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

Tue Oct 22 16:20:14 UTC 2019

Looks good

Maurizio

On 22/10/2019 16:27, Jan Lahoda wrote:
> Hi,
>
> A new draft of the pattern matching for instanceof has been published 
> here:
> https://mail.openjdk.java.net/pipermail/amber-dev/2019-October/004962.html 
>
>
> A significant change in this draft is that safe(*) reifiable types are 
> allowed in instanceof of. And updated version of the pattern matching 
> patch for javac which supports this change is here:
> http://cr.openjdk.java.net/~jlahoda/8231826/webrev.03/
>
> A diff from the previous revision:
> http://cr.openjdk.java.net/~jlahoda/8231826/webrev.delta.02-03/
>
> (*) reifiable types are allowed if (for "expr instanceof type"), 
> "expr" can be cast to "type" without unchecked warnings.
>
> Does this make sense?
>
> Thanks,
>     Jan
>
> On 15. 10. 19 15:02, Maurizio Cimadamore wrote:
>> Yep - that would be more helpful (at least to me)
>>
>> Thanks
>> Maurizio
>>
>> 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