8060036: C2: CmpU nodes can end up with wrong type information
Andreas Eriksson
andreas.eriksson at oracle.com
Wed May 20 11:04:11 UTC 2015
Comments added, thanks.
Can I have a Reviewer look at this as well?
New webrev:
http://cr.openjdk.java.net/~aeriksso/8060036/webrev.01/
- 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
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