On Monday 28 September 2009, Frank Mori Hess wrote: > The > strict aliasing rules allow compilers to make optimizations that assume > pointers to different types will not access the same memory. I'll try > to produce a test program soon.
Attached is an example program that shows undefined behavior due to aliasing from treating "float _Complex" and "float [2]" as interchangeable types. When compiled with on Debian Lenny (gcc 4.3.2) with "gcc -Wall -O2" the assertion fails. When compiled with "gcc -Wall -O2 -fno-strict-aliasing" the assertion passes.
#include <assert.h>
#include <complex.h>
#include <stdio.h>
#include <stdlib.h>
typedef float _Complex fftw_complex1;
typedef float fftw_complex2[2];
fftw_complex2 a;
int main()
{
fftw_complex1 *x = NULL;
fftw_complex2 **p = (fftw_complex2**)(&x);
*p = &a;
x = NULL;
assert(*p == NULL);
return 0;
}
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