Re: [PATCH] Fix C++ strict-aliasing issues with memcpy folding

[Jason Merrill <jason at redhat dot com>]

On 04/14/2010 04:29 PM, Richard Guenther wrote:
> On Wed, 14 Apr 2010, Jason Merrill wrote:

> > Note "or is copied as an array of character type".  Since struct assignment is
> > defined as memberwise assignment and union assignment is defined as assignment
> > of the value representation (which is an array of unsigned char), such an
> > assignment should transfer the effective type of a subobject that completely
> > overlaps either a union or an array of character type.

> Correct.  I don't remember off-hand if the code as written in
> libstdc++ and/or produced by the C++ frontend is recognizable
> as such.

The problematic code in std/functional looked like

union _Any_data
{
  // member functions
  _Nocopy_types _M_unused;
  char _M_pod_data[sizeof(_Nocopy_types)];
};

      void swap(function& __x)
      {
        _Any_data __old_functor = _M_functor;
        _M_functor = __x._M_functor;
        __x._M_functor = __old_functor;
        // ....
      }

Here we are doing an assignment of a union, which is defined as a copy 
of the value representation, which means copying it as an array of 
unsigned char.  So it should be equivalent to memcpy as far as TBAA is 
concerned.

> At least if we have an aggregate union assignment
> we do not use alias-set zero for it, but we only special
> case component-refs into union objects.  So, either we want
> to treat union assignment uniformly in the middle-end and
> have to adjust get_alias_set or the C/C++ langhooks need
> adjustment here.  Note that I am not sure we're not pessimizing
> much code that way.

Using alias set zero seems excessive; what we want is to use the 
existing memcpy semantics when copying a union or character array.

Jason