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[InstCombine] Pattern match minmax calls for unsigned saturation. #99250

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[InstCombine] Pattern match minmax calls for unsigned saturation. #99250

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fb11889
[InstCombine] Add test for unsigned addsub saturation. NFC.
huihzhang Jul 16, 2024
5010937
[InstCombine] Pattern match minmax calls for unsigned saturation.
huihzhang Jul 16, 2024
603655d
Fix pattern match for umin(UINT_MAX, BinOp(zext(A), zext(B))).
huihzhang Jul 17, 2024
6f86421
Simplify unsigned saturation pattern match rules:
huihzhang Jul 22, 2024
0044e69
Address review comments.
huihzhang Jul 24, 2024
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60 changes: 42 additions & 18 deletions llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1117,68 +1117,92 @@ static Instruction *moveAddAfterMinMax(IntrinsicInst *II,
return IsSigned ? BinaryOperator::CreateNSWAdd(NewMinMax, Add->getOperand(1))
: BinaryOperator::CreateNUWAdd(NewMinMax, Add->getOperand(1));
}
/// Match a sadd_sat or ssub_sat which is using min/max to clamp the value.
Instruction *InstCombinerImpl::matchSAddSubSat(IntrinsicInst &MinMax1) {
/// Match a [s|u]add_sat or [s|u]sub_sat which is using min/max to clamp the
/// value.
Instruction *InstCombinerImpl::matchAddSubSat(IntrinsicInst &MinMax1) {
Type *Ty = MinMax1.getType();

// We are looking for a tree of:
// max(INT_MIN, min(INT_MAX, add(sext(A), sext(B))))
// Where the min and max could be reversed
Instruction *MinMax2;
// 1. We are looking for a tree of signed saturation:
// smax(SINT_MIN, smin(SINT_MAX, add|sub(sext(A), sext(B))))
// Where the smin and smax could be reversed.
// 2. A tree of unsigned saturation:
// smax(UINT_MIN, smin(UINT_MAX, sub(zext(A), zext(B))))
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// Where the smin and smax could be reversed.
// Or umin(UINT_MAX, add(zext(A), zext(B)))
Instruction *MinMax2 = nullptr;
BinaryOperator *AddSub;
const APInt *MinValue, *MaxValue;
bool IsUnsignedSaturate = false;
if (match(&MinMax1, m_SMin(m_Instruction(MinMax2), m_APInt(MaxValue)))) {
if (!match(MinMax2, m_SMax(m_BinOp(AddSub), m_APInt(MinValue))))
return nullptr;
} else if (match(&MinMax1,
m_SMax(m_Instruction(MinMax2), m_APInt(MinValue)))) {
if (!match(MinMax2, m_SMin(m_BinOp(AddSub), m_APInt(MaxValue))))
return nullptr;
} else if (match(&MinMax1, m_UMin(m_BinOp(AddSub), m_APInt(MaxValue)))) {
// Bail out if AddSub could be negative.
if (!isKnownNonNegative(AddSub, SQ.getWithInstruction(AddSub)))
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Is this check necessary? I don't see any llvm.assume in the alive2 proof.

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This check was used to reject "umin(UINT_MAX, sub) -> usub_sat".
I pushed a new update to check for BinOp opcode 'Add', so that !isKnownNonNegative check can be deleted.

return nullptr;
IsUnsignedSaturate = true;
} else
return nullptr;

if (!IsUnsignedSaturate && MinValue && MinValue->isZero())
IsUnsignedSaturate = true;

// Check that the constants clamp a saturate, and that the new type would be
// sensible to convert to.
if (!(*MaxValue + 1).isPowerOf2() || -*MinValue != *MaxValue + 1)
if (!(*MaxValue + 1).isPowerOf2() ||
(!IsUnsignedSaturate && -*MinValue != *MaxValue + 1))
return nullptr;
// In what bitwidth can this be treated as saturating arithmetics?
unsigned NewBitWidth = (*MaxValue + 1).logBase2() + 1;
unsigned NewBitWidth =
(*MaxValue + 1).logBase2() + (IsUnsignedSaturate ? 0 : 1);
// FIXME: This isn't quite right for vectors, but using the scalar type is a
// good first approximation for what should be done there.
if (!shouldChangeType(Ty->getScalarType()->getIntegerBitWidth(), NewBitWidth))
return nullptr;

// Also make sure that the inner min/max and the add/sub have one use.
if (!MinMax2->hasOneUse() || !AddSub->hasOneUse())
if ((MinMax2 && !MinMax2->hasOneUse()) || !AddSub->hasOneUse())
return nullptr;

// Create the new type (which can be a vector type)
Type *NewTy = Ty->getWithNewBitWidth(NewBitWidth);

Intrinsic::ID IntrinsicID;
if (AddSub->getOpcode() == Instruction::Add)
IntrinsicID = Intrinsic::sadd_sat;
IntrinsicID =
IsUnsignedSaturate ? Intrinsic::uadd_sat : Intrinsic::sadd_sat;
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I don't see any logic thats checking the right binop for the given minmax pattern. I.e umin(UINT_MAX, add) -> uadd_sat is okay, but umin(UINT_MAX, sub) -> usub_sat isn.t

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@huihzhang huihzhang Jul 24, 2024 •
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"umin(UINT_MAX, sub) -> usub_sat" was previously rejected by "!isKnownNonNegative".

-  if (match(&MinMax1, m_UMin(m_BinOp(AddSub), m_APInt(MaxValue)))) {
-    // Bail out if AddSub could be negative.
-    if (!isKnownNonNegative(AddSub, SQ.getWithInstruction(AddSub)))
-      return nullptr;
+  if (match(&MinMax1, m_UMin(m_BinOp(AddSub), m_APInt(MaxValue))) &&
+      AddSub->getOpcode() == Instruction::Add) {

In the new update, I check for BinOp opcode equals 'Add', to make sure we don't accept umin(UINT_MAX, sub) case.

else if (AddSub->getOpcode() == Instruction::Sub)
IntrinsicID = Intrinsic::ssub_sat;
IntrinsicID =
IsUnsignedSaturate ? Intrinsic::usub_sat : Intrinsic::ssub_sat;
else
return nullptr;

// The two operands of the add/sub must be nsw-truncatable to the NewTy. This
// is usually achieved via a sext from a smaller type.
if (ComputeMaxSignificantBits(AddSub->getOperand(0), 0, AddSub) >
NewBitWidth ||
ComputeMaxSignificantBits(AddSub->getOperand(1), 0, AddSub) > NewBitWidth)
Value *Op0 = AddSub->getOperand(0);
Value *Op1 = AddSub->getOperand(1);
unsigned Op0MaxBitWidth =
IsUnsignedSaturate ? computeKnownBits(Op0, 0, AddSub).countMaxActiveBits()
: ComputeMaxSignificantBits(Op0, 0, AddSub);
unsigned Op1MaxBitWidth =
IsUnsignedSaturate ? computeKnownBits(Op1, 0, AddSub).countMaxActiveBits()
: ComputeMaxSignificantBits(Op1, 0, AddSub);
if (Op0MaxBitWidth > NewBitWidth || Op1MaxBitWidth > NewBitWidth)
return nullptr;

// Finally create and return the sat intrinsic, truncated to the new type
Function *F = Intrinsic::getDeclaration(MinMax1.getModule(), IntrinsicID, NewTy);
Value *AT = Builder.CreateTrunc(AddSub->getOperand(0), NewTy);
Value *BT = Builder.CreateTrunc(AddSub->getOperand(1), NewTy);
Value *Sat = Builder.CreateCall(F, {AT, BT});
return CastInst::Create(Instruction::SExt, Sat, Ty);
return CastInst::Create(
IsUnsignedSaturate ? Instruction::ZExt : Instruction::SExt, Sat, Ty);
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}


/// If we have a clamp pattern like max (min X, 42), 41 -- where the output
/// can only be one of two possible constant values -- turn that into a select
/// of constants.
Expand Down Expand Up @@ -1878,8 +1902,8 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
if (Instruction *Sel = foldClampRangeOfTwo(II, Builder))
return Sel;

if (Instruction *SAdd = matchSAddSubSat(*II))
return SAdd;
if (Instruction *AddSubSat = matchAddSubSat(*II))
return AddSubSat;

if (Value *NewMinMax = reassociateMinMaxWithConstants(II, Builder, SQ))
return replaceInstUsesWith(*II, NewMinMax);
Expand Down
2 changes: 1 addition & 1 deletion llvm/lib/Transforms/InstCombine/InstCombineInternal.h
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Original file line number Diff line number Diff line change
Expand Up @@ -392,7 +392,7 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
Instruction *narrowMathIfNoOverflow(BinaryOperator &I);
Instruction *narrowFunnelShift(TruncInst &Trunc);
Instruction *optimizeBitCastFromPhi(CastInst &CI, PHINode *PN);
Instruction *matchSAddSubSat(IntrinsicInst &MinMax1);
Instruction *matchAddSubSat(IntrinsicInst &MinMax1);
Instruction *foldNot(BinaryOperator &I);
Instruction *foldBinOpOfDisplacedShifts(BinaryOperator &I);

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