[PATCH v2] x86-64: Optimize bzero

[Wilco Dijkstra]

Hi,

>> The saving is in the lane-cross broadcast which is on the critical
>> path for memsets in [VEC_SIZE, 2 * VEC_SIZE] (think 32-64).

What is the speedup in eg. bench-memset? Generally the OoO engine will
be able to hide a small increase in latency, so I'd be surprised it shows up
as a significant gain.

If you can show a good speedup in an important application (or benchmark
like SPEC2017) then it may be worth pursuing. However there are other
optimization opportunities that may be easier or give a larger benefit.

>> Agreed it's not clear if it's worth it to start replacing memset calls with
>> bzero calls, but at the very least this will improve existing code that
>> uses bzero.

No code uses bzero, no compiler emits bzero. It died 2 decades ago...

> My point is this is a lot of code and infrastructure for a symbol marked
> as legacy for POSIX.1-2001 and removed on POSIX.1-2008 for the sake of
> marginal gains in specific cases.

Indeed, what we really should discuss is how to remove the last traces of
bcopy and bcmp from GLIBC. Do we need to keep a compatibility symbol
or could we just get rid of it altogether?

Cheers,
Wilco