8060036: C2: CmpU nodes can end up with wrong type information

[Andreas Eriksson]

On 2015-05-21 15:39, Roland Westrelin wrote:
>> http://cr.openjdk.java.net/~aeriksso/8060036/webrev.01/
> It looks good to me.
>
> 1607               if( p->bottom_type() != type(p) ) // If not already bottomed out
> 1608                 worklist.push(p); // Propagate change to user
>
>
> should be:
>
> if (p->bottom_type() != type(p)) { // If not already bottomed out
>   worklist.push(p); // Propagate change to user
> }
>
> to follow the hotspot style.
> https://wikis.oracle.com/display/HotSpotInternals/StyleGuide

I did it that way because all the other

if (...)
   worklist.push(p)

in that function omit the curly braces.
Should I have them in this change anyway?

>
> I can sponsor it if you need.

That would be very helpful, thanks.

- Andreas

>
> Roland.
>
>> - Andreas
>>
>> On 2015-05-19 20:24, Berg, Michael C wrote:
>>> Hi Andreas, could you add a comment to the push statement:
>>>
>>> // Propagate change to user
>>>
>>> And to Node* p declaration and init:
>>>
>>> // Propagate changes to uses
>>>
>>> So that the new code carries all similar information.
>>> Else the changes seem ok.
>>>
>>> -Michael
>>>
>>> -----Original Message-----
>>> From: hotspot-compiler-dev [mailto:hotspot-compiler-dev-bounces at openjdk.java.net] On Behalf Of Andreas Eriksson
>>> Sent: Tuesday, May 19, 2015 3:56 AM
>>> To: hotspot-compiler-dev at openjdk.java.net
>>> Subject: RFR: 8060036: C2: CmpU nodes can end up with wrong type information
>>>
>>> Hi,
>>>
>>> Could I please get two reviews for the following bug fix.
>>>
>>> https://bugs.openjdk.java.net/browse/JDK-8060036
>>> http://cr.openjdk.java.net/~aeriksso/8060036/webrev.00/
>>>
>>> The problem is with type propagation in C2.
>>>
>>> A CmpU can use type information from nodes two steps up in the graph, when input 1 is an AddI or SubI node.
>>> If the AddI/SubI overflows the CmpU node type computation will set up two type ranges based on the inputs to the AddI/SubI instead.
>>> These type ranges will then be compared to input 2 of the CmpU node to see if we can make any meaningful observations on the real type of the CmpU.
>>> This means that the type of the AddI/SubI can be bottom, but the CmpU can still use the type of the AddI/SubI inputs to get a more specific type itself.
>>>
>>> The problem with this is that the types of the AddI/SubI inputs can be updated at a later pass, but since the AddI/SubI is of type bottom the propagation pass will stop there.
>>> At that point the CmpU node is never made aware of the new types of the AddI/SubI inputs, and it is left with a type that is based on old type information.
>>>
>>> When this happens other optimizations using the type information can go very wrong; in this particular case a branch to a range_check trap that normally never happened was optimized to always be taken.
>>>
>>> This fix adds a check in the type propagation logic for CmpU nodes, which makes sure that the CmpU node will be added to the worklist so the type can be updated.
>>> I've not been able to create a regression test that is reliable enough to check in.
>>>
>>> Thanks,
>>> Andreas