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[InstCombine] fold fmul/fdiv with fabs operands

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fabs(X) * fabs(Y) --> fabs(X * Y)
fabs(X) / fabs(Y) --> fabs(X / Y)

If both operands of fmul/fdiv are positive, then the result must be positive.

There's a NAN corner-case that prevents removing the more specific fold just
above this one:
fabs(X) * fabs(X) -> X * X
That fold works even with NAN because the sign-bit result of the multiply is
not specified if X is NAN.

We can't remove that and use the more general fold that is proposed here
because once we convert to this:
fabs (X * X)
...it is not legal to simplify the 'fabs' out of that expression when X is NAN.
That's because fabs() guarantees that the sign-bit is always cleared - even
for NAN values.

So this patch has the potential to lose information, but it seems unlikely if
we do the more specific fold ahead of this one.

Differential Revision: https://reviews.llvm.org/D82277
This commit is contained in:
Sanjay Patel 2020-06-25 11:28:04 -04:00
parent b044a82270
commit c9e8c9e3ea
4 changed files with 31 additions and 15 deletions
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1
llvm/lib/Transforms/InstCombine/InstCombineInternal.h
View File

@ -635,6 +635,7 @@ private:
Value *getSelectCondition(Value *A, Value *B);
Instruction *foldIntrinsicWithOverflowCommon(IntrinsicInst *II);
Instruction *foldFPSignBitOps(BinaryOperator &I);
public:
/// Inserts an instruction \p New before instruction \p Old

15
llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
View File

@ -402,7 +402,7 @@ Instruction *InstCombiner::visitMul(BinaryOperator &I) {
return Changed ? &I : nullptr;
}
static Instruction *foldFPSignBitOps(BinaryOperator &I) {
Instruction *InstCombiner::foldFPSignBitOps(BinaryOperator &I) {
BinaryOperator::BinaryOps Opcode = I.getOpcode();
assert((Opcode == Instruction::FMul || Opcode == Instruction::FDiv) &&
"Expected fmul or fdiv");
@ -420,6 +420,19 @@ static Instruction *foldFPSignBitOps(BinaryOperator &I) {
if (Op0 == Op1 && match(Op0, m_Intrinsic<Intrinsic::fabs>(m_Value(X))))
return BinaryOperator::CreateWithCopiedFlags(Opcode, X, X, &I);
// fabs(X) * fabs(Y) --> fabs(X * Y)
// fabs(X) / fabs(Y) --> fabs(X / Y)
if (match(Op0, m_Intrinsic<Intrinsic::fabs>(m_Value(X))) &&
match(Op1, m_Intrinsic<Intrinsic::fabs>(m_Value(Y))) &&
(Op0->hasOneUse() || Op1->hasOneUse())) {
IRBuilder<>::FastMathFlagGuard FMFGuard(Builder);
Builder.setFastMathFlags(I.getFastMathFlags());
Value *XY = Builder.CreateBinOp(Opcode, X, Y);
Value *Fabs = Builder.CreateUnaryIntrinsic(Intrinsic::fabs, XY);
Fabs->takeName(&I);
return replaceInstUsesWith(I, Fabs);
}
return nullptr;
}

15
llvm/test/Transforms/InstCombine/fdiv.ll
View File

@ -598,9 +598,8 @@ define float @fabs_same_op_extra_use(float %x) {
define float @fabs_fabs(float %x, float %y) {
; CHECK-LABEL: @fabs_fabs(
; CHECK-NEXT: [[X_FABS:%.*]] = call float @llvm.fabs.f32(float [[X:%.*]])
; CHECK-NEXT: [[Y_FABS:%.*]] = call float @llvm.fabs.f32(float [[Y:%.*]])
; CHECK-NEXT: [[R:%.*]] = fdiv float [[X_FABS]], [[Y_FABS]]
; CHECK-NEXT: [[TMP1:%.*]] = fdiv float [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[R:%.*]] = call float @llvm.fabs.f32(float [[TMP1]])
; CHECK-NEXT: ret float [[R]]
;
%x.fabs = call float @llvm.fabs.f32(float %x)
@ -613,8 +612,8 @@ define float @fabs_fabs_extra_use1(float %x, float %y) {
; CHECK-LABEL: @fabs_fabs_extra_use1(
; CHECK-NEXT: [[X_FABS:%.*]] = call float @llvm.fabs.f32(float [[X:%.*]])
; CHECK-NEXT: call void @use_f32(float [[X_FABS]])
; CHECK-NEXT: [[Y_FABS:%.*]] = call float @llvm.fabs.f32(float [[Y:%.*]])
; CHECK-NEXT: [[R:%.*]] = fdiv ninf float [[X_FABS]], [[Y_FABS]]
; CHECK-NEXT: [[TMP1:%.*]] = fdiv ninf float [[X]], [[Y:%.*]]
; CHECK-NEXT: [[R:%.*]] = call ninf float @llvm.fabs.f32(float [[TMP1]])
; CHECK-NEXT: ret float [[R]]
;
%x.fabs = call float @llvm.fabs.f32(float %x)
@ -626,10 +625,10 @@ define float @fabs_fabs_extra_use1(float %x, float %y) {
define float @fabs_fabs_extra_use2(float %x, float %y) {
; CHECK-LABEL: @fabs_fabs_extra_use2(
; CHECK-NEXT: [[X_FABS:%.*]] = call fast float @llvm.fabs.f32(float [[X:%.*]])
; CHECK-NEXT: [[Y_FABS:%.*]] = call fast float @llvm.fabs.f32(float [[Y:%.*]])
; CHECK-NEXT: call void @use_f32(float [[Y_FABS]])
; CHECK-NEXT: [[R:%.*]] = fdiv reassoc ninf float [[X_FABS]], [[Y_FABS]]
; CHECK-NEXT: [[TMP1:%.*]] = fdiv reassoc ninf float [[X:%.*]], [[Y]]
; CHECK-NEXT: [[R:%.*]] = call reassoc ninf float @llvm.fabs.f32(float [[TMP1]])
; CHECK-NEXT: ret float [[R]]
;
%x.fabs = call fast float @llvm.fabs.f32(float %x)
@ -639,6 +638,8 @@ define float @fabs_fabs_extra_use2(float %x, float %y) {
ret float %r
}
; negative test - don't create an extra instruction
define float @fabs_fabs_extra_use3(float %x, float %y) {
; CHECK-LABEL: @fabs_fabs_extra_use3(
; CHECK-NEXT: [[X_FABS:%.*]] = call float @llvm.fabs.f32(float [[X:%.*]])

15
llvm/test/Transforms/InstCombine/fmul.ll
View File

@ -482,9 +482,8 @@ define float @fabs_squared_fast(float %x) {
define float @fabs_fabs(float %x, float %y) {
; CHECK-LABEL: @fabs_fabs(
; CHECK-NEXT: [[X_FABS:%.*]] = call float @llvm.fabs.f32(float [[X:%.*]])
; CHECK-NEXT: [[Y_FABS:%.*]] = call float @llvm.fabs.f32(float [[Y:%.*]])
; CHECK-NEXT: [[MUL:%.*]] = fmul float [[X_FABS]], [[Y_FABS]]
; CHECK-NEXT: [[TMP1:%.*]] = fmul float [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[MUL:%.*]] = call float @llvm.fabs.f32(float [[TMP1]])
; CHECK-NEXT: ret float [[MUL]]
;
%x.fabs = call float @llvm.fabs.f32(float %x)
@ -497,8 +496,8 @@ define float @fabs_fabs_extra_use1(float %x, float %y) {
; CHECK-LABEL: @fabs_fabs_extra_use1(
; CHECK-NEXT: [[X_FABS:%.*]] = call float @llvm.fabs.f32(float [[X:%.*]])
; CHECK-NEXT: call void @use_f32(float [[X_FABS]])
; CHECK-NEXT: [[Y_FABS:%.*]] = call float @llvm.fabs.f32(float [[Y:%.*]])
; CHECK-NEXT: [[MUL:%.*]] = fmul ninf float [[X_FABS]], [[Y_FABS]]
; CHECK-NEXT: [[TMP1:%.*]] = fmul ninf float [[X]], [[Y:%.*]]
; CHECK-NEXT: [[MUL:%.*]] = call ninf float @llvm.fabs.f32(float [[TMP1]])
; CHECK-NEXT: ret float [[MUL]]
;
%x.fabs = call float @llvm.fabs.f32(float %x)
@ -510,10 +509,10 @@ define float @fabs_fabs_extra_use1(float %x, float %y) {
define float @fabs_fabs_extra_use2(float %x, float %y) {
; CHECK-LABEL: @fabs_fabs_extra_use2(
; CHECK-NEXT: [[X_FABS:%.*]] = call fast float @llvm.fabs.f32(float [[X:%.*]])
; CHECK-NEXT: [[Y_FABS:%.*]] = call fast float @llvm.fabs.f32(float [[Y:%.*]])
; CHECK-NEXT: call void @use_f32(float [[Y_FABS]])
; CHECK-NEXT: [[MUL:%.*]] = fmul reassoc ninf float [[X_FABS]], [[Y_FABS]]
; CHECK-NEXT: [[TMP1:%.*]] = fmul reassoc ninf float [[X:%.*]], [[Y]]
; CHECK-NEXT: [[MUL:%.*]] = call reassoc ninf float @llvm.fabs.f32(float [[TMP1]])
; CHECK-NEXT: ret float [[MUL]]
;
%x.fabs = call fast float @llvm.fabs.f32(float %x)
@ -523,6 +522,8 @@ define float @fabs_fabs_extra_use2(float %x, float %y) {
ret float %mul
}
; negative test - don't create an extra instruction
define float @fabs_fabs_extra_use3(float %x, float %y) {
; CHECK-LABEL: @fabs_fabs_extra_use3(
; CHECK-NEXT: [[X_FABS:%.*]] = call float @llvm.fabs.f32(float [[X:%.*]])