I made a bubble sort 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:
time ./sort 30000./sort 30000 1.82s user 0.00s system 99% cpu 1.816 total-O2:
time ./sort 30000./sort 30000 1.00s user 0.00s system 99% cpu 1.005 total-O3:
time ./sort 30000./sort 30000 2.01s user 0.00s system 99% cpu 2.007 totalThe code:
#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):
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 .L5And the same for -O3:
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 .L5It seems like the only significant difference to me is the apparent attempt to use SIMD, 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 5 3600.
