2010/8/11 James Ladd <james_ladd@hotmail.com>

Hi Eliot,

That is a very nice blog you linked me to. Lots for me to read there.

I read this ...

The final bytecode is more interesting.
��143 ��10001111 llllkkkk jjjjjjjj iiiiiiii ��Push Closure Num Copied llll Num Args kkkk BlockSize jjjjjjjjiiiiiiii
This creates a closure, initializing it from the arguments and the current context.

I'm missing something, is "num copied� llll"� the number of temps copied from the containing method?

It is the number of values copied from the stack that become locals of the closure. �In�

Collection>>inject: thisValue into: binaryBlock�

"Accumulate a running value associated with evaluating the argument,�

binaryBlock, with the current value of the argument, thisValue, and the�

receiver as block arguments. For instance, to sum the numeric elements�

of a collection, aCollection inject: 0 into: [:subTotal :next | subTotal +�

next]."

| nextValue |

nextValue := thisValue.

self do: [:each | nextValue := binaryBlock value: nextValue value: each].

^nextValue

the closure�[:each | nextValue := binaryBlock value: nextValue value: each]�accesses two non-local variables binaryBlock & nextValue. �Because�binaryBlock is not modified its value can be copied directly but because nextValue is modified it cannot be copied and a shared location must be used to hold the value. �So the compiler arranges to create an (in this case one-element) array (an indirection vector) to hold nextValue. �The hidden variable holding the indirection vector is not modified so its value (the indirection vector) can be copied. �Hence�[:each | nextValue := binaryBlock value: nextValue value: each] has two copied values, one the value of binaryBlock and another a hidden Array that holds nextValue. �Hence the bytecode:

push: (Array new: 1) # create the indirection vector

popIntoTemp: 2 # store it as the 3rd temp (after firstValue & binaryBlock)

pushTemp: 0 # push firstValue

popIntoTemp: 0 inVectorAt: 2 # initialize nextValue with firstValue

self # push the receiver of #do:

pushTemp: 1 # push the first copied value binaryBlock

pushTemp: 2 # push the second copied value, the indirection vector holding nextValue�

closureNumCopied: 2 numArgs: 1 bytes 31 to 40 # create the closure with 1 arg and 2 copied values

pushTemp: 1 # push binaryBlock (the 1st copied value, the 2nd temp after each)

pushTemp: 0 inVectorAt: 2 # push nextValue, the 1st slot in the indirection vector which is the 3rd temp

pushTemp: 0 # push each

send: #value:value: (2 args) # evaluate�binaryBlock value: nextValue value: each�

storeIntoTemp: 0 inVectorAt: 2 # store the result into nextValue (1st slot in indirection vector)

blockReturn # return nextValue as the result of the block

send: #do: (1 arg) # evaluate self do: [:each|...]

pop # discard the result of�elf do: [:each|...]

pushTemp: 0 inVectorAt: 2 # push nextValue

returnTop # return nextValue as the result of inject:into:

If this still doesn't make sense look at the version in the first blog post that uses an explicit array and compare it with the above.

HTH

Eliot

Rgs, James.

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