[PATCH v2] x86-64: Optimize bzero

[Patrick McGehearty]

Just as another point of information, Solaris libc implemented
bzero as moving arguments around appropriately then jumping to
memset. Noone noticed enough to file a complaint. Of course,
short fixed-length bzero was handled with in line stores of zero
by the compiler. For long vector bzeroing, the overhead was
negligible.

When certain Sparc hardware implementations provided faster methods
for zeroing a cache line at a time on cache line boundaries,
memset added a single test for zero ifandonlyif the length of code
to memset was over a threshold that seemed likely to make it
worthwhile to use the faster method. The principal advantage
of the fast zeroing operation is that it did not require data
to move from memory to cache before writing zeros to memory,
protecting cache locality in the face of large block zeroing.
I was responsible for much of that optimization effort.
Whether that optimization was really worth it is open for debate
for a variety of reasons that I won't go into just now.

Apps still used bzero or memset(target,zero,length) according to
their preferences, but the code was unified under memset.

I am inclined to agree with keeping bzero in the API for
compatibility with old code/old binaries/old programmers. :-)

Using shared memset code for the implementation of bzero
is worthwhile for reducing future maintenance costs.

- Patrick McGehearty
former Sparc/Solaris performance tuning person



On 2/10/2022 2:42 PM, Adhemerval Zanella via Libc-alpha wrote:
>
> On 10/02/2022 17:27, Alejandro Colomar (man-pages) wrote:
>>> We are discussing different subjects here: what I want is to remove the
>>> glibc *internal* optimization for bzero, which is essentially an
>>> implementation detail.  In a first glance it would change performance,
>>> however gcc does a hard job replacing bzero/bcmp/bcopy with their
>>> str* counterparts, so it highly unlike that newer binaries will actually
>>> call bzero.
>> Okay, then yes, go ahead and remove bzero(3) from glibc if GCC will
>> continue supporting it.  Just remember that some users keep writing and
>> wanting to write bzero(3) instead of memset(3) in their .c files, so
>> it's far from being dead in source code.
> Again, I am not proposing to *remove* bzero, but rather the internal
> optimizations that currently only adds code complexity and maintenance
> burden.  My patchset [1] will keep the ABI as-is, the difference is
> bcopy and bzero will use the default implementation on all architectures.
>
> [1] https://patchwork.sourceware.org/project/glibc/list/?series=7243