[Bug rtl-optimization/30747] New: gcc should optimize x / (cond ? const1 : const2)

["ak at muc dot de" <gcc-bugzilla@gcc.gnu.org>]

(seen on other targets too) 

For 

x / (cond ? const1 : const2) 

gcc currently always generates a slow general purpose div. But it would
be much faster to rewrite it to

cond ? (x / const1 : x / const2) 

and then use the optimized sequences for division by constant

In the Linux kernel there are a few cases where this would help.
e.g. it is common in device drivers which manage a ring buffer and the ring
size depends on the hardware revision.

It would be also useful for some other cases where it is not easy
to rewrite the code by hand to the better form.

I made it rtl-optimization, although I presume it could be as well
done on tree level?


-- 
           Summary: gcc should optimize x / (cond ? const1 : const2)
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ak at muc dot de
GCC target triplet: x86_64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30747