[FFmpeg-devel] [PATCH] Mark C globals with small code model

[Pranav Kant]

I uploaded a new patch (v3) that addresses these concerns.

On Thu, Feb 27, 2025 at 5:14 PM Michael Niedermayer <michael at niedermayer.cc>
wrote:

> On Wed, Feb 26, 2025 at 07:44:37PM +0000, Pranav Kant via ffmpeg-devel
> wrote:
> > By default, all globals in C/C++ compiled by clang are allocated
> > in non-large data sections. See [1] for background on code models.
> > For PIC (Position independent code), this is fine as long as binary is
> > small but as binary size increases, users maybe want to use medium/large
> > code models (-mcmodel=medium) which moves data in to large sections.
> > As data in these large sections cannot be accessed using PIC code
> > anymore (as it may be too far away), compiler ends up using a different
> > instruction sequence when building C/C++ code -- using GOT to access
> > these globals (which can be relaxed by linker at link time if binary
> > ends up being smaller). However, assembly files continue to access these
> > globals defined in C/C++ files using older (and invalid instruction
> > sequence). So, we mark all such globals with an attribute that forces
> > them to be allocated in small sections allowing them to validly be
> > accessed from the assembly code.
> >
> > This patch should not have any affect on builds that use small code
> > model, which is the default mode.
> >
> > [1]
> https://eli.thegreenplace.net/2012/01/03/understanding-the-x64-code-models
> >
> > Signed-off-by: Pranav Kant <prka at google.com>
> > ---
> >  libavcodec/ac3dsp.c             |  2 ++
> >  libavcodec/cabac.c              |  2 ++
> >  libavcodec/x86/constants.c      |  8 ++++++++
> >  libavutil/attributes_internal.h | 15 +++++++++++++++
> >  4 files changed, 27 insertions(+)
>
> This produces many warnings:
>
> CC      libavcodec/svq1.o
> In file included from libavcodec/svq1.h:40,
>                  from libavcodec/svq1.c:35:
> ./libavutil/attributes_internal.h:43:5: warning: "ARCH_X86_64" is not
> defined, evaluates to 0 [-Wundef]
>    43 | #if ARCH_X86_64 && defined(__ELF__) && __has_attribute(model)
>       |     ^~~~~~~~~~~
>
> [...]
>
> --
> Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB
>
> If you fake or manipulate statistics in a paper in physics you will never
> get a job again.
> If you fake or manipulate statistics in a paper in medicin you will get
> a job for life at the pharma industry.
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