[Bug middle-end/90693] Missing popcount simplifications

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90693

Wilco  changed:

   What|Removed |Added

 Status|UNCONFIRMED |ASSIGNED
   Last reconfirmed||2019-08-09
   Assignee|unassigned at gcc dot gnu.org  |wilco at gcc dot gnu.org
 Ever confirmed|0   |1

[Bug middle-end/90693] Missing popcount simplifications

[david.bolvansky at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90693

--- Comment #3 from Dávid Bolvanský  ---
I measured it a bit..

(x-1) 

[Bug middle-end/90693] Missing popcount simplifications

[wilco at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90693

--- Comment #2 from Wilco  ---
(In reply to Dávid Bolvanský from comment #1)
> >> __builtin_popcount (x) == 1 into x == (x & -x)
> 
> 
> This will not work for x = 0.
> 
> Should work:
> x && x == (x & -x)
> x && (x & x-1) == 0

Good point, though that's not needed for (x & (x-1)) != 0 given you can only
have 2 or more bits set if x was non-zero to start with. It's worth finding a
branchless sequence, otherwise it may not be faster.

Maybe (x << clz (x)) == INT_MIN or (x-1) 

[Bug middle-end/90693] Missing popcount simplifications

[david.bolvansky at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90693

Dávid Bolvanský  changed:

   What|Removed |Added

 CC||david.bolvansky at gmail dot 
com

--- Comment #1 from Dávid Bolvanský  ---
>> __builtin_popcount (x) == 1 into x == (x & -x)


This will not work for x = 0.

Should work:
x && x == (x & -x)
x && (x & x-1) == 0