[FFmpeg-devel] Re: [DISCUSSION] Adding ARM64EC support to FFmpeg

[Stephen Hutchinson via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

On 10/3/25 12:39 PM, Rémi Denis-Courmont via ffmpeg-devel wrote:
> Le perjantaina 3. lokakuuta 2025, 18.11.55 Itä-Euroopan kesäaika Martin
> Storsjö a écrit :
>> Wanting to use the libav* libraries in a hybrid x86_64/arm64ec process
>> seems like something quite plausible though. Not something I'd go out of
>> my way to complicate other things to accomplish, but reasonable build
>> system tweaks to allow such builds sound tolerable to me.
> 
> Is it that plausible? That seems far less plausible than running a plain
> x86_64 application on an Arm system. Especially for something performance
> sensitive as FFmpeg, you would want a proper native port. Sure, you could very
> well end up with slow x86 code due to "non-technical challenges" (euphemism
> for the vendor not providing an Arm build or your IT department lagging
> behind).
> 
> But realistically, you're not going to be able to replace an x86
> libavcodec.dll with an AArch64 libavcodec.dll in such an x86 app. The chance
> of finding the right build options to get a compatible ABI are pretty much
> zero. In fact, they are very exactly zero because the libavutil ABI depends on
> the ISA due notably to the CPU feature flags, configuration parameters, etc.
> 
> So this can only work if you have Arm64EC application code calling an
> hypothetical Arm64EC libav*, but also calling x86 code on independent code
> paths. Why would you not port the x86 code to Arm too (or run them in a
> separate process)? That seems pretty fringe if not academic to me.
> 

While I haven't gotten through adding the more newly-added external
libraries to the tedious mingw build guide, IIRC most/all of them
already can be built natively for AArch64, so I can't really think of a
publicly-available set of circumstances where someone would be linking
an x86-64 dependency into a native Arm instance of FFmpeg.

I could see it as trying to use x86-64 builds of AviSynth+, though.
That's LoadLibrary, and I either can't remember or haven't seen
anything that describes how the Arm64EC concept behaves when using
dynamic loading.

Yes, there is a native AArch64 Windows release of AviSynth+,
but there won't be one for Arm64EC, because we dropped support
for building with MSVC outside of x86(-64).  An llvm-mingw-built
Arm64EC core wouldn't be able to load MSVC-built x86-64 plugins.

So it wouldn't surprise me if people stubbornly refuse to use
the AArch64 release because they don't want to let go of a build
of a plugin that the plugin author only published for x86-64
(even if said plugin is FOSS).
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