While out benchmarking today, I ran across code similar to this:
int *a;
int *b;
int *c;
const int ad[320];
const int bd[320];
const int cd[320];
void fill()
{
for (int i = 0; i < 320; i++)
{
a[i] = ad[i];
b[i] = bd[i];
c[i] = cd[i];
}
}
I was surprised and happy to see the vectoriser kick in for the copy.
The inner loop looks like:
add r5, r3, ip
adds r4, r3, r7
vldmia r2!, {d16-d17}
vldmia r1!, {d18-d19}
adds r0, r3, r6
vst1.32 {q9}, [r5]
vst1.32 {q8}, [r4]
vldmia r3, {d16-d17}
adds r3, r3, #16
cmp r3, r8
vst1.32 {q8}, [r0]
bne .L3
so r3 is the loop variable and {ip,r7} are the offsets from r3 to the
destination pointers. Adding a __restrict doesn't change the code.
Richard, will your auto-inc/dec changes combine the final vldmia r3,
add r3 into a vldmia r3! ?
Changing the int *a into in-file arrays like int a[320] gives:
vldmia r0!, {d16-d17}
vldmia r5!, {d18-d19}
vstmia r4!, {d18-d19}
vstmia r1!, {d16-d17}
vldmia r2!, {d16-d17}
vstmia r3!, {d16-d17}
cmp r3, r6
bne .L2
Marking them as extern int a[320] goes back to the first form.
Can we always use the second form? What optimisation is preventing it?
-- Michael
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