Question

When compiling C++ with the -fdump-tree-gimple option (GCC 4.6.1), I get code that has the following function in it:

std::_Vector_base<_Tp, _Alloc>::~_Vector_base() [with _Tp = int, _Alloc = std::allocator<int>] (struct _Vector_base * const this)
{
  int * D.8482;
  long int D.8483;
  int * D.8484;
  long int D.8485;
  long int D.8486;
  long int D.8487;
  long unsigned int D.8488;
  int * D.8489;
  struct _Vector_impl * D.8490;

  {
    try
      {
        D.8482 = this->_M_impl._M_end_of_storage;
        D.8483 = (long int) D.8482;
        D.8484 = this->_M_impl._M_start;
        D.8485 = (long int) D.8484;
        D.8486 = D.8483 - D.8485;
        D.8487 = D.8486 /[ex] 4;
        D.8488 = (long unsigned int) D.8487;
        D.8489 = this->_M_impl._M_start;
        std::_Vector_base<int, std::allocator<int> >::_M_deallocate(this, D.8489, D.8488);
      }
    finally
      {
        D.8490 = &this->_M_impl;
        std::_Vector_base<int, std::allocator<int>::_Vector_impl::~_Vector_impl (D.8490);
      }
  }
  <D.8393>:
}

You can get this code by making a simple program that uses std::vector<int>. In any case, the part of the code I do not understand is the line with D.8487 = D.8486 /[ex] 4;. I looked at the source code for /usr/include/c++/4.6.1/std_vector.h, and its destructor is a one-liner that calls _M_deallocate. Does anyone know what the operator /[ex] stands for? The only thing I have noticed so far is that the RHS operand is the size of the type the vector parameterizes over.

Answer 1

/[ex] means that it’s an exact divide expression.

From the GCC internals manual:

EXACT_DIV_EXPR

The EXACT_DIV_EXPR code is used to represent integer divisions where the numerator is known to be an exact multiple of the denominator. This allows the backend to choose between the faster of TRUNC_DIV_EXPR, CEIL_DIV_EXPR and FLOOR_DIV_EXPR for the current target.