[FFmpeg-devel] [PATCH] Hack around gcc 4.6 breaking asm using call.

[Reimar Döffinger]

On Tue, Sep 20, 2011 at 10:59:13PM +0400, Yuriy Kaminskiy wrote:
> Reimar Döffinger wrote:
> > gcc 4.6 no longer decrements esp to account for local variables.
> > Thus using call will end up overwriting some local variable.
> > So add an extra one it can safely clobber.
> > This is a huge hack because it's basically pure chance it works,
> > no idea how this is supposed to be done.
> > 
> > Fixes trac ticket #397.
> > 
> > Signed-off-by: Reimar Döffinger <Reimar.Doeffinger at gmx.de>
> > ---
> >  libswscale/x86/swscale_template.c |   10 ++++++++++
> >  1 files changed, 10 insertions(+), 0 deletions(-)
> > 
> > diff --git a/libswscale/x86/swscale_template.c b/libswscale/x86/swscale_template.c
> > index 6196e98..d65be48 100644
> > --- a/libswscale/x86/swscale_template.c
> > +++ b/libswscale/x86/swscale_template.c
> > @@ -2283,6 +2283,10 @@ static void RENAME(hyscale_fast)(SwsContext *c, int16_t *dst,
> >  #if defined(PIC)
> >      DECLARE_ALIGNED(8, uint64_t, ebxsave);
> >  #endif
> > +    // HACK: gcc 4.6 no longer decrements esp,
> > +    // use this to make it reserve space for the call
> > +    // return address
> > +    void *dummy;
> >  
> >      __asm__ volatile(
> >  #if defined(PIC)
> > @@ -2334,6 +2338,7 @@ static void RENAME(hyscale_fast)(SwsContext *c, int16_t *dst,
> >  #if defined(PIC)
> >            ,"m" (ebxsave)
> >  #endif
> > +          ,"m" (dummy)
> 
> Hmm, I'm not gcc/assembler expert, but here ebxsave is *input* parameter, but
> uninitialized before, not marked as *output* parameter, and there are no
> "memory" in clobbers (but it is actually modified/used in asm). So I won't even
> consider this gcc bug (or even "surprising behavior"). And it won't surprise me
> if your workaround will be broken again by next gcc version.

Well, what then is the correct way to tell gcc "hey, I will use the call
instruction here, don't break it". Not being able to use call at all in
asm code can hardly be considered sensible?
All your comments may well be bugs, but I don't see how they would be
related to call not being usable.