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

[Martin Storsjö]

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.

// Martin