Re: [PATCH v3] wcrtomb: Make behavior POSIX compliant

[Florian Weimer <fweimer@redhat.com>]

* Siddhesh Poyarekar:

> On 13/05/2022 10:58, Paul Eggert wrote:
>> On 5/12/22 21:56, Paul Eggert wrote:
>>> Hope you don't mind my bikeshedding.
>> Better yet, this:
>>    s[0] = buf[0];
>>    if (2 <= result && result <= 4)
>>      {
>>        s[1] = buf[1];
>>        memcpy (&s[result - 2], &buf[result - 2], 2);
>>      }
>>    else
>>      memcpy (s, buf, result);
>> On x86-64 with GCC 12.1 -O2 and a glibc-supplied charmap, this is
>> only 9 straight-line instructions, counting the compare insn and the 
>> conditional-branch insn that is never taken.
>> 
>
> Sorry I missed this one.  I tried it and with gcc 11 it seems to
> produce worse code, merging the two memcpys instead of inlining the
> first one.

Can we please use the simplified code?

“char buf[MB_LEN_MAX];” already gives GCC a strong hint this is a short
memcpy.  If it is beneficial on some architectures to inline such short,
but still variable memcpy calls, we should teach the compiler how to do
it, and not do it manually.

And wcrtomb would for sure benefit more from a fast path for ASCII and
UTF-8. 8-p

(Not that I think there are many users out there, given glibc's
historically poor performance in the multibyte/wide string conversion
space.)

Thanks,
Florian