[Bug rtl-optimization/55306] New: Make combine emit converted logical right shifts

["olegendo at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55306

             Bug #: 55306
           Summary: Make combine emit converted logical right shifts
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: olegendo@gcc.gnu.org


As of rev 193423 the combine pass can successfully convert arithmetic right
shifts into logical right shifts, but will not split / emit them as individual
insns if ...
* the target has arith. right shift patterns defined that match the shift
* the target does not have an insn that combine came up with, which inlcudes
  the logical shift.

There are some targets (SH, ARC, see PR 54089) where logical right shifts are
cheaper than arithmetic right shifts.
Since combine already has shift conversion integrated, it would probably make
sense to extend it, instead of replicating it in some separate pass or doing
fancy stuff in the target code.
Ideally, whenever combine converts a arith shift into a logical shift it should
look at the costs of the shifts and always split out the cheaper shift insn (if
the target has a pattern for the logical shift).