[FFmpeg-devel] Fix Altivec detection on Linux.

David Woodhouse dwmw2
Wed Mar 19 15:12:54 CET 2008

On Wed, 2008-03-19 at 15:03 +0100, Luca Barbato wrote:
> > I see we also build everything with -maltivec anyway, which with GCC now
> > doing autovectorisation means that we're likely to be using Altivec
> > anywhere, and this check is mostly pointless. 
> Since we do not like runtime cpu detection it is indeed pointless and 
> put there just to make happy the "usual suspects". The -maltivec stuff 
> should be triggered by default but you should be able to tune the cflags 
> to something useful (e.g G3 tuning) while still be able to build the 
> altivec stuff (they get -maltivec -mabi=altivec only in this case).

I want to build for a distribution, so I want it to work for users with
or with Altivec.

> > We should stop doing that. If GCC is managing to autovectorise any C
> > code and it actually makes a significant difference, then we should
> > think about building those components twice -- once with -maltivec and
> > once without -- and putting the Altivec version in /lib/altivec where
> > the dynamic linker will find it on appropriate systems.
> This part seems interesting, is there any standard/guideline about this?

It's the same thing which lets you put libraries in /lib/i686/ which
won't work on i586. Try this...

	LD_LIBRARY_PATH=/lib:/usr/lib LD_DEBUG=libs /bin/true

Not sure about proper documentation, but there are a few hints about it
at http://lkml.org/lkml/2007/4/24/3


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