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

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

[Nomis101]

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)?

Thanks and Regards
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Re: [FFmpeg-devel] [Question] Technical reason why FFmpeg sets a mstack-alignment value on compile time?

[Carl Eugen Hoyos]

Am Sa., 12. Nov. 2022 um 19:57 Uhr schrieb Nomis101 <Nomis101@web.de>:

> And here comes my questions. I was wondering, about the technical
> reason why FFmpeg does set mstack-alignment?

It is needed for x86_32.

Completely removing the option from all build scripts seems wrong to me.

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

[Ronald S. Bultje]

Hi,

On Sat, Nov 12, 2022 at 1:57 PM Nomis101 <Nomis101@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.

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

[Nomis101]

Am 12.11.22 um 23:09 schrieb Ronald S. Bultje:
> Hi,
>
> On Sat, Nov 12, 2022 at 1:57 PM Nomis101 <Nomis101@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.


>
> Ronald
> _______________________________________________
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> ffmpeg-devel-request@ffmpeg.org with subject "unsubscribe".

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Re: [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@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@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
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