Strict aliasing is an assumption, made by the C (or C++) compiler, that dereferencing pointers to objects of different types will never refer to the same memory location (i.e. alias each other.)

In the following example, int * and float * are different pointer types, so the assembly only loads %rdi once, but not for the case where both arguments have the same pointer type.

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void f(int *x, int *y)
{
    static int z;
    z += *x;
    *y = *y + 1;
    z += *x;
    // addl (%rdi), %eax
    // incl (%rsi)
    // addl (%rdi), %eax
}

void g(int *x, float *y)
{
    static int z;
    z += *x;
    *y = *y + 1;
    z += *x;
    // movl (%rdi), %eax
    // movl g.z(%rip), %ecx
    // addl %eax, %ecx
    // addl %eax, %ecx
}

http://cellperformance.beyond3d.com/articles/2006/06/understanding-strict-aliasing.html