[Vm-dev] Eliot's BlockClosure model questions

Clément Bera bera.clement at gmail.com
Fri Aug 2 05:12:05 UTC 2013

Hi Eliot.

So I changed the implementation according to what you've just said and it
works with Cog. I added a jump and a pushClosure byte code which is never
called but permits to be JIT-compatible.

^ [ 1  + 2 ]

17 <20> pushConstant: [...]
18 <72> pushConstant: false
19 <9F> jumpFalse: 28
20 <8F 00 00 04> closureNumCopied: 0 numArgs: 0 bytes 24 to 27
24  <76> pushConstant: 1
25  <77> pushConstant: 2
26  <B0> send: +
27  <7D> blockReturn
28 <7C> returnTop

Here the BlockClosure in the literals has a startpc of 24, therefore the
pushClosure bytecode cannot be called.

I will try to replace the jumpFalse by a jump, I didn't do it because Opal
then detects the block byte code as not reachable and removes it. I will
then check if it still works with the JIT (I don't know if the JIT has
these unreachable bytecode removal feature). I may earn some speed by not
having to push false.

Already now the clean block is definitely faster, at first look :
OCOpalExamples >>#exampleCleanBlock
^ [ 1  + 2 ]
foo := OCOpalExamples new.
[ foo exampleCleanBlock ] bench (5x faster)
[ foo exampleCleanBlock value ] bench (3.5 times faster)

I can prepare you an image so you can have a look, but
- Pharo 3 requires NativeBoost plugin to find environment variables so it
may not work on your Cog builds
- Pharo 3 is in alpha state which currently implies that the debugger is
not stable
- I need to clean it up before

Anyway I'm happy to have it working.

2013/8/1 Eliot Miranda <eliot.miranda at gmail.com>

> 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
> --
> best,
> Eliot
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