I have this simple functions:
int sum_vec(int c, ...) {
va_list argptr;
va_start(argptr, c);
int sum = 0;
while (c--) {
int x = va_arg(argptr, int);
sum += x;
}
va_end(argptr);
return sum;
}
When compiling with "-fomit-frame-pointer -Os -march=68000 -c -S
-mshort" I get this assembly (I have manually added comments with
clock cycles per instruction and a total for a count of 0, 8 and n>0):
.even
.globl _sum_vec
_sum_vec:
lea (6,%sp),%a0 | 8
move.w 4(%sp),%d1 | 12
clr.w %d0 | 4
jra .L1 | 12
.L2:
add.w (%a0)+,%d0 | 8
.L1:
dbra %d1,.L2 | 16,12
rts | 16
| c==0: 8+12+4+12+12+16=64
| c==8: 8+12+4+12+(16+8)*8+12+16=256
| c==n: =64+24n
When instead compiling with "-fomit-frame-pointer -O3 -march=68000 -c
-S -mshort" I expect to get more aggressive optimisation than -Os, or
at least just as performant, but instead I get this:
.even
.globl _sum_vec
_sum_vec:
move.w 4(%sp),%d0 | 12
jeq .L2 | 12,8
lea (6,%sp),%a0 | 8
subq.w #1,%d0 | 4
and.l #65535,%d0 | 16
add.l %d0,%d0 | 8
lea 8(%sp,%d0.l),%a1 | 16
clr.w %d0 | 4
.L1:
add.w (%a0)+,%d0 | 8
cmp.l %a0,%a1 | 8
jne .L1 | 12|8
rts | 16
.L2:
clr.w %d0 | 4
rts | 16
| c==0: 12+12+4+16=44
| c==8: 12+8+8+4+16+8+16+4+(8+8+12)*4-4+16=316
| c==n: =88+28n
The count==0 case is better. I can see what optimisation has been
tried for the loop, but it just not working since both the ini for the
loop and the loop itself becomes more costly.
Being a GCC beginner I would like a few pointers as to how I should go
about to fix this?
// Fredrik
