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[InstCombine] limit nsz: -(X - Y) --> Y - X to hasOneUse()

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As noted in the post-commit discussion for r329350, we shouldn't
generally assume that fsub is the same cost as fneg.

llvm-svn: 329429
This commit is contained in:
Sanjay Patel 2018-04-06 17:24:08 +00:00
parent 0b528e55e0
commit 6ad1591761
2 changed files with 16 additions and 15 deletions
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21
lib/Transforms/InstCombine/InstCombineAddSub.cpp
View File

@ -1698,21 +1698,18 @@ Instruction *InstCombiner::visitFSub(BinaryOperator &I) {
SQ.getWithInstruction(&I)))
return replaceInstUsesWith(I, V);
Value *X, *Y;
if (I.hasNoSignedZeros()) {
// Subtraction from -0.0 is the canonical form of fneg.
// fsub nsz 0, X ==> fsub nsz -0.0, X
if (match(Op0, m_PosZeroFP()))
return BinaryOperator::CreateFNegFMF(Op1, &I);
// Subtraction from -0.0 is the canonical form of fneg.
// fsub nsz 0, X ==> fsub nsz -0.0, X
if (I.hasNoSignedZeros() && match(Op0, m_PosZeroFP()))
return BinaryOperator::CreateFNegFMF(Op1, &I);
// With no-signed-zeros: -(X - Y) --> Y - X
if (match(Op0, m_NegZeroFP()) && match(Op1, m_FSub(m_Value(X), m_Value(Y))))
return BinaryOperator::CreateFSubFMF(Y, X, &I);
}
// More generally than above, if Op0 is not -0.0: Z - (X - Y) --> Z + (Y - X)
// If Op0 is not -0.0 or we can ignore -0.0: Z - (X - Y) --> Z + (Y - X)
// Canonicalize to fadd to make analysis easier.
// This can also help codegen because fadd is commutative.
// Note that if this fsub was really an fneg, the fadd with -0.0 will get
// killed later. We still limit that particular transform with 'hasOneUse'
// because an fneg is assumed better/cheaper than a generic fsub.
Value *X, *Y;
if (I.hasNoSignedZeros() || CannotBeNegativeZero(Op0, SQ.TLI)) {
if (match(Op1, m_OneUse(m_FSub(m_Value(X), m_Value(Y))))) {
Value *NewSub = Builder.CreateFSubFMF(Y, X, &I);

10
test/Transforms/InstCombine/fsub.ll
View File

@ -19,20 +19,24 @@ define float @test1(float %x, float %y) {
define float @neg_sub_nsz(float %x, float %y) {
; CHECK-LABEL: @neg_sub_nsz(
; CHECK-NEXT: [[T2:%.*]] = fsub nsz float [[Y:%.*]], [[X:%.*]]
; CHECK-NEXT: ret float [[T2]]
; CHECK-NEXT: [[TMP1:%.*]] = fsub nsz float [[Y:%.*]], [[X:%.*]]
; CHECK-NEXT: ret float [[TMP1]]
;
%t1 = fsub float %x, %y
%t2 = fsub nsz float -0.0, %t1
ret float %t2
}
; If the subtract has another use, we don't do the transform (even though it
; doesn't increase the IR instruction count) because we assume that fneg is
; easier to analyze and generally cheaper than generic fsub.
declare void @use(float)
define float @neg_sub_nsz_extra_use(float %x, float %y) {
; CHECK-LABEL: @neg_sub_nsz_extra_use(
; CHECK-NEXT: [[T1:%.*]] = fsub float [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[T2:%.*]] = fsub nsz float [[Y]], [[X]]
; CHECK-NEXT: [[T2:%.*]] = fsub nsz float -0.000000e+00, [[T1]]
; CHECK-NEXT: call void @use(float [[T1]])
; CHECK-NEXT: ret float [[T2]]
;