[9] RFR(M): 8040213: C2 does not put all modified nodes on IGVN worklist

[Vladimir Kozlov]

 > Thanks for the hint, the optimized build did not work. I'm now using
 > PRODUCT_RETURN and NOT_PRODUCT to guard the verification code. This is
 > consistent with other verification methods like
 > Compile::verify_graph_edges(..).

The logic which verifies modified nodes in phaseX.cpp is under #ifdef 
ASSERT. Yes, you are not collecting data in optimized VM by returning 
NULL from modified_nodes(). But it is very confusing. I would suggest to 
move all this code to debug build only. You can use NOT_DEBUG_RETURN 
instead of PRODUCT_RETURN and put definitions under #ifdef ASSERT in 
compile.cpp file.

Thanks,
Vladimir

On 7/21/14 2:50 AM, Tobias Hartmann wrote:
> Vladimir, John, thanks for the review.
>
> On 18.07.2014 01:32, John Rose wrote:
>> On Jul 17, 2014, at 3:32 PM, Vladimir Kozlov
>> <vladimir.kozlov at oracle.com> wrote:
>>
>>> Initial prototype was written by me so part of changes are mine.
>>>
>>> On 7/17/14 3:40 AM, Tobias Hartmann wrote:
>>>> Hi John,
>>>>
>>>> thanks a lot for the detailed review! Please see comments inline.
>>>>
>>>> On 16.07.2014 01:10, John Rose wrote:
>>>>>>> Done. New webrev:
>>>>>>> http://cr.openjdk.java.net/~thartmann/8040213/webrev.01/
>>>>> Loading C->_modified_nodes with a stack address is bad news, even if
>>>>> the code apparently cleans it up on exit.  (I say "apparently" because
>>>>> there are early exits from that block, and the cleanup is not RAII
>>>>> style.)  I strongly suggest heap allocation instead of stack
>>>>> allocation; it's expensive but it's for debug code.
>>> The list array allocation is done in compiler arena which is cleaned
>>> after compilation finished by ResourceMark, only descriptor is on stack:
>>>
>>> +   Unique_Node_List modified_nodes(comp_arena());
>>>
>>> It is our normal style. We do it a lot, see Compile::Code_Gen().
>> Yes, we do it when adding phase control blocks to the Compile object,
>> within the tree of calls rooted at the constructor.  I see that the
>> new code is arguably part of that pattern.  I hope that is an
>> exceptional use of this style?
>>
>> Note that C2Compiler::compile_method creates the Compile object and
>> then queries it.  When it queries it, the Compile object may contain
>> dangling pointers into phase control blocks stored on the stack
>> (during subroutine calls like Code_Gen) but now deallocated.
>>
>> It seems to me that we avoid touching those dangling pointers by
>> simple will power and discipline.   Such a style doesn't scale, since
>> it's too easy to make mistakes as the code gets complicated.
>>
>>> Tobias, if you want to allocate Unique_Node_List object on arena:
>>>
>>> _modified_nodes = new (comp_arena()) Unique_Node_List(comp_arena());
>> That makes me feel much better.
>>
>> I would prefer for us to do it this way for the other stack-chunks
>> also, but that would be outside the scope of this bug.
>
> I changed it to heap allocation.
>
>> Tobias, thanks for doing this work.
>>
>> — John
>>
>>>> There are other places in the code where this is done, for example in
>>>> Compile::Compile(..) the Unique_Node_List 'for_igvn' is created on the
>>>> stack. Is this a special case?
>>>>
>>>> I'm not sure how to correctly allocate the object on the heap. Do I
>>>> have
>>>> to use new/delete or is this handled by some magic in "ResourceObject'?
>>>>
>>>>
>>>>> You inserted '#ifdef ASSERT' after
>>>>> 'PhaseIterGVN::verify_PhaseIterGVN', but I don't think you need it,
>>>>> since all of the code (including the similar insertion a few lines up)
>>>>> is '#ifndef PRODUCT'.  See also suggestion above regarding
>>>>> 'modified_nodes'.
>>> #ifndef PRODUCT is also 'optimized' version of VM. n->dump() method
>>> is not defined in optimized build. That is why asserts are needed.
>>>
>>> Tobias, please, build 'optimized' VM to verify your changes.
>
> Thanks for the hint, the optimized build did not work. I'm now using
> PRODUCT_RETURN and NOT_PRODUCT to guard the verification code. This is
> consistent with other verification methods like
> Compile::verify_graph_edges(..).
>
> New webrev: http://cr.openjdk.java.net/~thartmann/8040213/webrev.03/
>
> Thanks,
> Tobias
>
>>> Thanks,
>>> Vladimir
>>>
>>>> True, I removed the #ifdef.
>>>>
>>>>> There are a few places where tests like 'while
>>>>> (modified_list->size())' appears but there is no null check of
>>>>> 'modified_list' itself.  This could cause a crash if we ever get the
>>>>> phasing wrong; consider an assert or explicit null check 'while
>>>>> (modified_list != NULL && ...)'.
>>>> I added explicit null checks.
>>>>
>>>>
>>>> New webrev: http://cr.openjdk.java.net/~thartmann/8040213/webrev.02/
>>>>
>>>> Thanks,
>>>> Tobias
>>>>
>>>>> — John