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[InstCombine] Add additional known bits info for self multiply #151202

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Merged
merged 6 commits into from
Aug 11, 2025
Merged

[InstCombine] Add additional known bits info for self multiply #151202

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4d0e990
pre-commit test
Jul 29, 2025
37dfb4b
Add additional known bits info for self multiply
Jul 29, 2025
116f88a
Generalize X * X info to all bit widths
Aug 2, 2025
fe3b0d4
replace TZ with TrailZero0
Aug 3, 2025
ec4791d
remove redundant setBit given new generalization
Aug 4, 2025
8c2754a
reword comments
Aug 9, 2025
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26 changes: 21 additions & 5 deletions llvm/lib/Support/KnownBits.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -888,11 +888,27 @@ KnownBits KnownBits::mul(const KnownBits &LHS, const KnownBits &RHS,
Res.Zero |= (~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) {
assert(Res.One[1] == 0 &&
"Self-multiplication failed Quadratic Reciprocity!");
Res.Zero.setBit(1);
// Self multiplying
if (NoUndefSelfMultiply) {
// bit[1] is guaranteed to be zero.
if (BitWidth > 1) {
assert(Res.One[1] == 0 &&
"Self-multiplication failed Quadratic Reciprocity!");
Res.Zero.setBit(1);
}
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Suggested change
// bit[1] is guaranteed to be zero.
if (BitWidth > 1) {
assert(Res.One[1] == 0 &&
"Self-multiplication failed Quadratic Reciprocity!");
Res.Zero.setBit(1);
}

It is covered by the logic below.


// If X has TZ trailing zeroes, then bit (2 * TZ + 1) must be zero.
unsigned TwoTZP1 = 2 * TrailZero0 + 1;
if (TwoTZP1 < BitWidth)
Res.Zero.setBit(TwoTZP1);

// If X has exactly TZ trailing zeros, then bit (2 * TZ + 2) must also be
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Suggested change
// If X has exactly TZ trailing zeros, then bit (2 * TZ + 2) must also be
// If X has exact TZ trailing zeros, then bit (2 * TZ + 2) must also be

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I don't understand the problem with the previous wording

// zero.
if (TrailZero0 < BitWidth && LHS.One[TrailZero0]) {
unsigned TwoTZP2 = TwoTZP1 + 1;
if (TwoTZP2 < BitWidth)
Res.Zero.setBit(TwoTZP2);
}
}

return Res;
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42 changes: 42 additions & 0 deletions llvm/test/Transforms/InstCombine/known-bits.ll
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Expand Up @@ -1244,6 +1244,48 @@ define i1 @extract_value_smul_fail(i8 %xx, i8 %yy) {
ret i1 %r
}

define i8 @known_self_mul_bit_0_set(i8 noundef %x) {
; CHECK-LABEL: @known_self_mul_bit_0_set(
; CHECK-NEXT: ret i8 0
;
%bit_0_set = or i8 %x, 1
%self_mul = mul i8 %bit_0_set, %bit_0_set
%r = and i8 %self_mul, 4
ret i8 %r
}

define i8 @known_self_mul_bit_0_unset(i8 noundef %x) {
; CHECK-LABEL: @known_self_mul_bit_0_unset(
; CHECK-NEXT: ret i8 0
;
%bit_0_unset = and i8 %x, -2
%self_mul = mul i8 %bit_0_unset, %bit_0_unset
%r = and i8 %self_mul, 8
ret i8 %r
}

define i8 @known_self_mul_bit_1_set_bit_0_unset(i8 noundef %x) {
; CHECK-LABEL: @known_self_mul_bit_1_set_bit_0_unset(
; CHECK-NEXT: ret i8 0
;
%lower_2_unset = and i8 %x, -4
%bit_1_set_bit_0_unset = or disjoint i8 %lower_2_unset, 2
%self_mul = mul i8 %bit_1_set_bit_0_unset, %bit_1_set_bit_0_unset
%r = and i8 %self_mul, 24
ret i8 %r
}

define i4 @known_self_mul_bit_1_set_bit_0_unset_i4(i4 noundef %x) {
; CHECK-LABEL: @known_self_mul_bit_1_set_bit_0_unset_i4(
; CHECK-NEXT: ret i4 0
;
%lower_2_unset = and i4 %x, -4
%bit_1_set_bit_0_unset = or disjoint i4 %lower_2_unset, 2
%self_mul = mul i4 %bit_1_set_bit_0_unset, %bit_1_set_bit_0_unset
%r = and i4 %self_mul, 24
ret i4 %r
}

define i8 @known_reduce_or(<2 x i8> %xx) {
; CHECK-LABEL: @known_reduce_or(
; CHECK-NEXT: ret i8 1
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