Re: RTL alias analysis

[Richard Guenther <richard dot guenther at gmail dot com>]

On 1/22/06, Alexandre Oliva <aoliva@redhat.com> 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.