Sponsor Request: 8241100: Make Boolean, Character, Byte, and Short implement Constable

[John Rose]

On Mar 20, 2020, at 10:49 AM, Jorn Vernee <JORN.VERNEE at ORACLE.COM> wrote:
> 
> W.r.t. the source type being needed, I see the following 4 major cases:
> 
> 1. src=Prim & dst=Prim -> cast convert. For boolean the least-significant-bit is used to convert it to/from an integer type.
> 2. src=Prim & dst=Ref   -> box the source value and potentially cast
> 3. src=Ref   & dst=Prim -> apply an unboxing conversion if possible and then a casting conversion (with same trick for boolean as (1))
> 4. src=Ref   & dst=Ref   -> reference cast
> 
> Without the source type we can't disambiguate between cases (2) and (4), or (1) and (3) because the bootstrap takes Object as an input.

I think that if you box the src=Prim from 1 or 2 and then
hand the resulting src=Ref to 3 or 4 you will get the same
thing.  The basic design here is that dst=Prim is the key
signal that unlocks the primitive conversions.  The type
of the src (Ref or Prim) doesn’t change the access to the
primitive conversions.

> For (2) and (4) the bootstrap invocation mechanism takes care of the boxing for us, and the cast is performed by (4). For (1) and (3) the conversion code for the latter handles conversion of wrapper types to Number and then to the target primitive per (1). In the end things seems to work out to the same result (though maybe I'm missing some subtle difference in a failure case).

Indeed they do.  That’s how I designed it.  The upshot is that
Object is a reliable carrier even of primitive values, for these
APIs.

> What I'm mostly worried about is that the source type already affects _how_ the conversion is done, and the fact that this difference is not observable seems somewhat incidental.... Coupled with the fact that asType and explicitCastArguments also have access to both the source and destination type, I think maybe the new bootstrap method should as well. After all, what if the set of cases is extended (valhalla?) and/or not being able to disambiguate starts to matter?

You shouldn’t be appealing to an internal function here for
designing the semantics.  The dual arguments to the internal
function are excess information; the shape of that function
predates the design decision described above.

Instead, appeal to some pseudocode, such as:

var id = identity(Object.class);
var mt = methodType(dst, Object.class);
var conv = explicitCastArguments(id, mt);
return conv.invoke(x);

I think that will give us all conversions we will need.

> 
> ---
> 
> I've written a more in-depth specification for the bootstrap, and re-implemented it using explicitCastArguments, since that helps to catch discrepancies between the input value and the source type. Here is the next iteration: http://cr.openjdk.java.net/~jvernee/8241100/webrev.01
> 
> I've kept the source type for now, if it should be removed the specification can be trimmed down (since there would be less cases to specify).
> 
> As for the name; I think "asType" might be confusing since the applied conversion is not quite the same as MethodHandle::asType. Since the bootstrap is implemented in terms of explicitCastArguments I went with "explicitCast", how is that?

Yes, I like that name.

— John