[Bug middle-end/48432] New: -Wstrict-overflow incorrectly warns for Emacs src/font.c

[eggert at gnu dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48432

           Summary: -Wstrict-overflow incorrectly warns for Emacs
                    src/font.c
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: middle-end
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: egg...@gnu.org


When compiling the GNU Emacs trunk with a GCC 4.6.0 that I built on
RHEL 5.6 (x86-64), I ran into a problem when compiling src/font.c that
I narrowed down to the following stripped-down test case.  When I
compile this test case with

  gcc -S -Wstrict-overflow -O2 f.c

the output is:

  f.c: In function 'font_list_entities':
  f.c:38:35: warning: assuming signed overflow does not occur when simplifying
co
nditional to constant [-Wstrict-overflow]

The warning is incorrect, since signed overflow is impossible here.
There are two 'int' variables, one of which is assigned only 0 or 1,
and the other which is assigned only values ranging from 6 through 12.
Changing either 'int' variable to 'unsigned' makes the warning go away.

I'm not sure whether this bug is the same as earlier bug reports I
filed in this area (PR48228, PR48267), but it's not obviously the same
so I figured I should be safe and file it.  It did take some time to
track down.

-----

extern unsigned foo0 (void);
extern unsigned foo2 (unsigned, unsigned);

static unsigned
font_delete_unmatched (unsigned vec, unsigned spec, unsigned int size)
{
  unsigned val = 0;

  while (1)
    {
      if (spec)
    {
      val = foo2 (vec, val);
      continue;
    }
    }
  return val;
}

unsigned
font_list_entities (unsigned frame, unsigned spec)
{
  unsigned val = 0;
  int need_filtering = 0;
  int i;

  for (i = 6; i < 13; i++)
    {
      if (foo0 ())
    need_filtering = 1;
      if (i == 10)
    i++;
    }

  while (foo0 ())
    if (need_filtering)
      val = font_delete_unmatched (val, need_filtering ? spec : 0, 0);
  return 0;
}