RFR [XS] 8041658: Use of -fdevirtualize on macroAssembler_x86.o (via -O2) with gcc 4.9.0 creates broken VM

[Andrew Haley]

On 04/25/2014 01:10 AM, John Rose wrote:
> On Apr 24, 2014, at 9:41 AM, Andrew Haley <aph at redhat.com> wrote:
> 
>> On 04/24/2014 05:21 PM, Volker Simonis wrote:
>>> And I'm not quite sure how to fix this in HotSpot.
>>>
>>> I first thought I could solve this with an anonymous union like:
>>>
>>> union {
>>>  void* _relocbuf[ _relocbuf_size ];
>>>  Relocation _reloc;
>>> }
>>> Relocation* reloc() const { return &_reloc; }
>>>
>>> but unfortunately I can't put a Relocation into a union because it is
>>> not a POD (at least not with C++98).
>>>
>>> Any other ideas how we could fool GCC 4.9?
>>>
>>> I more and more think this should be fixed in GCC because I can
>>> imagine this will also break other code.
>>
>> Only code that is not legal C++, and GCC developers have historically
>> been very reluctant to support that.  So, it might not happen.
>>
>> AFAICS there is no C++98 way to embed a non-POD object in an object in this
>> way.  It could be done via a pointer, but that wastes some space.
> 
> That design (RelocationHolder, bitwise non-PODS copy) was committed
> 1998-02-27.  (Full discl.—courtesy of yours truly.)

:-)

> Sorry it got in your way.  We've seen occasional problems with it
> before at high optimization levels.  And I agree that current C++
> standards make this doubtful usage.

I don't think that this transformation in GCC does anything useful.  I
have proposed a patch to GCC:

http://gcc.gnu.org/ml/gcc/2014-04/msg00236.html

Andrew.