[PATCH] libbb/last_char_is: rewrite for smaller and faster code

[Emmanuel Deloget]

Hello,

On Fri, Jul 3, 2020 at 9:15 PM Tito <farmatito at tiscali.it> wrote:
>
>
>
> On 7/3/20 6:13 PM, Xabier Oneca -- xOneca wrote:
> > Hi all,
> >
> >>> On Fri, 2020-07-03 at 08:12 +0200, Tito wrote:
> >>> [...]
> >>>> Improved version:
> >>>>
> >>>> char* last_char_is(const char *s, int c)  {
> >>>>      if (!s || !*s) return NULL;
> >>>>      while (*(s + 1))s++;
> >>>>      return (c == *s) ? (char *)s : NULL;
> >>>> }
> >
> > Let me "improve" the Tito's -11 bytes, probably introducing some subtle bug :)
> >
> > char* FAST_FUNC last_char_is(const char *s, int c)
> > {
> >     if (!s || !*s) return NULL;
> >     while (*(++s));
> >     return (*(--s) == c) ? s : NULL;
> > }
> >
> > function                                             old     new   delta
> > last_char_is                                          58      42     -16
> > ------------------------------------------------------------------------------
> > (add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-16)             Total: -16 bytes
> >    text    data     bss     dec     hex filename
> >  956449    4235    1904  962588   eb01c busybox_old
> >  956433    4235    1904  962572   eb00c busybox_unstripped
> >
> > Also, there's a warning compiling this function: return discards
> > ‘const’ qualifier from pointer target type
>
> Hi,
> cast it?
>
> char* FAST_FUNC last_char_is(const char *s, int c)
> {
>      if (!s || !*s) return NULL;
>      while (*(++s));
>      return (*(--s) == c) ? (char *)s : NULL;
> }
>

In favor of const-correctness, wouldn't it be better to return a const
char*? It's quite strange to promote a string to const and to a part
of it that is non-const (not to mention that if you feed the function
with a real const char* then the cast is not a good thing to do).

>
> > Fun to play with you.
> >
> > Cheers,
> >
> > Xabier Oneca_,,_

Best regards,

-- Emmanuel Deloget