[Ffmpeg-devel] building without optimizations, but with mmx enabled - II

Marco Manfredini mldb
Thu Aug 10 19:36:34 CEST 2006

On Thursday 10 August 2006 17:58, Luca Barbato wrote:
> Marco Manfredini wrote:
> > Theoretically, this should also lead to better optimisation, because the
> > compiler can arrange the loads. On the downside, "y" constraints work
> > only if the compiler gets at least the -fmmx switch. So this requires a
> > change to the
> -fmmx doesn't exists (on gcc-fsf at least)
> -mmmx is implied by -mcpu if the cpu supports it.

erm, -mmmx I meant to write. -mcpu= is the depreceated alias for -mtune=, it 
doesn't enable any features of the instruction set. -march= does that, but 
makes the code unrunnable of lesser platforms. The docs only say that -mmmx, 
-msse etc. just make the builtins for the selection extension available. This 
isn't true - the compiler starts to use sse2 instructions on his own behalf 
if -msse2 was given. But that still means that the MMX or SSE specific 
routines can be translated with the according -mmmx or -msse flag, giving 
access to the asm constraints.


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