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