RFR: 8297264: C2: Cast node is not processed again in CCP and keeps a wrong too narrow type which is later replaced by top

[Roberto Castañeda Lozano]

On Thu, 1 Dec 2022 09:37:57 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

> ![image](https://user-images.githubusercontent.com/17833009/205015579-1b6ac082-e992-4828-80f7-ff991964179b.png)
> 
> During CCP, we optimize the type of `348 CastII` in `CastIINode::Value()`: It matches the `CmpI/If` pattern because the current type of `119 Phi` is a constant int:
> https://github.com/openjdk/jdk/blob/9f24a6f43c6a5e1fa92275e0a87af4f1f0603ba3/src/hotspot/share/opto/castnode.cpp#L213-L215
> 
> Later in CCP, the type of `119 Phi` is updated and is no longer a constant but `348 CastII` is not processed anymore during CCP and keeps its wrong too narrow type. We apply more loop opts and at some point, the `CastII` is replaced by top because the input type is outside of the wrong type range of the `CastII`. Some data nodes are folded and the graph is left in a broken state and we assert during GCM.
> 
> I propose to add a `CastII` node back to the CCP worklist if we find such a `Cmp/If` pattern to ensure that the `CastII` type is correctly set during CCP.
> 
> Thanks,
> Christian

Looks good! Just a couple of questions/suggestions.

src/hotspot/share/opto/phaseX.cpp line 1967:

> 1965:         push_if_not_bottom_type(worklist, cast_ii);
> 1966:       }
> 1967:     }

Would it make sense to assume there is at most one `CastII` output and replace the loop with a call to the auxiliary function `Node* Node::find_out_with(int opcode)`?

test/hotspot/jtreg/compiler/c2/TestCastIIWrongTypeCCP.java line 1:

> 1: /*

This file might fit better under `test/hotspot/jtreg/compiler/ccp`.

test/hotspot/jtreg/compiler/c2/TestCastIIWrongTypeCCP.java line 27:

> 25:  * @test
> 26:  * @bug 8297264
> 27:  * @summary Test that CastII nodes are added to the CCP worklist if they could or could have been

Suggestion:

 * @summary Test that CastII nodes are added to the CCP worklist if they could have been

-------------

Marked as reviewed by rcastanedalo (Reviewer).

PR: https://git.openjdk.org/jdk/pull/11448