[llvm-dev] [test-suite] making polybench/symm succeed with "-Ofast" and "-ffp-contract=on"

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Sebastian Pop" <sebpop.llvm at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Renato Golin" <renato.golin at linaro.org>, "Sebastian Paul Pop" <s.pop at samsung.com>, "llvm-dev"
> <llvm-dev at lists.llvm.org>, "Matthias Braun" <matze at braunis.de>, "Clang Dev" <cfe-dev at lists.llvm.org>, "nd"
> <nd at arm.com>, "Abe Skolnik" <a.skolnik at samsung.com>
> Sent: Wednesday, October 12, 2016 9:43:37 AM
> Subject: Re: [test-suite] making polybench/symm succeed with "-Ofast" and "-ffp-contract=on"
> 
> On Wed, Oct 12, 2016 at 10:28 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> > ----- Original Message -----
> >> From: "Renato Golin" <renato.golin at linaro.org>
> >> To: "Hal Finkel" <hfinkel at anl.gov>
> >> Cc: "Sebastian Paul Pop" <s.pop at samsung.com>, "llvm-dev"
> >> <llvm-dev at lists.llvm.org>, "Matthias Braun"
> >> <matze at braunis.de>, "Clang Dev" <cfe-dev at lists.llvm.org>, "nd"
> >> <nd at arm.com>, "Abe Skolnik" <a.skolnik at samsung.com>,
> >> "Sebastian Pop" <sebpop.llvm at gmail.com>
> >> Sent: Wednesday, October 12, 2016 9:16:39 AM
> >> Subject: Re: [test-suite] making polybench/symm succeed with
> >> "-Ofast" and "-ffp-contract=on"
> >>
> >> On 12 October 2016 at 15:05, Hal Finkel <hfinkel at anl.gov> wrote:
> >> > This is something we need to understand. No, there's not always
> >> > an
> >> > error bar. With FMA formation and without non-IEEE-compliant
> >> > optimizations (i.e. fast-math), the optimized answer should be
> >> > identical to the non-optimized answer.
> >>
> >> What about architectures that this is never respected, like
> >> Darwin?
> >>
> >> In the general case, indeed, optimisation levels should not change
> >> the
> >> IEEE representation and the tests should be deterministic.
> >>
> >> But we can't guarantee this will always be the case.
> >>
> >>
> >> > We still do see cross-system discrepancies sometimes because of
> >> > differences in denormal handling, but on the same system that
> >> > should be consistent (aside, perhaps, from compiler-level
> >> > constant-folding issues).
> >>
> >> But the test-suite doesn't run on a single system, nor it has one
> >> reference_output for each system.
> >
> > I agree and understand, and we may need a tolerance in practice to
> > deal with differences from denormal handling, etc. However, if
> > Sebastian is seeing differences on the same system, we should
> > understand why. Is he running on an ARM Darwin system, or an x86
> > using fp80 arithmetic,
> 
> My dev machine is an x86_64-linux.  This is where I ran all my
> reported results.
> How do I determine whether I am using fp80 arithmetic?

I don't think that Clang/LLVM uses it by default on x86_64. If you're using -Ofast, however, that would explain it. I recommend looking at -O3 vs -O0 and make sure those are the same. -Ofast enables -ffast-math, which can legitimately cause differences.

 -Hal

> 
> Thanks,
> Sebastian
> 

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory