[Bug sanitizer/88479] New: sanitizer should provide an option to detect conversion to signed integer that overflows

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88479

            Bug ID: 88479
           Summary: sanitizer should provide an option to detect
                    conversion to signed integer that overflows
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org, marxin at gcc dot gnu.org
  Target Milestone: ---

The sanitizer should provide an option to detect the case where a conversion of
an integer to a signed integer overflows, whose behavior is
implementation-defined (thus not caught by -fsanitize=undefined) according to
ISO C 6.3.1.3p3.

More than for being an implementation-defined behavior, the reason is that such
an overflow may actually be a bug in the program (unnoticed yet, and/or
possibly noticeable only on some other platforms where the type sizes would be
different). Such an option would ease bug detection and debugging.

Once implemented, the -fsanitize=signed-integer-overflow description in the GCC
manual should be updated to mention this new option. It currently says:

    Note, integer promotion rules must be taken into account.  That is,
    the following is not an overflow:
         signed char a = SCHAR_MAX;
         a++;

as this just yields an overflow when the int SCHAR_MAX+1 is converted back to
signed char (thus this would be detected by this new feature).

Note: the addition of a conversion sanitizer has been suggested here:
  https://gcc.gnu.org/ml/gcc-help/2018-11/msg00078.html