[Bug target/17503] New: Huge performance regression in invalid_mode_change_p

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

Compiling insn-attrtab.i with compilers from a few weeks ago, this function is
way down the profile.  Now it's at the very top.

                               :bool
                               :invalid_mode_change_p (unsigned int regno, enum
reg_class class,
                               :                       enum machine_mode from_mode)
   625  0.1102    55  0.5528   :{ /* invalid_mode_change_p total: 117807 20.7631
 4747 47.7085 */
                               :  enum machine_mode to_mode;
                               :  int n;
   113  0.0199    17  0.1709   :  int start = regno * MAX_MACHINE_MODE;
                               :
116873 20.5985  4675 46.9849   :  EXECUTE_IF_SET_IN_BITMAP (&subregs_of_mode,
start, n,
                               :    if (n >= MAX_MACHINE_MODE + start)
                               :      return 0;
                               :    to_mode = n - start;
                               :    if (CANNOT_CHANGE_MODE_CLASS (from_mode,
to_mode, class))
                               :      return 1;
                               :  );
                               :  return 0;
   196  0.0345     0 0.0e+00   :}

I am not positive whether the function is being called more, or just spending
more time in the bitmap, but it looks like the latter - we're creating many more
entries in subregs_of_mode.  I selected a random call to invalid_mode_change_p,
and compared the size of the bitmap in both compilers.  In the older compiler,
the high entry in the bitmap was around 10,000 bits; in the newer, around 450,000.

The newer compiler was updated this morning.

-- 
           Summary: Huge performance regression in invalid_mode_change_p
           Product: gcc
           Version: 4.0.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: drow at gcc dot gnu dot org
                CC: gcc-bugs 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=17503