On 1/22/06, Alexandre Oliva <[EMAIL PROTECTED]> wrote:
> I don't think it is any different. GCC's exception for unions only
> applies if the object is accessed using the union type. So they are
> indeed equivalent. The scary thing is that I don't think they
> actually violate the ISO C strict aliasing rules, but they might still
> be mis-optimized by GCC, assuming I understand correctly what GCC
> does.
ISO C says that the following violates aliasing rules:
int foo(float f)
{
union { int i; float f; } u;
i.f = f;
return i.i;
}
because the memory at u is accessed as two different effective types.
Using a union doesn't change this. The correct solution for converting
a float to an integer (bit-representation) as in the expample is to use
two different memory locations and memcpy (which avoids aliasing
issues by means of using the special rules for access through 'char').
Richard.
