[cfe-dev] [llvm-dev] [test-suite] making the test-suite succeed with "-Ofast" and "-ffp-contract=on"

[Hal Finkel via cfe-dev]

----- Original Message -----

> From: "Matthias Braun" <matze at braunis.de>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Sebastian Paul Pop" <s.pop at samsung.com>, "llvm-dev"
> <llvm-dev at lists.llvm.org>, "Clang Dev" <cfe-dev at lists.llvm.org>,
> "nd" <nd at arm.com>, "Abe Skolnik" <a.skolnik at samsung.com>
> Sent: Friday, October 7, 2016 8:53:49 PM
> Subject: Re: [llvm-dev] [test-suite] making the test-suite succeed
> with "-Ofast" and "-ffp-contract=on"

> > On Oct 7, 2016, at 6:46 PM, Hal Finkel via llvm-dev <
> > llvm-dev at lists.llvm.org > wrote:
> 

> > ----- Original Message -----
> 

> > > From: "Hal Finkel via llvm-dev" < llvm-dev at lists.llvm.org >
> > 
> 
> > > To: "Sebastian Pop" < sebpop.llvm at gmail.com >
> > 
> 
> > > 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 >
> > 
> 
> > > Sent: Friday, October 7, 2016 7:56:53 PM
> > 
> 
> > > Subject: Re: [llvm-dev] [test-suite] making the test-suite
> > > succeed
> > > with "-Ofast" and "-ffp-contract=on"
> > 
> 

> > > ----- Original Message -----
> > 
> 

> > > > From: "Sebastian Pop" < sebpop.llvm at gmail.com >
> > > 
> > 
> 
> > > > To: "Renato Golin" < renato.golin at linaro.org >
> > > 
> > 
> 
> > > > Cc: "Kristof Beyls" < Kristof.Beyls at arm.com >, "Sebastian Paul
> > > > Pop"
> > > 
> > 
> 
> > > > < s.pop at samsung.com >, "llvm-dev"
> > > 
> > 
> 
> > > > < llvm-dev at lists.llvm.org >, "nd" < nd at arm.com >, "Abe Skolnik"
> > > 
> > 
> 
> > > > < a.skolnik at samsung.com >, "Clang Dev"
> > > 
> > 
> 
> > > > < cfe-dev at lists.llvm.org >, "Hal Finkel" < hfinkel at anl.gov >,
> > > > "Stephen
> > > 
> > 
> 
> > > > Canon" < scanon at apple.com >, "Matthias Braun"
> > > 
> > 
> 
> > > > < matze at braunis.de >
> > > 
> > 
> 
> > > > Sent: Friday, October 7, 2016 7:34:40 PM
> > > 
> > 
> 
> > > > Subject: [test-suite] making the test-suite succeed with
> > > > "-Ofast"
> > > 
> > 
> 
> > > > and "-ffp-contract=on"
> > > 
> > 
> 

> > > > Hi,
> > > 
> > 
> 

> > > > I would like to provide a summary of the different proposals on
> > > > how
> > > 
> > 
> 
> > > > to
> > > 
> > 
> 
> > > > fix the test-suite to make it succeed when specifying extra
> > > > CFLAGS
> > > 
> > 
> 
> > > > "-Ofast" and "-ffp-contract=on". I would like to expose the
> > > > issue
> > > 
> > 
> 
> > > > and
> > > 
> > 
> 
> > > > proposed ways to fix it to other potential reviewers that could
> > > 
> > 
> 
> > > > provide extra feedback. We also need to decide which proposal
> > > > (or
> > > 
> > 
> 
> > > > combination of) to implement and commit.
> > > 
> > 
> 

> > > > Proposal 1: https://reviews.llvm.org/D25277
> > > 
> > 
> 
> > > > modify the CMakes to compile and run each of these benchmarks
> > > 
> > 
> 
> > > > twice:
> > > 
> > 
> 
> > > > once with added CFLAGS -ffp-contract=off. Record on disk the
> > > > full
> > > 
> > 
> 
> > > > output of both runs and compare with FP_TOLERANCE. Hash the
> > > > output
> > > 
> > 
> 
> > > > of
> > > 
> > 
> 
> > > > the run with -ffp-contract=off and exact match against the
> > > 
> > 
> 
> > > > reference
> > > 
> > 
> 
> > > > output.
> > > 
> > 
> 

> > > > The good for Proposal 1:
> > > 
> > 
> 
> > > > - changes contained in the build system: no change to the code
> > > > of
> > > 
> > 
> 
> > > > the
> > > 
> > 
> 
> > > > benchmarks
> > > 
> > 
> 
> > > > - runs benchmarks under an extra configuration with CFLAGS +=
> > > 
> > 
> 
> > > > -ffp-contract=off
> > > 
> > 
> 

> > > > The bad for Proposal 1:
> > > 
> > 
> 
> > > > - compilation time will double
> > > 
> > 
> 
> > > > - running time on the device will double
> > > 
> > 
> 
> > > > - build system is more complex
> > > 
> > 
> 
> > > > - the build directory goes from 300M to 1.2G due to the extra
> > > 
> > 
> 
> > > > reference outputs recorded under -ffp-contract=off,
> > > 
> > 
> 
> > > > - when running test-suite over small devices it will cost 1G
> > > > more
> > > 
> > 
> 
> > > > transfer over the network.
> > > 
> > 
> 

> > > I prefer proposal 1 (although, to be fair, it was something I
> > 
> 
> > > suggested). Being the the business of trying to heavily modify
> > > every
> > 
> 
> > > benchmark that does floating-point computation, as in proposal 2,
> > 
> 
> > > does not seem to scale well, and can't always be done regardless.
> > 
> 

> > > We can make some effort to reduce the size of the problems being
> > 
> 
> > > computed by some of the benchmarks (e.g. pollybench); I think
> > > that
> > 
> 
> > > is reasonable and will help with the extra space requirements.
> > > That
> > 
> 
> > > having been said, functionally speaking, our test suite is at
> > > least
> > 
> 
> > > an order of magnitude too small, and so my sympathy is somewhat
> > 
> 
> > > limited. We're going to have to find a way to execute the test
> > > suite
> > 
> 
> > > in stages on smaller devices to limit the peak usage, if not
> > > because
> > 
> 
> > > of this then because we've added a lot more test applications and
> > 
> 
> > > benchmarks in the future.
> > 
> 

> > Another aspect to this is that we should have this kind of
> > infrastructure for other purposes as well. We have a similar lack
> > of
> > testing for -ffast-math. We don't even do a good job of (i.e. have
> > good buildbot coverage for) running the test suite @ -O1. -O2 and
> > -O3 are much better tested. More regular testing at -O0 (especially
> > with -g to pick up crashes in our debug-info generation logic) is
> > needed as well.
> 

> I think we should rather just setup more build jobs that run
> different confiruations rather than modifying the testsuite to
> compile multiple configurations in a single run, as that keeps
> things simpler.
Fair enough; however, for things like FP-contractions and fast-math optimizations, which can change the output, we need to figure how to to test these regularly, at least looking for compile-time failures (just disabling the tests for which these setting cause verification errors is unsatisfying because it is these very tests where we know the logic we want to test is actually firing). 

-Hal 

> Chris and me also started collecting typical configurations in the
> test-suite/cmake/caches/ directory, I hope this will become a
> popular thing with more bot owners as it nicely documents what
> people are doing IMO and it makes it easy to describe the
> configuration used in bugreports / when reproducing issues on a
> personal machine.

> - Matthias

-- 

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