r238389 - [omp] Re-work Clang's handling of -fopenmp and undo r237769.

[Chandler Carruth]

On Fri, May 29, 2015 at 11:40 AM İsmail Dönmez <ismail at donmez.ws> wrote:

> On Fri, May 29, 2015 at 9:32 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
> > On Fri, May 29, 2015 at 5:56 AM, İsmail Dönmez <ismail at donmez.ws> wrote:
> >>
> >> On Thu, May 28, 2015 at 11:48 PM, Chandler Carruth <chandlerc at gmail.com
> >
> >> wrote:
> >> > On Thu, May 28, 2015 at 5:09 AM İsmail Dönmez <ismail at donmez.ws>
> wrote:
> >> >>
> >> >> Hi,
> >> >>
> >> >> On Thu, May 28, 2015 at 4:52 AM, Chandler Carruth <
> chandlerc at gmail.com>
> >> >> wrote:
> >> >> > Author: chandlerc
> >> >> > Date: Wed May 27 20:52:38 2015
> >> >> > New Revision: 238389
> >> >> >
> >> >> > URL: http://llvm.org/viewvc/llvm-project?rev=238389&view=rev
> >> >> > Log:
> >> >> > [omp] Re-work Clang's handling of -fopenmp and undo r237769.
> >> >>
> >> >>
> >> >> This seems to break openmp tests with
> >> >> -DCLANG_DEFAULT_OPENMP_RUNTIME=libiomp5 see:
> >> >
> >> >
> >> > Yea, I wanted to have test coverage of the default, but it doesn't
> work
> >> > well
> >> > when the default is overridden.
> >> >
> >> > I looked into how to actually have test coverage of the default and
> >> > track
> >> > which default was selected, but its really hard and complex. Yuck.
> >> >
> >> > I'll look for a way to get at least *some* coverage of the default
> >> > without
> >> > making it break folks that are overriding it. Sorry for the trouble.
> >>
> >> Tests pass now but there is still one grave problem. With
> >> CLANG_DEFAULT_OPENMP_RUNTIME=libiomp5 if you do:
> >>
> >> λ clang -fopenmp=libgomp omp_hello.c
> >> λ ./a.out
> >> Hello World from thread = 0
> >> Number of threads = 1
> >>
> >> No OpenMP, lets go back to defaults:
> >>
> >> λ clang -fopenmp=libiomp5 omp_hello.c
> >> λ ./a.out
> >> Hello World from thread = 0
> >> Hello World from thread = 4
> >> Hello World from thread = 3
> >> Hello World from thread = 2
> >> Hello World from thread = 6
> >> Hello World from thread = 1
> >> Hello World from thread = 5
> >> Hello World from thread = 7
> >> Number of threads = 8
> >
> >
> > OK, and what's the one grave problem?
>
> tl;dr clang -fopenmp=libgomp compiles and links to libgomp but doesn't
> enable OpenMP.
>

I think that is actually the best behavior we can provide.

Clang doesn't support emitting code compatible with libgomp. There is
currently no support for actually enabling OpenMP in Clang and using
libgomp.
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