[PATCH v7] sysdeps/x86_64/multiarch/memmem-avx2.c: add memmem-avx2.c

[Rich Felker]

On Wed, Feb 21, 2024 at 11:30:16PM +0300, Alexander Monakov wrote:
> 
> On Wed, 21 Feb 2024, Noah Goldstein wrote:
> 
> > > +#ifndef TZCNT
> > > +#  define TZCNT(x) _tzcnt_u32 (x)
> > > +#endif
> > Use `__builtin_ctz`
> > > +#ifndef BLSR
> > > +#  define BLSR(x) _blsr_u32 (x)
> > > +#endif
> > 
> > Think you can drop the `BLSR` define (here and in the avx512)
> > and just replace with `((x) & ((x) - 1))`
> > any reasonable compiler will optimize that correctly.
> 
> I am really confused why review of such minor technical details is happening
> as if the proposed change is desirable and the goal is to include it in Glibc,
> and algorithm-wise it's all fine including the relevance of rarebyte_table to
> real-world uses of memmem and handling of page boundaries when iterating over
> the haystack. Not to mention the necessity of carrying SIMD variants of memmem
> in Glibc.

Same. I would really like to see glibc stop entertaining big-O
regressions in the form of magic tricks that happen to work well on
the submitter's test cases. It's reminiscent of the good ol' days of:

https://sourceware.org/git/?p=glibc.git;a=blob;f=string/strstr.c;hb=0ecb606cb6cf65de1d9fc8a919bceb4be476c602

It's also really not nice to people who do honestly want to contribute
to drag them along through revising something that's never going to
make sense to include. High-level "is this desirable to begin with?"
should really be resolved before code-review-for-inclusion.

Rich