On Fri, Dec 02, 2011 at 03:33:06PM +1300, Maxim Kuvyrkov wrote:
> I'm looking at a missed optimizations in combine and it is similar to the
> one you've fixed in PR18942
> (http://thread.gmane.org/gmane.comp.gcc.patches/81504).
>
> I'm trying to make GCC optimize
> (leu:SI
> (plus:SI (reg:SI) (const_int -1))
> (const_int 1))
>
> into
>
> (leu:SI
> (reg:SI)
> (const_int 2))
> .
>
> Your patch for PR18942 handles only EQ/NE comparisons, and I wonder if
> there is a reason not to handle LEU/GEU, LTU/GTU comparisons as well. I'm
> a bit fuzzy whether signed comparisons can be optimized here as well, but
> I can't see the problem with unsigned comparisons.
Consider reg:SI being 0? Then (leu:SI (plus:SI (reg:SI) (const_int -1))
(const_int 1))
is 0, but (leu:SI (reg:SI) (const_int 2)) is 1.
You could transform this if you have a guarantee that reg:SI will not be 0
(and, in your general
> Regarding the testcase, the general pattern
>
> (set (tmp1) (plus:SI (reg:SI) (const_int A))
> (set (tmp2) (leu:SI (tmp1) (const_int B))
case that reg:SI isn't 0 .. A-1).
Jakub
