New x86-64 micro-architecture levels

[Florian Weimer]

* Allan Sandfeld Jensen:

> On Freitag, 10. Juli 2020 19:30:09 CEST Florian Weimer via Gcc wrote:
>> glibc (or an alternative loader implementation) would search for
>> libraries starting at level D, going back to level A, and finally the
>> baseline implementation in the default library location.
>> 
>> I expect that some distributions will also use these levels to set a
>> baseline for the entire distribution (i.e., everything would be built to
>> level A or maybe even level C), and these libraries would then be
>> installed in the default location.
>> 
>> I'll be glad if I can get any feedback on this proposal.  I plan to turn
>> it into a merge request for the x86-64 psABI document eventually.

> Sounds good, though if I could dream I would also love a partial
> replacement option. So that you could have a generic x86-64 binary
> that only had some AVX2 optimized replacement functions in a
> supplementary library.
>
> Perhaps implemented by marked the library as a partial replacement, so
> the dynamic linker would also load the base or lower libraries except
> for functions already resolved.

I think you can do something like it today, at least from the glibc
dynamic loader perspective.  Programs link against the soname of the
optimized shared object (which can be empty), and that shared object
depends on the object with the fallback implementation.  A special
link-only shared object containing just the ABI under the front soname
(that of the optimized library) would be used via a linker object, so
that it is not possible to accidentally link against the wrong soname.

For non-versioned symbols, this setup has worked since forever.  For
versioned symbols, delegating from the optimized to the unoptimized
library needs at least glibc 2.30, with commit f0b2132b35248c1f4a80
("ld.so: Support moving versioned symbols between sonames [BZ #24741]"),
although some of us have backported this commit into earlier releases.

Where this falls flat is support for LTO and
-fno-semantic-interposition.  Some care is needed to make precisely the
right set of symbols interposable.  But to honest, I'm not sure if this
entire mechanism is a big improvement over function multi-versioning.

Thanks,
Florian