[Bug middle-end/53104] New: convert_move on MODE_PARTIAL_INT does not look for existing conversion

["bigotp at acm dot org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53104

             Bug #: 53104
           Summary: convert_move on MODE_PARTIAL_INT does not look for
                    existing conversion
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bigotp@acm.org


convert_move has special code to handle conversion to and from
MODE_PARTIAL_INT.  Above this code is a comment stating that targets are
expected to provide conversion insns for the MODE_PARTIAL_INT modes they
use.

When presented with a move from HImode to PSImode, however, this code will
unconditionally convert HImode to SImode, then truncate back down to
PSImode.  For cases where both these operations are fairly expensive, it
should instead first check to see whether the target provided the desired
conversion directly.