See the repro below on x86-64 Linux (Ubuntu 17.10) I think the -O2 version is returning the wrong result; in the loop in memswp it appears to be writing to *q before reading from it.
$ cat main.c
#include <stdio.h>
extern void memswp(int *p, int *q, int n);
int main()
{
__attribute__((aligned(4))) char a[] = {'a','b','c','d','e','f','g','h',0};
memswp((int *)a, (int *)a + 1, 1);
puts(a);
return 0;
}
$ cat move.c
void memswp(int *p, int *q, int n)
{
for (int i = 0; i < n; i++)
{
int t = ((struct { int x; } *)p + i)->x;
((struct { int x; } *)p + i)->x = ((struct { int x; } *)q + i)->x;
((struct { int x; } *)q + i)->x = t;
}
}
$ icc -O0 main.c move.c -o swp0 && ./swp0
efghabcd
$ icc -O2 main.c move.c -o swp2 && ./swp2
efghefgh
$ icc --version
icc (ICC) 18.0.1 20171018
Copyright (C) 1985-2017 Intel Corporation. All rights reserved.
