[Bug libstdc++/30204] New: std::vector operator[] 10x speedup (patch)

["charles at rebelbase dot com" <gcc-bugzilla@gcc.gnu.org>]

vector::operator[](size_type) in bits/stl_vector.h is currently implemented as

      reference
      operator[](size_type __n)
      { return *(begin() + __n); }

      const_reference
      operator[](size_type __n) const
      { return *(begin() + __n); }

A faster implementation would be:

      reference
      operator[](size_type __n)
      { return _M_impl._M_start[__n]; }

      const_reference
      operator[](size_type __n) const
      { return _M_impl._M_start[__n]; }

I tried a simple timing test on both implementations,
and the latter appears to be 10x faster:

(11:59:43)(charles xyzzy)(~): cat test.cc
#include <vector>
int main () {
  std::vector<int> x (100);
  unsigned long l = 0;
  const unsigned long iterations = 100000000;
  for (unsigned long i=0; i<iterations; ++i)
    l += x[50];
  return 0;
}
(12:00:14)(charles xyzzy)(~): g++ -o test test.cc -lstdc++
(12:00:22)(charles xyzzy)(~): time ./test

real    0m2.956s
user    0m2.948s
sys     0m0.008s
(12:00:27)(charles xyzzy)(~): cat test2.cc
#include <vector>
int main () {
  std::vector<int> x (100);
  unsigned long l = 0;
  const unsigned long iterations = 100000000;
  for (unsigned long i=0; i<iterations; ++i)
    l += x._M_impl._M_start[50];
  return 0;
}
(12:00:31)(charles xyzzy)(~): g++ -o test2 test2.cc -lstdc++
(12:00:39)(charles xyzzy)(~): time ./test2

real    0m0.228s
user    0m0.228s
sys     0m0.000s


-- 
           Summary: std::vector operator[] 10x speedup (patch)
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: charles at rebelbase dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30204