Mark,
I'll walk into the trap of offering a solution. While I'm not familiar with
your implementation of SmallTalk, the array example might work like this
(generic/loose java)
// language stack
public static TaggedArray stack = TaggedArrays.allocate(1024*1024);
public static int TOS = -1;
// value conversions
long toValue(int i) { return ((long)i << TaggedArrays.SHIFT) |
TaggedArrays.TAG; }
int toInt(long v) { return (int)(v >> TaggedArrays.SHIFT) ; }
// stack manipulation
void pushObject(Object o) { stack.setReference(++TOS, o); }
void pushInt(int i) { stack.setValue(++TOS, toValue(i)); }
Object popObject() { stack.getReference(TOS--); }
int popInt() { toInt(stack.getValue(TOS--)); }
....
// push array on stack
pushObject(new byte[10000]);
// push index on stack
pushInt(10);
// get the array element, leaving the result on the stack
invoke("getArray");
// work with the result
int b = popInt();
The code for getArray might be something like (loose error handling)
int i;
Object a;
try {
i = popInt();
} catch (TaggedArrayException | IndexOutOfBoundsException ex) {
// handle the fact it wasn't a value or underflow
}
try {
a = popObject();
} catch (TaggedArrayException | IndexOutOfBoundsException ex) {
// handle the fact it wasn't an object or underflow
}
...
if (a instanceof byte[]) {
// get the byte array element
long b = ((byte[])a)[i];
// push result on the stack
pushInt(b);
} else ...
Not the best of examples, but
- there are no boxed values involved here
- the JVM compiler would convert setValue/setReference/getValue/getReference to
simple memory access, so will perform well
- the net result is very simple native code without a lot of runtime handling
- note: you can choose to encrypt "values" suitable for your language needs
I'll see if we can set up a TaggedArray breakout session at the summit and run
thru other possibilities.
Cheers,
-- Jim
On 2012-07-02, at 2:23 PM, Mark Roos wrote:
> From Jim
>
> It occurred to me on that sleepless Monday night, that the solution for most
> dynamic languages could be so much simpler. First, we have to look at what
> it is we really need. Ultimately it's about boxing. We want to avoid
> allocating memory whenever we need to store a primitive value in an object.
> Concerning ourselves with passing around tagged values in registers and
> storing in stack frames is all red herring. All that is needed is a
> mechanism for storing tagged values (or compressed values) in a no-type slot
> of a generic object. Thinking about it in these terms isolates all issues to
> a single array-like class, and thus simplifies implementation and simplifies
> testing. Instances of this class can be used as objects, as stack frames and
> even full stacks. A good percentage of a dynamic language needs are covered.
>
> Just having spent the last year implementing Smalltalk on the JVM the issue
> of boxing ( particularly for integers ) is of interest for me. While I agree
> with
> your statement that allocating memory is the big issue, I don't really
> understand your comment about 'when we store a primitive'. My most visible
> issue
> is in a 'for loop' like construct where I generate an index and use it to
> access a byte array. In this case I need to create both the index and the
> byte accessed.
> For instance scanning a million byte array requires that I create a million
> indexes and then another million ( assuming no cache ) one for each byte
> accessed.
> These are all then discarded. Its not clear to me how your proposal would
> help here.
>
> As I have thought about the boxing issue ( the Smalltalk I am using as a
> reference has tagged integers ) I keep thinking that any jvm solution is
> probably
> going to have some 'java' or other target driven characteristics that make it
> hard to use. In my case I have a java class that all of my objects are
> instances of.
> If there were a 'tagged' object type then it would have to be able to
> substitute as one of my instances, hold or reference the same information
> (like method lookups, class, shape etc ), exist on the stack or in a temp,
> be testable in a GWT ....
>
> Here I agree with you that making this work in the JVM is probably too
> difficult.
>
> So my thoughts are back to how to have a boxed primitive with no allocation
> overhead unless it is saved or accessed outside of a thread, in other words
> how to reuse the box. I can do this for some situations where I can prove
> the scope but I have yet to figure out a general solution.
>
> So while I can see a use for mixing references and primitives in an array it
> has not shown up in my work as amajor issue. Perhaps this is due to my not
> keeping parallel stacks?
>
> In any case hope to hear more on this at the summit
>
> regards
> mark
>
>
>
>
>
>
>
>
>
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