Simplification of relational operations (was [patch for PR18942])

[Zdenek Dvorak]

Hi,

> 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.
> 
> Any reason why this optimization would be unsafe?

the reason I handled only EQ/NE patterns is that you do not need to worry about
overflows there.  For ordering predicates, x - 1 <= 1 is not equivalent to x <= 2
if x = 0 (the former becomes MAX_UNSIGNED_INT <= 1, which is false),

Zdenek