Request for review(M): 6627983: G1: Bad oop deference during marking

Vladimir Kozlov vladimir.kozlov at
Tue Mar 1 13:48:22 PST 2011



Igor Veresov wrote:
> On 3/1/11 12:23 PM, Vladimir Kozlov wrote:
>> I have only comments about library_call.cpp:
>> Why "false" in copy_to_clone()? It seems, you get double barriers in
>> this method as well:
>> 4085 /*dest_uninitialized*/false);
> It doesn't matter here, because we're treating it all as T_LONGs, so 
> there's going to be no barriers. But for consistency I suppose it could 
> be "true" here.
>> 4596 // so the the pre-barriers wouldn't peek into the old values. See
>> CR 6627983.
>> ^ 'the' 2 times ^ unitialized
> Fixed. Thanks!
>> 4597 const bool& dest_uninitialized = must_clear_dest;
>> ^ why reference?
> John wanted a separate variable to inform arraycopy of whether the 
> target is initialized. This variable should in fact follow the state of 
> the must_clear_dest, so I decided to make it a reference, so that when 
> must_clear_dest changes so does dest_uninitialized. If you don't like 
> that I can make it a regular variable.
> Although I would rather just used must_clear_dest for everything and not 
> replicate the state.
> Thanks,
> igor
>> The rest changes seems fine.
>> Thanks,
>> Vladimir
>> Igor Veresov wrote:
>>> John, all,
>>> I've updated the change, making corrections that John recommended.
>>> Also, now the decision about what to do with the barrier is deferred
>>> to the barrier emitting procedure. Thus, if a new pre-barrier is added
>>> that would need to do something even in case if the destination array
>>> is uninitialized it can be handled properly.
>>> New webrev:
>>> Thanks,
>>> igor
>>> On 2/25/11 8:25 PM, Igor Veresov wrote:
>>>> I think my fix is not good enough for the case when we'll need
>>>> prebarriers that will not require the previous value and these could be
>>>> a reality in the future. So, it is generally incorrect to elide
>>>> prebarriers but is only ok for a specific flavor. I'm working to
>>>> alleviate this problem a little bit and will post the updated version
>>>> early next week. John, I will also address your recommendations.
>>>> Thanks,
>>>> igor
>>>> On 2/24/11 6:28 PM, John Rose wrote:
>>>>> On Feb 24, 2011, at 6:02 PM, Igor Veresov wrote:
>>>>>> On 2/24/11 5:27 PM, John Rose wrote:
>>>>>>> On Feb 24, 2011, at 4:49 PM, Igor Veresov wrote:
>>>>>>> If it is simply foo(..., !must_clear_dest), it becomes hard for
>>>>>>> maintainers to see what the argument means.
>>>>>> I can see your point but I just didn't want to introduce additional
>>>>>> clutter. For example generate_block_arraycopy() never requires
>>>>>> barrier suppression, so why do we need to add an argument there?
>>>>> There's not a strong need. An extra "true" or "false" would be a
>>>>> little clutter, but it would also hint to programmers what's going on.
>>>>> It's a esthetic call mostly...
>>>>> (Here's some meandering on the subject. While reading the code, if I
>>>>> am tracing the flow from caller definition to callee definition, when
>>>>> arguments are explicit I can observe them directly. But if they are
>>>>> defaulted, then I have to visit the callee's declaration as well. I
>>>>> have to visit three locations in the source instead of two. I
>>>>> personally find this awkward unless the meaning of the defaulted
>>>>> argument is immediately and intuitively obvious. In this case the
>>>>> meaning of the argument is subtly related to other parts of the
>>>>> system.)
>>>>>> From the debugging standpoint it's way better to emit a barrier than
>>>>>> not to. Missing barriers will be a nightmare to find. So, I'd rather
>>>>>> have a barrier erroneously emitted in some path by default, which
>>>>>> would make the marking crash almost immediately.
>>>>> Good point.
>>>>>> As for using !must_clear_dest as a gate, I thought it would more
>>>>>> clearly convey to the reader the reason why the barrier is suppressed
>>>>>> - because the dest array is not cleared and contains garbage, so the
>>>>>> reader won't have to go back to see in which cases use_pre_barrier is
>>>>>> set. Perhaps I should add more comments.
>>>>> The bug number would help a lot, I think. It's a subtle interaction
>>>>> between (a) zeroing removal, (b) the arraycopy stubs, and (c) G1
>>>>> invariants.
>>>>>>> In the stub generator the optional boolean is OK as is, although I
>>>>>>> generally prefer optional booleans to default to 'false'. Given the
>>>>>>> way the bug is structured, I keep wanting to call it
>>>>>>> 'suppress_pre_barrier', meaning "just this once, trust me when I
>>>>>>> tell you not to emit the pre-barrier, if there is one."
>>>>>> That's pretty much equivalent. I don't have strong feelings about
>>>>>> this and will change it if you think it's more readable.
>>>>> I do think it would be more readable. It could be just an old Lisper
>>>>> bias, though: Optional arguments default to NULL, which is the false
>>>>> value in Lisp.
>>>>> -- John

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