[Vm-dev] Eliot's BlockClosure model questions

Thu Aug 1 08:21:35 UTC 2013

Hello Eliot,

So I implemented clean blocks with Opal in Pharo 3. I didn't know where to
put the byte code of the clean block, so I put it at the end of the method.

ex:
exampleCleanBlock
^ [ 1  + 2 ]

17 <20> pushConstant: [...]
18 <7C> returnTop
19 <76> pushConstant: 1
20 <77> pushConstant: 2
21 <B0> send: +
22 <7D> blockReturn

having in the literal Array:
[ 1 + 2 ]
#exampleCleanBlock
OCOpalExamples

The startpc of the block is 19.
Its outerContext is a context with nil as receiver and the method
OCOpalExamples>>#exampleCleanBlock.
Its numArgs is 0 and it has no copiedValues.

But it does not work with the JIT.
If I run:
OCOpalExamples new exampleCleanBlock value
I got 3 all the time, it's fine. Now
1 to: 5 do: [ :i |
OCOpalExamples new exampleCleanBlock value ]
Works on Stack VM, but crashes Cog VM. I don't know why (not enough
knowledge about the Cog JIT).

Do you have any clue ?

2013/7/31 Eliot Miranda <eliot.miranda at gmail.com>

>
>
>
> On Tue, Jul 30, 2013 at 1:56 PM, Clément Bera <bera.clement at gmail.com>wrote:
>
>>
>> Thanks for the answer it was very helpful. I got it now.
>>
>> I had a look at the first posts of your blog (Closures I & II) when I was
>> working on the Opal compiler. Today I was looking at Under Cover
>> Contexts and the Big Frame-Up<http://www.mirandabanda.org/cogblog/2009/01/14/under-cover-contexts-and-the-big-frame-up/> and
>> I think I should read all your blog.
>>
>> That is really nice that you wrote this blog it is the main documentation
>> about an efficient Smalltalk VM. I learnt by looking at Cog's source
>> mostly. VW VM source is closed so... I will have a look at Strongtalk
>> implementation instead it seems it is open source.
>>
>> Why are the clean blocks of VW much faster ? Are they activated like
>> method ? I didn't find it in your blog (probably because it is not in Cog).
>> Is it possible to implement clean blocks in Pharo/Squeak ? (I think that
>> 53% of blocks non optimized by the compiler are clean in Pharo 3) Would it
>> worth it ?
>>
>
> Clean blocks are faster because they don't access their outer environment
> and hence their outer context does not have to be created.  So there is no
> allocation associated with a clean block.  It exists already as a literal
> and its outer context does not have to be reified.  Normal closures are
> created when the point at which they are defined in method execution is
> reached (the pushClosure bytecode) and if the current context does not yet
> exist that must be instantiated too, so creating a closure usually takes
> two allocations.
>
> Clean blocks are activated like blocks.  Block and method activation is
> different in the first phase (the send side) but quite similar in the
> second phase (frame building).  In VW for example, finding the machine code
> method associated with a block involves a cache lookup which can be slow.
>  In Cog, it involves following a pointer in the method header (inside, the
> VM replaces the header of a method with a pointer to its machine code) and
> then jumping to a hard-coded binary search which jumps to the correct
> block's entry-point depending on the closure's startpc.  If a method
> contains a single block then this is a direct jump.  As a result, block
> dispatch in Cog is typically faster than in VW.
>
> Yes, it is possible to implement clean blocks.  It is only an issue to do
> with the representation of closures.  Ideally they need a method inst var,
> making the outerContext inst var optional (or at least nil in a clean
> block).  But that would require a change to BlockClosure's class definition
> and a VM change.  To avoid having to change the class definition of
> BlockClosure and the VM, the compiler could create an empty context to hold
> onto the method, and that would work fine.  So to implement clean blocks
> the compiler would instantiate a BlockClosure literal for each clean block
> and a MethodContext whose receiver was nil shared between all the clean
> blocks in a method.  There are tricky issues such as setting breakpoints in
> methods (toggle break on entry), or copying methods, which would require
> scanning the literals for clean blocks and duplicating them and their
> outerCOntext too.  But that's just detail.  Some time I must try this for
> Squeak.  Let me know if you try if=t for Opal.  (and of course I'm very
> happy to help with advice).
>
> I expect that in certain cases the speedup would be noticeable, but it is
> a micro-optimization.  You'd of course only notice the difference in tight
> loops that used clean blocks.
>
>
> 2013/7/30 Eliot Miranda <eliot.miranda at gmail.com>
>>
>>>
>>> http://www.mirandabanda.org/cogblog/2008/06/07/closures-part-i/
>>> Hi Clément,
>>>
>>> On Mon, Jul 29, 2013 at 1:54 AM, Clément Bera <bera.clement at gmail.com>wrote:
>>>
>>>>
>>>> Hello guys,
>>>>
>>>> I was looking recently at the blockClosure model of Eliot in
>>>> Pharo/Squeak and the blockClosure model of VisualWorks and I have a few
>>>> questions.
>>>>
>>>> - Why Pharo/Squeak does not have compiled block as in VW and has the
>>>> block byte code in the enclosing method ? Is it to save memory ? Would it
>>>> worth it to implement CompiledBlock in term of speed and memory consumption
>>>> ?
>>>>
>>>
>>> Squeak derives directly from the "blue book" Smalltalk-80 implementation
>>> in which CompiledMethod is a hybrid object, half pointers (method header
>>> and literals) and half bytes (bytecode and source pointer).  This format
>>> was chosen to save space in the original 16-bit Smalltalk implementations
>>> on the Xerox D machines (Alto & Dorado).  VisualWorks has a few extra steps
>>> in between,  In ObjectWorks 2.4 and ObjectWorks 2.5 Peter Deutsch both
>>> introduced closures and eliminated the hybrid CompiledMethod format,
>>> introducing CompiledBlock.
>>>
>>> IMO adding CompiledBlock, while simplifying the VM a little would not
>>> improve performance, especially in the interpreter, essentially because
>>> activating and retuning form methods now requires an ecxtra level of
>>> indirection to get from the CompiledMethod object to its bytecodes in its
>>> bytecode object.
>>>
>>> However, adding CompiledBlock (or rather eliminating the hybrid
>>> CompiledMethod format) would definitely *not* save space.  The hybrid
>>> format is more compact (one less object per method).  One can try and
>>> improve this as in VisualWorks by encoding the bytecodes of certain methods
>>> as SmallIntegers in the literal frame, but this is only feasible in a pure
>>> JIT VM.  Squeak still has an interpreter, and Cog is a hybrid JIT and
>>> Interpreter.  In an interpreter it is costly in performance to be able to
>>> interpret this additional form of bytecodes.
>>>
>>> So IMO while the hybrid CompiledMethod isn't ideal it is acceptable,
>>> having important advantages to go along with its disadvantages.
>>>
>>>  - Why Pharo/Squeak context have this variable closureOrNil instead of
>>>> having the closure in the receiver field as in VW ? Is it an optimization
>>>> because there are a lot of access to self and instance variables in the
>>>> blocks in Pharo/Squeak ? Because if I'm correct it uses 1 more slot per
>>>> stack frame to have this.
>>>>
>>>
>>> I did this because I think its simpler and more direct.  I don't like
>>> VW's access to the receiver and inst vars having to use different bytecodes
>>> within a block to within a method.  There are lots of complexities
>>> resulting from this (e.g. in scanning code for inst var refs, the
>>> decompiler, etc).
>>>
>>> But in fact there isn't really an additional stack slot because the
>>> frame format in the VM does not use the stacked receiver (the 0'th
>>> argument) as accessing the receiver in this position requires knowing the
>>> method's argument count.  So in both methods and blocks the receiver is
>>> pushed on the stack immediately before allocating space for, and nilling,
>>> any temporaries.  This puts the receiver in a known place relative to the
>>> frame pointer, making it accessible to the bytecodes without having to know
>>> the method's argument count.  So the receiver always occurs twice on the
>>> stack in a method anyway.  In a block, the block is on the stack in the
>>> 0'th argument position.  The actual receiver is pushed after the temps.
>>>
>>> - Lastly, does VW have the tempVector optimization for escaping write
>>>> temporaries in their blockClosure ? It seems they have not (I don't see any
>>>> reference to it in VW 7). Did Pharo/Squeak blocks earns a lot of speed or
>>>> memory with this optimization ?
>>>>
>>>
>>> Yes, VW has this same organization.  I implemented it in VisualWorks 5i
>>> in ~ 2000.  It resulted in a significant increase in performance (for
>>> example, factors of two improvement in block-intensive code such as
>>> exception handling).  This is because of details in the context-to-stack
>>> mapping machinery which mean that if an activation of a closure can update
>>> the temporaries of its outer contexts then keeping contexts and stack
>>> frames in sync is much more complex and costly.  The 5i/Cog organization
>>> (which in fact derives from some Lisp implementations) results in much
>>> simpler context-to0stack mapping such that no tests need be done when
>>> returning from a method to keep frames and contexts in sync.
>>>
>>>
>>>
>>>> Thank you for any answer.
>>>>
>>>
>>> You're most welcome.  Have you read my blog post on the design?  It is "Under
>>> Cover Contexts and the Big Frame-Up<http://www.mirandabanda.org/cogblog/2009/01/14/under-cover-contexts-and-the-big-frame-up/>",
>>> with additional information in "Closures Part I" & "Closures Part II –
>>> the Bytecodes<http://www.mirandabanda.org/cogblog/2008/07/22/closures-part-ii-the-bytecodes/>
>>> ".
>>> --
>>> best,
>>> Eliot
>>>
>>>
>>
>>
>
>
> --
> best,
> Eliot
>
>
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