[Vm-dev] A BlockClosure optimization

[Igor Stasenko]

On 15 May 2010 20:12, Eliot Miranda <eliot.miranda at gmail.com> wrote:
>
>
>
> On Sat, May 15, 2010 at 10:02 AM, Igor Stasenko <siguctua at gmail.com> wrote:
>>
>> On 15 May 2010 19:35, Eliot Miranda <eliot.miranda at gmail.com> wrote:
>> >
>> > Hi Igor,
>> > On Fri, May 14, 2010 at 10:14 PM, Igor Stasenko <siguctua at gmail.com> wrote:
>> >>
>> >> Hello,
>> >>
>> >> i just thought, that we could optimize a closure-copy mechanism
>> >> to reuse a closure (BlockClosure instance), which were created before
>> >> for same context.
>> >
>> >     that's a good one.  Also good is precomputing closures for blocks that don't capture their dynamic environment (don't close over any variables and don't include an ^-return; VW parlance "clean blocks").  Another one, but this requires a new bytecode set/vm is to not reify the current context for blocks that don't contain ^-returns (VW parlance "copying blocks").  But these last two should be preferences since they affect debugging (within a block so optimized one can't discover its origin).
>> > (VW parlance for normal blocks is "full blocks"; all blocks in my closure compiler are full, so the current context must be reified, not an issue in the non-Cog VMs as its already there, but it is an issue in a faster VM, it often means two allocations instead of one).
>> >
>> >
>> >>
>> >> A mechanism of optimization can illustrated by following code.
>> >>
>> >> Suppose , you having a method, which using a loop:
>> >>
>> >> myMethod
>> >>
>> >>  1 to: 100 do: [:i |
>> >>   dict at: i ifAbsent: [ foo bar ] ]
>> >>
>> >> The idea is to copy a closure from method's literal frame just once,
>> >> and store it into temp,
>> >> and then reuse it like following:
>> >>
>> >> myMethod
>> >> | closure |
>> >>  1 to: 100 do: [:i |
>> >>   dict at: i ifAbsent: (closure ifNil: [ closure := [ foo bar ] ] ) ]
>> >>
>> >> ----------
>> >>
>> >> A simple benchmark shows that we could gain from it:
>> >>
>> >> [ 1000000 timesRepeat: [ [ 1+1] value ] ] timeToRun
>> >> 670
>> >>
>> >> [
>> >> | closure |  closure := nil.
>> >> 1000000 timesRepeat: [
>> >>        (closure ifNil: [ closure := [ 1+1] ]) value ]
>> >> ] timeToRun
>> >> 518
>> >>
>> >> As you can see, even implemented in smalltalk (without any changes to
>> >> VM) it shows
>> >> a significant performance boost.
>> >
>> > That's what's nice about this optimization.  It doesn't require any VM modifications ;)
>> >
>>
>> Yes, it doesn't.  A compiler can detect, if a closure push bytecode found
>> in a loop (a bytecode block which having a backwards jump)
>> and then add a temp and emit the bytecodes to (re)use that temp.
>>
>> A potential VM modification could be to add a bytecode like
>> "pushBlockClosure: litIndex orReuseTemp: tempIndex ",
>> which should be equivalent to given piece: closure ifNil: [ closure := [ 1+1] ].
>>
>> Too bad, this technique can't be used for a nested blocks, since they
>> have to use different 'outerContext'
>> per each activation of outer closure. :(
>
> Is that true?  Isn't the real issue that if any niladic block contains an ^-return then that block any any blocks it is nested within cannot be shared?  It would be good to try and state the invariants in formal English before you implement.  The implementation will be much better as a result.

My English is not good enough to explain what i mean :)

I can show this with a code:

1 to: 1000 do: [ :i |    dict at: i ifAbsent: [  | x |  x := self foo:
i. dict2 at: i ifAbsent: [ x ] ] ]

here, the outer closure "[  | x | ... ]" can use caching.
While inner " [ x ] " can't, because its using a temp, created in an
outer block.
This is because each activation of outer block creating a new context.

Sure thing, for blocks which don't refer to the state of outer
context, it still should be possible.
And even if nested block refers to the state of method's context (such
as self or its temps):

1 to: 1000 do: [ :i |    dict at: i ifAbsent: [ dict2 at: i ifAbsent:
[ self foo: i ] ] ]

could be written as:

| {i} closureInner closureOuter |

closureInner := [self foo: i].
closureOuter := [ dict2 at: i ifAbsent: closureInner ].
1 to: 1000 do: [ :i |  dict at: i ifAbsent: closureOuter ].


yes, i know , that the above is incorrect code,
but take into account that temp 'i' actually moved to method's scope,
because #to:do: loop is inlined by compiler,
hence i placed {i}  to indicate that.


>>
>> >>
>> >> Of course, if we put something, which loads processor by real work,
>> >> instead of just [1+1],
>> >> the difference will be less significant.
>> >>
>> >> But apart from this, lying the fact, that copying closure object each
>> >> time means memory allocation,
>> >> and hence leads to more frequent GC.
>> >
>> > What real codes have you seen the costs in?  I think they're there (VisualWorks went to some effort to reduce costs using the two other optimizations I described) but how big?  In any case you should implement this and see whether any useful benchmarks (e.g. system recompilation) show measurable speedups.
>>
>> I never went to exploring a bytecode emission code in
>> Encoder / MethodNode / BytecodeAgnosticMethodNode.
>> It is hellishly complex and its easy to get lost there. :)
>> But i could try.
>
> That's not what I meant.  I simply meant that you need to measure the effect of your optimization (caching blocks used in loops in a temp) on real use cases such as e.g. recompiling the entire system, which is a decent benchmark.

Please, elaborate, what you mean then. Do you mean, manually rewrite
the code in appropriate places
in Compiler toolchain, where its possible?

>>
>> >>
>> >> --
>> >> Best regards,
>> >> Igor Stasenko AKA sig.
>>
>>
>> --
>> Best regards,
>> Igor Stasenko AKA sig.
>
>
>



-- 
Best regards,
Igor Stasenko AKA sig.