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

[Tobias Hartmann]

Hi Vladimir,

thanks for the review.

On 22.07.2014 19:20, Vladimir Kozlov wrote:
> C->modified_nodes() is called only from debug code so you can simplify 
> it by putting whole method inside DEBUG_ONLY() instead of different 
> return:
>
> DEBUG_ONLY( Unique_Node_List* modified_nodes() const { return 
> _modified_nodes; } )

Done: http://cr.openjdk.java.net/~thartmann/8040213/webrev.05/

Thanks,
Tobias

>
> Otherwise look good.
>
> Thanks,
> Vladimir
>
> On 7/22/14 1:15 AM, Tobias Hartmann wrote:
>> Hi Vladimir,
>>
>> On 21.07.2014 19:13, Vladimir Kozlov wrote:
>>> > 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.
>>
>> Okay, that's true. I missed that there is a NOT_DEBUG_RETURN. Now 
>> using it.
>>
>> New webrev: http://cr.openjdk.java.net/~thartmann/8040213/webrev.04/
>>
>> Thanks,
>> Tobias
>>
>>>
>>> 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
>>