Hello Ralf,
> It seems to me this macro does not actually check whether OpenMP code
> would be accepted by the compiler, nor whether any necessary runtime
> libraries are actually linked in. Using an OpenMP function (or, even
> better, linking a small portable self-contained OpenMP program) could
> achieve this.
A "small portable self-contained OpenMP program" looks like this:
int main ()
{
char buf[1000];
int i;
#pragma omp parallel for schedule(dynamic)
for (i = 0; i < 1000; i++)
buf[i] = i*i;
return 0;
}
But such a program is useless for an autoconf test, because the program
without the #pragma must be well-formed, and the #pragma can be ignored by
any compiler.
However, the test can be made a little sharper by testing for the presence
of <omp.h> and one of the simplest library functions.
> What if I want to create Fortran or C++ code that uses OpenMP?
For C++, the test should work the same. For Fortran, you have a separate
macro. I named the macro 'AC_C_...' to make it clear that it does not
apply to Fortran, Java, or Lisp.
> While I understand why you choose to set OPENMP_CFLAGS rather than
> amending CFLAGS, and it makes sense to me, too, it does not fit the other
> modes of operation of Autoconf macros. Not sure if this
> inconsistency in Autoconf weighs less than the advantage. Intruding into
> users' namespace is always a double-edged sword.
OPENMP_CFLAGS seems like a natural name; other autoconf macros set the
variable X_CFLAGS. I don't see an inconsistency.
> > In the macro, it would have been simpler to blindly test for the first
> > supported option in a list "-fopenmp -xopenmp -openmp -mp -omp -qsmp=omp",
> > but many compilers accept several of these options and do something
> > unrelated - like to create an output file called 'penmp' or to warn
> > about an unsupported compiler option. The code therefore applies each
> > option only to the brand of compiler that is known to be likely to
> > support it.
>
> Another way to address this would be to enter a subdir `conftest' and
> ensure no file with such a name is created.
This catches the case of -openmp, but does not the case of options that
lead to a compiler warning at each compiler invocation.
> > In the doc, I'm not sure whether one should write "The
> > @code{AC_C_OPENMP} macro" or "The macro @code{AC_C_OPENMP}". Please
> > correct if needed.
>
> FWIW, I'd slightly prefer the latter.
OK.
> > 2007-05-17 Bruno Haible <bruno@xxxxxxxxx>
> >
> > * lib/autoconf/c.m4 (AC_C_OPENMP): New macro.
> > * doc/autoconf.texi (C Compiler): Document AC_C_OPENMP.
>
> This change would need to be mentioned in NEWS.
Included below.
> > + AC_DEFUN([AC_C_OPENMP],
> > + [
> > + AC_MSG_CHECKING([whether to use OpenMP])
> > + AC_ARG_ENABLE(openmp,
> > + [ --disable-openmp do not use OpenMP],
>
> The enable switch is not documented in the manual.
> Please use AS_HELP_STRING.
OK.
> > + [ac_openmp_choice="$enableval"],
>
> Superfluous double-quotes.
OK.
> > + [ac_openmp_choice=yes])
> > + AC_MSG_RESULT([$ac_openmp_choice])
> > + OPENMP_CFLAGS=
> > + if test "$ac_openmp_choice" = yes; then
> > + AC_MSG_CHECKING([for $CC option to support OpenMP])
> > + AC_CACHE_VAL([ac_cv_prog_cc_openmp], [
> > + ac_cv_prog_cc_openmp=no
> > + AC_COMPILE_IFELSE([
> > + #ifndef _OPENMP
> > + Unlucky
> > + #endif
> > + ], [ac_cv_prog_cc_openmp=none])
> > + if test "$ac_cv_prog_cc_openmp" = no; then
> > + dnl Try these flags:
> > + dnl GCC >= 4.2 -fopenmp
> > + dnl SunPRO C -xopenmp
> > + dnl Intel C -openmp
> > + dnl SGI C, PGI C -mp
> > + dnl Tru64 Compaq C -omp
> > + dnl AIX IBM C -qsmp=omp
>
> Doesn't AIX have a compiler for GNU/Linux as well? Does it support OpenMP?
You mean, IBM? Yes, they sell a port of XLC for Linux [1]. But I have no
info about the command-line option for OpenMP in this compiler.
[1] http://www-306.ibm.com/software/info/ecatalog/en_US/products/Q860134T18351F02.html?&S_TACT=none&S_CMP=none
> The Intel compiler can fool the $GCC test with suitable compiler options
Does autoconf support this configuration? In the sense that all of autoconf
so far works fine with an Intel icc that defines __GNUC__ ?
> (no arms race between the Intel people making icc more like GCC and
> Autoconf detecting non-GCC compilers, please; this holds for some other
> compilers as well), but at least the version I just tried (8.1) does not
> understand -fopenmp, warns about it, but does not fail. Dunno if that
> would be enough of a reason to try Intel first?
Yes, if a masqueraded Intel icc is supported, then feel free to move the
Intel C test before the GNU C test.
> > + if test "$GCC" = yes; then
> > + dnl --- Test for GCC.
> > + gt_save_CFLAGS="$CFLAGS"
>
> s/gt_/ac_/g here and elsewhere. Superfluous double-quotes here.
OK.
> > + CFLAGS="$CFLAGS -fopenmp"
> > + AC_COMPILE_IFELSE([
> > + #ifndef _OPENMP
> > + Unlucky
>
> Please use 'choke me'.
OK.
> Can we eliminate some of the redundancy in the code by a loop like this,
> if we decide to go the way of using preprocessor defines?
> Besides cutting down the size of the expanded macro quite a bit it would
> also help avoid inconsistencies when the macro is bugfixed later.
Yes, that's good for maintainability.
> > + case $ac_cv_prog_cc_openmp in
> > + none)
>
> Why not just initialize $ac_cv_prog_cc_openmp with "none needed",
> "unsupported" etc. right away?
Indeed, that simplifies the code a bit.
> > + AC_MSG_RESULT([none needed]) ;;
> > + no)
> > + AC_MSG_RESULT([unsupported]) ;;
> > + *)
> > + AC_MSG_RESULT([$ac_cv_prog_cc_openmp]) ;;
> [...]
>
> Please ensure that the generated test for this new macro succeeds.
> You can run it verbosely like this:
> make check TESTSUITEFLAGS='-k AC_C_OPENMP -v -d -x'
I don't understand what you mean, as I'm not familiar with the autoconf
test suite. Can you take over this part, please?
>> + [ac_openmp_choice=yes])
>> + AC_MSG_RESULT([$ac_openmp_choice])
> Why not eliminate ac_openmp_choice and use $enable_openmp instead?
Indeed, this simplifies it a bit more.
Here is an updated patch.
2007-05-17 Bruno Haible <bruno@xxxxxxxxx>
* lib/autoconf/c.m4 (AC_C_OPENMP): New macro.
* doc/autoconf.texi (C Compiler): Document AC_C_OPENMP.
* NEWS: Mention AC_C_OPENMP.
*** NEWS 14 May 2007 16:54:55 -0000 1.428
--- NEWS 17 May 2007 16:19:13 -0000
***************
*** 1,5 ****
--- 1,7 ----
* Major changes in Autoconf 2.61b (????-??-??)
+ ** New macro AC_C_OPENMP.
+
** AC_C_BIGENDIAN now supports universal binaries a la Mac OS X.
** AC_C_RESTRICT now prefers to #define 'restrict' to a variant spelling
*** doc/autoconf.texi 14 May 2007 16:54:55 -0000 1.1154
--- doc/autoconf.texi 17 May 2007 16:19:22 -0000
***************
*** 6593,6598 ****
--- 6593,6622 ----
if it accepts one of those, otherwise define @code{inline} to be empty.
@end defmac
+ @defmac AC_C_OPENMP
+ @acindex{C_OPENMP}
+ @cvindex _OPENMP
+ OpenMP (@url{http://www.openmp.org/}) is an extension of the C language
+ that makes it possible to optimize programs for Shared Multiprocessing
+ architectures, such as multi-core CPUs, with very small effort.
+
+ The macro @code{AC_C_OPENMP} sets the variable @code{OPENMP_CFLAGS} to the
+ C compiler flags needed for supporting OpenMP. If the compiler already
+ supports OpenMP or if it has no way to activate OpenMP support,
+ @code{OPENMP_CFLAGS} is set to empty. Also, the user can reject OpenMP
+ support if it is not the default, by invoking @samp{configure} with the
+ @samp{--disable-openmp} option; then @code{OPENMP_CFLAGS} is set to empty
+ as well.
+
+ The @code{OPENMP_CFLAGS} need to be used when compiling programs and when
+ linking programs, like the @code{CFLAGS}. The presence of OpenMP support
+ at compile time is revealed by the preprocessor macro @code{_OPENMP}.
+
+ Linking a program with @code{OPENMP_CFLAGS} typically adds one more shared
+ library to the program's dependencies, therefore its use is recommended only
+ on programs that actually use code conditional on @code{#ifdef _OPENMP}.
+ @end defmac
+
@defmac AC_C_CHAR_UNSIGNED
@acindex{C_CHAR_UNSIGNED}
@cvindex __CHAR_UNSIGNED__
*** lib/autoconf/c.m4 14 May 2007 16:54:55 -0000 1.247
--- lib/autoconf/c.m4 17 May 2007 16:19:23 -0000
***************
*** 1841,1843 ****
--- 1841,1943 ----
fi
fi
])
+
+
+ # AC_C_OPENMP
+ # -----------
+ # Check which options need to be passed to the C compiler to support OpenMP.
+ # Set the OPENMP_CFLAGS variable to these options.
+ # The options are necessary at compile time (so the #pragmas are understood)
+ # and at link time (so the appropriate library is linked with).
+ # This macro takes care to not produce redundant options if $CC $CFLAGS already
+ # supports OpenMP. It also is careful to not pass options to compilers that
+ # misinterpret them; for example, most compilers accept "-openmp" and create
+ # an output file called 'penmp' rather than activating OpenMP support.
+ AC_DEFUN([AC_C_OPENMP],
+ [
+ AC_MSG_CHECKING([whether to use OpenMP])
+ AC_ARG_ENABLE(openmp,
+ [AS_HELP_STRING([--disable-openmp], [do not use OpenMP])],
+ [],
+ [enable_openmp=yes])
+ AC_MSG_RESULT([$enable_openmp])
+ OPENMP_CFLAGS=
+ if test "$enable_openmp" = yes; then
+ AC_MSG_CHECKING([for $CC option to support OpenMP])
+ AC_CACHE_VAL([ac_cv_prog_cc_openmp], [
+ ac_cv_prog_cc_openmp=unsupported
+ AC_LINK_IFELSE([
+ #ifndef _OPENMP
+ choke me
+ #endif
+ #include <omp.h>
+ int main () { return omp_get_num_threads (); }
+ ], [ac_cv_prog_cc_openmp="none needed"])
+ if test "$ac_cv_prog_cc_openmp" = unsupported; then
+ dnl Try these flags:
+ dnl GCC >= 4.2 -fopenmp
+ dnl SunPRO C -xopenmp
+ dnl Intel C -openmp
+ dnl SGI C, PGI C -mp
+ dnl Tru64 Compaq C -omp
+ dnl AIX IBM C -qsmp=omp
+ for brand in GCC SunPRO Intel SGI/PGI Compaq AIX; do
+ case $brand in
+ GCC)
+ ac_conditional='defined __GNUC__'
+ ac_option='-fopenmp' ;;
+ SunPRO)
+ ac_conditional='defined __SUNPRO_C || defined __SUNPRO_CC'
+ ac_option='-xopenmp' ;;
+ Intel)
+ ac_conditional='defined __INTEL_COMPILER'
+ ac_option='-openmp' ;;
+ SGI/PGI)
+ ac_conditional='defined __sgi || defined __PGI || defined __PGIC__'
+ ac_option='-mp' ;;
+ Compaq)
+ ac_conditional='defined __DECC || defined __DECCXX'
+ ac_option='-omp' ;;
+ AIX)
+ ac_conditional='defined _AIX'
+ ac_option='-qsmp=omp' ;;
+ esac
+ if test $brand = GCC; then
+ if test "$GCC" = yes; then
+ ac_openmp_result=yes
+ else
+ ac_openmp_result=no
+ fi
+ else
+ AC_EGREP_CPP([Brand], [
+ #if $ac_conditional
+ Brand
+ #endif
+ ], [ac_openmp_result=yes], [ac_openmp_result=no])
+ fi
+ if test $ac_openmp_result = yes; then
+ ac_save_CFLAGS=$CFLAGS
+ CFLAGS="$CFLAGS $ac_option"
+ AC_LINK_IFELSE([
+ #ifndef _OPENMP
+ choke me
+ #endif
+ #include <omp.h>
+ int main () { return omp_get_num_threads (); }
+ ], [ac_cv_prog_cc_openmp=$ac_option])
+ CFLAGS=$ac_save_CFLAGS
+ break
+ fi
+ done
+ fi
+ ])
+ AC_MSG_RESULT([$ac_cv_prog_cc_openmp])
+ case $ac_cv_prog_cc_openmp in
+ "none needed" | unsupported)
+ OPENMP_CFLAGS= ;;
+ *)
+ OPENMP_CFLAGS=$ac_cv_prog_cc_openmp ;;
+ esac
+ fi
+ AC_SUBST([OPENMP_CFLAGS])
+ ])
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