The problem reported is a compile bug,
reported by the development GCC team with GCC 4.2.

The original issue can no longer be reproduced in MySQL 5.1,
since the configure script no longer define HAVE_ATOMIC_ADD,
which caused the Linux atomic functions to be used (and cause a problem
with an invalid cast).

This patch implements some code cleanup for 5.1 only, which was identified
during the investigation of this issue.

With this patch, statistics maintained in THD::status_var are by definition
owned by the running thread, and do not need to be protected against race
conditions. These statistics are maintained by the status_var_* helpers,
which do not require any lock.