Memory Size Load and Store penalty analysis? - c

Question

Profiling the code with ocount shows more cycles with penalty on and lesser cycles with penalty off. I'm trying to understand why there is more penalty when the penalty flag is on?
uint16_t arr[1010];
uint32_t r[500];
void func()
{
uint32_t i = 0;
for (i = 0; i < 1000; i+=2)
{
arr[i] = i;
arr[i+1] = i+10;
#ifdef PENALTY_ON
r[i/2] = *(uint32_t *)((uint16_t *)&arr[i+1]);
#endif
}
#ifndef PENALTY_ON
for (i = 0; i < 1000; i+=2)
{
r[i/2] = *(uint32_t *)((uint16_t *)&arr[i+1]);
}
#endif
}

Answer 1

Compiling both with gcc on a 32-bit machine with -O3
With PENALTY_ON
00000000 <func>:
0: 31 c0 xor %eax,%eax
2: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
8: 8d 50 0a lea 0xa(%eax),%edx
b: 66 89 94 00 02 00 00 mov %dx,0x2(%eax,%eax,1)
12: 00
13: 8b 8c 00 02 00 00 00 mov 0x2(%eax,%eax,1),%ecx
1a: 89 c2 mov %eax,%edx
1c: 66 89 84 00 00 00 00 mov %ax,0x0(%eax,%eax,1)
23: 00
24: 83 c0 02 add $0x2,%eax
27: d1 ea shr %edx
29: 3d e8 03 00 00 cmp $0x3e8,%eax
2e: 89 0c 95 00 00 00 00 mov %ecx,0x0(,%edx,4)
35: 75 d1 jne 8 <func+0x8>
37: f3 c3 repz ret
Without PENALTY_ON
00000000 <func>:
0: 31 c0 xor %eax,%eax
2: 8d b6 00 00 00 00 lea 0x0(%esi),%esi
8: 8d 50 0a lea 0xa(%eax),%edx
b: 66 89 84 00 00 00 00 mov %ax,0x0(%eax,%eax,1)
12: 00
13: 66 89 94 00 02 00 00 mov %dx,0x2(%eax,%eax,1)
1a: 00
1b: 83 c0 02 add $0x2,%eax
1e: 3d e8 03 00 00 cmp $0x3e8,%eax
23: 75 e3 jne 8 <func+0x8>
25: 66 31 c0 xor %ax,%ax
28: 8b 8c 00 02 00 00 00 mov 0x2(%eax,%eax,1),%ecx
2f: 89 c2 mov %eax,%edx
31: 83 c0 02 add $0x2,%eax
34: d1 ea shr %edx
36: 3d e8 03 00 00 cmp $0x3e8,%eax
3b: 89 0c 95 00 00 00 00 mov %ecx,0x0(,%edx,4)
42: 75 e4 jne 28 <func+0x28>
44: f3 c3 repz ret
I think the reason is that a Read-after-Write stall occurs with PENALTY_ON
b: 66 89 94 00 02 00 00 mov %dx,0x2(%eax,%eax,1)
12: 00
13: 8b 8c 00 02 00 00 00 mov 0x2(%eax,%eax,1),%ecx