[Bug tree-optimization/63464] compare one character to many: faster

[rguenther at suse dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63464

--- Comment #2 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 6 Oct 2014, jakub at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63464
> 
> Jakub Jelinek <jakub at gcc dot gnu.org> changed:
> 
>            What    |Removed                     |Added
> ----------------------------------------------------------------------------
>              Status|UNCONFIRMED                 |NEW
>    Last reconfirmed|                            |2014-10-06
>                  CC|                            |jakub at gcc dot gnu.org,
>                    |                            |rguenth at gcc dot gnu.org,
>                    |                            |steven at gcc dot gnu.org
>      Ever confirmed|0                           |1
> 
> --- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> We have this optimization implemented for switches, if you compile
> char*f3(char*s){
>   do
>     {
>       switch (*s)
>         {
>         case ' ':
>         case ',':
>         case '\r':
>         case '\n':
>           ++s;
>           continue;
>         default:
>           return s;
>         }
>     }
>   while (1);
> }
> 
> then it will do the bit test, see r189173 (and various follow-up fixes for
> that).
> Now, we can argue whether in this case it is beneficial to perform the
> MINUS_EXPR or if maxval is small enough (e.g. when maxval is smaller than
> BITS_PER_WORD), just assume minval is 0.
> 
> And then the question is, if we should teach reassoc range optimizations to
> reuse emit_case_bit_tests, or convert such tests into a GIMPLE_SWITCH and
> expand as such.
> 
> Richard/Steven, thoughts about this?

Factoring out the code-gen part of switch-conversion and use it
from reassoc range optimization?

I don't think creating a GIMPLE_SWITCH from reassoc is a good idea
(given that switch conversion runs early).

Richard.