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

[Yuriy Kaminskiy]

Reimar Döffinger wrote:
> 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?

Oops, looked at assembler output, above is unrelated, sorry for noise.
But found WTF this breaks: google://"amd64 red-zone".
I think compiling this file (and any other that have call inside asm) with
-mno-red-zone option would be safer.

> All your comments may well be bugs, but I don't see how they would be
> related to call not being usable.