Re: [Boost-bugs] [Boost C++ Libraries] #11312: smart_ptr::detail::as_allocator::allocate() adds alignment padding when it's not needed

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #11312: smart_ptr::detail::as_allocator::allocate() adds alignment padding when it's not needed
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-05-16 00:31:47

#11312: smart_ptr::detail::as_allocator::allocate() adds alignment padding when
it's not needed
----------------------------------+-----------------------
  Reporter:  David Aue <david@…>  |      Owner:  glenfe
      Type:  Bugs                 |     Status:  new
 Milestone:  To Be Determined     |  Component:  smart_ptr
   Version:  Boost 1.57.0         |   Severity:  Problem
Resolution:                       |   Keywords:
----------------------------------+-----------------------

Comment (by David Aue <david@…>):

 {{{
 pointer allocate(size_type count, const_void_pointer = 0) {
     enum {
         M = boost::alignment_of<type>::value
     };
     std::size_t n1 = count * sizeof(value_type);
     std::size_t n2 = data.size * sizeof(type);
     std::size_t n3 = n2 + M;
     CA ca(allocator());
     void* p1 = ca.allocate(n1 + n3);
     void* p2 = static_cast<char*>(p1) + n1;
     (void)boost::alignment::align(M, n2, p2, n3);
     *data.result = static_cast<type*>(p2);
     return static_cast<value_type*>(p1);
 }


 I call:
 boost::allocate_shared<int[]>(allocator, 4);

 Inside allocate() I see this:

 n1 == 20 for ms_init_tag
 n2 == 16 for the 4 ints
 M == 4
 n3 == 20

 ca.allocate is called with n1+n3 for a total of 40 bytes.

 p1 == some_address
 p2 == some_address + n1
 boost::alignment::align called to fix p2 for 4 byte alignment
 p2 is unchanged by align

 So we allocated 40 bytes and we are using the first 20 for ms_init and the
 next 16 for the int[4] and then there are 4 unused bytes that are not
 affecting our 4 byte alignment requirement.

 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11312#comment:3>
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