On Fri Nov 7 14:09:09 2014 GMT, Siavash Eliasi wrote: > > On 11/07/2014 03:14 PM, Steven Newbury wrote: > > On Thu, 2014-11-06 at 21:00 -0800, Matt Turner wrote: > >> On Thu, Nov 6, 2014 at 8:56 PM, Siavash Eliasi < > >> siavashser...@gmail.com> wrote: > >>> Then I do recommend removing the "if (cpu_has_sse4_1)" from this > >>> patch and similar places, because there is no runtime CPU > >>> dispatching happening for SSE optimized code paths in action and > >>> just adds extra overhead (unnecessary branches) to the generated > >>> code. > >> No. Sorry, I realize I misread your previous question: > >> > >>>> I guess checking for "cpu_has_sse4_1" is unnecessary if it isn't > >>>> controllable by user at runtime; because "USE_SSE41" is a > >>>> compile time check and requires the target machine to be SSE 4.1 > >>>> capable already. > >> USE_SSE41 is set if the *compiler* supports SSE 4.1. This allows you > >> to build the code and then use it only on systems that actually > >> support it. > >> > >> All of this could have been pretty easily answered by a few greps > >> though... > > I wonder what difference it would make to have an option to compile > > out the run-time check code to avoid the additional overhead in cases > > where the builder *knows* at compile time what the run-time system is? > > (ie Gentoo) > I think that's possible. Since "cpu_has_sse4_1" and friends are simply > macros, one can set them to "true" or "1" during compile time if it's > going to be built for an SSE 4.1 capable target so your smart compiler > will totally get rid of the unnecessary runtime check. > > I guess "common_x86_features.h" should be modified to something like this: > > #ifdef __SSE4_1__ > #define cpu_has_sse4_1 1 > #else > #define cpu_has_sse4_1 (_mesa_x86_cpu_features & X86_FEATURE_SSE4_1) > #endif > Yes, this was what I was thinking. Then perhaps an option for disabling run-time detection, with the available cpu features then determined during configuration setting appropriate defines.
Whether it's worth it I don't know. I can imagine the compiler having an easier job optimizing the code. -- _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org http://lists.freedesktop.org/mailman/listinfo/mesa-dev
