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[KnownBits] Make avg{Ceil,Floor}S optimal #110329

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[KnownBits] Make avg{Ceil,Floor}S optimal #110329

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57 changes: 43 additions & 14 deletions llvm/lib/Support/KnownBits.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -766,32 +766,61 @@ KnownBits KnownBits::usub_sat(const KnownBits &LHS, const KnownBits &RHS) {
static KnownBits avgCompute(KnownBits LHS, KnownBits RHS, bool IsCeil,
bool IsSigned) {
unsigned BitWidth = LHS.getBitWidth();
LHS = IsSigned ? LHS.sext(BitWidth + 1) : LHS.zext(BitWidth + 1);
RHS = IsSigned ? RHS.sext(BitWidth + 1) : RHS.zext(BitWidth + 1);
LHS =
computeForAddCarry(LHS, RHS, /*CarryZero*/ !IsCeil, /*CarryOne*/ IsCeil);
LHS = LHS.extractBits(BitWidth, 1);
return LHS;
KnownBits ExtLHS = IsSigned ? LHS.sext(BitWidth + 1) : LHS.zext(BitWidth + 1);
KnownBits ExtRHS = IsSigned ? RHS.sext(BitWidth + 1) : RHS.zext(BitWidth + 1);
KnownBits Res = computeForAddCarry(ExtLHS, ExtRHS, /*CarryZero=*/!IsCeil,
/*CarryOne=*/IsCeil);
Res = Res.extractBits(BitWidth, 1);

// If we have only 1 known signbit between LHS/RHS we can try to figure
// out result signbit.
// NB: If we know both signbits `computeForAddCarry` gets the optimal result
// already.
if (IsSigned && Res.isSignUnknown() &&
LHS.isSignUnknown() != RHS.isSignUnknown()) {
if (LHS.isSignUnknown())
std::swap(LHS, RHS);
KnownBits UnsignedLHS = LHS;
KnownBits UnsignedRHS = RHS;
UnsignedLHS.One.clearSignBit();
UnsignedLHS.Zero.setSignBit();
UnsignedRHS.One.clearSignBit();
UnsignedRHS.Zero.setSignBit();
KnownBits ResOf =
computeForAddCarry(UnsignedLHS, UnsignedRHS, /*CarryZero=*/!IsCeil,
/*CarryOne=*/IsCeil);
// Assuming no overflow (which is the case since we extend the addition when
// taking the average):
// Neg + Neg -> Neg
// Neg + Pos -> Neg if the signbit doesn't overflow
if (LHS.isNegative() && ResOf.isNonNegative())
Res.makeNegative();
// Pos + Pos -> Pos
// Pos + Neg -> Pos if the signbit does overflow
else if (LHS.isNonNegative() && ResOf.isNegative())
Res.makeNonNegative();
}
return Res;
}

KnownBits KnownBits::avgFloorS(const KnownBits &LHS, const KnownBits &RHS) {
return avgCompute(LHS, RHS, /* IsCeil */ false,
/* IsSigned */ true);
return avgCompute(LHS, RHS, /* IsCeil=*/false,
/* IsSigned=*/true);
}

KnownBits KnownBits::avgFloorU(const KnownBits &LHS, const KnownBits &RHS) {
return avgCompute(LHS, RHS, /* IsCeil */ false,
/* IsSigned */ false);
return avgCompute(LHS, RHS, /* IsCeil=*/false,
/* IsSigned=*/false);
}

KnownBits KnownBits::avgCeilS(const KnownBits &LHS, const KnownBits &RHS) {
return avgCompute(LHS, RHS, /* IsCeil */ true,
/* IsSigned */ true);
return avgCompute(LHS, RHS, /* IsCeil=*/true,
/* IsSigned=*/true);
}

KnownBits KnownBits::avgCeilU(const KnownBits &LHS, const KnownBits &RHS) {
return avgCompute(LHS, RHS, /* IsCeil */ true,
/* IsSigned */ false);
return avgCompute(LHS, RHS, /* IsCeil=*/true,
/* IsSigned=*/false);
}

KnownBits KnownBits::mul(const KnownBits &LHS, const KnownBits &RHS,
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7 changes: 3 additions & 4 deletions llvm/unittests/Support/KnownBitsTest.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -521,16 +521,15 @@ TEST(KnownBitsTest, BinaryExhaustive) {
[](const APInt &N1, const APInt &N2) { return APIntOps::mulhu(N1, N2); },
/*CheckOptimality=*/false);

testBinaryOpExhaustive("avgFloorS", KnownBits::avgFloorS, APIntOps::avgFloorS,
/*CheckOptimality=*/false);
testBinaryOpExhaustive("avgFloorS", KnownBits::avgFloorS,
APIntOps::avgFloorS);

testBinaryOpExhaustive("avgFloorU", KnownBits::avgFloorU,
APIntOps::avgFloorU);

testBinaryOpExhaustive("avgCeilU", KnownBits::avgCeilU, APIntOps::avgCeilU);

testBinaryOpExhaustive("avgCeilS", KnownBits::avgCeilS, APIntOps::avgCeilS,
/*CheckOptimality=*/false);
testBinaryOpExhaustive("avgCeilS", KnownBits::avgCeilS, APIntOps::avgCeilS);
}

TEST(KnownBitsTest, UnaryExhaustive) {
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