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

[Maxim Kuvyrkov]

On 2/12/2011, at 9:45 PM, Jakub Jelinek wrote:

> 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).

Jacub,
Zdenek,

Thank you for explaining the overflow in comparisons.  In fact, the unsigned overflow is intentionally used in expanding of switch statements to catch the 'default:' case in certain switch statements with just one conditional branch.

--
Maxim Kuvyrkov
CodeSourcery / Mentor Graphics