[Bug tree-optimization/49873] New: Optimizer regression

[Bug tree-optimization/49873] New: Optimizer regression

[abenea at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49873

           Summary: Optimizer regression
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: abenea@gmail.com


Test code:

typedef struct {
    char *ptr;
} S;

void f1(char* data) {
    S s;
    s.ptr = 0;
    char *same_data = s.ptr + (data - s.ptr);
    same_data[0] = 1;
}

int main() {
    char c;
    f1(&c);
    if (c != 1)
        return 1;
    return 0;
}

Compiled with gcc version 4.4.6, the program above returns 0 regardless of the
optimization level.

Compiled with gcc versions 4.5.3 or 4.6.1, the return value is 0 only with -O0.
Using -O1, -O2, -O3 or -Os, the return value is 1.

The correct value (0) is returned if s.ptr is replaced with a local variable or
the code from the function f1 is moved inside main.

[Bug tree-optimization/49873] Optimizer regression

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49873

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-07-27 17:00:06 UTC ---
I think this code is undefined because you are subtracting two "arrays" which
produces an undefined result in C.

[Bug tree-optimization/49873] Optimizer regression

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49873

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-07-28 09:55:58 UTC ---
    char *same_data = s.ptr + (data - s.ptr);
    same_data[0] = 1;

same_data now points to s.ptr plus some offset that isn't computed in any
standard compliant way.  As s.ptr is NULL the store is dead.

With

  char *same_data = (char *)((uintptr_t) s.ptr + (uintptr_t) data - (uintptr_t)
s.ptr));

you would get the correct answer, avoiding all the undefinedness in your code.