[Bug c/61240] [4.8/4.9/4.10 Regression] Incorrect warning "integer overflow in expression" on pointer-pointer subtraction

["rguenth at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-05-20
   Target Milestone|---                         |4.8.3
            Summary|Incorrect warning "integer  |[4.8/4.9/4.10 Regression]
                   |overflow in expression" on  |Incorrect warning "integer
                   |pointer-pointer subtraction |overflow in expression" on
                   |                            |pointer-pointer subtraction
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We warn for

 <integer_cst 0x7ffff6d54e28 type <integer_type 0x7ffff6c407e0 long int>
constant public overflow 1>

via c-common.c:overflow_warning called from

#1  0x000000000065b80e in parser_build_binary_op (location=5653, 
    code=MINUS_EXPR, arg1=..., arg2=...)
    at /space/rguenther/src/svn/trunk/gcc/c/c-typeck.c:3411
#2  0x000000000068f5ed in c_parser_binary_expression (parser=0x7ffff6d67000, 
    after=0x0, omp_atomic_lhs=<tree 0x0>)
    at /space/rguenther/src/svn/trunk/gcc/c/c-parser.c:6282
#3  0x000000000068dfee in c_parser_conditional_expression (
    parser=0x7ffff6d67000, after=0x0, omp_atomic_lhs=<tree 0x0>)
    at /space/rguenther/src/svn/trunk/gcc/c/c-parser.c:5934
#4  0x000000000068dd75 in c_parser_expr_no_commas (parser=0x7ffff6d67000, 
    after=0x0, omp_atomic_lhs=<tree 0x0>)
    at /space/rguenther/src/svn/trunk/gcc/c/c-parser.c:5852

when building p - (p + -1U) which gets simplified to - -1U -> 1U (with overflow
set - as it's sizetype arithmetic).  pointer_diff is guilty here which calls

  /* First do the subtraction as integers;
     then drop through to build the divide operator.
     Do not do default conversions on the minus operator
     in case restype is a short type.  */

  op0 = build_binary_op (loc,
                         MINUS_EXPR, convert (inttype, op0),
                         convert (inttype, op1), 0);

doing 0 - -1U, converting them to inttype (long int) first.  I suggest
to do that conversion and strip overflow bits in the POINTER_PLUS_EXPR
decomposition part.