[FFmpeg-devel] [PATCH] configure: make sure LTO does not optimize out the test functions

[Michael Niedermayer]

On Wed, Oct 26, 2016 at 01:16:13AM +0200, Andreas Cadhalpun wrote:
> On 25.10.2016 23:34, Carl Eugen Hoyos wrote:
> > 2016-10-25 19:19 GMT+02:00 Andreas Cadhalpun <andreas.cadhalpun at googlemail.com>:
> > 
> >> +        # LTO could optimize out the test functions without this
> >> +        echo "#if defined(__GNUC__) && __GNUC__ >= 4"
> >> +        echo "  #define USED __attribute__((used))"
> >> +        echo "#else"
> >> +        echo "  #define USED"
> >> +        echo "#endif"
> > 
> > Why is the ugly #if - #else - #endif necessary?
> 
> I'm under the impression that __attribute__((used)) is not available for all compilers,
> see e.g. libavutil/attributes.h.
> But thinking more about it, a better way is to actually use the functions so that
> LTO can't optimize them out. This avoids the ugly preprocessor checks.
> 
> > Please mention ticket #5909 if it is related.
> 
> Sure, new patch attached.
> 
> By the way, this is not a regression in 3.1.5, but a fix included in that
> release allowed them to drop their distro-specific patch, exposing the problem
> with LTO.
> 
> Best regards,
> Andreas
> 

>  configure |    7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 742684cf379693d08075d43fdfb75ed5e2e936c6  0001-configure-make-sure-LTO-does-not-optimize-out-the-te.patch
> From bb289a0b2b0948afa99227bcff188301c1143624 Mon Sep 17 00:00:00 2001
> From: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
> Date: Tue, 25 Oct 2016 19:09:46 +0200
> Subject: [PATCH] configure: make sure LTO does not optimize out the test
>  functions
> 
> Fixes trac ticket #5909
> 
> Bud-Id: https://bugs.gentoo.org/show_bug.cgi?id=598054
> Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun at googlemail.com>
> ---
>  configure | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/configure b/configure
> index 481f692..54faef1 100755
> --- a/configure
> +++ b/configure
> @@ -1150,7 +1150,12 @@ check_func_headers(){
>          for func in $funcs; do
>              echo "long check_$func(void) { return (long) $func; }"
>          done
> -        echo "int main(void) { return 0; }"
> +        echo "int main(void) { int ret = 0;"
> +        # LTO could optimize out the test functions without this
> +        for func in $funcs; do
> +            echo " ret |= ((intptr_t)check_$func) & 0xFFFF;"
> +        done
> +        echo "return ret; }"

breaks configure

i get this:

ERROR: LoadLibrary/dlopen not found for avisynth

If you think configure made a mistake, make sure you are using the latest
version from Git.  If the latest version fails, report the problem to the
ffmpeg-user at ffmpeg.org mailing list or IRC #ffmpeg on irc.freenode.net.
Include the log file "config.log" produced by configure as this will help
solve the problem.

[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Many that live deserve death. And some that die deserve life. Can you give
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