RFR: JDK-8231826: Implement javac changes for pattern matching for instanceof

Maurizio Cimadamore maurizio.cimadamore at oracle.com
Tue Oct 15 13:02:19 UTC 2019

Yep - that would be more helpful (at least to me)


On 15/10/2019 13:56, Jan Lahoda wrote:
> Would this be better?
> Full patch:
> http://cr.openjdk.java.net/~jlahoda/8231826/webrev.02/
> Diff from previous:
> http://cr.openjdk.java.net/~jlahoda/8231826/webrev.delta.01-02/
> Thanks,
>     Jan
> On 15. 10. 19 13:59, Maurizio Cimadamore wrote:
>> Hi,
>> the flow changes look good - I think the TransPattern documentation 
>> should contain less text and more code snippet examples which show 
>> what the generated code looks like (as I've tried to do in my email, 
>> and as you've done for visitTypeTest). In other words, what is 
>> missing here is "the big picture" which shows what are the main ideas 
>> behind the translation strategy.
>> Specific example: decorateExpression:
>> + //if there are binding variables defined and used only in this 
>> expression,
>> + //which are not confined to a specific sub-expression,
>> + //a let expression is created which replaces the statement, and
>> + //the binding variables are hoisted into this let expression:
>> This kind of illustrates my point:
>> * "if there are binding variables defined and used only in this 
>> expression, which are not confined to a specific sub-expression" is 
>> very convoluted, and will be almost meaningless when we pick up this 
>> code again in 6 months
>> * "a let expression is created which replaces the statement" - 
>> statement? Probably cut and paste error
>> Thanks
>> Maurizio
>> On 15/10/2019 10:46, Jan Lahoda wrote:
>>> Hi,
>>> I've updated the patch with the Flow changes and with additional 
>>> comments in TransPatterns here:
>>> http://cr.openjdk.java.net/~jlahoda/8231826/webrev.01/
>>> diff from previous:
>>> http://cr.openjdk.java.net/~jlahoda/8231826/webrev.delta.00-01/
>>> An additional patch (that would apply on top of this one) which 
>>> makes all instanceof instances to be modelled as instanceof <pattern>:
>>> http://cr.openjdk.java.net/~jlahoda/8231826/webrev.01.unify.instanceof/
>>> Some more comment inline.
>>> On 10. 10. 19 17:33, Maurizio Cimadamore wrote:
>>>> Hi Jan,
>>>> the code looks generally very clean, kudos.
>>>> Some general comments:
>>>> * looking at the spec, it seems like both "instanceof T" and 
>>>> "instanceof T t" are cases of type test patterns. I guess I'm fine 
>>>> with the implementation doing what it always did in terms of plain 
>>>> "instanceof T", but I'm worried about the intersection between this 
>>>> and e.g. the tree API - InstanceofTree::getPattern returns null in 
>>>> cases like "instanceof T"; now, I know I know that we're speaking 
>>>> about a JDK specific API, but I think this issue reveals some 
>>>> modelling issues in the way we treat instanceof, and I'm worried 
>>>> that some of these issues might pop up in the future. I'd prefer to 
>>>> either rectify the spec so that plain 'instanceof T' is not a 
>>>> pattern matching instanceof, or rectify javac so that these tests 
>>>> are internally also represented with patterns (at the expense of 
>>>> some extra allocation, perhaps).
>>>> * If I'm not mistaken the only use for the "MATCH_BINDING_TO_OUTER" 
>>>> flag is to be able to distinguish between regular 'expression-wide' 
>>>> bindings, and bindings which 'leaked' outside a statement (e.g. an 
>>>> if statement). And the reason you need to distinguish between these 
>>>> is that you don't want Check::checkUnique to flag duplicate errors 
>>>> between regular 'expression-wide' bindings, which are reported 
>>>> elsewhere (MatchBindingsComputer). But this is also, more 
>>>> crucially, used in TransPattern, where the 'isPreserved' flag is 
>>>> used to control whether a variable decl for the binding variable 
>>>> should be 'lifted' to the enclosing statement context or not. Is my 
>>>> understanding correct here?
>>> Yes, the primary intent is to mark variables that need to be hoisted 
>>> to the parent of the current statement.
>>>> * The idea behind TransPatterns seems to be: when we process a 
>>>> statement, or an expression, we attempt to add all declaration for 
>>>> the bindings that are used inside the statements/expression 
>>>> upfront. If we are processing a statement, then we surround the 
>>>> results in a block; e.g.
>>>> if (obj instanceof Foo f) {
>>>>    ...
>>>> }
>>>> becomes
>>>> {
>>>>     Foo f$;
>>>>     if (let Object temp = obj in (obj instanceof Foo && (f$ = 
>>>> (Foo)temp) == temp) {
>>>>        ...
>>>> }
>>>> If we are processing an expression, we instead generate a LetExpr, 
>>>> e.g.
>>>> boolean b = obj instanceof Foo t && t.equals(foo);
>>>> becomes:
>>>> boolean b = let Foo f$ = null in ((let Object temp = obj in (obj 
>>>> instanceof Foo && (f$ = (Foo)temp) == temp) && f$.equals(foo))
>>>> So, sometimes the hoisted var is a real var decl in a block, other 
>>>> times is a var decl inside a let expression. In these cases we have 
>>>> to generate an initializer, to set the value (which might be used 
>>>> later). 
>>> The hoisted vars do not have an initializer (they used to have one, 
>>> but it is both unnecessary and was masking out bugs, so I have 
>>> removed it). But I see I've forgot the initializer code commented 
>>> out in TransPatterns, removed in the updated version to avoid 
>>> confusion.
>>>> On top of that, instanceof generates its own letExpr to cache the 
>>>> target of the test (to avoid double computation).
>>>> It also seems to me that the code handles cases where the binding 
>>>> variable is not used, neither hoisted - e.g.
>>>> boolean field = obj instanceof Foo t;
>>>> In this case we generate a plain instanceof w/o the init (because 
>>>> the 't' variable hasn't been hoisted anywhere).
>>>> Finally, we use the 'isPreserved()' flag to ensure that variables 
>>>> are hoisted correctly - for instance, if something is to be 
>>>> preserved (and the enclosing context allows for it) we push things 
>>>> in the enclosing context instead.
>>>> Am I getting the correct picture here?
>>> Yes, I think it is correct.
>>>> It would be nice to have more javadoc spread around to help the 
>>>> reader understand what's the rationale and show some snippet of 
>>>> generated code.
>>>> * Flow, I wonder if, like you had created SnippetAliveAnalyzer, 
>>>> creating a SnippetBreakAnalyzer would help you avoid the 
>>>> breaksOut[0] trick (that could become a field in the child visitor)
>>> I tried to do these two in the updated patch.
>>> Thanks for the comments!
>>> Jan
>>>> Other than that, it looks very good.
>>>> Maurizio
>>>> On 10/10/2019 13:12, Jan Lahoda wrote:
>>>>> Hi,
>>>>> As part of the effort to prepare JEP 305: Pattern Matching for 
>>>>> instanceof (Preview) for Propose to Target, I would like to ask 
>>>>> for a code review for the corresponding javac changes.
>>>>> The webrev is here:
>>>>> http://cr.openjdk.java.net/~jlahoda/8231826/webrev.00/
>>>>> The patch applies on top of:
>>>>> https://mail.openjdk.java.net/pipermail/compiler-dev/2019-October/013727.html 
>>>>> The current spec the patch is striving to implements is here:
>>>>> http://cr.openjdk.java.net/~gbierman/jep305/jep305-20190918/specs/patterns-instanceof-jls.html 
>>>>> As far as I know, there is one (significant) open issue in the 
>>>>> spec, and that is whether non-reifiable types should be allowed in 
>>>>> "instanceof <type-test-pattern>". Currently (AFAIK), the spec does 
>>>>> not allow non-reifiable types in the type test pattern in 
>>>>> instanceof, and the javac implementation should be consistent with 
>>>>> the spec. Should the spec change, the corresponding update to the 
>>>>> javac code should have a very limited impact.
>>>>> I'll be preparing a CSR for this change in the coming days.
>>>>> The JBS issue for this code change is:
>>>>> https://bugs.openjdk.java.net/browse/JDK-8231826
>>>>> Any feedback is welcome!
>>>>> Thanks!
>>>>>     Jan

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