[FFmpeg-devel] [PATCH] ARM: remove useless stack push/pop

[Rafaël Carré]

On Wed, 09 Jun 2010 00:43:54 +0100
M?ns Rullg?rd <mans at mansr.com> wrote:

> Rafa?l Carr? <rafael.carre at gmail.com> writes:

> > While I'm here, did anyone try to build FFmpeg with -mthumb yet ?
> 
> Yes, gcc generated invalid asm. 

Do you have more information about this ?

It would be useful if the same thing happens to me and I know what to
look for

> For that reason, and others, we force -marm. 

Ok

> > "grep -Er '(pop|ldm).*pc' libavcodec/arm" shows that there is a lot
> > of functions which can't be called from thumb on armv4t : using
> > ldm ...,pc will not perform the switch from arm to thumb on these
> > CPU.
> 
> So use interworking if you need to.  Any decent linker support that.

interworking modifies function calls by inserting veneers but returning
from ASM code while setting the T bit correctly is your responsibility

Also "The AAPCS requires that all sub-routine call and return sequences
support inter-working between ARM and Thumb states." but it doesn't
really makes sense in internal functions so unless you export assembly
functions you're good

> See also a blog post I did some time ago on the topic.  Perhaps I
> should revisit that.

I found http://hardwarebug.org/2009/03/25/thumbs-up/ which looks
promising, although it's about thumb-2, not thumb

> BTW, many, if not most, Cortex-A8 chips in the field have hardware
> bugs rendering any mixing of Thumb and ARM code unreliable.  Older
> cores work, but most of those are pre-Thumb2 and the speed penalty
> there is too great for FFmpeg.

I wouldn't know, i only have really old CPUs ;)

> > diff --git a/libavcodec/arm/jrevdct_arm.S

> Does this function call any other functions?  If so, the stack must
> maintain 8-byte alignment, and this is the easiest way to accomplish
> that.  Not that you'd want to use that DCT implementation anyway.

No it doesn't (stack pointer is modified a few lines below entry anyway)

-- 
Rafa?l Carr?
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