[llvm-dev] Condition code in DAGCombiner::visitFADDForFMACombine?

Ryan Taylor via llvm-dev
Wed, 22 Aug 2018 11:29:53 -0000

The example starts as SPIR-V with the NoContraction decoration flag on the
fmul.

I think what you are saying seems valid in that if the user had put the
flag on the fadd instead of the fmul it would not contract and so in this
example the user needs to put the NoContraction on the fadd though I'm not
sure that's a good expectation of the user. On the surface, I think that if
an operation didn't have the contract flag than it wouldn't be
contracted,
regardless of what flags any other operation has.

On Wed, Aug 22, 2018 at 3:55 AM Nicolai Hähnle via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 21.08.2018 16:08, Ryan Taylor via llvm-dev wrote:
> > So I have a test case where:
> >
> > %20 = fmul nnan arcp float %15, %19
> > %21 = fadd reassoc nnan arcp contract float %20, -1.000000e+00
> >
> > is being contracted in DAG to fmad. Is this correct since the fmul has
> > no reassoc or contract fast math flag?
>
> By having the reassoc and contract flags on fadd, the frontend is
> essentially saying "different rounding on the value produced by the
fadd
> is okay".
>
> So I'd say contracting this to fma is correct.
>
> Where does this code come from, and why do you think contracting to fma
> is wrong?
>
> Cheers,
> Nicolai
>
>
>
>
> >
> > Thanks.
> >
> > On Mon, Aug 20, 2018 at 12:56 PM Ryan Taylor <ryta1203 at gmail.com
> > <mailto:ryta1203 at gmail.com>> wrote:
> >
> >     I'm curious why the condition to fuse is this:
> >
> >     // Floating-point multiply-add with intermediate rounding.
> >        bool HasFMAD = (LegalOperations &&
> >     TLI.isOperationLegal(ISD::FMAD, VT));
> >
> >     static bool isContractable(SDNode *N) {
> >        SDNodeFlags F = N->getFlags();
> >        return F.hasAllowContract() || F.hasAllowReassociation();
> >     }
> >
> >     bool CanFuse = Options.UnsafeFPMath || isContractable(N);
> >     bool AllowFusionGlobally = (Options.AllowFPOpFusion => >
FPOpFusion::Fast || CanFuse || HasFMAD);
> >     // If the addition is not contractable, do not combine.
> >     if (!AllowFusionGlobally && !isContractable(N))
> >          return SDValue();
> >
> >     Specifically the AllowFusionGlobally, I would have expected
> >     something more like:
> >
> >     bool AllowFusionGlobally = (Options.AllowFPOpFusion => >
FPOpFusion::Fast && CanFuse && HasFMAD);
> >
> >     or at the very least:
> >
> >     bool AllowFusionGlobally = ((Options.AllowFPOpFusion => >
FPOpFusion::Fast || CanFuse) && HasFMAD);
> >
> >     It seems that as long as the target can do fmad it does do fmad
> >     since HasFMAD is true.
> >
> >     Thanks.
> >
> >
> >
> >
> >
> > _______________________________________________
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> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
>
> --
> Lerne, wie die Welt wirklich ist,
> Aber vergiss niemals, wie sie sein sollte.
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