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[InstCombine] generalize 2 LSB of demanded bits for X*X 

This is a follow-up suggested in D119060.
Instead of checking each of the bottom 2 bits individually,
we can check them together and handle the possibility that
we demand both together.

https://alive2.llvm.org/ce/z/C2ihC2

Differential Revision: https://reviews.llvm.org/D119139
This commit is contained in:
Sanjay Patel 2022-02-07 11:04:22 -05:00
parent d42765b3be
commit 897d92faef
2 changed files with 28 additions and 17 deletions
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19
llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
View File

@ -546,16 +546,6 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
}
case Instruction::Mul: {
if (DemandedMask.isPowerOf2()) {
if (I->getOperand(0) == I->getOperand(1)) {
// X * X is odd iff X is odd.
if (DemandedMask == 1)
return I->getOperand(0);
// 'Quadratic Reciprocity': mul(x,x) -> 0 if we're only demanding bit[1]
if (DemandedMask == 2)
return ConstantInt::getNullValue(VTy);
}
// The LSB of X*Y is set only if (X & 1) == 1 and (Y & 1) == 1.
// If we demand exactly one bit N and we have "X * (C' << N)" where C' is
// odd (has LSB set), then the left-shifted low bit of X is the answer.
@ -568,6 +558,15 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
return InsertNewInstWith(Shl, *I);
}
}
// For a squared value "X * X", the bottom 2 bits are 0 and X[0] because:
// X * X is odd iff X is odd.
// 'Quadratic Reciprocity': X * X -> 0 for bit[1]
if (I->getOperand(0) == I->getOperand(1) && DemandedMask.ult(4)) {
Constant *One = ConstantInt::get(VTy, 1);
Instruction *And1 = BinaryOperator::CreateAnd(I->getOperand(0), One);
return InsertNewInstWith(And1, *I);
}
computeKnownBits(I, Known, Depth, CxtI);
break;
}

26
llvm/test/Transforms/InstCombine/mul-masked-bits.ll
View File

@ -113,8 +113,8 @@ define i67 @one_demanded_low_bit(i67 %x) {
define i33 @squared_one_demanded_low_bit(i33 %x) {
; CHECK-LABEL: @squared_one_demanded_low_bit(
; CHECK-NEXT: [[AND:%.*]] = and i33 [[X:%.*]], 1
; CHECK-NEXT: ret i33 [[AND]]
; CHECK-NEXT: [[TMP1:%.*]] = and i33 [[X:%.*]], 1
; CHECK-NEXT: ret i33 [[TMP1]]
;
%mul = mul i33 %x, %x
%and = and i33 %mul, 1
@ -133,9 +133,8 @@ define <2 x i8> @squared_one_demanded_low_bit_splat(<2 x i8> %x) {
define i33 @squared_demanded_2_low_bits(i33 %x) {
; CHECK-LABEL: @squared_demanded_2_low_bits(
; CHECK-NEXT: [[MUL:%.*]] = mul i33 [[X:%.*]], [[X]]
; CHECK-NEXT: [[AND:%.*]] = and i33 [[MUL]], 3
; CHECK-NEXT: ret i33 [[AND]]
; CHECK-NEXT: [[TMP1:%.*]] = and i33 [[X:%.*]], 1
; CHECK-NEXT: ret i33 [[TMP1]]
;
%mul = mul i33 %x, %x
%and = and i33 %mul, 3
@ -144,11 +143,24 @@ define i33 @squared_demanded_2_low_bits(i33 %x) {
define <2 x i8> @squared_demanded_2_low_bits_splat(<2 x i8> %x) {
; CHECK-LABEL: @squared_demanded_2_low_bits_splat(
; CHECK-NEXT: [[MUL:%.*]] = mul <2 x i8> [[X:%.*]], [[X]]
; CHECK-NEXT: [[AND:%.*]] = or <2 x i8> [[MUL]], <i8 -4, i8 -4>
; CHECK-NEXT: [[TMP1:%.*]] = and <2 x i8> [[X:%.*]], <i8 1, i8 1>
; CHECK-NEXT: [[AND:%.*]] = or <2 x i8> [[TMP1]], <i8 -4, i8 -4>
; CHECK-NEXT: ret <2 x i8> [[AND]]
;
%mul = mul <2 x i8> %x, %x
%and = or <2 x i8> %mul, <i8 252, i8 252>
ret <2 x i8> %and
}
; negative test
define i33 @squared_demanded_3_low_bits(i33 %x) {
; CHECK-LABEL: @squared_demanded_3_low_bits(
; CHECK-NEXT: [[MUL:%.*]] = mul i33 [[X:%.*]], [[X]]
; CHECK-NEXT: [[AND:%.*]] = and i33 [[MUL]], 7
; CHECK-NEXT: ret i33 [[AND]]
;
%mul = mul i33 %x, %x
%and = and i33 %mul, 7
ret i33 %and
}