One of the common defenses of the abuse of “undefined behavior” by C compilers, and the stupid aliasing rules in the standard is that those things are necessary for optimizations, like using vector operations. Here’s an example of where gcc ignores the standard and does what the C language semantics require while taking advantage of Intel SIMD instructions. The C code is an example of a common technique in network code.
#include <stdlib.h>
#include <stdio.h>
#include <limits.h>
struct message { int code; int value; float data[20]; int checkval; };
int checksum(struct message *m){
int i, *p = (int *)m;
int lim = sizeof(struct message) / sizeof i;
int c = 0;
for (i=0 ; i < lim; i++) { c ^= *(p+i); }
return c;
}
int main (int argc, char **argv)
{
struct message m;
int v;
fread((char *)&m, sizeof(char), sizeof(m), stdin);
v = checksum(&m);
printf("Checksum was %d\n",v);
}
The translation.
checksum: movdquxmm0, XMMWORDPTR [rdi+32] movdquxmm2, XMMWORDPTR [rdi+48] movdquxmm1, XMMWORDPTR [rdi] movdquxmm3, XMMWORDPTR [rdi+16] pxorxmm0, xmm2 movdquxmm4, XMMWORDPTR [rdi+64] moveax, DWORDPTR [rdi+80] pxorxmm1, xmm3 xoreax, DWORDPTR [rdi+84] xoreax, DWORDPTR [rdi+88] pxorxmm0, xmm1 movedx, eax pxorxmm0, xmm4 movdqaxmm1, xmm0 psrldqxmm1, 8 pxorxmm0, xmm1 movdqaxmm1, xmm0 psrldqxmm1, 4 pxorxmm0, xmm1 movdeax, xmm0 xoreax, edx ret
See the godbolt page
Vector optimization with C aliasing