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

[Paul Eggert <eggert@cs.ucla.edu>]

On 5/13/22 06:42, Siddhesh Poyarekar wrote:
> Ugh, both of you just kill all the fun.  I'll push a fix with just the 
> memcpy ;)

Just as well, as the code I sent before is slow for ASCII anyway. I 
shouldn't let that stand as my suggestion, even if just for fun.


> with gcc 11 it seems to produce worse code, merging the two memcpys instead of inlining the first one.

Here's a better version if someone ever wants to tune this stuff. In 
this version there's only one memcpy so GCC won't merge it. On x86-64 
with GCC 12.1 -O2, if RESULT<=2 this executes 6 instructions (including 
1 comparison and 1 conditional branch); if 3<=RESULT<=4 this executes 8 
instructions total (including 2 comparisons and 2 conditional branches).

   if (result <= 2)
     {
       s[0] = buf[0];
       s[result - 1] = buf[result - 1];
     }
   else if (result <= 4)
     {
       s[0] = buf[0];
       s[1] = buf[1];
       s[result - 2] = buf[result - 2];
       s[result - 1] = buf[result - 1];
     }
   else
     memcpy (s, buf, result);

There are of course other variants....