RFR: JDK-8160354: uninitialized value warning and VM crash are occurred with GCC 6

[Kim Barrett]

> On Jun 27, 2016, at 6:12 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
>> On Jun 27, 2016, at 10:29 AM, Yasumasa Suenaga <yasuenag at gmail.com> wrote:
>> 
>> Hi all,
>> 
>> This review request relates to JDK-8160310: HotSpot cannot be built with GCC 6 .
>> 
>> I encountered 2 compiler warnings and 2 VM crashes when I compiled OpenJDK 9 with
>> GCC 6 on Fedora 24 x64.
>> I think these error should be fixed.
>> 
>> I uploaded webrev.
>> Could you review it?
>> 
>> http://cr.openjdk.java.net/~ysuenaga/JDK-8160354/webrev.00/
> 

> src/cpu/x86/vm/assembler_x86.cpp
> 191   RelocationHolder rspec = (disp_reloc == relocInfo::none)
> 192                                   ? RelocationHolder::none
> 193                                   : rspec = Relocation::spec_simple(disp_reloc);
> 
> I have no idea what is being attempted by this change, but I really
> doubt this is correct. The precedence of ?: is higher than the
> precedence of =.
> 
> I think I see what might be going wrong with the original code.
> 
> RelocationHolder has a _relocbuf member, which is really just storage
> for a Relocation object.  The constructors for RelocationHolder are
> both problematic, but the no-arg constructor is the one at fault here.
> 
> RelocationHolder::RelocationHolder() {
>  new(*this) Relocation();
> }
> 
> This is constructing a different object over the current, which is
> undefined behavior, so gcc 6 is perhaps eliding it, leading to the
> failure.  What this should actually be doing is using the start of the
> _relocbuf member as the placement new location.

No, that analysis is wrong.

That isn't a placement new, it's new with another argument to the
allocator, specifically this holder.

I suspect the initialization of _relocbuf below might be related.

> src/share/vm/code/relocInfo.hpp
> 495   void* _relocbuf[ _relocbuf_size ] = {0};
> 
> I'm not sure why this might be needed, but I don't think this is valid
> C++98 code.  I think this is actually using a C++14 feature.