[Bug libstdc++/23425] New: vector::clear should be manually inlined

[tkho at ucla dot edu]

vector::clear calls erase(begin(), end()), which has an unnecessary call to
memmove, in addition to a function call overhead. The fix results in a
2-instruction clear for simple types.

Compiling the following (-O2 -save-temps) shows the difference:

#include <vector>

using std::vector;

int main() {
  vector<int> v;

  asm("#start1");
  v.clear();
  asm("#end1");

  asm("#start2");
  v.erase(v.begin(), v.end());
  asm("#end2");
}

This was a notable performance difference between libstdc++ stl and stlport I
found in profiling a large application.

-- 
           Summary: vector::clear should be manually inlined
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tkho at ucla dot edu
                CC: dank at kegel dot com,gcc-bugs at gcc dot gnu dot org
  GCC host triplet: i686-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23425