Re: [PATCH v2] NEWS: Add entry for x86-64 ISA level build

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 27/07/22 13:22, Noah Goldstein via Libc-alpha wrote:
> On Thu, Jul 28, 2022 at 12:12 AM Joseph Myers <joseph@codesourcery.com> wrote:
>>
>> On Wed, 27 Jul 2022, Noah Goldstein via Libc-alpha wrote:
>>
>>> +* Support building x86-64 with multiple ISA levels has been added.
>>> +  Higher ISA levels will use better optimized functions in the dynamic
>>> +  loader and non-multiarch build as well as reduce the size of libc.so.
>>
>> I don't think "with multiple ISA levels" is very meaningful to users.
>> How is "with multiple ISA levels" different from the multi-arch feature
>> that's been present for many years - I think it could easily be read as a
>> description of that longstanding existing feature?  You need a description
>> that makes clear to users who aren't following glibc development what the
>> actual new feature is (i.e. that if you build with a compiler that
>> defaults to a newer ISA level such as -march=x86-64-v2, the resulting libc
>> will use corresponding optimized implementations and omit the versions
>> only relevant on older processors, if I'm understanding the feature
>> correctly).
> 
> That's fair.
> 
> Not really sure this needs a NEWS entry at the end of the day. It's not really
> directly visible. The only meaningful change is that there won't be any
> legacy sse code so potentially (with great care) a user could entirely
> omit `vzeroupper`.
> 
> Adhemerval do you think this needs an entry or is it too behind the scenes?

I had the impression that reorganization would allow an user visible
optimization (due the code ifunc selection change), but in the end
I agree that is should be transparent to users.  I don't have a strong
opinion here, maybe it is would be better to omit.