Comment 6 for bug 2004264

(In reply to Noah Goldstein from comment #2)
> (In reply to Simon Chopin from comment #0)
> > I did some tests on master (2f39e44a84) for the upcoming 2.37 release, and I
> > found a regression in libunistring test suite on amd64 with AVX-2
> > instructions, more specifically on gnulib's test-strncat. It can be
> > reproduced using these instructions:
> >
> > https://sourceware.org/glibc/wiki/Testing/Gnulib
> >
> >
> > I bisected the issue to
> > commit 642933158e7cf072d873231b1a9bb03291f2b989
> > Author: Noah Goldstein <email address hidden>
> > Date: Tue Nov 8 17:38:39 2022 -0800
> >
> > x86: Optimize and shrink st{r|p}{n}{cat|cpy}-avx2 functions
> >
> > Optimizations are:
> > 1. Use more overlapping stores to avoid branches.
> > 2. Reduce how unrolled the aligning copies are (this is more of a
> > code-size save, its a negative for some sizes in terms of
> > perf).
> > 3. For st{r|p}n{cat|cpy} re-order the branches to minimize the
> > number that are taken.
> >
> > I get the following backtrace:
> >
> > #0 __strncat_avx2 () at ../sysdeps/x86_64/multiarch/strncat-avx2.S:76
>
> Do you know what instruction its segfaulting at?
> > #1 0x00005555555555d7 in strncat (__len=0, __src=0x7ffff7de4000 "",
> > __dest=0x55555555c2a1 "") at
> > /tmp/glibc-dev/include/bits/string_fortified.h:138
> > #2 check_single (input=input@entry=0x7ffff7de4000 "", n=n@entry=0,
> > length=90) at unistr/test-strncat.h:41
> > #3 0x0000555555555352 in check (input=0x555555559100 <input> "Grüß Gott.
> > Здравствуйте! x=(-b±sqrt(b²-4ac))/(2a) 日本語,中文,한글", input_length=91)
> > at unistr/test-strncat.h:86
> > #4 main () at test-strncat.c:58
> >
> > I'm out of my depth in the assembler code.

Reproduced. Changed the `malloc` to mmap and put it at the end of the code.

Issue is:
```
 test %rdx, %rdx
 jl L(zero_len)
```

Needs to be `jle` (was `decq` at some point, when changed didn't update flag).
I think the test is actually UB b.c `dst` is not a valid null-terminated string (even though zero length), but will fix.