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KnownBits: refine high-bits of mul in signed case #113051

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KnownBits: refine high-bits of mul in signed case #113051

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9459ba9
ValueTracking/test: cover known bits of mul
artagnon Oct 19, 2024
b6b0cfd
KnownBits: refine high-bits of mul in signed case
artagnon Oct 18, 2024
901b6e5
KnownBitsTest: cover in unittests; address review
artagnon Oct 23, 2024
603ec71
KnownBits: address review; more concise
artagnon Oct 23, 2024
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32 changes: 20 additions & 12 deletions llvm/lib/Support/KnownBits.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -796,19 +796,26 @@ KnownBits KnownBits::mul(const KnownBits &LHS, const KnownBits &RHS,
assert((!NoUndefSelfMultiply || LHS == RHS) &&
"Self multiplication knownbits mismatch");

// Compute the high known-0 bits by multiplying the unsigned max of each side.
// Conservatively, M active bits * N active bits results in M + N bits in the
// result. But if we know a value is a power-of-2 for example, then this
// computes one more leading zero.
// TODO: This could be generalized to number of sign bits (negative numbers).
APInt UMaxLHS = LHS.getMaxValue();
APInt UMaxRHS = RHS.getMaxValue();

// For leading zeros in the result to be valid, the unsigned max product must
// Compute the high known-0 or known-1 bits by multiplying the max of each
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I think you can wrap the whole of this section in if (!LHS.isSignUnknown() && !RHS.isSignUnknown). If either sign was unknown then we would not get any useful high-zeros or high-ones info from this calculation. That would resolve my confusion about your use of isNegative below by making it clear that there is no "unknown sign" case to worry about - both operand are known negative or known positive.

// side. Conservatively, M active bits * N active bits results in M + N bits
// in the result. But if we know a value is a power-of-2 for example, then
// this computes one more leading zero or one.
APInt MaxLHS = LHS.isNegative() ? LHS.getMinValue().abs() : LHS.getMaxValue(),
MaxRHS = RHS.isNegative() ? RHS.getMinValue().abs() : RHS.getMaxValue();
Comment on lines +803 to +804
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Can you remove the .abs() calls here and use smul_ov below instead?


// For leading zeros or ones in the result to be valid, the max product must
// fit in the bitwidth (it must not overflow).
bool HasOverflow;
APInt UMaxResult = UMaxLHS.umul_ov(UMaxRHS, HasOverflow);
unsigned LeadZ = HasOverflow ? 0 : UMaxResult.countl_zero();
APInt Result = MaxLHS.umul_ov(MaxRHS, HasOverflow);
bool NegResult = LHS.isNegative() ^ RHS.isNegative();
unsigned LeadZ = 0, LeadO = 0;
if (!HasOverflow) {
// Do not set leading ones unless the result is known to be non-zero.
if (NegResult && LHS.isNonZero() && RHS.isNonZero())
LeadO = (-Result).countLeadingOnes();
else if (!NegResult)
LeadZ = Result.countLeadingZeros();
}

// The result of the bottom bits of an integer multiply can be
// inferred by looking at the bottom bits of both operands and
Expand Down Expand Up @@ -873,8 +880,9 @@ KnownBits KnownBits::mul(const KnownBits &LHS, const KnownBits &RHS,

KnownBits Res(BitWidth);
Res.Zero.setHighBits(LeadZ);
Res.One.setHighBits(LeadO);
Res.Zero |= (~BottomKnown).getLoBits(ResultBitsKnown);
Res.One = BottomKnown.getLoBits(ResultBitsKnown);
Res.One |= BottomKnown.getLoBits(ResultBitsKnown);

// If we're self-multiplying then bit[1] is guaranteed to be zero.
if (NoUndefSelfMultiply && BitWidth > 1) {
Expand Down
143 changes: 143 additions & 0 deletions llvm/test/Analysis/ValueTracking/knownbits-mul.ll
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@@ -0,0 +1,143 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; RUN: opt < %s -passes=instcombine -S | FileCheck %s

define i8 @mul_low_bits_know(i8 %xx, i8 %yy) {
; CHECK-LABEL: define i8 @mul_low_bits_know(
; CHECK-SAME: i8 [[XX:%.*]], i8 [[YY:%.*]]) {
; CHECK-NEXT: ret i8 0
;
%x = and i8 %xx, 2
%y = and i8 %yy, 4
%mul = mul i8 %x, %y
%r = and i8 %mul, 6
ret i8 %r
}

define i8 @mul_low_bits_know2(i8 %xx, i8 %yy) {
; CHECK-LABEL: define i8 @mul_low_bits_know2(
; CHECK-SAME: i8 [[XX:%.*]], i8 [[YY:%.*]]) {
; CHECK-NEXT: ret i8 0
;
%x = or i8 %xx, -2
%y = and i8 %yy, 4
%mul = mul i8 %x, %y
%r = and i8 %mul, 2
ret i8 %r
}

define i8 @mul_low_bits_partially_known(i8 %xx, i8 %yy) {
; CHECK-LABEL: define i8 @mul_low_bits_partially_known(
; CHECK-SAME: i8 [[XX:%.*]], i8 [[YY:%.*]]) {
; CHECK-NEXT: [[Y:%.*]] = or i8 [[YY]], 2
; CHECK-NEXT: [[MUL:%.*]] = sub nsw i8 0, [[Y]]
; CHECK-NEXT: [[R:%.*]] = and i8 [[MUL]], 2
; CHECK-NEXT: ret i8 [[R]]
;
%x = or i8 %xx, -4
%x.notsmin = or i8 %x, 3
%y = or i8 %yy, -2
%mul = mul i8 %x.notsmin, %y
%r = and i8 %mul, 6
ret i8 %r
}

define i8 @mul_low_bits_unknown(i8 %xx, i8 %yy) {
; CHECK-LABEL: define i8 @mul_low_bits_unknown(
; CHECK-SAME: i8 [[XX:%.*]], i8 [[YY:%.*]]) {
; CHECK-NEXT: [[X:%.*]] = or i8 [[XX]], 4
; CHECK-NEXT: [[Y:%.*]] = or i8 [[YY]], 6
; CHECK-NEXT: [[MUL:%.*]] = mul i8 [[X]], [[Y]]
; CHECK-NEXT: [[R:%.*]] = and i8 [[MUL]], 6
; CHECK-NEXT: ret i8 [[R]]
;
%x = or i8 %xx, -4
%y = or i8 %yy, -2
%mul = mul i8 %x, %y
%r = and i8 %mul, 6
ret i8 %r
}

define i8 @mul_high_bits_know(i8 %xx, i8 %yy) {
; CHECK-LABEL: define i8 @mul_high_bits_know(
; CHECK-SAME: i8 [[XX:%.*]], i8 [[YY:%.*]]) {
; CHECK-NEXT: ret i8 0
;
%x = and i8 %xx, 2
%y = and i8 %yy, 4
%mul = mul i8 %x, %y
%r = and i8 %mul, 16
ret i8 %r
}

define i8 @mul_high_bits_know2(i8 %xx, i8 %yy) {
; CHECK-LABEL: define i8 @mul_high_bits_know2(
; CHECK-SAME: i8 [[XX:%.*]], i8 [[YY:%.*]]) {
; CHECK-NEXT: ret i8 -16
;
%x = or i8 %xx, -2
%y = and i8 %yy, 4
%y.nonzero = or i8 %y, 1
%mul = mul i8 %x, %y.nonzero
%r = and i8 %mul, -16
ret i8 %r
}

define i8 @mul_high_bits_know3(i8 %xx, i8 %yy) {
; CHECK-LABEL: define i8 @mul_high_bits_know3(
; CHECK-SAME: i8 [[XX:%.*]], i8 [[YY:%.*]]) {
; CHECK-NEXT: ret i8 0
;
%x = or i8 %xx, -4
%y = or i8 %yy, -2
%mul = mul i8 %x, %y
%r = and i8 %mul, -16
ret i8 %r
}

define i8 @mul_high_bits_unknown(i8 %xx, i8 %yy) {
; CHECK-LABEL: define i8 @mul_high_bits_unknown(
; CHECK-SAME: i8 [[XX:%.*]], i8 [[YY:%.*]]) {
; CHECK-NEXT: [[X:%.*]] = and i8 [[XX]], 2
; CHECK-NEXT: [[Y:%.*]] = and i8 [[YY]], 4
; CHECK-NEXT: [[MUL:%.*]] = mul nuw nsw i8 [[X]], [[Y]]
; CHECK-NEXT: ret i8 [[MUL]]
;
%x = and i8 %xx, 2
%y = and i8 %yy, 4
%mul = mul i8 %x, %y
%r = and i8 %mul, 8
ret i8 %r
}

define i8 @mul_high_bits_unknown2(i8 %xx, i8 %yy) {
; CHECK-LABEL: define i8 @mul_high_bits_unknown2(
; CHECK-SAME: i8 [[XX:%.*]], i8 [[YY:%.*]]) {
; CHECK-NEXT: [[X:%.*]] = or i8 [[XX]], -2
; CHECK-NEXT: [[Y:%.*]] = and i8 [[YY]], 4
; CHECK-NEXT: [[MUL:%.*]] = mul nsw i8 [[X]], [[Y]]
; CHECK-NEXT: [[R:%.*]] = and i8 [[MUL]], -16
; CHECK-NEXT: ret i8 [[R]]
;
%x = or i8 %xx, -2
%y = and i8 %yy, 4
%mul = mul i8 %x, %y
%r = and i8 %mul, -16
ret i8 %r
}

; TODO: This can be reduced to zero.
define i8 @mul_high_bits_unknown3(i8 %xx, i8 %yy) {
; CHECK-LABEL: define i8 @mul_high_bits_unknown3(
; CHECK-SAME: i8 [[XX:%.*]], i8 [[YY:%.*]]) {
; CHECK-NEXT: [[X:%.*]] = or i8 [[XX]], 28
; CHECK-NEXT: [[Y:%.*]] = or i8 [[YY]], 30
; CHECK-NEXT: [[MUL:%.*]] = mul i8 [[X]], [[Y]]
; CHECK-NEXT: [[R:%.*]] = and i8 [[MUL]], 16
; CHECK-NEXT: ret i8 [[R]]
;
%x = or i8 %xx, -4
%y = or i8 %yy, -2
%mul = mul i8 %x, %y
%r = and i8 %mul, 16
ret i8 %r
}
52 changes: 51 additions & 1 deletion llvm/unittests/Support/KnownBitsTest.cpp
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Expand Up @@ -815,7 +815,7 @@ TEST(KnownBitsTest, ConcatBits) {
}
}

TEST(KnownBitsTest, MulExhaustive) {
TEST(KnownBitsTest, MulLowBitsExhaustive) {
for (unsigned Bits : {1, 4}) {
ForeachKnownBits(Bits, [&](const KnownBits &Known1) {
ForeachKnownBits(Bits, [&](const KnownBits &Known2) {
Expand Down Expand Up @@ -849,4 +849,54 @@ TEST(KnownBitsTest, MulExhaustive) {
}
}

TEST(KnownBitsTest, MulHighBits) {
unsigned Bits = 8;
SmallVector<std::pair<int, int>, 4> TestPairs = {
{2, 4}, {-2, -4}, {2, -4}, {-2, 4}};
for (auto [K1, K2] : TestPairs) {
KnownBits Known1(Bits), Known2(Bits);
if (K1 > 0) {
// If we only set the zeros of ~K1, Known1 could be zero. Avoid this case,
// as we can only set leading ones in the case where LHS and RHS have
// different signs, when the result is known non-zero.
Known1.Zero |= ~(K1 | 1);
Known1.One |= 1;
} else {
Known1.One |= K1;
}
if (K2 > 0) {
// If we only set the zeros of ~K1, Known1 could be zero. Avoid this case,
// as we can only set leading ones in the case where LHS and RHS have
// different signs, when the result is known non-zero.
Known2.Zero |= ~(K2 | 1);
Known2.One |= 1;
} else {
Known2.One |= K2;
}
KnownBits Computed = KnownBits::mul(Known1, Known2);
KnownBits Exact(Bits);
Exact.Zero.setAllBits();
Exact.One.setAllBits();

ForeachNumInKnownBits(Known1, [&](const APInt &N1) {
ForeachNumInKnownBits(Known2, [&](const APInt &N2) {
APInt Res = N1 * N2;
Exact.One &= Res;
Exact.Zero &= ~Res;
});
});

// Check that the high bits are optimal, with the caveat that mul_ov of LHS
// and RHS doesn't overflow, which is the case for our TestPairs.
APInt Mask = APInt::getHighBitsSet(
Bits, (Exact.Zero | Exact.One).countLeadingOnes());
Exact.Zero &= Mask;
Exact.One &= Mask;
Computed.Zero &= Mask;
Computed.One &= Mask;
EXPECT_TRUE(checkResult("mul", Exact, Computed, {Known1, Known2},
/*CheckOptimality=*/true));
}
}

} // end anonymous namespace
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