[Bug c/82802] New: Potential UBSAN error with pointer difference (32-bits mode)

["yann.collet.73 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82802

            Bug ID: 82802
           Summary: Potential UBSAN error with pointer difference (32-bits
                    mode)
           Product: gcc
           Version: 7.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yann.collet.73 at gmail dot com
  Target Milestone: ---

As part of our CI test suite,
we compile and run fuzzer tests every day.

The UBSAN test has been failing for some time now.
I suspect it's related to our provider having updated at some point the gcc
version.


The failure happens in this situation :
presuming we have 2 pointers : highPtr > lowPtr,
if I request the distance in 32-bits mode,
both pointers being associated to the same object 
(one is the upper limit, another is a cursor into the object)

`highPtr - lowPtr` generates this UBSAN error : 
runtime error: signed integer overflow: -2147452928 - 1879078921 cannot be
represented in type 'int'

The values of these pointers are :
highPtr : 0x80007800
lowPtr  : 0x70018AAB

As can be seen, there is no overflow : highPtr is > lowPtr, and the distance is
~256 MB, well within the limits of ptrdiff_t in 32-bits.
Nonetheless, UBSAN consider it an error, likely because it crosses the
0x80000000 threshold.
I suspect the pointer addresses are converted into `int` type *before* the
substraction, which leads to UBSAN conclusion.


The same code on clang doesn't trigger any error.