[FFmpeg-devel] [Question] Technical reason why FFmpeg sets a mstack-alignment value on compile time?

[Ronald S. Bultje]

Hi,

On Sun, Nov 13, 2022 at 8:47 AM Nomis101 <Nomis101 at web.de> wrote:

> Am 12.11.22 um 23:09 schrieb Ronald S. Bultje:
> > Hi,
> >
> > On Sat, Nov 12, 2022 at 1:57 PM Nomis101 <Nomis101 at web.de> wrote:
> >
> >> Hi.If building FFmpeg, configure does set a value for mstack-alignment,
> >> for example on macOS
> >> -mstack-alignment=16. On HandBrake we found there is an issue with
> current
> >> Clang in Xcode about
> >> conflicting 'override-stack-alignment' values if we build with ThinLTO.
> >>
> >> "ld: linking module flags 'override-stack-alignment': IDs have
> conflicting
> >> values"
> >>
> >> I found out why this is. x264 does set -mstack-alignment=64 in
> configure,
> >> FFmpeg does set
> >> -mstack-alignment=16 and this seems to be a conflict when linking. If I
> >> modify both configure files
> >> and remove the mstack-alignment part, then the build does finish with no
> >> error.
> >>
> >> And here comes my questions. I was wondering, about the technical reason
> >> why FFmpeg does set
> >> mstack-alignment? And if it would be safe to disable it in case FFmpeg
> is
> >> compiled with ThinLTO (for
> >> HandBrake)?
> >>
> >
> > It can safely be set to the higher of the two values, it has the same
> > meaning. Alignment of 64 implies alignment of 16.
> Thanks. I found out, that its enough to disable the mstack-alignment part
> for x264 in case of
> ThinLTO. This will also fix the error.
>

For people looking at this in the archives after a google search, trying
this out and wondering why their x264 crashes, please see the following
reference:

https://mailman.videolan.org/pipermail/x264-devel/2022-November/012963.html

(Summary: if you remove the stack alignment in x264, please also remove the
assumption in x264's build that the stack *is* in fact aligned, and then
everything will work fine (although a few instructions slower).)

Ronald