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

[Adam Nemet via cfe-dev]

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.

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.

Adam

> On Mar 15, 2017, at 6:27 AM, Renato Golin via cfe-dev <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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