On Wed, Feb 1, 2012 at 12:28 AM, Eliot Miranda <eliot.miranda@gmail.com> wrote:

On Tue, Jan 31, 2012 at 3:13 PM, Mariano Martinez Peck <marianopeck@gmail.com> wrote:

On Tue, Jan 31, 2012 at 11:42 PM, Eliot Miranda <eliot.miranda@gmail.com> wrote:

On Tue, Jan 31, 2012 at 12:41 PM, Mariano Martinez Peck <marianopeck@gmail.com> wrote:

On Tue, Jan 31, 2012 at 9:28 PM, Eliot Miranda <eliot.miranda@gmail.com> wrote:

On Tue, Jan 31, 2012 at 12:19 PM, Igor Stasenko <siguctua@gmail.com> wrote:

On 31 January 2012 20:50, Eliot Miranda <eliot.miranda@gmail.com> wrote:
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> On Tue, Jan 31, 2012 at 11:22 AM, Mariano Martinez Peck <marianopeck@gmail.com> wrote:
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>> Hi Eliot. Me again :) I was checking the changes Igor did some time ago for the fast become where he basically swapped the bytes contents between the objects when they were the same size and same header type. He put such code in separate primtives and some changes in the image side to call them. I have just played with them and they seem to work. I have 2 questions for you:
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>> 1) Do you think that this new fast become can have problems when becoming CompiledMethods? I am asking because of the JIT/Pic. Maybe I need a flushCache or something?
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> Yes, almost certainly. You'd want to do a flushCache on both methods.
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are there other object types which we need to be careful with?

There are a few. e.g. the Array literals in named primitives (because they hold target function pointers). CompiledMethods (because they may have associated machine code). Contexts (because they may have associated stack frames).

Eliot, I don't understand why we have these problems with the "fast become" but not with the normal one. What happens wich each of your examples with the normal become? how are they solved?

The "slow" become is implemented in terms of the GC's pointer-forwarding mechanism, which is used in normal garbage collection, not just become. This machinery is the ObjectMemory>remap: machinery. The JIT implements the same mapping machinery for literal objects embedded in machine code. These include not just literals but also classes in inline-caches. So it would seem that implementing markObject: and remap: for literals in jitted methods is all one needs to support GC and become:. In fact, life is more complex because there is an optimization in the JIT to avoid scanning all of machine code on incremental GC. The jit maintains a list of those methods that contain references to young objects and only scans this list on an incremental GC, and this list must be maintained correctly. Hence there are three different remap routines in the jit,

Cogit>mapObjectReferencesInMachineCodeForIncrementalGC
"Update all references to objects in machine code for an incremental gc.

Avoid scanning all code by using the youngReferrers list. In an incremental
GC a method referring to young may no longer refer to young, but a method

not referring to young cannot and will not refer to young afterwards."

Cogit>mapObjectReferencesInMachineCodeForFullGC

"Update all references to objects in machine code for a full gc. Since
the current (New)ObjectMemory GC makes everything old in a full GC

a method not referring to young will not refer to young afterwards"

Cogit>mapObjectReferencesInMachineCodeForBecome

"Update all references to objects in machine code for a become.
Unlike incrementalGC or fullGC a method that does not refer to young

may refer to young as a result of the become operation."

Aha. Ok. Now I see. So, let me see if I understand. So the problem of CompiledMethod gets fixed if we flush its cache. Right?

I hope so. The current implementation reads

CoInterpreterPrimitives>primitiveFlushCacheByMethod
"The receiver is a compiledMethod. Clear all entries in the method lookup cache that

refer to this method, presumably because it has been redefined, overridden or removed.
Override to flush appropriate machine code caches also."

super primitiveFlushCacheByMethod.
cogit unlinkSendsTo: self stackTop

That may not be enough. The VM may have to throw the method away. Tests will show whether merely unlinking is sufficient.

Eliot, I have to admit that the discussion is going further than my understandings :(

The issue is that if the method is being used then throwing it away may involve flushing the stack activations of the method, and that makes the implementation much more complex.

Ok, I understand that, but again I don't understand why
a) this does not happen with the "slow become". Is it again because of the forwarding table?
b) it doesn't happen when we flush methods. For example, when using TestCoverage that we put objects as methods and we use the #run:with:in: we flush the method cache... why that doens't need to flush stack activations? just becuase in that case we are sure that the method is not being used?

Now...if we always send #mapObjectReferencesInMachineCodeForBecome after the "fast become" we will be updating all literals from machine code methods.

Um, will it? The mapping is done only to references to objects that are forwarded. If that's going to do the trick then great. But I don't know enough about your fast become to know.

I think I was wrong. Obviously, #mapObjectReferencesInMachineCodeForBecome is based in the forwarding table. Igor's solution just swaps contents... it does nothing regarding forwarding table.

Here I attach a slighly modified version of Igor code so that you can take a look, at leat to get an idea of what we are talking about. I attach the VMMaker changs and the image side. Personally, I think that performing the become with arrays is just becuause the become is slow. If become were fast, we would do the loop in image side, right? Because of that, I think it is nice to have it in a separate primitive which fallbacks in the array.

I didn't understand this one " e.g. the Array literals in named primitives (because they hold target function pointers)"

Look at a method containing a named primitive that's in use and look at its first literal. e.g.

(StandardFileStream >> #primRead:into:startingAt:count:) literalAt: 1 #(#FilePlugin #primitiveFileRead 0 12)

That 12 is meaningful to the VM. See primitiveExternalCall:

External primitive methods first literals are an array of

* The module name (String | Symbol)
* The function name (String | Symbol)
* The session ID (SmallInteger) [OBSOLETE]
* The function index (Integer) in the externalPrimitiveTable

Yes, I know that, what I don't understand is what that can be affected by the fast become. The external primitive table doesn't have a pointer to the method, but to the function address. So even if I become a named prmitive, wouldn't the table still be correct?
In any case, I guess we can do a #flushExternalPrimitives or #flushExternalPrimitives to avoid possible problems. Would that help?

What happens with "Contexts (because they may have associated stack frames)." ? should we need to flush somehow or update stack frames?

Yes. If you smash the state of a context that has an associated stack frame then the VM will likely crash. See senders of externalDivorceFrame:andContext: to see where the VM disassociates contexts and their stack frames when an access to a context (e.g. changing its stack pointer or pc) necessitates it.

ok, and so that can happens if we become contexts?

Thanks Eliot.

Thanks!

Sorry for the noob question.

It's a good question :)

because i was thinking to just put a check in fast-become prim and
simply fail the prim if object type(s) to be swapped are not
supported, so user will be forced to use slow good-old #become:

I agree. But you can do even better, by checking that the compiled method has a machine-code version, and/or checking that a context is "single" (has no associated stack state). It doesn't need to fail if there isn't any special state. Identifying the named primitive linking literals is more difficult...

Ideally, I would love to be able to do the fast become for all of them, even if that implies doing something extra for special cass (like flushing method cache).

As they say, don't get caught.

--
Best regards,
Igor Stasenko.

--
best,
Eliot

--
Mariano
http://marianopeck.wordpress.com

--
best,
Eliot

--
Mariano
http://marianopeck.wordpress.com

--
best,
Eliot