[Bug c/61240] New: Incorrect warning "integer overflow in expression" on pointer-pointer subtraction

["Keith.S.Thompson at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61240

            Bug ID: 61240
           Summary: Incorrect warning "integer overflow in expression" on
                    pointer-pointer subtraction
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: Keith.S.Thompson at gmail dot com

Credit goes to "Lumbering Lummox", the author of this Stack Overflow post:
http://stackoverflow.com/q/23747641/827263

I see this problem with gcc versions 4.8.0 and 4.9.0, both compiled from
source, on Linux Mint 14 on x86_64.

Source program:

int main(void) {
    int i;
    int *p = &i;
    int *q = &i + 1;
    p - (p - 1);
    q - (q - 1);
}

Compiler output:

% /usr/local/apps/gcc-4.8.0/bin/gcc gcc-bug-integer-overflow.c
gcc-bug-integer-overflow.c: In function ‘main’:
gcc-bug-integer-overflow.c:5:7: warning: integer overflow in expression
[-Woverflow]
     p - (p - 1);
       ^
gcc-bug-integer-overflow.c:6:7: warning: integer overflow in expression
[-Woverflow]
     q - (q - 1);
       ^

A warning would be appropriate for "p - (p - 1)", since (p - 1) has undefined
behavior -- but since it's pointer arithmetic, not integer arithmetic, the
"integer overflow" warning is at least incorrectly worded. Furthermore, the
error message points to the first "-", which is not the problem.

As for "q - (q - 1)", no warning should be issued at all, since both "(q - 1)"
and "q - (q - 1)" are valid expressions with well defined behavior (yielding &i
and (ptrdiff_t)1, respectively).

This might be related to bug #48267.
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From: "ian at airs dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug go/61232] [4.10 Regression] link errors building libgo
Date: Mon, 19 May 2014 23:53:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?ida232

--- Comment #10 from Ian Lance Taylor <ian at airs dot com> ---
Yes, I think that would be clearer.

Your patch does seem to fix the problem building libgo.  Thanks.