[cfe-dev] [llvm-dev] [RFC] FP Contract = fast?

[Adam Nemet via cfe-dev]

> On Mar 15, 2017, at 2:00 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> 
> On 03/15/2017 01:47 PM, Adam Nemet wrote:
>> 
>>> On Mar 15, 2017, at 11:36 AM, Mehdi Amini <mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>> 
>>> 
>>>> On Mar 15, 2017, at 10:13 AM, Hal Finkel via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>>>> 
>>>> 
>>>> On 03/15/2017 12:10 PM, Adam Nemet via llvm-dev wrote:
>>>>> Relevant to this discussion is http://bugs.llvm.org/show_bug.cgi?id=25721 <http://bugs.llvm.org/show_bug.cgi?id=25721> (-ffp-contract=fast does not work with LTO).  I am working on adding function attributes for fp-contract=fast which should fix this.
>>>> 
>>>> Great!
>>>> 
>>> 
>>> A function attribute would be a strict improvement over today: LLVM can’t do contraction today. But actually I’m not sure if it is the long term right choice: attributes don’t combine well with inlining for instance. You mentioned FMF earlier, why don’t we have a FMF to allow contraction?
>> 
>> OK, I thought that the prerequisite for that was a fast-math pragma which I don’t think is something we have (I want to be able to specify contract=fast on smaller granularity than module).  But now that I think more about, we should be able to turn a user function attribute into FMF in the front-end which is the most flexible. 
> 
> I agree, a FMF is the way to go and we can then control it with the pragma. We can use the STDC FP_CONTRACT pragma for contraction;

Just to confirm, do you mean to introduce a “fast” option to the pragma, e.g.:

#pragma STDC FP_CONTRACT FAST

Thanks,
Adam

> I also think that having a "fast math" pragma is also a good idea (the fact that we can currently only specify fast-math settings on a translation-unit level is somewhat problematic).
> 
>> 
>>> 
>>> Also, IIUC, the function attribute as well as a FMF wouldn’t apply to the “ON” setting but only to the “FAST” mode (no way to distinguish source level statement in llvm IR).
> 
> Right. We still have the existing fmuladd intrinsic method for dealing with the "ON" setting.
> 
>> 
>> Yes.
>> 
>> Adam
>> 
>>> 
>>> — 
>>> Mehdi
>>> 
>>> 
>>> 
>>> 
>>> 
>>>>> 
>>>>> Also now that we have backend optimization remarks, I am planning to report missed optimization when we can’t fuse FMAs due “fast” not being on.  This will show up in the opt-viewer.  Then the user can opt in either with the command-line switch or the new function attribute.
>>>> 
>>>> That seems useful.
>>>> 
>>>> Thanks again,
>>>> Hal
>>>> 
>>>>> 
>>>>> Adam
>>>>> 
>>>>>> On Mar 15, 2017, at 6:27 AM, Renato Golin via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> wrote:
>>>>>> 
>>>>>> Folks,
>>>>>> 
>>>>>> I've been asking around people about the state of FP contract, which
>>>>>> seems to be "on" but it's not really behaving like it, at least not as
>>>>>> I would expect:
>>>>>> 
>>>>>> int foo(float a, float b, float c) { return a*b+c; }
>>>>>> 
>>>>>> $ clang -target aarch64-linux-gnu -O2 -S fma.c -ffp-contract=on -o -
>>>>>> (...)
>>>>>> fmul s0, s0, s1
>>>>>> fadd s0, s0, s2
>>>>>> (...)
>>>>>> 
>>>>>> $ clang -target aarch64-linux-gnu -O2 -S fma.c -ffp-contract=fast -o -
>>>>>> (...)
>>>>>> fmadd s0, s0, s1, s2
>>>>>> (...)
>>>>>> 
>>>>>> I'm not sure this works in Fortran either, but defaulting to "on" when
>>>>>> (I believe) the language should allow contraction and not doing it is
>>>>>> not a good default.
>>>>>> 
>>>>>> i haven't worked out what would be necessary to make it work on a
>>>>>> case-by-case basis (what kinds of fusions does C allow?) to make sure
>>>>>> we don't do all or nothing, but if we don't want to start that
>>>>>> conversation now, then I'd recommend we just turn it all the way to 11
>>>>>> (like GCC) and let people turn it off if they really mean it.
>>>>>> 
>>>>>> The rationale is that:
>>>>>> 
>>>>>> * Contracted operations increase precision (less rounding steps)
>>>>>> * It performs equal or faster on all architectures I know (true everywhere?)
>>>>>> * Users already expect that (certainly, GCC users do)
>>>>>> * Makes us look good on benchmarks :)
>>>>>> 
>>>>>> A recent SPEC2k6 comparison Linaro did for AArch64, enabling
>>>>>> -ffp-contract=fast took the edge of GCC in a number of cases and in
>>>>>> some of them made them comparable in performance. So, any reasons not
>>>>>> to?
>>>>>> 
>>>>>> If we go with it, we need to first finish the job that Sebastian was
>>>>>> dong on the test-suite, then just turn it on by default. A second
>>>>>> stage would be to add tests/benchmarks that explicitly test FP
>>>>>> precision, so that we have some extra guarantee that we're doing the
>>>>>> right thing.
>>>>>> 
>>>>>> Opinions?
>>>>>> 
>>>>>> cheers,
>>>>>> --renato
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>>>>> 
>>>>> 
>>>>> 
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>>>> 
>>>> -- 
>>>> Hal Finkel
>>>> Lead, Compiler Technology and Programming Languages
>>>> Leadership Computing Facility
>>>> Argonne National Laboratory
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>>> 
>> 
> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory

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