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[KnownBits] Make {s,u}{add,sub}_sat optimal #113096

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Nov 5, 2024
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[KnownBits] Make {s,u}{add,sub}_sat optimal #113096

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[KnownBits] Make `{s,u}{add,sub}_sat` optimal
goldsteinn Sep 27, 2024
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goldsteinn Nov 1, 2024
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142 changes: 77 additions & 65 deletions llvm/lib/Support/KnownBits.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -610,28 +610,82 @@ static KnownBits computeForSatAddSub(bool Add, bool Signed,
const KnownBits &RHS) {
// We don't see NSW even for sadd/ssub as we want to check if the result has
// signed overflow.
KnownBits Res =
KnownBits::computeForAddSub(Add, /*NSW=*/false, /*NUW=*/false, LHS, RHS);
unsigned BitWidth = Res.getBitWidth();
auto SignBitKnown = [&](const KnownBits &K) {
return K.Zero[BitWidth - 1] || K.One[BitWidth - 1];
};
std::optional<bool> Overflow;
unsigned BitWidth = LHS.getBitWidth();

std::optional<bool> Overflow;
// Even if we can't entirely rule out overflow, we may be able to rule out
// overflow in one direction. This allows us to potentially keep some of the
// add/sub bits. I.e if we can't overflow in the positive direction we won't
// clamp to INT_MAX so we can keep low 0s from the add/sub result.
bool MayNegClamp = true;
bool MayPosClamp = true;
if (Signed) {
// If we can actually detect overflow do so. Otherwise leave Overflow as
// nullopt (we assume it may have happened).
if (SignBitKnown(LHS) && SignBitKnown(RHS) && SignBitKnown(Res)) {
// Easy cases we can rule out any overflow.
if (Add && ((LHS.isNegative() && RHS.isNonNegative()) ||
(LHS.isNonNegative() && RHS.isNegative())))
Overflow = false;
else if (!Add && (((LHS.isNegative() && RHS.isNegative()) ||
(LHS.isNonNegative() && RHS.isNonNegative()))))
Overflow = false;
else {
// Check if we may overflow. If we can't rule out overflow then check if
// we can rule out a direction at least.
KnownBits UnsignedLHS = LHS;
KnownBits UnsignedRHS = RHS;
// Get version of LHS/RHS with clearer signbit. This allows us to detect
// how the addition/subtraction might overflow into the signbit. Then
// using the actual known signbits of LHS/RHS, we can figure out which
// overflows are/aren't possible.
UnsignedLHS.One.clearSignBit();
UnsignedLHS.Zero.setSignBit();
UnsignedRHS.One.clearSignBit();
UnsignedRHS.Zero.setSignBit();
KnownBits Res =
KnownBits::computeForAddSub(Add, /*NSW=*/false,
/*NUW=*/false, UnsignedLHS, UnsignedRHS);
if (Add) {
// sadd.sat
Overflow = (LHS.isNonNegative() == RHS.isNonNegative() &&
Res.isNonNegative() != LHS.isNonNegative());
if (Res.isNegative()) {
// Only overflow scenario is Pos + Pos.
MayNegClamp = false;
// Pos + Pos will overflow with extra signbit.
if (LHS.isNonNegative() && RHS.isNonNegative())
Overflow = true;
} else if (Res.isNonNegative()) {
// Only overflow scenario is Neg + Neg
MayPosClamp = false;
// Neg + Neg will overflow without extra signbit.
if (LHS.isNegative() && RHS.isNegative())
Overflow = true;
}
// We will never clamp to the opposite sign of N-bit result.
if (LHS.isNegative() || RHS.isNegative())
MayPosClamp = false;
if (LHS.isNonNegative() || RHS.isNonNegative())
MayNegClamp = false;
} else {
// ssub.sat
Overflow = (LHS.isNonNegative() != RHS.isNonNegative() &&
Res.isNonNegative() != LHS.isNonNegative());
if (Res.isNegative()) {
// Only overflow scenario is Neg - Pos.
MayPosClamp = false;
// Neg - Pos will overflow with extra signbit.
if (LHS.isNegative() && RHS.isNonNegative())
Overflow = true;
} else if (Res.isNonNegative()) {
// Only overflow scenario is Pos - Neg.
MayNegClamp = false;
// Pos - Neg will overflow without extra signbit.
if (LHS.isNonNegative() && RHS.isNegative())
Overflow = true;
}
// We will never clamp to the opposite sign of N-bit result.
if (LHS.isNegative() || RHS.isNonNegative())
MayPosClamp = false;
if (LHS.isNonNegative() || RHS.isNegative())
MayNegClamp = false;
}
}
// If we have ruled out all clamping, we will never overflow.
if (!MayNegClamp && !MayPosClamp)
Overflow = false;
} else if (Add) {
// uadd.sat
bool Of;
Expand All @@ -656,52 +710,8 @@ static KnownBits computeForSatAddSub(bool Add, bool Signed,
}
}

if (Signed) {
if (Add) {
if (LHS.isNonNegative() && RHS.isNonNegative()) {
// Pos + Pos -> Pos
Res.One.clearSignBit();
Res.Zero.setSignBit();
}
if (LHS.isNegative() && RHS.isNegative()) {
// Neg + Neg -> Neg
Res.One.setSignBit();
Res.Zero.clearSignBit();
}
} else {
if (LHS.isNegative() && RHS.isNonNegative()) {
// Neg - Pos -> Neg
Res.One.setSignBit();
Res.Zero.clearSignBit();
} else if (LHS.isNonNegative() && RHS.isNegative()) {
// Pos - Neg -> Pos
Res.One.clearSignBit();
Res.Zero.setSignBit();
}
}
} else {
// Add: Leading ones of either operand are preserved.
// Sub: Leading zeros of LHS and leading ones of RHS are preserved
// as leading zeros in the result.
unsigned LeadingKnown;
if (Add)
LeadingKnown =
std::max(LHS.countMinLeadingOnes(), RHS.countMinLeadingOnes());
else
LeadingKnown =
std::max(LHS.countMinLeadingZeros(), RHS.countMinLeadingOnes());

// We select between the operation result and all-ones/zero
// respectively, so we can preserve known ones/zeros.
APInt Mask = APInt::getHighBitsSet(BitWidth, LeadingKnown);
if (Add) {
Res.One |= Mask;
Res.Zero &= ~Mask;
} else {
Res.Zero |= Mask;
Res.One &= ~Mask;
}
}
KnownBits Res = KnownBits::computeForAddSub(Add, /*NSW=*/Signed,
/*NUW=*/!Signed, LHS, RHS);

if (Overflow) {
// We know whether or not we overflowed.
Expand All @@ -714,7 +724,7 @@ static KnownBits computeForSatAddSub(bool Add, bool Signed,
APInt C;
if (Signed) {
// sadd.sat / ssub.sat
assert(SignBitKnown(LHS) &&
assert(!LHS.isSignUnknown() &&
"We somehow know overflow without knowing input sign");
C = LHS.isNegative() ? APInt::getSignedMinValue(BitWidth)
: APInt::getSignedMaxValue(BitWidth);
Expand All @@ -735,8 +745,10 @@ static KnownBits computeForSatAddSub(bool Add, bool Signed,
if (Signed) {
// sadd.sat/ssub.sat
// We can keep our information about the sign bits.
Res.Zero.clearLowBits(BitWidth - 1);
Res.One.clearLowBits(BitWidth - 1);
if (MayPosClamp)
Res.Zero.clearLowBits(BitWidth - 1);
if (MayNegClamp)
Res.One.clearLowBits(BitWidth - 1);
} else if (Add) {
// uadd.sat
// We need to clear all the known zeros as we can only use the leading ones.
Expand Down
9 changes: 1 addition & 8 deletions llvm/test/Analysis/ValueTracking/knownbits-sat-addsub.ll
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Expand Up @@ -142,14 +142,7 @@ define i1 @ssub_sat_low_bits(i8 %x, i8 %y) {

define i1 @ssub_sat_fail_may_overflow(i8 %x, i8 %y) {
; CHECK-LABEL: @ssub_sat_fail_may_overflow(
; CHECK-NEXT: [[XX:%.*]] = and i8 [[X:%.*]], 15
; CHECK-NEXT: [[YY:%.*]] = and i8 [[Y:%.*]], 15
; CHECK-NEXT: [[LHS:%.*]] = or i8 [[XX]], 1
; CHECK-NEXT: [[RHS:%.*]] = and i8 [[YY]], -2
; CHECK-NEXT: [[EXP:%.*]] = call i8 @llvm.ssub.sat.i8(i8 [[LHS]], i8 [[RHS]])
; CHECK-NEXT: [[AND:%.*]] = and i8 [[EXP]], 1
; CHECK-NEXT: [[R:%.*]] = icmp eq i8 [[AND]], 0
; CHECK-NEXT: ret i1 [[R]]
; CHECK-NEXT: ret i1 false
;
%xx = and i8 %x, 15
%yy = and i8 %y, 15
Expand Down
12 changes: 4 additions & 8 deletions llvm/unittests/Support/KnownBitsTest.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -383,26 +383,22 @@ TEST(KnownBitsTest, BinaryExhaustive) {
"sadd_sat", KnownBits::sadd_sat,
[](const APInt &N1, const APInt &N2) -> std::optional<APInt> {
return N1.sadd_sat(N2);
},
/*CheckOptimality=*/false);
});
testBinaryOpExhaustive(
"uadd_sat", KnownBits::uadd_sat,
[](const APInt &N1, const APInt &N2) -> std::optional<APInt> {
return N1.uadd_sat(N2);
},
/*CheckOptimality=*/false);
});
testBinaryOpExhaustive(
"ssub_sat", KnownBits::ssub_sat,
[](const APInt &N1, const APInt &N2) -> std::optional<APInt> {
return N1.ssub_sat(N2);
},
/*CheckOptimality=*/false);
});
testBinaryOpExhaustive(
"usub_sat", KnownBits::usub_sat,
[](const APInt &N1, const APInt &N2) -> std::optional<APInt> {
return N1.usub_sat(N2);
},
/*CheckOptimality=*/false);
});
testBinaryOpExhaustive(
"shl",
[](const KnownBits &Known1, const KnownBits &Known2) {
Expand Down
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