07-24-2023, 02:45 AM
I made a [bubble sort][1] implementation in C, and was testing its performance when I noticed that the `-O3` flag made it run even slower than no flags at all! Meanwhile `-O2` was making it run a lot faster as expected.
Without optimisations:
```none
time ./sort 30000
./sort 30000 1.82s user 0.00s system 99% cpu 1.816 total
```
`-O2`:
```none
time ./sort 30000
./sort 30000 1.00s user 0.00s system 99% cpu 1.005 total
```
`-O3`:
```none
time ./sort 30000
./sort 30000 2.01s user 0.00s system 99% cpu 2.007 total
```
The code:
```c
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <time.h>
int n;
void bubblesort(int *buf)
{
bool changed = true;
for (int i = n; changed == true; i--) { /* will always move at least one element to its rightful place at the end, so can shorten the search by 1 each iteration */
changed = false;
for (int x = 0; x < i-1; x++) {
if (buf[x] > buf[x+1]) {
/* swap */
int tmp = buf[x+1];
buf[x+1] = buf[x];
buf[x] = tmp;
changed = true;
}
}
}
}
int main(int argc, char *argv[])
{
if (argc != 2) {
fprintf(stderr, "Usage: %s <arraysize>\n", argv[0]);
return EXIT_FAILURE;
}
n = atoi(argv[1]);
if (n < 1) {
fprintf(stderr, "Invalid array size.\n");
return EXIT_FAILURE;
}
int *buf = malloc(sizeof(int) * n);
/* init buffer with random values */
srand(time(NULL));
for (int i = 0; i < n; i++)
buf[i] = rand() % n + 1;
bubblesort(buf);
return EXIT_SUCCESS;
}
```
The assembly language generated for `-O2` (from [godbolt.org][2]):
```none
bubblesort:
mov r9d, DWORD PTR n[rip]
xor edx, edx
xor r10d, r10d
.L2:
lea r8d, [r9-1]
cmp r8d, edx
jle .L13
.L5:
movsx rax, edx
lea rax, [rdi+rax*4]
.L4:
mov esi, DWORD PTR [rax]
mov ecx, DWORD PTR [rax+4]
add edx, 1
cmp esi, ecx
jle .L2
mov DWORD PTR [rax+4], esi
mov r10d, 1
add rax, 4
mov DWORD PTR [rax-4], ecx
cmp r8d, edx
jg .L4
mov r9d, r8d
xor edx, edx
xor r10d, r10d
lea r8d, [r9-1]
cmp r8d, edx
jg .L5
.L13:
test r10b, r10b
jne .L14
.L1:
ret
.L14:
lea eax, [r9-2]
cmp r9d, 2
jle .L1
mov r9d, r8d
xor edx, edx
mov r8d, eax
xor r10d, r10d
jmp .L5
```
And the same for `-O3`:
```none
bubblesort:
mov r9d, DWORD PTR n[rip]
xor edx, edx
xor r10d, r10d
.L2:
lea r8d, [r9-1]
cmp r8d, edx
jle .L13
.L5:
movsx rax, edx
lea rcx, [rdi+rax*4]
.L4:
movq xmm0, QWORD PTR [rcx]
add edx, 1
pshufd xmm2, xmm0, 0xe5
movd esi, xmm0
movd eax, xmm2
pshufd xmm1, xmm0, 225
cmp esi, eax
jle .L2
movq QWORD PTR [rcx], xmm1
mov r10d, 1
add rcx, 4
cmp r8d, edx
jg .L4
mov r9d, r8d
xor edx, edx
xor r10d, r10d
lea r8d, [r9-1]
cmp r8d, edx
jg .L5
.L13:
test r10b, r10b
jne .L14
.L1:
ret
.L14:
lea eax, [r9-2]
cmp r9d, 2
jle .L1
mov r9d, r8d
xor edx, edx
mov r8d, eax
xor r10d, r10d
jmp .L5
```
It seems like the only significant difference to me is the apparent attempt to use [SIMD][3], which *seems* like it should be a large improvement, but I also can't tell what on earth it's attempting with those `pshufd` instructions... is this just a failed attempt at SIMD? Or maybe the couple of extra instructions is just about edging out my instruction cache?
Timings were done on an AMD [Ryzen][4] 5 3600.
[1]:
Without optimisations:
```none
time ./sort 30000
./sort 30000 1.82s user 0.00s system 99% cpu 1.816 total
```
`-O2`:
```none
time ./sort 30000
./sort 30000 1.00s user 0.00s system 99% cpu 1.005 total
```
`-O3`:
```none
time ./sort 30000
./sort 30000 2.01s user 0.00s system 99% cpu 2.007 total
```
The code:
```c
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <time.h>
int n;
void bubblesort(int *buf)
{
bool changed = true;
for (int i = n; changed == true; i--) { /* will always move at least one element to its rightful place at the end, so can shorten the search by 1 each iteration */
changed = false;
for (int x = 0; x < i-1; x++) {
if (buf[x] > buf[x+1]) {
/* swap */
int tmp = buf[x+1];
buf[x+1] = buf[x];
buf[x] = tmp;
changed = true;
}
}
}
}
int main(int argc, char *argv[])
{
if (argc != 2) {
fprintf(stderr, "Usage: %s <arraysize>\n", argv[0]);
return EXIT_FAILURE;
}
n = atoi(argv[1]);
if (n < 1) {
fprintf(stderr, "Invalid array size.\n");
return EXIT_FAILURE;
}
int *buf = malloc(sizeof(int) * n);
/* init buffer with random values */
srand(time(NULL));
for (int i = 0; i < n; i++)
buf[i] = rand() % n + 1;
bubblesort(buf);
return EXIT_SUCCESS;
}
```
The assembly language generated for `-O2` (from [godbolt.org][2]):
```none
bubblesort:
mov r9d, DWORD PTR n[rip]
xor edx, edx
xor r10d, r10d
.L2:
lea r8d, [r9-1]
cmp r8d, edx
jle .L13
.L5:
movsx rax, edx
lea rax, [rdi+rax*4]
.L4:
mov esi, DWORD PTR [rax]
mov ecx, DWORD PTR [rax+4]
add edx, 1
cmp esi, ecx
jle .L2
mov DWORD PTR [rax+4], esi
mov r10d, 1
add rax, 4
mov DWORD PTR [rax-4], ecx
cmp r8d, edx
jg .L4
mov r9d, r8d
xor edx, edx
xor r10d, r10d
lea r8d, [r9-1]
cmp r8d, edx
jg .L5
.L13:
test r10b, r10b
jne .L14
.L1:
ret
.L14:
lea eax, [r9-2]
cmp r9d, 2
jle .L1
mov r9d, r8d
xor edx, edx
mov r8d, eax
xor r10d, r10d
jmp .L5
```
And the same for `-O3`:
```none
bubblesort:
mov r9d, DWORD PTR n[rip]
xor edx, edx
xor r10d, r10d
.L2:
lea r8d, [r9-1]
cmp r8d, edx
jle .L13
.L5:
movsx rax, edx
lea rcx, [rdi+rax*4]
.L4:
movq xmm0, QWORD PTR [rcx]
add edx, 1
pshufd xmm2, xmm0, 0xe5
movd esi, xmm0
movd eax, xmm2
pshufd xmm1, xmm0, 225
cmp esi, eax
jle .L2
movq QWORD PTR [rcx], xmm1
mov r10d, 1
add rcx, 4
cmp r8d, edx
jg .L4
mov r9d, r8d
xor edx, edx
xor r10d, r10d
lea r8d, [r9-1]
cmp r8d, edx
jg .L5
.L13:
test r10b, r10b
jne .L14
.L1:
ret
.L14:
lea eax, [r9-2]
cmp r9d, 2
jle .L1
mov r9d, r8d
xor edx, edx
mov r8d, eax
xor r10d, r10d
jmp .L5
```
It seems like the only significant difference to me is the apparent attempt to use [SIMD][3], which *seems* like it should be a large improvement, but I also can't tell what on earth it's attempting with those `pshufd` instructions... is this just a failed attempt at SIMD? Or maybe the couple of extra instructions is just about edging out my instruction cache?
Timings were done on an AMD [Ryzen][4] 5 3600.
[1]:
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