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

[Michael Niedermayer]

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.
> 
> Note that one of recent commits (b4a224c5e4109cb2cca8bac38628673d685fe763) added
> other code with input parameters modified in asm, but not marked as clobbered;
> it won't surprise me if this also will be broken by some gcc version (if not
> already).
> 

> And there are places in postproc where asm uses stack space below esp, that also
> does not look future-proof (I posted patch more than year ago; it went nowhere).

iam sorry about this, ive applied it now and will push it after testing

for reference:
  Re: [FFmpeg-devel] [patch v4, v5] libpostproc: mmx code uses stack below %esp

last message in the thread (from me) said:
"then patch ok"

but it seems noone applied it


[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

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