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

[Andreas Cadhalpun]

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

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