[Bug tree-optimization/45216] New: Rotate expressions not recognized at tree level

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

We have RROTATE_EXPR and LROTATE_EXPR, but the patterns are not reliably
detected at the tree level.  I'm attaching a testcase reduced from the Linux
kernel, which has at least one sequence that can be rewritten using a rolw
instruction:

        movzwl  %cx, %edx
        movzwl  %ax, %eax
        shrw    $8, %cx
        movl    %edx, -44(%ebp)
        sall    $8, %edx
        movw    %cx, -50(%ebp)
        orw     %dx, -50(%ebp)

Other opportunities exist.  At the most basic level, a function like

unsigned short rol8 (unsigned short word, unsigned int shift)
{
  return (word << 8) | (word >> 8);
}

should be transformed at the tree level by recognizing the rotate.


-- 
           Summary: Rotate expressions not recognized at tree level
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bernds at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45216