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[KnownBits] Implement accurate unsigned and signed max and min 

Use the new implementation in ValueTracking, SelectionDAG and
GlobalISel.

Differential Revision: https://reviews.llvm.org/D87034
This commit is contained in:
Jay Foad 2020-09-02 16:01:48 +01:00
parent 04ea680a8c
commit 5350e1b509
7 changed files with 169 additions and 90 deletions
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16
llvm/include/llvm/Support/KnownBits.h
View File

@ -173,6 +173,10 @@ public:
One.extractBits(NumBits, BitPosition));
}
/// Return KnownBits based on this, but updated given that the underlying
/// value is known to be greater than or equal to Val.
KnownBits makeGE(const APInt &Val) const;
/// Returns the minimum number of trailing zero bits.
unsigned countMinTrailingZeros() const {
return Zero.countTrailingOnes();
@ -241,6 +245,18 @@ public:
static KnownBits computeForAddSub(bool Add, bool NSW, const KnownBits &LHS,
KnownBits RHS);
/// Compute known bits for umax(LHS, RHS).
static KnownBits umax(const KnownBits &LHS, const KnownBits &RHS);
/// Compute known bits for umin(LHS, RHS).
static KnownBits umin(const KnownBits &LHS, const KnownBits &RHS);
/// Compute known bits for smax(LHS, RHS).
static KnownBits smax(const KnownBits &LHS, const KnownBits &RHS);
/// Compute known bits for smin(LHS, RHS).
static KnownBits smin(const KnownBits &LHS, const KnownBits &RHS);
/// Insert the bits from a smaller known bits starting at bitPosition.
void insertBits(const KnownBits &SubBits, unsigned BitPosition) {
Zero.insertBits(SubBits.Zero, BitPosition);

70
llvm/lib/Analysis/ValueTracking.cpp
View File

@ -1212,59 +1212,41 @@ static void computeKnownBitsFromOperator(const Operator *I,
if (SelectPatternResult::isMinOrMax(SPF)) {
computeKnownBits(RHS, Known, Depth + 1, Q);
computeKnownBits(LHS, Known2, Depth + 1, Q);
} else {
computeKnownBits(I->getOperand(2), Known, Depth + 1, Q);
computeKnownBits(I->getOperand(1), Known2, Depth + 1, Q);
switch (SPF) {
default:
llvm_unreachable("Unhandled select pattern flavor!");
case SPF_SMAX:
Known = KnownBits::smax(Known, Known2);
break;
case SPF_SMIN:
Known = KnownBits::smin(Known, Known2);
break;
case SPF_UMAX:
Known = KnownBits::umax(Known, Known2);
break;
case SPF_UMIN:
Known = KnownBits::umin(Known, Known2);
break;
}
break;
}
unsigned MaxHighOnes = 0;
unsigned MaxHighZeros = 0;
if (SPF == SPF_SMAX) {
// If both sides are negative, the result is negative.
if (Known.isNegative() && Known2.isNegative())
// We can derive a lower bound on the result by taking the max of the
// leading one bits.
MaxHighOnes =
std::max(Known.countMinLeadingOnes(), Known2.countMinLeadingOnes());
// If either side is non-negative, the result is non-negative.
else if (Known.isNonNegative() || Known2.isNonNegative())
MaxHighZeros = 1;
} else if (SPF == SPF_SMIN) {
// If both sides are non-negative, the result is non-negative.
if (Known.isNonNegative() && Known2.isNonNegative())
// We can derive an upper bound on the result by taking the max of the
// leading zero bits.
MaxHighZeros = std::max(Known.countMinLeadingZeros(),
Known2.countMinLeadingZeros());
// If either side is negative, the result is negative.
else if (Known.isNegative() || Known2.isNegative())
MaxHighOnes = 1;
} else if (SPF == SPF_UMAX) {
// We can derive a lower bound on the result by taking the max of the
// leading one bits.
MaxHighOnes =
std::max(Known.countMinLeadingOnes(), Known2.countMinLeadingOnes());
} else if (SPF == SPF_UMIN) {
// We can derive an upper bound on the result by taking the max of the
// leading zero bits.
MaxHighZeros =
std::max(Known.countMinLeadingZeros(), Known2.countMinLeadingZeros());
} else if (SPF == SPF_ABS) {
computeKnownBits(I->getOperand(2), Known, Depth + 1, Q);
computeKnownBits(I->getOperand(1), Known2, Depth + 1, Q);
// Only known if known in both the LHS and RHS.
Known.One &= Known2.One;
Known.Zero &= Known2.Zero;
if (SPF == SPF_ABS) {
// RHS from matchSelectPattern returns the negation part of abs pattern.
// If the negate has an NSW flag we can assume the sign bit of the result
// will be 0 because that makes abs(INT_MIN) undefined.
if (match(RHS, m_Neg(m_Specific(LHS))) &&
Q.IIQ.hasNoSignedWrap(cast<Instruction>(RHS)))
MaxHighZeros = 1;
Known.Zero.setSignBit();
}
// Only known if known in both the LHS and RHS.
Known.One &= Known2.One;
Known.Zero &= Known2.Zero;
if (MaxHighOnes > 0)
Known.One.setHighBits(MaxHighOnes);
if (MaxHighZeros > 0)
Known.Zero.setHighBits(MaxHighZeros);
break;
}
case Instruction::FPTrunc:

36
llvm/lib/CodeGen/GlobalISel/GISelKnownBits.cpp
View File

@ -308,11 +308,24 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
Known, DemandedElts, Depth + 1);
break;
}
case TargetOpcode::G_SMIN:
case TargetOpcode::G_SMIN: {
// TODO: Handle clamp pattern with number of sign bits
KnownBits KnownRHS;
computeKnownBitsImpl(MI.getOperand(1).getReg(), Known, DemandedElts,
Depth + 1);
computeKnownBitsImpl(MI.getOperand(2).getReg(), KnownRHS, DemandedElts,
Depth + 1);
Known = KnownBits::smin(Known, KnownRHS);
break;
}
case TargetOpcode::G_SMAX: {
// TODO: Handle clamp pattern with number of sign bits
computeKnownBitsMin(MI.getOperand(1).getReg(), MI.getOperand(2).getReg(),
Known, DemandedElts, Depth + 1);
KnownBits KnownRHS;
computeKnownBitsImpl(MI.getOperand(1).getReg(), Known, DemandedElts,
Depth + 1);
computeKnownBitsImpl(MI.getOperand(2).getReg(), KnownRHS, DemandedElts,
Depth + 1);
Known = KnownBits::smax(Known, KnownRHS);
break;
}
case TargetOpcode::G_UMIN: {
@ -321,13 +334,7 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
DemandedElts, Depth + 1);
computeKnownBitsImpl(MI.getOperand(2).getReg(), KnownRHS,
DemandedElts, Depth + 1);
// UMIN - we know that the result will have the maximum of the
// known zero leading bits of the inputs.
unsigned LeadZero = Known.countMinLeadingZeros();
LeadZero = std::max(LeadZero, KnownRHS.countMinLeadingZeros());
Known &= KnownRHS;
Known.Zero.setHighBits(LeadZero);
Known = KnownBits::umin(Known, KnownRHS);
break;
}
case TargetOpcode::G_UMAX: {
@ -336,14 +343,7 @@ void GISelKnownBits::computeKnownBitsImpl(Register R, KnownBits &Known,
DemandedElts, Depth + 1);
computeKnownBitsImpl(MI.getOperand(2).getReg(), KnownRHS,
DemandedElts, Depth + 1);
// UMAX - we know that the result will have the maximum of the
// known one leading bits of the inputs.
unsigned LeadOne = Known.countMinLeadingOnes();
LeadOne = std::max(LeadOne, KnownRHS.countMinLeadingOnes());
Known.Zero &= KnownRHS.Zero;
Known.One &= KnownRHS.One;
Known.One.setHighBits(LeadOne);
Known = KnownBits::umax(Known, KnownRHS);
break;
}
case TargetOpcode::G_FCMP:

27
llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
View File

@ -3390,29 +3390,13 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
case ISD::UMIN: {
Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
Known2 = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
// UMIN - we know that the result will have the maximum of the
// known zero leading bits of the inputs.
unsigned LeadZero = Known.countMinLeadingZeros();
LeadZero = std::max(LeadZero, Known2.countMinLeadingZeros());
Known.Zero &= Known2.Zero;
Known.One &= Known2.One;
Known.Zero.setHighBits(LeadZero);
Known = KnownBits::umin(Known, Known2);
break;
}
case ISD::UMAX: {
Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
Known2 = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
// UMAX - we know that the result will have the maximum of the
// known one leading bits of the inputs.
unsigned LeadOne = Known.countMinLeadingOnes();
LeadOne = std::max(LeadOne, Known2.countMinLeadingOnes());
Known.Zero &= Known2.Zero;
Known.One &= Known2.One;
Known.One.setHighBits(LeadOne);
Known = KnownBits::umax(Known, Known2);
break;
}
case ISD::SMIN:
@ -3446,12 +3430,13 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
}
}
// Fallback - just get the shared known bits of the operands.
Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
if (Known.isUnknown()) break; // Early-out
Known2 = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
Known.Zero &= Known2.Zero;
Known.One &= Known2.One;
if (IsMax)
Known = KnownBits::smax(Known, Known2);
else
Known = KnownBits::smin(Known, Known2);
break;
}
case ISD::FrameIndex:

62
llvm/lib/Support/KnownBits.cpp
View File

@ -83,6 +83,68 @@ KnownBits KnownBits::computeForAddSub(bool Add, bool NSW,
return KnownOut;
}
KnownBits KnownBits::makeGE(const APInt &Val) const {
// Count the number of leading bit positions where our underlying value is
// known to be less than or equal to Val.
unsigned N = (Zero | Val).countLeadingOnes();
// For each of those bit positions, if Val has a 1 in that bit then our
// underlying value must also have a 1.
APInt MaskedVal(Val);
MaskedVal.clearLowBits(getBitWidth() - N);
return KnownBits(Zero, One | MaskedVal);
}
KnownBits KnownBits::umax(const KnownBits &LHS, const KnownBits &RHS) {
// If we can prove that LHS >= RHS then use LHS as the result. Likewise for
// RHS. Ideally our caller would already have spotted these cases and
// optimized away the umax operation, but we handle them here for
// completeness.
if (LHS.getMinValue().uge(RHS.getMaxValue()))
return LHS;
if (RHS.getMinValue().uge(LHS.getMaxValue()))
return RHS;
// If the result of the umax is LHS then it must be greater than or equal to
// the minimum possible value of RHS. Likewise for RHS. Any known bits that
// are common to these two values are also known in the result.
KnownBits L = LHS.makeGE(RHS.getMinValue());
KnownBits R = RHS.makeGE(LHS.getMinValue());
return KnownBits(L.Zero & R.Zero, L.One & R.One);
}
KnownBits KnownBits::umin(const KnownBits &LHS, const KnownBits &RHS) {
// Flip the range of values: [0, 0xFFFFFFFF] <-> [0xFFFFFFFF, 0]
auto Flip = [](KnownBits Val) { return KnownBits(Val.One, Val.Zero); };
return Flip(umax(Flip(LHS), Flip(RHS)));
}
KnownBits KnownBits::smax(const KnownBits &LHS, const KnownBits &RHS) {
// Flip the range of values: [-0x80000000, 0x7FFFFFFF] <-> [0, 0xFFFFFFFF]
auto Flip = [](KnownBits Val) {
unsigned SignBitPosition = Val.getBitWidth() - 1;
APInt Zero = Val.Zero;
APInt One = Val.One;
Zero.setBitVal(SignBitPosition, Val.One[SignBitPosition]);
One.setBitVal(SignBitPosition, Val.Zero[SignBitPosition]);
return KnownBits(Zero, One);
};
return Flip(umax(Flip(LHS), Flip(RHS)));
}
KnownBits KnownBits::smin(const KnownBits &LHS, const KnownBits &RHS) {
// Flip the range of values: [-0x80000000, 0x7FFFFFFF] <-> [0xFFFFFFFF, 0]
auto Flip = [](KnownBits Val) {
unsigned SignBitPosition = Val.getBitWidth() - 1;
APInt Zero = Val.One;
APInt One = Val.Zero;
Zero.setBitVal(SignBitPosition, Val.Zero[SignBitPosition]);
One.setBitVal(SignBitPosition, Val.One[SignBitPosition]);
return KnownBits(Zero, One);
};
return Flip(umax(Flip(LHS), Flip(RHS)));
}
KnownBits &KnownBits::operator&=(const KnownBits &RHS) {
// Result bit is 0 if either operand bit is 0.
Zero |= RHS.Zero;

4
llvm/unittests/CodeGen/GlobalISel/KnownBitsTest.cpp
View File

@ -719,9 +719,9 @@ TEST_F(AArch64GISelMITest, TestKnownBitsUMax) {
KnownBits KnownUmax = Info.getKnownBits(CopyUMax);
EXPECT_EQ(64u, KnownUmax.getBitWidth());
EXPECT_EQ(0u, KnownUmax.Zero.getZExtValue());
EXPECT_EQ(0xffu, KnownUmax.Zero.getZExtValue());
EXPECT_EQ(0xffffffffffffff00, KnownUmax.One.getZExtValue());
EXPECT_EQ(0u, KnownUmax.Zero.getZExtValue());
EXPECT_EQ(0xffu, KnownUmax.Zero.getZExtValue());
EXPECT_EQ(0xffffffffffffff00, KnownUmax.One.getZExtValue());
}

44
llvm/unittests/Support/KnownBitsTest.cpp
View File

@ -103,13 +103,15 @@ TEST(KnownBitsTest, BinaryExhaustive) {
unsigned Bits = 4;
ForeachKnownBits(Bits, [&](const KnownBits &Known1) {
ForeachKnownBits(Bits, [&](const KnownBits &Known2) {
KnownBits KnownAnd(Bits), KnownOr(Bits), KnownXor(Bits);
KnownBits KnownAnd(Bits);
KnownAnd.Zero.setAllBits();
KnownAnd.One.setAllBits();
KnownOr.Zero.setAllBits();
KnownOr.One.setAllBits();
KnownXor.Zero.setAllBits();
KnownXor.One.setAllBits();
KnownBits KnownOr(KnownAnd);
KnownBits KnownXor(KnownAnd);
KnownBits KnownUMax(KnownAnd);
KnownBits KnownUMin(KnownAnd);
KnownBits KnownSMax(KnownAnd);
KnownBits KnownSMin(KnownAnd);
ForeachNumInKnownBits(Known1, [&](const APInt &N1) {
ForeachNumInKnownBits(Known2, [&](const APInt &N2) {
@ -126,6 +128,22 @@ TEST(KnownBitsTest, BinaryExhaustive) {
Res = N1 ^ N2;
KnownXor.One &= Res;
KnownXor.Zero &= ~Res;
Res = APIntOps::umax(N1, N2);
KnownUMax.One &= Res;
KnownUMax.Zero &= ~Res;
Res = APIntOps::umin(N1, N2);
KnownUMin.One &= Res;
KnownUMin.Zero &= ~Res;
Res = APIntOps::smax(N1, N2);
KnownSMax.One &= Res;
KnownSMax.Zero &= ~Res;
Res = APIntOps::smin(N1, N2);
KnownSMin.One &= Res;
KnownSMin.Zero &= ~Res;
});
});
@ -140,6 +158,22 @@ TEST(KnownBitsTest, BinaryExhaustive) {
KnownBits ComputedXor = Known1 ^ Known2;
EXPECT_EQ(KnownXor.Zero, ComputedXor.Zero);
EXPECT_EQ(KnownXor.One, ComputedXor.One);
KnownBits ComputedUMax = KnownBits::umax(Known1, Known2);
EXPECT_EQ(KnownUMax.Zero, ComputedUMax.Zero);
EXPECT_EQ(KnownUMax.One, ComputedUMax.One);
KnownBits ComputedUMin = KnownBits::umin(Known1, Known2);
EXPECT_EQ(KnownUMin.Zero, ComputedUMin.Zero);
EXPECT_EQ(KnownUMin.One, ComputedUMin.One);
KnownBits ComputedSMax = KnownBits::smax(Known1, Known2);
EXPECT_EQ(KnownSMax.Zero, ComputedSMax.Zero);
EXPECT_EQ(KnownSMax.One, ComputedSMax.One);
KnownBits ComputedSMin = KnownBits::smin(Known1, Known2);
EXPECT_EQ(KnownSMin.Zero, ComputedSMin.Zero);
EXPECT_EQ(KnownSMin.One, ComputedSMin.One);
});
});
}