RFR: 8065585: Change ShouldNotReachHere() to never return

[Stefan Karlsson]

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(...)
- 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.

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

Thanks,
StefanK