogX, LogY);
});

// log2(umin(X, Y)) -> umin(log2(X), log2(Y))
// log2(umax(X, Y)) -> umax(log2(X), log2(Y))
auto *MinMax = dyn_cast<MinMaxIntrinsic>(Op);
if (MinMax && MinMax->hasOneUse() && !MinMax->isSigned()) {
// Use AssumeNonZero as false here. Otherwise we can hit case where
// log2(umax(X, Y)) != umax(log2(X), log2(Y)) (because overflow).
if (Value *LogX = takeLog2(Builder, MinMax->getLHS(), Depth,
/*AssumeNonZero*/ false, DoFold))
if (Value *LogY = takeLog2(Builder, MinMax->getRHS(), Depth,
/*AssumeNonZero*/ false, DoFold))
return IfFold([&]() {
return Builder.CreateBinaryIntrinsic(MinMax->getIntrinsicID(), LogX,
LogY);
});
}

return nullptr;
}

/// If we have zero-extended operands of an unsigned div or rem, we may be able
/// to narrow the operation (sink the zext below the math).
static Instruction *narrowUDivURem(BinaryOperator &I,
Expand Down Expand Up @@ -1614,13 +1500,9 @@ Instruction *InstCombinerImpl::visitUDiv(BinaryOperator &I) {
}

// Op1 udiv Op2 -> Op1 lshr log2(Op2), if log2() folds away.
if (takeLog2(Builder, Op1, /*Depth*/ 0, /*AssumeNonZero*/ true,
/*DoFold*/ false)) {
Value *Res = takeLog2(Builder, Op1, /*Depth*/ 0,
/*AssumeNonZero*/ true, /*DoFold*/ true);
if (Value *Res = tryGetLog2(Op1, /*AssumeNonZero=*/true))
return replaceInstUsesWith(
I, Builder.CreateLShr(Op0, Res, I.getName(), I.isExact()));
}

return nullptr;
}
Expand Down
99 changes: 99 additions & 0 deletions llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
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Expand Up @@ -2565,6 +2565,105 @@ Instruction *InstCombinerImpl::visitGEPOfGEP(GetElementPtrInst &GEP,
return nullptr;
}

Value *InstCombiner::takeLog2(Value *Op, unsigned Depth, bool AssumeNonZero,
bool DoFold) {
auto IfFold = [DoFold](function_ref<Value *()> Fn) {
if (!DoFold)
return reinterpret_cast<Value *>(-1);
return Fn();
};

// FIXME: assert that Op1 isn't/doesn't contain undef.

// log2(2^C) -> C
if (match(Op, m_Power2()))
return IfFold([&]() {
Constant *C = ConstantExpr::getExactLogBase2(cast<Constant>(Op));
if (!C)
llvm_unreachable("Failed to constant fold udiv -> logbase2");
return C;
});

// The remaining tests are all recursive, so bail out if we hit the limit.
if (Depth++ == MaxAnalysisRecursionDepth)
return nullptr;

// log2(zext X) -> zext log2(X)
// FIXME: Require one use?
Value *X, *Y;
if (match(Op, m_ZExt(m_Value(X))))
if (Value *LogX = takeLog2(X, Depth, AssumeNonZero, DoFold))
return IfFold([&]() { return Builder.CreateZExt(LogX, Op->getType()); });

// log2(trunc x) -> trunc log2(X)
// FIXME: Require one use?
if (match(Op, m_Trunc(m_Value(X)))) {
auto *TI = cast<TruncInst>(Op);
if (AssumeNonZero || TI->hasNoUnsignedWrap())
if (Value *LogX = takeLog2(X, Depth, AssumeNonZero, DoFold))
return IfFold([&]() {
return Builder.CreateTrunc(LogX, Op->getType(), "",
/*IsNUW=*/TI->hasNoUnsignedWrap());
});
}

// log2(X << Y) -> log2(X) + Y
// FIXME: Require one use unless X is 1?
if (match(Op, m_Shl(m_Value(X), m_Value(Y)))) {
auto *BO = cast<OverflowingBinaryOperator>(Op);
// nuw will be set if the `shl` is trivially non-zero.
if (AssumeNonZero || BO->hasNoUnsignedWrap() || BO->hasNoSignedWrap())
if (Value *LogX = takeLog2(X, Depth, AssumeNonZero, DoFold))
return IfFold([&]() { return Builder.CreateAdd(LogX, Y); });
}

// log2(X >>u Y) -> log2(X) - Y
// FIXME: Require one use?
if (match(Op, m_LShr(m_Value(X), m_Value(Y)))) {
auto *PEO = cast<PossiblyExactOperator>(Op);
if (AssumeNonZero || PEO->isExact())
if (Value *LogX = takeLog2(X, Depth, AssumeNonZero, DoFold))
return IfFold([&]() { return Builder.CreateSub(LogX, Y); });
}

// log2(X & Y) -> either log2(X) or log2(Y)
// This requires `AssumeNonZero` as `X & Y` may be zero when X != Y.
if (AssumeNonZero && match(Op, m_And(m_Value(X), m_Value(Y)))) {
if (Value *LogX = takeLog2(X, Depth, AssumeNonZero, DoFold))
return IfFold([&]() { return LogX; });
if (Value *LogY = takeLog2(Y, Depth, AssumeNonZero, DoFold))
return IfFold([&]() { return LogY; });
}

// log2(Cond ? X : Y) -> Cond ? log2(X) : log2(Y)
// FIXME: Require one use?
if (SelectInst *SI = dyn_cast<SelectInst>(Op))
if (Value *LogX = takeLog2(SI->getOperand(1), Depth, AssumeNonZero, DoFold))
if (Value *LogY =
takeLog2(SI->getOperand(2), Depth, AssumeNonZero, DoFold))
return IfFold([&]() {
return Builder.CreateSelect(SI->getOperand(0), LogX, LogY);
});

// log2(umin(X, Y)) -> umin(log2(X), log2(Y))
// log2(umax(X, Y)) -> umax(log2(X), log2(Y))
auto *MinMax = dyn_cast<MinMaxIntrinsic>(Op);
if (MinMax && MinMax->hasOneUse() && !MinMax->isSigned()) {
// Use AssumeNonZero as false here. Otherwise we can hit case where
// log2(umax(X, Y)) != umax(log2(X), log2(Y)) (because overflow).
if (Value *LogX = takeLog2(MinMax->getLHS(), Depth,
/*AssumeNonZero*/ false, DoFold))
if (Value *LogY = takeLog2(MinMax->getRHS(), Depth,
/*AssumeNonZero*/ false, DoFold))
return IfFold([&]() {
return Builder.CreateBinaryIntrinsic(MinMax->getIntrinsicID(), LogX,
LogY);
});
}

return nullptr;
}

Value *InstCombiner::getFreelyInvertedImpl(Value *V, bool WillInvertAllUses,
BuilderTy *Builder,
bool &DoesConsume, unsigned Depth) {
Expand Down
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