------- Comment #19 from steven at gcc dot gnu dot org 2010-02-10 23:47 -------
In r118474, cse.c:find_best_addr makes the replacement here:
if ((addr_folded_cost < addr_cost
|| (addr_folded_cost == addr_cost
/* ??? The rtx_cost comparison is left over from an older
version of this code. It is probably no longer
helpful.*/
&& (rtx_cost (folded, MEM) > rtx_cost (addr, MEM)
|| approx_reg_cost (folded) < approx_reg_cost (addr))))
&& validate_change (insn, loc, folded, 0))
addr = folded;
All the costs are the same, except the approx_reg_cost tests:
(gdb) p debug_rtx(addr)
(plus:SI (reg/f:SI 102)
(const_int 4 [0x4]))
$35 = void
(gdb) p debug_rtx(folded)
(plus:SI (reg/f:SI 25 sfp)
(const_int -8 [0xfffffffffffffff8]))
$36 = void
(gdb) p approx_reg_cost(addr)
$37 = 1
(gdb) p approx_reg_cost(folded)
$38 = 0
(gdb)
The cost difference comes from approx_reg_cost_1, which uses CHEAP_REGNO for
regs. CHEAP_REGNO prefers the frame pointer reg over normal pseudos:
/* Compute cost of X, as stored in the `cost' field of a table_elt. Fixed
hard registers and pointers into the frame are the cheapest with a cost
of 0. Next come pseudos with a cost of one and other hard registers with
a cost of 2. Aside from these special cases, call `rtx_cost'. */
#define CHEAP_REGNO(N) \
(REGNO_PTR_FRAME_P(N) \
|| (HARD_REGISTER_NUM_P (N) \
&& FIXED_REGNO_P (N) && REGNO_REG_CLASS (N) != NO_REGS))
This shouldn't be very difficult to teach fwprop about, too.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39871
