[Bug c/88301] New: Optimization regression with undefined unsigned overflow

[pskocik at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88301

            Bug ID: 88301
           Summary: Optimization regression with undefined unsigned
                    overflow
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pskocik at gmail dot com
  Target Milestone: ---

I noticed gcc 7.* did a really nice optimization that allowed me to communicate
I want even some unsigned overflows to be
undefined:

        #define ADD_NW(A,B) (__extension__({ __typeof(A+B) R;
if(__builtin_add_overflow(A,B,&R)) __builtin_unreachable(); R ;}))
        _Bool a_b(unsigned A,  unsigned B) { return A+B >= B; }
        _Bool a_b2(unsigned A,  unsigned B) { return ADD_NW(A,B) >= B; }

resulted in:

        a_b:
                        add     edi, esi
                        setnc   al
                        ret
        a_b2:
                        mov     eax, 1
                        ret

But on gcc 8.* it's

        a_b:
                add     edi, esi
                setnc   al
                ret
        a_b2:
                add     edi, esi
                setnc   al
                ret

again.