[FFmpeg-devel] [PATCH v2 1/5] configure: aarch64: Support assembling the dotprod and i8mm arch extensions

[Martin Storsjö]

On Tue, 30 May 2023, Martin Storsjö wrote:

> Current clang versions fail to support the dotprod and i8mm
> features in the .arch_extension directive, but do support them
> if enabled with -march=armv8.4-a on the command line. (Curiously,
> lowering the arch level with ".arch armv8.2-a" doesn't make the
> extensions unavailable if they were enabled with -march; if that
> changes, Clang should also learn to support these extensions via
> .arch_extension for them to remain usable here.)

FWIW, since today, Clang does support enabling these extensions with both 
these extensions, see 
https://github.com/llvm/llvm-project/commit/4b8d9abca7d0280878fb12de331e688ee85d7cd8 
and 
https://github.com/llvm/llvm-project/commit/4b8d9abca7d0280878fb12de331e688ee85d7cd8.

It turns out that it is possible to enable these extensions with older 
Clang via assembly too, but due to a bug, it would require using e.g. 
".arch armv8.6-a+crc" (it requires using a "+<ext>" for any random 
unrelated extension). I won't try to support using that in our assembly, 
as the proper mechanism should be supported going forward.

As there was no further opposition, I'll push this patchset now with the 
last modifications that were suggested.

// Martin