RFR: JDK-8211073 Remove -Wno-extra from Hotspot

[Kim Barrett]

> On Oct 22, 2019, at 3:17 AM, Magnus Ihse Bursie <magnus.ihse.bursie at oracle.com> wrote:
> 
> I have tested that this compiles without warnings on gcc 4.8, 5.5, 6.5, 7.4 and 8.3 on x64. I have also tried building zero on x64, aarch64 and arm32 with gcc 8.3.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8211073
> WebRev: http://cr.openjdk.java.net/~ihse/JDK-8211073-enable-extra-on-hotspot/webrev.01

Change looks good.

I remain somewhat leary of omnibus warning options in a code base like
this, which is large and needs to support multiple compiler versions
on multiple platforms. But at least most can be disabled if needed.

Note that gcc9 adds:
-Wdeprecated-copy
  Warn that the implicit declaration of a copy constructor or copy
  assignment operator is deprecated if the class has a user-provided
  copy constructor or copy assignment operator, in C++11 and up. This
  warning is enabled by -Wextra. With -Wdeprecated-copy-dtor, also
  deprecate if the class has a user-provided destructor.

-Wredundant-move 
  This warning warns about redundant calls to std::move; that is, when a
  move operation would have been performed even without the std::move
  call.

Neither of these are an issue for us yet, because we're not yet using
C++11 or later.  But I expect -Wdeprecated-copy to cause problems for
that update, and we might need to (temporarily) globally disable it as
part of JEP 347.

It's annoying that some of its warnings have no associated -Wno-xxx,
in particular the one about mixing integral types and enum types in
conditional expressions, because of the history of our code base and
because it's perfectly well-formed usage. But apparently we don't have
any of those right now.

> On Oct 22, 2019, at 4:28 AM, Magnus Ihse Bursie <magnus.ihse.bursie at oracle.com> wrote:
> I wouldn't count on gcc 9 bringing in anything new to -Wextra. In general, gcc is *extremely* conservative about changing -Wextra nowadays. Virtually all new warnings that are added are added as indivudual warnings with explicit names to be turned on or turned off. Only after a long vetting process does any of these gets added to -Wextra. I think the last time anything was added to -Wextra was in like gcc 6..? So the -Wextra is in a way a bit of a legacy system in gcc for handling warnings.

This depends on what you mean by additions or changes to -Wextra. As
mentioned above, gcc9 adds two new warnings, but they can be
individually controlled. The set of warnings only provided by -Wextra
and not individually controlled has indeed not changed in a long time.
I checked, and gcc4.9 and gcc9.2 have the same set of those.