[Bug rtl-optimization/39871] [4.3/4.4/4.5 regression] Code size increase on ARM due to poor register allocation

["steven at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #16 from steven at gcc dot gnu dot org  2010-02-10 22:50 -------
In fwprop.c of r118475, we get to propagate_rtx_1 (fwprop.c:334):

          /* Copy propagations are always ok.  Otherwise check the costs.  */
          if (!(REG_P (old) && REG_P (new))
              && !should_replace_address (op0, new_op0, GET_MODE (x)))
            return true;

At this point the simplified address has been found, but fwprop decides not to
substitute the new address:

(gdb) p debug_rtx(op0)
(plus:SI (reg/f:SI 102)
    (const_int 4 [0x4]))
$58 = void
(gdb) p debug_rtx(new_op0)
(plus:SI (reg/f:SI 25 sfp)
    (const_int -8 [0xfffffffffffffff8]))
$59 = void
(gdb) p should_replace_address(op0,new_op0,SImode)
$60 = 0 '\000'

The replacement isn't done because fwprop sees no benefit in doing the
transformation. Stepping through should_replace_address we get:

202       gain = address_cost (old, mode) - address_cost (new, mode);
(gdb) next
208       if (gain == 0)
(gdb) p gain
$64 = 0
(gdb) next
209         gain = rtx_cost (new, SET) - rtx_cost (old, SET);
(gdb) 
211       return (gain > 0);
(gdb) p gain
$65 = 0

Perhaps we should prefer addresses based on the frame pointer over other
addresses?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39871