[Bug rtl-optimization/31101] Problem with -funroll-all-loops (postreload CSE problems)

["ubizjak at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #2 from ubizjak at gmail dot com  2007-03-09 18:21 -------
No, this is not a target problem. I have traced the problem down to
reload_cse_simplify(), where insn that loads flags (either test or sahf) gets
cleared. This is highly suspicious part of code (why value is set to 0 ?):

      if (!count && reload_cse_noop_set_p (body))
        {
          rtx value = SET_DEST (body);
          if (REG_P (value)
              && ! REG_FUNCTION_VALUE_P (value))
            value = 0;
          delete_insn_and_edges (insn);
          return;
        }

So, reload_cse_noop_set_p() through rtx_equal_for_cse_lib_p() finds many copies
of:

(set (reg:CCNO 17 flags)
    (compare:CCNO (and:SI (zero_extract:SI (reg:HI 0 ax [73])
                (const_int 8 [0x8])
                (const_int 8 [0x8]))
            (const_int 4 [0x4]))
        (const_int 0 [0x0])))

And since postreload CSE pass thinks it can copy them from previous flag
register (neglecting the fact that ax is changed in a preceeding insn), it
clears all but the first two occurences of flags loading insn. Following this,
second postreload CSE pass removes also fnstsw instructions.


-- 

ubizjak at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|target                      |rtl-optimization
            Summary|Problem with -funroll-all-  |Problem with -funroll-all-
                   |loops                       |loops (postreload CSE
                   |                            |problems)


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31101