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[InstCombine] FP: Z - (X - Y) --> Z + (Y - X)

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This restores what was lost with rL73243 but without
re-introducing the bug that was present in the old code.

Note that we already have these transforms if the ops are 
marked 'fast' (and I assume that's happening somewhere in
the code added with rL170471), but we clearly don't need 
all of 'fast' for these transforms.

llvm-svn: 329362
This commit is contained in:
Sanjay Patel 2018-04-05 23:21:15 +00:00
parent fb0967ea63
commit 5d3b82944d
2 changed files with 17 additions and 8 deletions
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13
lib/Transforms/InstCombine/InstCombineAddSub.cpp
View File

@ -1698,6 +1698,7 @@ 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
@ -1705,11 +1706,20 @@ Instruction *InstCombiner::visitFSub(BinaryOperator &I) {
return BinaryOperator::CreateFNegFMF(Op1, &I);
// With no-signed-zeros: -(X - Y) --> Y - X
Value *X, *Y;
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)
// Canonicalize to fadd to make analysis easier.
// This can also help codegen because fadd is commutative.
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);
return BinaryOperator::CreateFAddFMF(Op0, NewSub, &I);
}
}
if (isa<Constant>(Op0))
if (SelectInst *SI = dyn_cast<SelectInst>(Op1))
if (Instruction *NV = FoldOpIntoSelect(I, SI))
@ -1721,7 +1731,6 @@ Instruction *InstCombiner::visitFSub(BinaryOperator &I) {
return BinaryOperator::CreateFAddFMF(Op0, ConstantExpr::getFNeg(C), &I);
// X - (-Y) --> X + Y
Value *Y;
if (match(Op1, m_FNeg(m_Value(Y))))
return BinaryOperator::CreateFAddFMF(Op0, Y, &I);

12
test/Transforms/InstCombine/fsub.ll
View File

@ -27,12 +27,12 @@ define float @neg_sub_nsz(float %x, float %y) {
ret float %t2
}
; FIXME: With nsz: Z - (X - Y) --> Z + (Y - X)
; With nsz: Z - (X - Y) --> Z + (Y - X)
define float @sub_sub_nsz(float %x, float %y, float %z) {
; CHECK-LABEL: @sub_sub_nsz(
; CHECK-NEXT: [[T1:%.*]] = fsub float [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[T2:%.*]] = fsub nsz float [[Z:%.*]], [[T1]]
; CHECK-NEXT: [[TMP1:%.*]] = fsub nsz float [[Y:%.*]], [[X:%.*]]
; CHECK-NEXT: [[T2:%.*]] = fadd nsz float [[TMP1]], [[Z:%.*]]
; CHECK-NEXT: ret float [[T2]]
;
%t1 = fsub float %x, %y
@ -40,12 +40,12 @@ define float @sub_sub_nsz(float %x, float %y, float %z) {
ret float %t2
}
; FIXME: Same as above: if 'Z' is not -0.0, swap fsub operands and convert to fadd.
; Same as above: if 'Z' is not -0.0, swap fsub operands and convert to fadd.
define float @sub_sub_known_not_negzero(float %x, float %y) {
; CHECK-LABEL: @sub_sub_known_not_negzero(
; CHECK-NEXT: [[T1:%.*]] = fsub float [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[T2:%.*]] = fsub float 4.200000e+01, [[T1]]
; CHECK-NEXT: [[TMP1:%.*]] = fsub float [[Y:%.*]], [[X:%.*]]
; CHECK-NEXT: [[T2:%.*]] = fadd float [[TMP1]], 4.200000e+01
; CHECK-NEXT: ret float [[T2]]
;
%t1 = fsub float %x, %y