[Vm-dev] Eliot's BlockClosure model questions

Eliot Miranda eliot.miranda at gmail.com
Thu Aug 1 20:17:08 UTC 2013

On Thu, Aug 1, 2013 at 10:15 AM, Eliot Miranda <eliot.miranda at gmail.com>wrote:

> On Thu, Aug 1, 2013 at 1:21 AM, Clément Bera <bera.clement at gmail.com>wrote:
>> 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.
Thinking about it I'm pretty sure the problem is that the JIT scans for and
counts pushClosure: bytecodes to know how many blocks a method contains,
but clean blocks don't need pushClosure: bytecodes.  So the JIT needs to
look for clean blocks, e.g. either by scanning a method's literals or by
looking at the arguments of pushLiteral: bytecodes.  In any case the image
will allow me to develop a fix.

> 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 ?
> no.  send me an image?
>> 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
> --
> best,
> Eliot

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