[Openmp-dev] Build is terribly slow on Arm

[Michael Kruse via Openmp-dev]

How do you compile LLVM (your cmake ... command line)?

What flags do you use to compile your application?

Michael

Am Mi., 10. Juni 2020 um 18:26 Uhr schrieb Itaru Kitayama
<itaru.kitayama at gmail.com>:
>
>  Michael, David,
>
> Sorry I wasn't clear; I am building LLVM on Arm with CUDA Toolkit 11 RC.
>
> On Thu, Jun 11, 2020 at 8:23 AM Michael Kruse <llvm at meinersbur.de> wrote:
> >
> > Itaru could mean compiling his application with debug symbols (-O0 -g)
> > using a release build of clang (it's always good to mention which
> > version of clang you are using) or an optimized version of his
> > application using an CMAKE_BUILD_TYPE=Debug build of clang. Maybe he
> > could clarify?
> >
> > If you are running linux, a performance trace (flame graph / Linux
> > perf record) would be useful, especially if you are using a debug
> > build of clang.
> >
> > Michael
> >
> > Am Mi., 10. Juni 2020 um 04:55 Uhr schrieb Jonas Hahnfeld via
> > Openmp-dev <openmp-dev at lists.llvm.org>:
> > >
> > > You wrote "They were both Debug build". Maybe I misunderstand?
> > >
> > > Am Mittwoch, den 10.06.2020, 18:52 +0900 schrieb Itaru Kitayama:
> > > > Why do you think I build in Debug? I try to
> > > > avoid that all time.
> > > >
> > > > On Wed, Jun 10, 2020 at 18:38 Jonas Hahnfeld <hahnjo at hahnjo.de> wrote:
> > > > > You should not use Debug build to compare the performance of compilers.
> > > > > As you're targeting different architectures, you might run into
> > > > > different checks in the two backends. I believe it's possible this is
> > > > > triggered by OpenMP code generation which might hit some corner cases.
> > > > > This could also happen in Release builds, but might be less probable.
> > > > >
> > > > > Also keep in mind that AArch64 (which you're probably talking about in
> > > > > the context of HPC) now uses GlobalISel in some configurations. I don't
> > > > > remember if it's with or without optimizations (or maybe even both
> > > > > nowadays?), but you can try to deactivate this on the command line to
> > > > > rule out this area.
> > > > >
> > > > > Regards
> > > > > Jonas
> > > > >
> > > > > Am Dienstag, den 09.06.2020, 08:12 +0900 schrieb Itaru Kitayama via
> > > > > Openmp-dev:
> > > > > > They were both Debug build, but it took 20-25 minutes to finish on Arm.
> > > > > >
> > > > > > On Tue, Jun 9, 2020 at 7:50 AM Itaru Kitayama <
> > > > > > itaru.kitayama at gmail.com
> > > > > > > wrote:
> > > > > > > My OpenMP offloading app builds within a minute or so on
> > > > > > > x86, but the same app takes longer than 5 minutes on Arm.
> > > > > > > How do I investigate this further, supplying -v and post
> > > > > > > the log here?
> > > >
> > > >
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