Bug 595610 – sys-devel/gcc: Support building against sys-libs/libomp

• Product: Gentoo Linux
• Status: RESOLVED NEEDINFO

[Description Michał Górny 2016-09-30 07:11:13 UTC]

I've been approached by Gentoo user who told me that he's been able to achieve superior performance when building OpenMP programs using gfortran against libomp (instead of libgomp). I'll link or attach his results when I get a confirmation that it's fine to do so.

However, doing that seems non-trivial. It seems that gcc strongly prefers files from /usr/lib/gcc..., so in order to enforce libomp we'd either have to replace libgomp.so or libgomp.spec. Does anyone have a better idea? Maybe it'd be possible to override openmp link libraries via Gentoo .spec files?

[Comment 1 Ryan Hill (RETIRED) 2016-10-01 01:38:51 UTC]

I thought that the openmp stuff in gcc was heavily tied to libgomp and other implementations weren't compatible (in that to use them you didn't pass -fopenmp but just linked in the wanted library, eg. intel's libiomp worked that way).  That was several years ago though so things may have changed?

[Comment 2 Michał Górny 2016-10-01 06:46:14 UTC]

I think you need -fopenmp to get the needed pragmas.

$ gfortran omptry.f90 -lomp
$ ./a.out 
 max threads:            0
  20000

$ gfortran omptry.f90 -fopenmp
$ ./a.out 
 max threads:            2
  20000

I could do '-fopenmp -lomp' but that links both libraries in, so probably not a good idea at all.

However, the following seems to work:

$ gfortran omptry.f90 -fopenmp -c
$ gfortran omptry.o -lomp
$ ./a.out 
 max threads:            2
  20000

However, it's a little inconvenient ;-).

[Comment 3 Andreas K. Hüttel 2018-06-23 20:20:07 UTC]

(In reply to Michał Górny from comment #0)
> I've been approached by Gentoo user who told me that he's been able to
> achieve superior performance when building OpenMP programs using gfortran
> against libomp (instead of libgomp). I'll link or attach his results when I
> get a confirmation that it's fine to do so.

Any news here?

> 
> However, doing that seems non-trivial. It seems that gcc strongly prefers
> files from /usr/lib/gcc..., so in order to enforce libomp we'd either have
> to replace libgomp.so or libgomp.spec. Does anyone have a better idea? Maybe
> it'd be possible to override openmp link libraries via Gentoo .spec files?

No clue, but if it's worthwhile researching...

[Comment 4 Michał Górny 2018-06-23 20:56:00 UTC]

I don't really know.  I probably never got any reply, and I don't even really recall where to look for it.