[Bug c++/90089] New: Missing optimization, elimination of empty data dependant loops

[gareth@ignition-web.co.uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90089

            Bug ID: 90089
           Summary: Missing optimization, elimination of empty data
                    dependant loops
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gareth@ignition-web.co.uk
  Target Milestone: ---

void * x;
void test(){ for(void * p = x; p; p=*(void**)p); }

With -O3 gives:

test():
        mov     rax, QWORD PTR x[rip]
        test    rax, rax
        je      .L1
.L3:
        mov     rax, QWORD PTR [rax]
        test    rax, rax
        jne     .L3
.L1:
        ret

As you can see it currently traverses pointers even though it could be
optimized away. 

6.8.2.2 Forward progress
The implementation may assume that any thread will eventually do one of the
following:

    (1.1) terminate,
    (1.2) make a call to a library I/O function,
    (1.3) perform an access through a volatile glvalue, or
    (1.4) perform a synchronization operation or an atomic operation.

[ Note: This is intended to allow compiler transformations such as removal of
empty loops, even when termination cannot be proven. — end note ]

MSVC and ICC performs this optimization.