[code-reflection] RFR: Update the model of pattern matching when pattern variable identifier is underscore [v2]

[Mourad Abbay]

On Wed, 11 Sep 2024 00:59:30 GMT, Paul Sandoz <psandoz at openjdk.org> wrote:

>> Yep - this PR is about a type test pattern whose binding name is left unspecified (`_`). I was observing that unspecified names can occur in many places (method parameters, local var decls, etc.) so we probably need some uniform way to handle with "missing names" ? What I'm objecting to, in this PR, is that we're creating a binding, where the binding name is `_` - which strikes me as incorrect. The name is missing/absent, `_` is just the Java way of denoting missing names.
>
> Right, they are interconnected. I proposed to Mourad we should use `null` to signal no binding/name. The corresponding Var modeling the pattern variable can have `null` for its name, which we can do generally for variables with no name.
> 
> For example for:
> 
> `boolean x = o instanceof String s`
> 
> the model is:
> 
> 
>     %4 : java.lang.String = constant @null;
>     %5 : Var<java.lang.String> = var %4 @"s";
>     %6 : boolean = pattern.match %3
>         ()java.lang.reflect.code.ExtendedOp$Pattern$Binding<java.lang.String> -> {
>             %7 : java.lang.reflect.code.ExtendedOp$Pattern$Binding<java.lang.String> = pattern.binding @"s";
>             yield %7;
>         }
>         (%8 : java.lang.String)void -> {
>             var.store %5 %8;
>             yield;
>         };
>     %9 : Var<boolean> = var %6 @"x";
> 
> 
> For:
> 
> `boolean _ = o instanceof String _`
> 
> we could generate:
> 
> 
>     %4 : java.lang.String = constant @null;
>     %5 : Var<java.lang.String> = var %4;
>     %6 : boolean = pattern.match %3
>         ()java.lang.reflect.code.ExtendedOp$Pattern$Binding<java.lang.String> -> {
>             %7 : java.lang.reflect.code.ExtendedOp$Pattern$Binding<java.lang.String> = pattern.binding;
>             yield %7;
>         }
>         (%8 : java.lang.String)void -> {
>             var.store %5 %8;
>             yield;
>         };
>     %9 : Var<boolean> = var %6;
> 
> 
> When lowering it might be possible to remove unnamed variables, they are never loaded.

I suggest to not have Var that correspond to unnamed pattern variable

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PR Review Comment: https://git.openjdk.org/babylon/pull/227#discussion_r1753065370