RFR: 8317376: Minor improvements to the 'this' escape analyzer [v5]

[Vicente Romero]

On Tue, 16 Apr 2024 19:15:14 GMT, Archie Cobbs <acobbs at openjdk.org> wrote:

>> Please review several fixes and improvements to the `this-escape` lint warning analyzer.
>> 
>> The goal here is to apply some relatively simple logical fixes that improve the precision and accuracy of the analyzer, and capture the remaining low-hanging fruit so we can consider the analyzer relatively complete with respect to what's feasible with its current design.
>> 
>> Although the changes are small from a logical point of view, they generate a fairly large patch due to impact of refactoring (sorry!). Most of the patch derives from the first two changes listed below.
>> 
>> The changes are summarized here:
>> 
>> #### 1. Generalize how we categorize references
>> 
>> The `Ref` class hierarchy models the various ways in which, at any point during the execution of a constructor or some other method/constructor that it invokes, there can be live references to the original object under construction lying around. We then look for places where one of these `Ref`'s might be passed to a subclass method. In other words, the analyzer keeps track of these references and watches what happens to them as the code executes so it can catch them trying to "escape".
>> 
>> Previously the `Ref` categories were:
>> * `ThisRef` - The current instance of the (non-static) method or constructor being analyzed
>> * `OuterRef` - The current outer instance of the (non-static) method or constructor being analyzed
>> * `VarRef` - A local variable or method parameter currently in scope
>> * `ExprRef` - An object reference sitting on top of the Java execution stack
>> * `YieldRef` - The current switch expression's yield value(s)
>> * `ReturnRef` - The current method's return value(s)
>> 
>> For each of those types, we further classified the "indirection" of the reference, i.e., whether the reference was direct (from the thing itself) or indirect (from something the thing referenced).
>> 
>> The problem with that hierarchy is that we could only track outer instance references that happened to be associated with the current instance. So we might know that `this` had an outer instance reference, but if we said `var x = this` we wouldn't know that `x` had an outer instance reference.
>> 
>> In other words, we should be treating "via an outer instance" as just another flavor of indirection along with "direct" and "indirect".
>> 
>> As a result, with this patch the `OuterRef` class goes away and a new `Indirection` enum has been created, with values `DIRECT`, `INDIRECT`, and `OUTER`.
>> 
>> #### 2. Track the types...
>
> Archie Cobbs has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Synchronize TreeInfo.isExplicitThisReference() with PR #18088.

looks sensible to me

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Marked as reviewed by vromero (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/16208#pullrequestreview-2006363434