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

[Hal Finkel via cfe-dev]

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 (-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; 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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