[Bug c/48228] New: incorrect signed overflow warning when only 0 and 1 are used

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48228

           Summary: incorrect signed overflow warning when only 0 and 1
                    are used
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: egg...@gnu.org


Created attachment 23747
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23747
"gcc -v -save-temps -S -Wstrict-overflow -O2 scroll.c" output

I ran into this problem when compiling the GNU Emacs trunk with a GCC
4.5.2 that I built on RHEL 5.5 (x86-64).  I narrowed it down to the
following stripped-down test case.

When I compile the following program with "gcc -S -Wstrict-overflow -O2"
GCC reports "warning: assuming signed overflow does not occur when
simplifying conditional to constant".  This warning is bogus, since
signed overflow is obviously impossible in this function: all the
types and values are unsigned, except for one variable that is only
assigned 0 and 1 to.


unsigned int
do_scrolling (unsigned int window_size, unsigned int writecost)
{
  unsigned int i = window_size;

  int terminal_window_p = 0;
  unsigned int queue = 0;

  for (i = window_size; i; i--)
    {
      if (writecost < i)
    ++queue;
      else if (writecost & 1)
    terminal_window_p = 1;
    }

  if (queue > 0)
    {
      if (!terminal_window_p)
    {
      terminal_window_p = 1;
    }
    }

  if (terminal_window_p)
    return 100;
  return 0;
}