RE: Classpath build process and VM-specific issues

[Jeroen Frijters]

Andrew Haley wrote:
>  > "jbyte" must have a single platform-specific definition, as all
>  > JVMs on that platform should be able to execute the same JNI
>  > library code (no recompilation required).
> 
> I didn't know that.  Is that requirement documented anywhere?  I can't
> see how you'd do it on a machine with 32-bit and 64-bit modes, for
> example.

Platform = Machine + OS. I don't have any reference, but I believe that
Etienne is right in saying that the same library should be usuable with
all JVMs on a specific platform.

> Clearly the highest performance comes from using a naked long.

No, the highest performance comes from using a platform specific
pointer. I think that for a JIT it is easier to replace a typed object
reference with a native pointer then a long. That's why I'm very much in
favor of using RawData. It can even be abstract, that way the JNI layer
can allocate sub types that contain the appropriate number of bits to
contain a native pointer.

For example:

public abstract class RawData {}

public final class RawData32 extends RawData
{
  private int pointer;
}

public final class RawData64 extends RawData
{
  private long pointer;
}

All the Java code would ever see are RawData references, but the JNI
layer knows that all RawData references are actually 32 or 64 bit (or
whatever that platform requires).

Regards,
Jeroen