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

Nicolai Hähnle via llvm-dev
Wed, 22 Aug 2018 15:41:45 -0000

On 22.08.2018 13:29, Ryan Taylor wrote:
> 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.

Okay, I see that the SPIR-V spec specifically calls out this example.

Unless there are conflicting requirements with another frontend, I'd say
we should make sure LLVM is aligned with SPIR-V here. Something along
the lines of (in LangRef):

``contract``
    Allow floating-point contraction (e.g. fusing a multiply followed by
    an addition into a fused multiply-and-add). This flag must be present
    on all affected instruction.

And we should probably say the same about ``reassoc`` as well.

Cheers,
Nicolai




>
> On Wed, Aug 22, 2018 at 3:55 AM Nicolai Hähnle via llvm-dev
> <llvm-dev at lists.llvm.org <mailto: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>
>      > <mailto: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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>
>
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--
Lerne, wie die Welt wirklich ist,
Aber vergiss niemals, wie sie sein sollte.