Hi,
We were using GCC 3.4.0 to generate Thumb code for ARM processor,
switching to GCC 4.1.1 has improved our code size (we always use -Os switch),
but has severely altered the execution speed.
After further investigation, we isolate one the problem in the
following example:
Source code:
void foo(int *a)
{ int i;
for (i = 0; i < 1000000; i++)
a[0] += a[1];
}
The result with GCC 3.4.0 with -mthumb -Os was:
00000000 <foo>:
0: b500 push {lr}
2: 6803 ldr r3, [r0, #0]
4: 4a03 ldr r2, [pc, #12] (14 <.text+0x14>)
6: 6841 ldr r1, [r0, #4]
8: 3a01 sub r2, #1
a: 185b add r3, r3, r1
c: 2a00 cmp r2, #0
e: d1fb bne 8 <foo+0x8>
10: 6003 str r3, [r0, #0]
12: bd00 pop {pc}
14: 4240 neg r0, r0
16: 000f lsl r7, r1, #0
when compiled for ARM with GCC 4.1.1 (and mainline too) with -mthumb
-O1, we get:
00000000 <foo>:
0: b510 push {r4, lr}
2: 1c04 adds r4, r0, #0
4: 2200 movs r2, #0
6: 6841 ldr r1, [r0, #4]
8: 4803 ldr r0, [pc, #12] (18 <.text+0x18>)
a: 6823 ldr r3, [r4, #0]
c: 185b adds r3, r3, r1
e: 3201 adds r2, #1
10: 4282 cmp r2, r0
12: d1fb bne.n c <foo+0xc>
14: 6023 str r3, [r4, #0]
16: bd10 pop {r4, pc}
18: 4240 negs r0, r0
1a: 000f lsls r7, r1, #0
-> No so bad but slower than 3.4.0
when compiled with -mthumb -Os, we get:
00000000 <foo>:
0: b510 push {r4, lr}
2: 6802 ldr r2, [r0, #0]
4: 6844 ldr r4, [r0, #4]
6: 2100 movs r1, #0
8: 4b03 ldr r3, [pc, #12] (18 <.text+0x18>)
a: 3101 adds r1, #1
c: 1912 adds r2, r2, r4
e: 4299 cmp r1, r3
10: d1fa bne.n 8 <foo+0x8>
12: 6002 str r2, [r0, #0]
14: bd10 pop {r4, pc}
16: 0000 lsls r0, r0, #0
18: 4240 negs r0, r0
1a: 000f lsls r7, r1, #0
-> The Load of the loop end value is performed within the loop !
when compiled with -mthumb -O3, we get:
00000000 <foo>:
0: b530 push {r4, r5, lr}
2: 6802 ldr r2, [r0, #0]
4: 4d05 ldr r5, [pc, #20] (1c <.text+0x1c>)
6: 1d04 adds r4, r0, #4
8: 2100 movs r1, #0
a: 6823 ldr r3, [r4, #0]
c: 3101 adds r1, #1
e: 18d3 adds r3, r2, r3
10: 1c1a adds r2, r3, #0
12: 6003 str r3, [r0, #0]
14: 42a9 cmp r1, r5
16: d1f8 bne.n a <foo+0xa>
18: bd30 pop {r4, r5, pc}
1a: 0000 lsls r0, r0, #0
1c: 4240 negs r0, r0
1e: 000f lsls r7, r1, #0
-> Amazingly slow !
Does anybody has a magic set of options to generate an efficient and
small code as 3.4.0 did.
Thanks in advance for any hints on this problem.
David
