8065585: Change ShouldNotReachHere() to never return

[David Holmes]

Hi Stefan,

Stefan Karlsson wrote:
> Hi,
>
> Today the it's possible for the code to return out from
> ShouldNotReachHere() calls. This sometimes forces us to add return
> statements and assignments to parts of the code where it they don't make
> sense. By telling the compiler that ShouldNotReachHere is a dead end, we
> can get rid of these unnecessary constructs.
>
> For example:
>
> 1) We could get rid of return statements after ShouldNotReachHere():
>
> bool is_marked() {
>     // actual code here
>
>     // Execution path that "should not" happen.
>     ShouldNotReachHere();
>     return false;
> }
>
> 2) The following code will actually use an uninitialized value today.
> The compiler will find this if we turn on -Wuninitialized. But if we
> change ShouldNotReachHere() to not return,  the compiler will stop
> complaining because report(type) will never be called with an
> uninitialized value:
>
> int type;
> switch (value) {
>     case TYPE_OOP: type = 0; break;
>     case TYPE_KLASS: type = 1; break;
>     default: ShouldNotReachHere();
> }
> report(type)
>
>
> The patch changes the following functions and defines to never return:
> - fatal(...)
> - ShouldNotCallThis()
> - ShouldNotReachHere()
> - report_vm_out_of_memory(...)

You changed the behaviour of this one in the Debugging case - it no 
longer returns.

> - vm_exit_out_of_memory(...)
>
> but leaves the following unchanged:
> - Unimplemented()
> - Untested(...)
> - guarantee(...)
> We might want to change the the behavior of Unimplemented() and
> Untested(...), but they are used a lot in compiler code, so I've not
> changed them for this patch. There has been request to leave
> guarantee(...) unchanged so that they can be turned off in production code.
>
> I had to change some instance of ShouldNotReachHere() in destructors,
> because the VS C++ compiler complained about potential memory leaks.

The approach seems inconsistent though - sometimes a switch to a 
guarantee, sometimes removal of the destructor and making it private 
(which doesn't quite give the same level of protection).

> http://cr.openjdk.java.net/~stefank/8065585/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8065585

In summary:

The BREAKPOINTs have been moved out of the macros into the functions, 
and the macros then simplified to direct calls. Ok.

Some calls now have the NORETURN_ATTRIBUTE applied. Ok.

Calls to functions that have NORETURN_ATTRIBUTE can be cleaned up if 
they previously allowed for the function returning. Ok.

So first question: is this attribute handled correctly by all the 
compilers we need to consider?

Second, more important question: have you examined how this attribute 
affects the ability to walk the stack? We have already seen issues on 
some platforms where library functions, like abort(), have the noreturn 
attribute and as a result the call is optimized in a way that prevents 
the stack from being walked - see eg:

https://git.matricom.net/Firmware/bionic/commit/5f32207a3db0bea3ca1c7f4b2b563c11b895f276

though this:

https://www.raspberrypi.org/forums/viewtopic.php?t=60540&p=451729

suggests that problem may have been addressed by the libc folk. But it 
still raises the question as to how our own noreturn functions will be 
handled and how they will affect stacktrace generation in hs_err logs or 
via gdb.

Thanks,
David

>
> Thanks,
> StefanK
>