Trust final fields in records

[Mandy Chung]

I really like to see "final fields truly final" at least start with the 
new features such as inline classes and records.

Final fields of hidden classes have no write access [1] regardless of 
the accessible flag.  I'd propose to make final fields of records and 
inline classes non-modifiable in a similar fashion as hidden classes.

Mandy

[1] 
https://download.java.net/java/early_access/jdk15/docs/api/java.base/java/lang/reflect/Field.html#set(java.lang.Object,java.lang.Object)

On 6/11/20 1:38 PM, Christoph Dreis wrote:
> Hi,
>
> I’ve played around with records the other day and wondered if their (final) fields could be maybe trusted.
> This would allow further optimizations to kick in.
>
> E.g. with the following benchmark:
>
> @BenchmarkMode(Mode.AverageTime)
> @OutputTimeUnit(TimeUnit.NANOSECONDS)
> @State(Scope.Benchmark)
> public class MyBenchmark {
> 	static final Rectangle rectangle;
> 	static {
> 		rectangle = new Rectangle(1, 1);
> 	}
>
> 	record Rectangle(int length, int width) {
> 		public int size() {
> 			return length * width;
> 		}
> 	}
>
> 	@Benchmark public int testSize() { return 1000 / rectangle.size(); }
> }
>
> I see the following results when I apply the attached patch:
>
> Benchmark                                        Mode  Cnt   Score    Error   Units
> MyBenchmark.testSizeBefore       avgt   10   3,873 ±  0,044   ns/op
> MyBenchmark.testSizePatched     avgt   10   1,740 ±  0,058   ns/op
>
> After all, records state that they are "shallowly immutable" - whatever " shallowly" means here.
> The risk that I see here is that people could still use reflection on records to change fields - for reasons.
> Maybe that aspect could be tightened though before records go non-experimental in favor of the optimization?
>
> I wonder if this could be considered. If so, I would highly appreciate it if someone can sponsor the patch.
>
> Let me know what you think.
>
> Cheers,
> Christoph
>
> ===== PATCH =====
> diff -r 984fde9a0b7f src/hotspot/share/ci/ciField.cpp
> --- a/src/hotspot/share/ci/ciField.cpp	Tue Jun 09 16:22:54 2020 +0000
> +++ b/src/hotspot/share/ci/ciField.cpp	Thu Jun 11 22:25:02 2020 +0200
> @@ -231,6 +231,9 @@
>     // Trust final fields in all boxed classes
>     if (holder->is_box_klass())
>       return true;
> +  // Trust final fields in records
> +  if (holder->is_record())
> +    return true;
>     // Trust final fields in String
>     if (holder->name() == ciSymbol::java_lang_String())
>       return true;
> diff -r 984fde9a0b7f src/hotspot/share/ci/ciInstanceKlass.cpp
> --- a/src/hotspot/share/ci/ciInstanceKlass.cpp	Tue Jun 09 16:22:54 2020 +0000
> +++ b/src/hotspot/share/ci/ciInstanceKlass.cpp	Thu Jun 11 22:25:02 2020 +0200
> @@ -64,6 +64,7 @@
>     _has_nonstatic_concrete_methods = ik->has_nonstatic_concrete_methods();
>     _is_unsafe_anonymous = ik->is_unsafe_anonymous();
>     _is_hidden = ik->is_hidden();
> +  _is_record = ik->is_record();
>     _nonstatic_fields = NULL; // initialized lazily by compute_nonstatic_fields:
>     _has_injected_fields = -1;
>     _implementor = NULL; // we will fill these lazily
> @@ -125,6 +126,7 @@
>     _has_injected_fields = -1;
>     _is_unsafe_anonymous = false;
>     _is_hidden = false;
> +  _is_record = false;
>     _loader = loader;
>     _protection_domain = protection_domain;
>     _is_shared = false;
> diff -r 984fde9a0b7f src/hotspot/share/ci/ciInstanceKlass.hpp
> --- a/src/hotspot/share/ci/ciInstanceKlass.hpp	Tue Jun 09 16:22:54 2020 +0000
> +++ b/src/hotspot/share/ci/ciInstanceKlass.hpp	Thu Jun 11 22:25:02 2020 +0200
> @@ -57,6 +57,7 @@
>     bool                   _has_nonstatic_concrete_methods;
>     bool                   _is_unsafe_anonymous;
>     bool                   _is_hidden;
> +  bool                   _is_record;
>   
>     ciFlags                _flags;
>     jint                   _nonstatic_field_size;
> @@ -200,6 +201,10 @@
>       return _is_hidden;
>     }
>   
> +  bool is_record() const {
> +    return _is_record;
> +  }
> +
>     ciInstanceKlass* get_canonical_holder(int offset);
>     ciField* get_field_by_offset(int field_offset, bool is_static);
>     ciField* get_field_by_name(ciSymbol* name, ciSymbol* signature, bool is_static);
>
>