[Android] [PATCH 5/8] contrib: ffmpeg: Use --extra-cflags='-mincoming-stack-boundary=4'

[Rafaël Carré]

Le 2012-07-25 11:53, Martin Storsjö a écrit :
> On Wed, 25 Jul 2012, Rafaël Carré wrote:
> 
>> Le 2012-07-25 11:44, Martin Storsjö a écrit :
>>> On Wed, 25 Jul 2012, Rafaël Carré wrote:
>>>
>>>> Le 2012-07-25 04:51, Edward Wang a écrit :
>>>>> The issue here is that android-x86 doesn't guarantee an aligned stack, which means some of the inline asm functions will waste an extra register on realigning the stack.
>>>>>
>>>>> libavcodec does however have stuff in place in all entrypoints that realign the stack on all calls into the library. If you build with --extra-cflags='-mincoming-stack-boundary=4', gcc assumes that the stack actually is aligned already, which the entry points ensure, so this inline asm builds fine.
>>>>>
>>>>> Pointed-out-by: Martin Storsjö <martin at martin.st>
>>>>> ---
>>>>>  contrib/src/ffmpeg/rules.mak |    9 +++++++++
>>>>>  1 files changed, 9 insertions(+), 0 deletions(-)
>>>>>
>>>>> diff --git a/contrib/src/ffmpeg/rules.mak b/contrib/src/ffmpeg/rules.mak
>>>>> index ed6d8e8..fb069a3 100644
>>>>> --- a/contrib/src/ffmpeg/rules.mak
>>>>> +++ b/contrib/src/ffmpeg/rules.mak
>>>>> @@ -82,6 +82,15 @@ endif
>>>>>  # Linux
>>>>>  ifdef HAVE_LINUX
>>>>>  FFMPEGCONF += --target-os=linux --enable-pic
>>>>> +
>>>>> +ifeq ($(ANDROID_ABI), x86)
>>>>> +ifdef HAVE_ANDROID
>>>>> +# Android-x86 doesn't guarantee an aligned stack, so we tell gcc to
>>>>> +# assume that the stack actually is aligned already
>>>>
>>>> The comment is a bit misleading, if Android-x86 doesn't guarantee
>>>> alignement, then we can't assume that the stack is aligned (since it is
>>>> not).
>>>
>>> Not sure if this is an objection against the patch or just the description
>>> here. The info in the commit message has all the details - libavcodec has
>>> entry point stuff which makes sure it is aligned on entry, and hopefully
>>> gcc keeps it aligned across the calls within the library then.
>>
>> This was against the description, I think this option forces gcc to
>> align stack, rather than assume an alignement
> 
> Perhaps it does both, but for the out of register conditions in inline 
> asm, the critical part is that this tells gcc that the stack is aligned 
> already on entry into the functions, so that gcc doesn't need to waste one 
> register on aligning local stack arrays.

What about this:

# Android-x86 gcc doesn't guarantee an aligned stack, so we tell gcc to
# align the stack on 16 bytes, so we don't need to waste a precious
# register in assembly functions to realign it.

?