Re: Intel AVX10.1 Compiler Design and Support

[Phoebe Wang <phoebe.pf.w@gmail.com>]

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>  Changing ABIs like that for existing code that has worked for some time
on
>  existing hardware is a bad idea.

I agree, so Proposal 3 is the last choice.

The target of the proposals is to solve the ABI incompatible issue between
AVX10-256 and AVX10-512 when passing/returning 512 vectors. So we are
discussing the default ABI rather than other vector variants.

If you believe that changing 512-bit ABI (the 512-bit version) is a bad
idea, how about Proposal 1 and 2? I don't want to call the non 512-bit
version an ABI because it doesn't provide the interaction between 256-bit
and 512-bit targets. Besides, LLVM also behaves differently with GCC on non
512-bit targets. It is a good time to solve the problem together if we make
the 512-bit ABI consistent and target independent. WDYT?

Thanks
Phoebe

Joseph Myers <joseph@codesourcery.com> 于2023年8月10日周四 04:43写道：

> On Wed, 9 Aug 2023, Wang, Phoebe via Gcc-patches wrote:
>
> > Proposal 3: Change the ABI of 512-bit vector and always be
> > passed/returned from memory.
>
> Changing ABIs like that for existing code that has worked for some time on
> existing hardware is a bad idea.
>
> At this point it seems appropriate to remind people of another ABI
> consideration for vector extensions.  glibc's libmvec defines vector
> versions of various functions, including AVX512 ones (of course those
> function versions only work on hardware with the relevant instructions).
> glibc's headers use both _Pragma ("omp declare simd notinbranch") and
> __attribute__ ((__simd__ ("notinbranch"))) to declare, to the compiler
> including those headers, what function variants are available in glibc.
>
> Existing glibc versions need to continue to work with new compiler
> versions.  That is, it's part of the ABI, which must remain stable,
> exactly which function versions the above pragma and attribute imply are
> available - and of course the details of how those functions versions take
> arguments / return results are also part of the ABI (it would be OK for a
> new compiler to choose not to use some of those vector versions, but not
> to start calling them with a different ABI).
>
> Maybe you'll want to add new vector function versions, with different
> interfaces, to libmvec in future.  If so, you need a *different* pragma or
> attribute to declare to the compiler that the libmvec version using that
> pragma or attribute has the additional functions - so new compilers using
> the existing header will not try to generate calls to new function
> versions that don't exist in that glibc version (but new compilers using a
> new header version from new glibc will see the new pragma or attribute and
> so be able to generate the relevant calls to new functions).  And once
> you've defined the ABI for such a new pragma or attribute, that itself
> then becomes a stable interface - so if you end up with vector extensions
> involving yet another set of interfaces, they need another corresponding
> new pragma / attribute for libmvec to declare to the compiler that the new
> interfaces exist.
>
> --
> Joseph S. Myers
> joseph@codesourcery.com
>
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