Re: Undefined behavior due to 6.5.16.1p3

[Martin Sebor]

On 03/09/2015 01:26 PM, Robbert Krebbers wrote:

I was wondering whether GCC uses 6.5.16.1p3 of the C11 standard as a
license to perform certain optimizations. If so, could anyone provide me
an example program.

In particular, I am interested about the "then the overlap shall be
exact" part of 6.5.16.1p3:

   If the value being stored in an object is read from another
   object that overlaps in any way the storage of the first
   object, then the overlap shall be exact and the two objects
   shall have qualified or unqualified versions of a compatible
   type; otherwise, the behavior is undefined.

I suspect every compiler relies on this requirement in certain
cases otherwise copying would require making use of temporary
storage. Here's an example:

  struct A {
    int a [32];
  };

  union {
    struct A a;
    struct {
      char b1;
      struct A b2;
    } b;
  } u;

  void foo (void) {
    u.b.b2 = u.a;
  }

Martin