RFR: 8317376: Minor improvements to the 'this' escape analyzer [v4]

[Vicente Romero]

On Tue, 9 Apr 2024 15:36:27 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 with a new target base due to a merge or a rebase. The pull request now contains eight commits:
> 
>  - Merge branch 'master' into JDK-8317376
>  - Merge branch 'master' into JDK-8317376
>  - Merge branch 'master' into JDK-8317376
>  - Merge branch 'master' into JDK-8317376
>  - Merge branch 'master' into JDK-8317376
>  - Javadoc++
>  - Merge branch 'master' into JDK-8317376
>  - Several improvements to the 'this' escape analyzer.
>    
>    - Track direct, indirect, and outer references for all Ref types.
>    - Keep type information about all references to improve tracking precision.
>    - Track enhanced for() invocations of iterator(), hasNext(), and next().
>    - Don't report an escape of a non-public outer instances as a leak.
>    - Fix omitted tracking of references from newly instantiated instances.
>    - Fix omitted tracking of leaks via lambda return values.
>    - Remove unneccesary suppressions of this-escape lint warning.

src/jdk.compiler/share/classes/com/sun/tools/javac/comp/ThisEscapeAnalyzer.java line 1343:

> 1341:      *    but also NOT an enclosing outer class of 'currentClass'.
> 1342:      */
> 1343:     private boolean isExplicitThisReference(Types types, Type.ClassType currentClass, JCFieldAccess select) {

suggestion: why not moving this method to TreeInfo already in preparation for the changes coming with flexible constructor bodies?

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PR Review Comment: https://git.openjdk.org/jdk/pull/16208#discussion_r1567757547