[FFmpeg-devel] Re: [GASPP PATCH 1/2] Pass "-machine" options through to armasm

["Martin Storsjö via ffmpeg-devel" <ffmpeg-devel@ffmpeg.org>]

On Tue, 20 Jan 2026, Rémi Denis-Courmont via ffmpeg-devel wrote:

> Do you have a plan to address the outstanding shortcomings? What would 
> the actual performance look like in such case, seen as it would disable 
> most or all of the assembler optimisations?

It is possible to use the current aarch64 assembler as such - even 
though it does violate the arm64ec ABI.

I just did a set of test builds of ffmpeg with MSVC, natively for arm64, 
one for arm64ec (with an unpatched ffmpeg) and one for x86_64. For a 
simple testcase of decoding VP9, both the proper arm64 and the less proper 
arm64ec build end up decoding at a similar speed, ~390 fps. The x86_64 
build, running emulated runs at 171 fps.

The fact that the arm64ec ABI is violated by the assembly is mostly not a 
practical problem, as long as it only is called directly by the arm64ec 
code, not by emulated x86_64. There are also some other limitations 
(if someone is suspending/resuming the process, the register state would 
be limited to what is expressible in arm64ec mode). There's of course no 
guarantees that this violation will be tolerated in the same way in future 
versions of Windows either.

The cpuflags part of the library API/ABI is indeed a problem - but for a 
caller user application which doesn't call that (and, practically 
speaking, I would expect that most user apps don't do that), that issue 
doesn't really crop up either.

I'm not arguing for us to support this in any greater form than we 
currently do - anyone wanting to use this can really build it for 
themselves. But in the current form, with ABI violations at all, it does 
practically work quite fine in many cases, and it does give essentially 
the same performance as a native build. But those who want to do that will 
need to worry about the limitations - and I wouldn't expect those who 
currently distribute binary builds of ffmpeg to care to add arm64ec 
builds.

(Do note that the ffmpeg project itself doesn't distribute binary builds, 
that is up to third party volunteers. So at this point, it's not us that 
would need to be convinced, it's those third parties that do binary 
builds.)

// Martin
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