[Bug sanitizer/106885] New: -(a-b) is folded to b-a before the UBSAN pass is run

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106885

            Bug ID: 106885
           Summary: -(a-b) is folded to b-a before the UBSAN pass is run
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kristerw at gcc dot gnu.org
                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: ---

GCC is folding -(a-b) to b-a before the UBSAN pass is run, which may hide
undefined behavior from the sanitizer.

This can be seen by the following program, which invokes undefined behavior
that is not detected by -fsanitize=undefined

int main(void)
{
  volatile int a = 0;
  volatile int b = 0x80000000;
  return -(a - b);
}