Warn when comparing nonnull arguments to NULL in a function.

Warn when comparing nonnull arguments to NULL in a function.

If an argument is marked as nonnull then passing in a NULL argument
will produce bad results even if the code checks against NULL. GCC
might optimize such checks away so warn the user when the function
contains such comparisons.

nn.c: In function ‘foo’:
nn.c:6:27: warning: nonnull argument ‘bar’ compared to NULL [-Wnonnull]
 void foo(void *bar) { if (!bar) abort(); }
                           ^
gcc/ChangeLog

	* doc/invoke.texi (Wnonnull): Also warns when comparing against NULL.

gcc/c/ChangeLog

       * c-typeck.c (build_binary_op): Check and warn when nonnull arg
       parm against NULL.

gcc/cp/ChangeLog

       * typeck.c (cp_build_binary_op): Check and warn when nonnull arg
       parm against NULL.

gcc/testsuite/ChangeLog

       * c-c++-common/nonnull-1.c: New test.

From-SVN: r227649