disabling -O on gcc platforms

[Christoph Rupp]

Hi!

This weekend, i migrated my project
(http://freshmeat.net/projects/hamsterdb/) from scons to autotools. It
was quite easy.

However, i have one problem, and after searching the web and the
autoconf mailing list, i did found some results, but it was not exactly
what i needed. The problem is that if my library is compiled with gcc <
4.1.1, then EVERY optimization flag (even -O0) breaks the library. I
therefore have to completely get rid of optimization if an older gcc is
used.

I have written a macro to recognize such gcc compilers; but how do i get
rid of -O? It seems that per default, CFLAGS is "-O2 -g"; i don't know
why -g is enabled, i'm not making debug builds. I therefore *could*
completely clear CFLAGS. But i don't know if that's a good idea - maybe
other unix platforms have other flags which would then be missed.

Here's the macro for configure.in:

if test "$GCC" = yes ; then
    AC_MSG_CHECKING(whether gcc version is older than 4.1.1)
    AC_TRY_COMPILE(,
        [#define GCC_VERSION (__GNUC__ * 10000 \
                            + __GNUC_MINOR__ * 100 \
                            + __GNUC_PATCHLEVEL__)
        #if GCC_VERSION<40101
        #   error "gcc too old, disabling optimizations"
        #endif
        int main(void) { return 0; }],
                   gcc_version_ok=yes,
                   gcc_version_ok=no)
    if test $gcc_version_ok != yes; then
        AC_MSG_RESULT(Yes, disabling optimizations)
    else
        AC_MSG_RESULT(No, enabling optimizations)
    fi
fi
AM_CONDITIONAL(ENABLE_GCC_OPTIMIZATION, test x$gcc_version_ok = xyes)

And here's the code from Makefile.am. Currently, it completely deletes
CFLAGS if GCC is used, and then enables -O3, if we're not in DEBUG mode
and ENABLE_GCC_OPTIMIZATION is set:

if USING_GCC
    CFLAGS     =
endif
if DEBUG
    AM_CFLAGS += -DDEBUG -DHAM_DEBUG
else
    AM_CFLAGS += -DHAM_RELEASE -ffast-math
if ENABLE_GCC_OPTIMIZATION
    AM_CFLAGS += -O3 -ffast-math
endif
endif

Thanks,
Chris