[Bug middle-end/85929] New: _GLIBCXX_ASSERTIONS, subscript type mismatch, and std::vector bounds check elimination

[fw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85929

            Bug ID: 85929
           Summary: _GLIBCXX_ASSERTIONS, subscript type mismatch, and
                    std::vector bounds check elimination
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fw at gcc dot gnu.org
  Target Milestone: ---

Compile this with “gcc -O2” on a 64-bit platform:

#define _GLIBCXX_ASSERTIONS
#include <vector>

typedef unsigned int size_type;

void
increment (std::vector<double> &vec)
{
   size_type count = vec.size();
   for (size_type i = 0; i < count; ++i)
     vec[i] += 1;
}

The expectation is that there is no call to abort.  The bounds check is
optimized away if the (global) size_type is itself size_t/unsigned long
(matching the index type of the vector).

A loop like this appears in the dealII benchmark from SPEC2006, and it is a
major source of performance regression with _GLIBCXX_ASSERTIONS.