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Re: MPI recognition

From: Eric Blake
Subject: Re: MPI recognition
Date: Tue, 21 Dec 2010 08:57:04 -0700
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On 12/21/2010 07:16 AM, Olaf Lenz wrote:
> I have attached my simplified attempt do this. When I run autoconf, I
> get the warning
> warning: AC_REQUIRE: `AC_PROG_CC' was expanded before
> it was required
> and configure doesn't seem to be looking for mpicc, but always finds gcc
> first.
> The problem seems to be that AC_PROG_CC doesn't work when using it in a
> conditional, is that right? Is there any simple way to do what I want?

The problem is that the autoconf AC_REQUIRE machinery can only be used
to require a macro with default arguments.  When you invoke AC_PROG_CC
inside AS_IF, the AS_IF ends up hoisting the AC_PROG_CC body to occur
early, but in the process, loses the arguments that you specified.
Whether this can be fixed in future autoconf is another matter, but in
general, AC_PROG_CC is special enough that if you ever want to pass
non-default arguments, you must invoke it outside of conditionals.

> After some trial and error I have found a solution for my problem that
> is very simple. Now I wonder whether this is actually the correct way to
> do it, or are there some hidden pitfalls?
> AC_ARG_WITH(mpi,,, [with_mpi=guess])
> if test x"$with_mpi" != xno; then
>   COMPILERS="mpicc hcc mpxlc_r mpxlc mpcc cmpicc gcc cc"
> else
>   COMPILERS="gcc cc"
> fi

Yes, this is actually a quite decent solution, although you may be
artificially limiting the set of $COMPILERS to something less than what
AC_PROG_CC() would support by default.

Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library

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