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

Thu Aug 23 10:36:01 PDT 2018

If we have this:
r = (X * Y) + Z

And we want that to become an fma op/node, 'contract' is checked on the
fadd because it is the fadd that's being altered to form the (possibly
non-standard) result. As I think was noted earlier, whether 'contract' is
set on the fmul is irrelevant in our current implementation. This allows
the scenario where a strict fmul was inlined into code with looser FP
semantics, and we're still free to create an fma. If the end value allows
non-standard behavior, then we assume that intermediate ops leading up to
that end value can use non-standard behavior too. (cc'ing Michael Berg who
did a lot of the DAG FMF work recently)

I'm not familiar with SPIR-V, but it sounds like it has an inverse flag
system to what we have in IR and DAG - ops are presumed contract-able
unless specified with 'no-contract'? Not sure how to resolve that.

If we want to change the LLVM FMF semantics, then there will be breakage in
the IR optimizer too (at least for 'reassoc'; not sure about 'contract').
Either way, I agree that we should try to clarify the LangRef about this
because you can't tell how things are supposed to work from the current
description.

On Wed, Aug 22, 2018 at 9:41 AM, Nicolai Hähnle via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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>>     Aber vergiss niemals, wie sie sein sollte.
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>
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