RFR: javac support for Parametric VM, first stab [v3]
Vicente Romero
vromero at openjdk.java.net
Fri Mar 12 17:33:57 UTC 2021
> This patch is the initial support for Parametric VM in javac. This is the first iteration of the prototype, so still a work in progress. In order to test the new code users need to pass the hidden option: `-XDsupportParametricVM` to javac. See the examples below for an illustration of the bytecode the compiler can generate now. Lets assume we have the following class annotated with the `@Parametric` annotation:
>
> import java.lang.annotation.*;
>
> @Parametric(id="PAIR")
> class Pair<X, Y> {
> @Parametric(id="PAIR")
> X fst;
> @Parametric(id="PAIR")
> Y snd;
>
> Pair(X fst, Y snd) {
> this.fst = fst;
> this.snd = snd;
> }
>
> Pair<Y, X> swap() {
> return new Pair<>(snd, fst);
> }
>
> @Parametric(id="make", kind=ParamKind.METHOD_ONLY)
> static <U, W> Pair<U, W> make(U fst, W snd) {
> return new Pair<>(fst, snd);
> }
>
> @Parametric(id="setFst", kind=ParamKind.METHOD_AND_CLASS)
> <Z> Pair<Z, Y> setFst(Z newFst) {
> return null;
> }
> }
>
> After compilation this class will contain the following new entries in the constant pool:
>
> #22 = Parameter CLASS:0 // CLASS:0
> #37 = Parameter METHOD_ONLY:0 // METHOD_ONLY:0
> #44 = Parameter METHOD_AND_CLASS:0 // METHOD_AND_CLASS:0
> also there will be several elements with the new attribute `Parametric` which will appear in both fields, the `Pair` class per-se and methods: `make` and `setFst`.
>
> Let's now assume we define a Client willing to use class `Pair`, it will need to define values to bound to the "holes" defined in class `Pair` using the `Parametric` annotation. For this class Client should use annotations `@LinkageClass` and or `@LinkageMethod` depending on the kind of hole in class `Pair` it is intending to bound to. Let's define class `Client` as:
>
> import java.lang.annotation.*;
>
> class Client {
> @LinkageClass("linkageClass")
> void linkClassParams() {
> Pair<String, Integer> psi = new Pair<>("first", 2);
> }
>
> @LinkageClass("linkageClass")
> void linkClassParamsFieldAccess(Pair<String, Integer> psi) {
> psi.fst = "field";
> }
>
>
> @LinkageMethod("linkageMethod1")
> void linkMethodParams(Pair<String, Integer> psi) {
> Pair<String, Integer> pair = Pair.make("hello", 1);
> }
>
> @LinkageClass("linkageClass")
> @LinkageMethod("linkageMethod2")
> void linkClassAndMethodParams(Pair<String, Integer> psi) {
> Pair<String, Integer> pss = psi.setFst("b");
> }
> }
>
> again compiling this class with the hidden option `-XDsupportParametricVM` we can find these new entries in the CP:
>
> #7 = Linkage #8:#9 // linkageClass:Pair
> #8 = String #10 // linkageClass
> #9 = Class #11 // Pair
>
> this Linkage CP entry is referred from method: `linkClassParams` as:
>
> void linkClassParams();
> descriptor: ()V
> flags: (0x0000)
> Code:
> stack=4, locals=2, args_size=1
> 0: new #7 // Linkage linkageClass:Pair
>
> also:
>
> #25 = Linkage #8:#26 // linkageClass:Pair.fst:Ljava/lang/Object;
> #26 = Fieldref #7.#27 // Pair.fst:Ljava/lang/Object;
> #27 = NameAndType #28:#29 // fst:Ljava/lang/Object;
>
> referred from method: `linkClassParamsFieldAccess` the bytecode looks like:
>
> void linkClassParamsFieldAccess(Pair<java.lang.String, java.lang.Integer>);
> descriptor: (LPair;)V
> flags: (0x0000)
> Code:
> stack=2, locals=2, args_size=2
> 0: aload_1
> 1: ldc #23 // String field
> 3: putfield #25 // Linkage linkageClass:Pair.fst:Ljava/lang/Object;
> 6: return
>
> also:
>
> #32 = Linkage #33:#34 // linkageMethod1:Pair.make:(Ljava/lang/Object;Ljava/lang/Object;)LPair;
> #33 = String #35 // linkageMethod1
> #34 = Methodref #9.#36 // Pair.make:(Ljava/lang/Object;Ljava/lang/Object;)LPair;
>
> this time referred by method: `linkMethodParams` with bytecode:
>
> void linkMethodParams(Pair<java.lang.String, java.lang.Integer>);
> descriptor: (LPair;)V
> flags: (0x0000)
> Code:
> stack=2, locals=3, args_size=2
> 0: ldc #30 // String hello
> 2: iconst_1
> 3: invokestatic #14 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
> 6: invokestatic #32 // Linkage linkageMethod1:Pair.make:(Ljava/lang/Object;Ljava/lang/Object;)LPair;
> 9: astore_2
> 10: return
>
> and finally:
>
> #41 = Linkage #42:#43 // linkageMethod2:Pair.setFst:(Ljava/lang/Object;)LPair;
> #42 = String #44 // linkageMethod2
> #43 = Methodref #7.#45 // Pair.setFst:(Ljava/lang/Object;)LPair;
>
> in this last example it can be seen that Methodref at #43 is also pointing to #7 which, as commented above, is another Linkage_info entry in the CP.
>
> TIA, for the feedback and comments,
> Vicente
Vicente Romero has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains one additional commit since the last revision:
associate an id to the Parameter CP
-------------
Changes:
- all: https://git.openjdk.java.net/valhalla/pull/364/files
- new: https://git.openjdk.java.net/valhalla/pull/364/files/20a7f16e..5b6afe9f
Webrevs:
- full: https://webrevs.openjdk.java.net/?repo=valhalla&pr=364&range=02
- incr: https://webrevs.openjdk.java.net/?repo=valhalla&pr=364&range=01-02
Stats: 16 lines in 4 files changed: 12 ins; 0 del; 4 mod
Patch: https://git.openjdk.java.net/valhalla/pull/364.diff
Fetch: git fetch https://git.openjdk.java.net/valhalla pull/364/head:pull/364
PR: https://git.openjdk.java.net/valhalla/pull/364
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