[Bug c/81876] New: [7 Regression] bogus -Wstrict-overflow warning with -O3

["bunk at stusta dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81876

            Bug ID: 81876
           Summary: [7 Regression] bogus -Wstrict-overflow warning with
                    -O3
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bunk at stusta dot de
  Target Milestone: ---

Testcase based on an xbubble build error on Debian:

$ cat test.c
struct _Bubble {
  int color;
};
typedef struct _Bubble * Bubble;

typedef enum {
  EAST=0,
  NORTH_EAST,
  NORTH_WEST,
  WEST,
  SOUTH_WEST,
  SOUTH_EAST
} Quadrant;

struct _CellArray {
  Bubble cell[(8 * 12)];
  int first_row;
};
typedef struct _CellArray * CellArray;

void cell_array_lower( CellArray ca ) {
  int i;

  for ( i = ( ca->first_row*8 ); i < ( ca->first_row*8 ) + 8; i++ )
    ca->cell[i] = ((void *)0);
}
$ gcc-6 -O2 -Wall -Werror -c test.c
$ gcc-6 -O3 -Wall -Werror -c test.c
$ gcc -O2 -Wall -Werror -c test.c
$ gcc -O3 -Wall -Werror -c test.c
test.c: In function ‘cell_array_lower’:
test.c:25:17: error: assuming signed overflow does not occur when assuming that
(X + c) >= X is always true [-Werror=strict-overflow]
     ca->cell[i] = ((void *)0);
     ~~~~~~~~~~~~^~~~~~~~~~~~~
cc1: all warnings being treated as errors
$


It works when replacing the "+ 8" in the loop condition with a variable whose
value is not known at compile time.