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

[Adam Nemet via llvm-dev]

> On Mar 15, 2017, at 11:36 AM, Mehdi Amini <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. 

> 
> 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).

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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