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

[Mehdi Amini via cfe-dev]

> On Mar 15, 2017, at 10:13 AM, Hal Finkel via cfe-dev <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?

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).

— 
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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