Re: detection and support of OpenMP

[Steven G. Johnson]

On Tue, 9 Oct 2007, Bruno Haible wrote:
> The usual way to handle new compilers or new platforms in autoconf is
> through patching the autoconf macros. AC_PROG_CC_STDC, AC_PROG_CC_C99
> cannot be extended by environment variables either.

I don't think that's true.

First, of course autoconf will have to eventually be patched if new flags 
are found, but that doesn't help the users with old configure scripts and 
older versions of autoconf.  Environment variables are the "usual" way to 
extend configure scripts for new systems, not patching autoconf macros, in 
my experience.

Second, it is common for autoconf tests that have output variables to be 
overridable by the user.  For example, AC_CHECK_PROG etc can be 
overridden, AC_FC_LIBRARY_LDFLAGS, AC_PROG_CPP, etc.  Also, *FLAGS 
variables in autoconf are nearly all user-extensible.  Here, we output 
OPENMP_*FLAGS, so it should be overridable.

Third, in the examples you gave, the user *can* override/extend by setting 
CC appropriately (note that CC is modified by those macros).  With the 
current AC_OPENMP, you could of course add an OpenMP flag to CC/CFLAGS, 
but that would not be the same because it would force that flag to be used 
for *all* compilations, not just those using OPENMP_*FLAGS

> Good point. How about a macro AC_DISABLE_OPENMP which changes the default
> setting to off (like AM_DISABLE_SHARED in libtool)?

That seems okay to me, I suppose, although I'm more inclined just to have 
a single OpenMP macro that behaves like most other autoconf macros (not 
adding its own --enable/--disable option, leaving this up to the user). 
Entia non multiplicanda sunt.

> I would reserve a first argument to denote the OpenMP version number (to
> cope with future developments of that spec), i.e. I would actually propose
>   AC_OPENMP(VERSION, ACTION-IF-FOUND, ACTION-IF-NOT-FOUND)

The more standard autoconf style would be to have separate tests for 
particular features in new versions, e.g. an AC_OPENMP_FOOBAR test to see 
if OpenMP has the FOOBAR feature from OpenMP version 12, rather than 
testing for specific versions.

Regards,
Steven G. Johnson