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Stoop 300-block optimizationinvw

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    Stoop 300-block optimizationinvw Stoop 300-block optimizationinvw Presentation Transcript

    • Blocks and Optimization in VW
    •  
      • [ :x :y | |tmp| ...]
      • value
      • value:
      • value: value:
      • value: value: value:
      • valueWithArguments:
      • In VisualWorks there are four types of blocks:
        • Full Blocks
        • Copying Blocks
        • Clean Blocks
        • Inlined Blocks
      • The programmer does not have to explicitly mention.
      • Inferred by the compiler. However, knowing the subtle differences allows the programmer to write more efficient code.
      Blocks and Optimization in VW
      • Read and assign temporary variables.
      • Block containing explicit return ^.
      • Compiled in a BlockClosure.
      • Evaluation by the creation of an explicit MethodContext or BlockContext object instead of using a pseudo-object contained in the stack.
      • Most costly
      • Instead of:
      • m1: arg1 m1: arg1
      • arg1 isNil ^ arg1 isNil
      • ifTrue: [^ 1] ifTrue: [1]
      • ifTrue: [^ 2] ifTrue: [2]
      Full Blocks
      • Read temporary variables but do not assign them.
      • No explicit return.
      • Access instance variables of self and assign them.
      • Not compiled into a BlockClosure.
      • They are compiled by copying every access into the block, thus avoiding explicit references to a context where the copied variables appear.
      • Their arguments and temporaries are merged into the enclosing method’s context as “compiler-generated temporaries”.
      Copying Blocks
      • Contain only reference block temporary variables or global variables.
      • No reference to self or to instance variables.
      • nodes do: [:each | each name = #stef]
      • nodes select: [:each | each isLocal]
      Clean Blocks
      • Code of certain methods, like whileFalse: ifTrue:, is directly inlined into the code of the calling method.
      • The literal blocks (without arguments) passed as argument to such methods are also inlined in the byte-code of the calling method.
      • Inlined methods are whileTrue, whileTrue:, whileFalse, whileFalse:, and: or:, ifTrue:, ifFalse:, ifTrue:ifFalse:, ifFalse:ifTrue:, to:do:, to:do:by:
      • Look in MessageNode>>transform* methods to see the inlining
      Inlined Blocks
      • testInLined
      • 1 to: 5 do: [:x| ]
      • Compiled into :
      • | t1 |
      • t1 := 1.
      • [t1 <= 5] whileTrue: [t1 := t1 + 1].
      • But no BlockClosure is created (look into the byte codes)
      Inlined Blocks
      • Instead of:
      • |t|
      • [:x | t := x foo] value: 1.
      • t := t * 2.
      • ^t
      • The reference to t inside the block makes it at least a copying block.
      • t := makes it full.
      • With the following we have a clean block.
      • |t|
      • t := [:x | x foo] value:1.
      • t := t * 2.
      • ^t
      From Full to Copy
      • Full blocks are evaluated in a separate context.
      • The following code evaluates to false:
      • |outerContext answer|
      • outerContext := thisContext.
      • (1 to: 1) do: [:i | answer := thisContext == outerContext].
      • ^answer
      • But the following evaluates to true because: to:do: is an inlined block
      • |outerContext answer|
      • outerContext := thisContext.
      • 1 to: 1 do: [:i | answer := thisContext == outerContext].
      • ^answer
      • So it is better to use to:do: than (to:) do:
      Contexts
      • Instead of:
      • |maxNumber|
      • maxNumber := 0.
      • #(1 2 43 56 2 49 3 2 0 ) do: [:each| maxNumber := each max: maxNumber].
      • ^maxNumber
      • Write
      • #(1 2 43 56 2 49 3 2 0 ) inject: 0 into: [:maxNumber :ele| maxNumber max: ele]
      • no need for a temporary variable
      • full block becomes a clean block
      inject:into:
      • str1 , str2 creates a new structure in which str1 and str2 elements are stored
      • SequenceableCollection>>, aSequenceableCollection
      • &quot;Answer a copy of the receiver concatenated with the argument,
      • a SequenceableCollection.&quot;
      • ^self copyReplaceFrom: self size + 1
      • to: self size
      • with: aSequenceableCollection
      • SequenceableCollection>>copyReplaceFrom: start to: stop with: replacementCollection
      • &quot;Answer a copy of the receiver satisfying the following conditions:
      • ..&quot;
      About String Concatenation
      • Suppose that we want to concatenate a pretty long list of strings, for example the keys of the Smalltalk dictionary.
      • |bigString|
      • bigString := String new.
      • Smalltalk keys do: [:aString | bigString := bigString, aString].
      • Here the assignment of bigString leads to a Full Block
      • We can suppress the assignment like that and thus obtain a clean block
      • |aStream|
      • aStream:= WriteStream on: String new.
      • Smalltalk keys do: [:aString | aStream nextPutAll: aString].
      Streams, Blocks, and Optimization
      • inject:into: allows us to suppress the reference to variables that are outside the block and to obtain a clean block.
      • |aStream|
      • aStream:= WriteStream on: String new.
      • Smalltalk keys inject: aStream
      • into: [:cumul :aString| cumul nextPutAll: aString. cumul]
      Streams, Blocks, and Optimization (ii)
      • Instance Variables:
      • method <CompiledBlock>
      • outerContext <Context | nil>
      • copiedValues <Object | Array | nil>
      • &quot;Clean&quot; closure with no references to anything from outer scopes. A clean closure has outerContext = nil and copiedValues = empty Array.
      • &quot;Copying&quot; closure that copies immutable values from outer scopes when the closure is created. A copying closure has outerContext = nil and copiedValues = Object or Array.
      • &quot;Full&quot; closure that retains a reference to the next outer scope. A full closure has outerContext ~= nil and copiedValues = nil.
      • As an optimization, copiedValues holds the single copied value if there is exactly one, or an Array of values if there is more than one. Note that if there is a single copied value, the value being copied can be nil, so testing for nil in copiedValues is not a reliable means of classifying closures. The way to check whether a closure has copied values is to ask its method whether numCopiedValues > 0.
      BlockClosure Class Comments
      • Now if we use a stream for the Smalltalk keys we can avoid an iteration method. With whileFalse: that is inlined the block itself will be inlined.
      • |aReadStream aWriteStream|
      • aReadStream := ReadStream on: Smalltalk keys asArray.
      • aWriteStream := WriteStream on: String new.
      • [aReadStream atEnd] whileFalse: [aWriteStream nextPutAll: a ReadStream next].
      • Optimization Yes, but Readibility First
      Streams, Blocks, and Optimization (iii)
    • Summary
      • Blocks
      • Try to define clean blocks