Java frontend: add support for lambdas that require generating boxing, unboxing, widening casts and upcasts #5359
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Turns out the usual lambda metafactory can apply boxing/unboxing, so you might see a situation like
int myMethod(int i) { ... }
interface MyLambdaType { Integer f(Integer i); }
MyLambdaType mlt = SomeType::myMethod;
And contrary to my prior expectations, the compiler doesn't generate a method stub to do the type conversion,
it just directly requests an invokestatic lambda pointed at myMethod, regardless of the int vs. Integer clash.
The lambda metafactory (which re-implement in JBMC) is then responsible for adding the necessary conversions.
This broadens our support for lambdas with implicit boxing/unboxing conversions to include those that use a method-reference to a method with type Object -> x to satisfy a functional interface of type primitive -> x (requiring boxing the parameter and then upcasting to Object), or a method with type x -> primitive satisfying an interface of type x -> Object (requiring the same conversion on the return value).
…type widening For example, an int -> int method is a valid instantiation of a functional interface that expects a byte -> int, or one that expects an int -> long. In both cases it's down to the metafactory / our code that emulates its behaviour to add the widening cast.
smowton
requested review from
cesaro,
cristina-david,
peterschrammel,
pkesseli and
thk123
as code owners
…d boxing/unboxing
…from a symbol-table id These users start with an irep_idt, so it's cheaper to look up by id than to create a derived string and then look up by that.
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…to an interface method returning void
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Codecov Report
@@ Coverage Diff @@
## develop #5359 +/- ##
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+ Coverage 68.17% 68.19% +0.01%
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Files 1170 1171 +1
Lines 96540 96590 +50
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+ Hits 65819 65866 +47
- Misses 30721 30724 +3
Continue to review full report at Codecov.
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thk123
approved these changes
May 28, 2020
| } | ||
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| const java_primitive_type_infot * | ||
| get_java_primitive_type_info(const typet &maybe_primitive_type) |
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⛏️ optionalt rather than a pointer
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| if(original_is_pointer == required_is_pointer) | ||
| { | ||
| if(original_is_pointer && original_type != required_type) | ||
| return typecast_exprt{expr, required_type}; | ||
| else | ||
| return expr; | ||
| } | ||
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I think the answer is byte to int and Derived -> Base?
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Gains a comment in a later commit (it's for pointer or integer type clashes without boxing, where plain casting always does the trick (forcing pointers and widening integers, so long as only valid Java casts are fed in in the first place)
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The Java lambda metafactory, per https://docs.oracle.com/javase/8/docs/api/java/lang/invoke/LambdaMetafactory.html, can require adapting the type of the referenced method to that of the target functional interface by introducing boxing, unboxing, primitive type widening and pointer upcasting conversions. This adds tests for a wide variety of such cases and implements the required suppport.