[Bug middle-end/52919] New: shift expansion mishandles SHIFT_COUNT_TRUNCATED on partial integer modes

[bigotp at acm dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52919

             Bug #: 52919
           Summary: shift expansion mishandles SHIFT_COUNT_TRUNCATED on
                    partial integer modes
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: big...@acm.org


The code for SHIFT_COUNT_TRUNCATED in expmed.c:expand_shift_1 improperly uses
GET_MODE_BITSIZE instead of GET_MODE_PRECISION when calculating the truncation
value.  This can result in failure to identify an acceptable shift operation;
e.g. with a PSImode with 20 bits of precision, a shift of 20 may be requested.