Structure of the HotSpot Interpreter

[Andrew Haley]

On 5/31/24 13:24, Julian Waters wrote:
> Thanks for the overview. I unfortunately can't do +PrintInterpreter at
> the moment since my JDK is experiencing compilation failures

That doesn't matter: the Java you're using as a bootstrap compiler will
do it.

> everywhere (It's in a bit of a mess right now), but I will try doing
> that once I've gotten everything fixed. However, I've been digging
> through the code a little, and I think I see a bit of a pattern. The
> methods in the Template Table files are all geared towards emitting
> the executable code into memory, and each of their methods are passed
> as a pointer to the corresponding bytecode definition to actually emit
> code into memory. The dispatch mechanism is still a bit of a mystery
> to me,

Why? It's the four instructions that I quoted last time:


    0x0000ffff785428cc:   ldrb		w8, [x22, #1]!      ;;  403:     __ dispatch_epilog(tos_out, step);

Offset to the dispatch table:

    0x0000ffff785428d0:   add		w9, w8, #0x500

Load the address of the next action, and jump to it:

    0x0000ffff785428d4:   ldr		x9, [x21, w9, uxtw #3]
    0x0000ffff785428d8:   br		x9

> but from what I can see the code that dispatches to the next
> bytecode is emitted by dispatch_next. Did I get all of that right, or
> is there anything I am missing?

By the dispatch_XXX methods, yes.

-- 
Andrew Haley  (he/him)
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
https://keybase.io/andrewhaley
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