[Bug c/60090] New: For expression without ~, gcc -O1 emits "comparison of promoted ~unsigned with unsigned"

[chengniansun at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60090

            Bug ID: 60090
           Summary: For expression without ~, gcc -O1 emits "comparison of
                    promoted ~unsigned with unsigned"
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chengniansun at gmail dot com

For the expression "(l ^ a) != b", gcc -O0 emits no warning, but gcc -O1 emits
a warning "comparison of promoted ~unsigned with unsigned". After checking the
gimple code, I found the optimization transforms the expression to a bitwise
not expression. 

However, in a syntactic perspective, the warning message is confusing, given
the fact that there is no '~' in this expression. 

$: cat s.c
int fn1(unsigned char a, unsigned char b) {
  const unsigned l = 4294967295u;
  return (l ^ a) != b;
}
$: gcc-trunk -c -Wsign-compare s.c
$: gcc-trunk -c -Wsign-compare s.c -O1
s.c: In function ‘fn1’:
s.c:3:18: warning: comparison of promoted ~unsigned with unsigned
[-Wsign-compare]
   return (l ^ a) != b;
                  ^