Re: [musl] strcmp() guarantees and assumptions

[Markus Wichmann <nullplan@gmx.net>]

Am Sun, Jul 16, 2023 at 07:59:57PM +0200 schrieb Robert Clausecker:
> That's good to hear.  Any idea on the “what do existing libc
> implementations permit” bit?
>

So I quickly checked musl, dietlibc, bionic, and glibc, and
unsurprisingly, all of the implementations I looked at allow the strings
to be unterminated if they mismatch before access becomes restricted.
This is, of course, an implementation detail that applications must not
rely on, but it nevertheless is the case.

The problem in your implementation is that the calls to strlen() will
iterate over both input strings to the end, causing basically a cache
flush for large inputs, only to then iterate over both inputs a second
time. Iterating only once is a major benefit, since it avoids half of
the cache misses.

Also, glibc already has an SSE strcmp implementation you may want to
look at.

Ciao,
Markus