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

[Roland Westrelin]

> 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 can sponsor it if you need.

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
>