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

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.


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