RFR: JDK-8203157: Object equals abstraction for BarrierSetAssembler

Andrew Haley aph at redhat.com
Tue Jun 12 09:11:01 UTC 2018

On 06/11/2018 08:17 PM, Roman Kennke wrote:
> Am 11.06.2018 um 19:11 schrieb Andrew Haley:
>> On 06/11/2018 04:56 PM, Andrew Haley wrote:
>>> On 06/08/2018 09:17 PM, Roman Kennke wrote:
>>>> Why is it better? And how would I do that? It sounds like a fairly
>>>> complex undertaking for a special case. Notice that if the oop doesn't
>>>> qualify as immediate operand (quite likely for an oop?) it used to be
>>>> moved into rscratch1 anyway a few lines below.
>>> Sorry for the slow reply.  I'm looking now.
>> OK.  The problem is that this is a very bad code smell:
>>       case T_ARRAY:
>>         jobject2reg(opr2->as_constant_ptr()->as_jobject(), rscratch1);
>>         __ cmpoop(reg1, rscratch1);
>> I can't tell that this is correct.  rscratch1 is used by assembler
>> macros, and I don't know if some other GC (e.g. ZGC) might need to use
>> rscratch1 inside cmpoop.  The risk here is obvious.  The Right Thing
>> to do IMO is to generate a scratch register for pointer comparisons.
>> Unless, I guess, we know that cmpoop never ever needs a scratch
>> register for any forseeable garbage collector.
> I do know that Shenandoah does not require a tmp reg. I also do know
> that no other collector currently needs equals-barriers at all.

So cmpoop() is literally useless.  It does nothing except add a layer
of obfuscation in the name of some possible future collector.

> I cannot see into the future. I prefer to be pragmatic and solve
> existing problems.

Perhaps, but this change you're asking me to review doesn't solve a
problem, it creates one.  This is how technical debt happens.

> How about I add a comment to the obj_equals() API that says 'don't
> use tmp reg X, and if you really need to, push/pop it or let the
> compiler generate one for you' ?

It's awkward, isn't it?  I know that this is the wrong way to solve
the problem, but I'm as vulnerable to social pressure as anyone else.
Perhaps I should give up and choose an easy life.  :-)

Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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