[PATCH v2] x86-64: Optimize bzero

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi,

> Is there any way it can be setup so that one C impl can cover all the
> arch that want to just leave `__memsetzero` as an alias to `memset`?
> I know they have incompatible interfaces that make it hard but would
> a weak static inline in string.h work?

No that won't work. A C implementation similar to current string/bzero.c
adds unacceptable overhead (since most targets just implement memset and
will continue to do so). An inline function in string.h would introduce target
hacks in our headers, something we've been working hard to remove over the
years.

The only reasonable option is a target specific optimization in GCC and LLVM
so that memsetzero is only emitted when it is known an optimized GLIBC
implementation exists (similar to mempcpy).

> It's worth noting that between the two `memset` is the cold function
> and `__memsetzero` is the hot one. Based on profiles of GCC11 and
> Python3.7.7 setting zero covers 99%+ cases.

There is no doubt memset of zero is by far the most common. What is in doubt
is whether micro-optimizing is worth it on modern cores. Does Python speed up
by a measurable amount if you use memsetzero?

Cheers,
Wilco