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

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[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
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27 changes: 22 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 +888,28 @@ 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);
}

// If X has TZ trailing zeroes, then bit (2 * TZ + 1) must be zero.
unsigned TZ = (~LHS.Zero).countr_zero();
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Use TrailZero0 directly.

unsigned TwoTZP1 = 2 * TZ + 1;
if (TwoTZP1 < BitWidth)
Res.Zero.setBit(TwoTZP1);

// If X has exactly TZ trailing zeros, then bit (2 * TZ + 2) must also be
// zero.
if (TZ < BitWidth && LHS.One[TZ]) {
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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Original file line number Diff line number Diff line change
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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