alias question

[Patrick Horgan]

On 02/18/2011 09:35 AM, xorbe wrote:
> #include<stdio.h>
> char b[4] = {0,0,0,0};
> const short*w((short*)b);
> const int*d((  int*)b);
> void print() {printf("%02x%02x%02x%02x %04x%04x %08x\n",
>                       b[3],b[2],b[1],b[0],w[1],w[0],d[0]);}
> int main() {
>    print();
>    for (int i(0); i<4; ++i) { b[i] = i+1; print(); }
> }
You're right.  This might make it clearer, it's a version that's valid C 
and looks like it has an optimization opportunity (which would give you 
output you _didn't_ want;) but actually doesn't because of the special 
case char& or char* can alias anything rule.

#include <stdio.h>

char b[4] = {0,0,0,0};
const short *w;
const int   *d;

int main()
{
     size_t i;
     w = ((short*)b);
     d = ((  int*)b);
     /* the next line makes explicit endianess assumptions */
     printf("%02x%02x%02x%02x %04x%04x 
%08x\n",b[3],b[2],b[1],b[0],w[1],w[0],d[0]);
     for (i=0; i<4; ++i) {
         b[i] = (char)(i+1);
         printf("%02x%02x%02x%02x %04x%04x 
%08x\n",b[3],b[2],b[1],b[0],w[1],w[0],d[0]);
     }
     return 0;
}

A compiler might want to optimize the references to w and d since 
they're unchanged inside the loop, and just remember their values from 
outside the list.  Unfortunately, since access through a char reference 
or pointer can alias anything, the compiler is unable to make this 
assumption and cannot do any optimizations.  In general, doing any 
accesses of things through a char* or char& in a method or function can 
make a lot of optimizations impossible.

Patrick

p.s. Please read my aliasing whitepaper and make any suggestions for 
improvement that occur to you.  http://dbp-consulting.com/StrictAliasing.pdf