On 15 May 2010 20:12, Eliot Miranda <
eliot.miranda@gmail.com> wrote:
>
>
>
> On Sat, May 15, 2010 at 10:02 AM, Igor Stasenko <
siguctua@gmail.com> wrote:
>>
>> On 15 May 2010 19:35, Eliot Miranda <
eliot.miranda@gmail.com> wrote:
>> >
>> > Hi Igor,
>> > On Fri, May 14, 2010 at 10:14 PM, Igor Stasenko <
siguctua@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.