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Reassoc FMF should not optimize FMA(a, 0, b) to (b) 

Optimizing (a * 0 + b) to (b) requires assuming that a is finite and not
NaN. DAGCombiner will do this optimization when the reassoc fast math
flag is set, which is not correct. Change DAGCombiner to only consider
UnsafeMath for this optimization.

Differential Revision: https://reviews.llvm.org/D130232

Co-authored-by: Andrea Faulds <andrea.faulds@arm.com>
This commit is contained in:
Sven van Haastregt 2022-07-26 09:39:12 +01:00
parent 1bc7b06ffd
commit c8d91b07bb
2 changed files with 19 additions and 4 deletions
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9
llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
View File

@ -15002,7 +15002,7 @@ SDValue DAGCombiner::visitFMA(SDNode *N) {
// FMA nodes have flags that propagate to the created nodes.
SelectionDAG::FlagInserter FlagsInserter(DAG, N);
bool UnsafeFPMath =
bool CanReassociate =
Options.UnsafeFPMath || N->getFlags().hasAllowReassociation();
// Constant fold FMA.
@ -15026,7 +15026,8 @@ SDValue DAGCombiner::visitFMA(SDNode *N) {
CostN1 == TargetLowering::NegatibleCost::Cheaper))
return DAG.getNode(ISD::FMA, DL, VT, NegN0, NegN1, N2);
if (UnsafeFPMath) {
// FIXME: use fast math flags instead of Options.UnsafeFPMath
if (Options.UnsafeFPMath) {
if (N0CFP && N0CFP->isZero())
return N2;
if (N1CFP && N1CFP->isZero())
@ -15043,7 +15044,7 @@ SDValue DAGCombiner::visitFMA(SDNode *N) {
!DAG.isConstantFPBuildVectorOrConstantFP(N1))
return DAG.getNode(ISD::FMA, SDLoc(N), VT, N1, N0, N2);
if (UnsafeFPMath) {
if (CanReassociate) {
// (fma x, c1, (fmul x, c2)) -> (fmul x, c1+c2)
if (N2.getOpcode() == ISD::FMUL && N0 == N2.getOperand(0) &&
DAG.isConstantFPBuildVectorOrConstantFP(N1) &&
@ -15084,7 +15085,7 @@ SDValue DAGCombiner::visitFMA(SDNode *N) {
}
}
if (UnsafeFPMath) {
if (CanReassociate) {
// (fma x, c, x) -> (fmul x, (c+1))
if (N1CFP && N0 == N2) {
return DAG.getNode(

14
llvm/test/CodeGen/AArch64/neon-fma-FMF.ll
View File

@ -54,3 +54,17 @@ define <2 x float> @no_fma_sub_1(<2 x float> %A, <2 x float> %B, <2 x float> %C)
%tmp2 = fsub <2 x float> %C, %tmp1;
ret <2 x float> %tmp2
}
; Regression test: contract FMF allows folding (A * 0 + B) to FMA(A, 0, B), but
; reassoc FMF must not allow further folding to just (B) without additional
; FMFs (ninf, nnan)
define float @fma_zero(float %A, float %B) {
; CHECK-LABEL: fma_zero:
; CHECK: // %bb.0:
; CHECK-NEXT: movi d2, #0000000000000000
; CHECK-NEXT: fmadd s0, s0, s2, s1
; CHECK-NEXT: ret
%tmp1 = fmul contract reassoc float %A, 0.0e+0;
%tmp2 = fadd contract reassoc float %B, %tmp1;
ret float %tmp2
}