------- Comment #5 from cltang at pllab dot cs dot nthu dot edu dot tw
2010-02-25 16:01 -------
The cause seems to be the subreg lowering pass, the returning of the DImode
value is split into:
(insn 46 32 47 7 c.c:23 (set (reg:SI 0 r0)
(subreg:SI (reg:DI 133 [ D.2015 ]) 0)) 167 {*thumb1_movsi_insn} (nil))
(insn 47 46 40 7 c.c:23 (set (reg:SI 1 r1 [+4 ])
(subreg:SI (reg:DI 133 [ D.2015 ]) 4)) 167 {*thumb1_movsi_insn}
(expr_list:REG_DEAD (reg:DI 133 [ D.2015 ])
(nil)))
As the IRA dumps show, this splitting into two insns causes r0 to conflict with
the lifetime of r133, preventing it to be assigned to r0-r1 and coalesced.
I don't have a patch for this, but I found turning off the subreg lowering by
-fno-split-wide-types retains the DImode return:
(insn 37 32 40 7 c.c:23 (set (reg/i:DI 0 r0)
(reg:DI 133 [ D.2015 ])) 164 {*thumb1_movdi_insn} (expr_list:REG_DEAD
(reg:DI 133 [ D.2015 ])
(nil)))
and generates:
push {r4, r5, r6, lr}
sub sp, sp, #16
mov r5, r0
mov r0, sp
mov r6, r1
mov r4, sp
bl func
cmp r5, #1
bne .L2
ldr r0, [sp]
ldr r1, [sp, #4]
b .L3
.L2:
ldr r0, [r4, #8]
ldr r1, [r4, #12]
cmp r6, #2
beq .L3
ldr r3, [r4]
ldr r4, [r4, #4]
sub r0, r0, r3
sbc r1, r1, r4
.L3:
add sp, sp, #16
@ sp needed for prologue
pop {r4, r5, r6, pc}
which should be similar to what 4.2.1 generated.
The sp duplication in r4, as mentioned by others, is another problem.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42502
