In case someone would like to defend the performance of #~~ and #notNil, 
here's a little benchmark I just wrote:

| base ifNilIfNotNil notIdentical notEqual notNil |
Smalltalk garbageCollect.
base := (1 to: 5) collect: [ :run |
 	[ 1 to: 100000000 do: [ :i | ] ] timeToRun ].

ifNilIfNotNil := (1 to: 5) collect: [ :run |
 	[ 1 to: 100000000 do: [ :i | i ifNil: [ false ] ifNotNil: [ true ] ] ] timeToRun ].

notIdentical := (1 to: 5) collect: [ :run |
 	[ 1 to: 100000000 do: [ :i | i ~~ nil ] ] timeToRun ].

notEqual := (1 to: 5) collect: [ :run |
 	[ 1 to: 100000000 do: [ :i | i ~= nil ] ] timeToRun ].


notNil := (1 to: 5) collect: [ :run |
 	[ 1 to: 100000000 do: [ :i | i notNil ] ] timeToRun ].

{
 	#ifNilIfNotNil -> ifNilIfNotNil.
 	#notIdentical -> notIdentical.
 	#notEqual -> notEqual.
 	#notNil -> notNil
}
 	replace: [ :each |
 		each key -> (each value average - base average) asFloat ];
 	sort: #value ascending

"{#ifNilIfNotNil->73.6 . #notNil->336.8 . #notIdentical->511.6 . 
#notEqual->3354.0}"

So it's 4.5x faster than #notNil and 7x faster than #~~ on 64-bit Spur on 
Linux. Once we have jitted #~~, I'll change the code again.

Levente


On Tue, 28 Feb 2017, commits at source.squeak.org wrote:

> Levente Uzonyi uploaded a new version of Collections to project The Trunk:
> http://source.squeak.org/trunk/Collections-ul.739.mcz
>
> ==================== Summary ====================
>
> Name: Collections-ul.739
> Author: ul
> Time: 28 February 2017, 7:34:55.711173 pm
> UUID: 66c6ecf2-71bf-4e7d-80e6-5ace72fa19ba
> Ancestors: Collections-ul.738
>
> - use micro optimization for #~~ and #notNil in common collection methods
> - reuse one of the OrderedCollections in RunArray >> #scanFrom:
> - two other minor tweaks in KeyedSet
> - removed accidentally commited Set >> #includes2:
>
> =============== Diff against Collections-ul.738 ===============
>
> Item was changed:
>  ----- Method: Dictionary>>includesKey: (in category 'testing') -----
>  includesKey: key
>  	"Answer whether the receiver has a key equal to the argument, key."
> 
> + 	(array at: (self scanFor: key)) ifNil: [ ^false ] ifNotNil: [ ^true ]!
> - 	 ^(array at: (self scanFor: key)) notNil!
>
> Item was changed:
>  ----- Method: KeyedSet>>includesKey: (in category 'testing') -----
>  includesKey: key
> 
> + 	(array at: (self scanFor: key)) ifNil: [ ^false ] ifNotNil: [ ^true ]!
> - 	^ (array at: (self scanFor: key)) notNil!
>
> Item was changed:
>  ----- Method: KeyedSet>>remove:ifAbsent: (in category 'removing') -----
>  remove: oldObject ifAbsent: aBlock
>
>  	| index |
>  	index := self scanFor: (keyBlock value: oldObject).
> + 	(array at: index) ifNil: [ ^ aBlock value ].
> - 	(array at: index) == nil ifTrue: [ ^ aBlock value ].
>  	array at: index put: nil.
>  	tally := tally - 1.
>  	self fixCollisionsFrom: index.
>  	^ oldObject!
>
> Item was changed:
>  ----- Method: KeyedSet>>removeKey:ifAbsent: (in category 'removing') -----
>  removeKey: key ifAbsent: aBlock
>
>  	| index obj |
>  	index := self scanFor: key.
> + 	obj := (array at: index) ifNil: [ ^ aBlock value ].
> - 	(obj := array at: index) == nil ifTrue: [ ^ aBlock value ].
>  	array at: index put: nil.
>  	tally := tally - 1.
>  	self fixCollisionsFrom: index.
>  	^ obj enclosedSetElement!
>
> Item was changed:
>  ----- Method: RunArray class>>scanFrom: (in category 'instance creation') -----
>  scanFrom: strm
>  	"Read the style section of a fileOut or sources file.  nextChunk has already been done.  We need to return a RunArray of TextAttributes of various kinds.  These are written by the implementors of writeScanOn:"
>  	| runs values attrList char |
>  	(strm peekFor: $( ) ifFalse: [^ nil].
>  	runs := OrderedCollection new.
>  	[strm skipSeparators.
>  	 strm peekFor: $)] whileFalse:
>  		[runs add: (Number readFrom: strm)].
>  	values := OrderedCollection new.	"Value array"
>  	attrList := OrderedCollection new.	"Attributes list"
>  	[(char := strm peek) == nil] whileFalse: [
>  		(char isSeparator or: [ char = $!! ])
>  			ifTrue: [ "n.b. Skip $!! to meet expectations of RunArrayTest>>testScanFromTrailer.
>  					The example string used in that test does not seem to match the implemention
>  					of the fileOut serialization, but the test may be right and the implementation
>  					wrong. In any case, relax the parsing here to meet the test expectations, and to
>  					be more consistent with the original version of this method that assumed any
>  					unexpected charater to be a separator. -dtl Jan 2014"
>  				strm next "space, cr do nothing"]
>  			ifFalse: [char == $,
>  					ifTrue: [strm next.
>  						values add: attrList asArray.
> + 						attrList reset ]
> - 						attrList := OrderedCollection new]
>  					ifFalse: [attrList add:  (TextAttribute newFrom: strm)]
>  				]
>  		].
>  	values add: attrList asArray.
>  	^ self runs: runs asArray values: (values copyFrom: 1 to: runs size) asArray
>  "
>  RunArray scanFrom: (ReadStream on: '(14 50 312)f1,f1b,f1LInteger +;i')
>  "!
>
> Item was removed:
> - ----- Method: Set>>includes2: (in category 'testing') -----
> - includes2: anObject 
> - 
> - 	^((array at: (self scanFor: anObject)) == nil) not!
>
> Item was changed:
>  ----- Method: WeakIdentityDictionary>>includesKey: (in category 'testing') -----
>  includesKey: key
>  	"Answer whether the receiver has a key equal to the argument, key."
> 
> + 	(array at: (self scanFor: key))
> + 		ifNil: [
> + 			"it just has been reclaimed"
> + 			^false]
> + 		ifNotNil: [ :element |
> + 			element == vacuum
> + 				ifTrue: [ ^false ]
> + 				ifFalse: [ ^true ] ]!
> - 	^(array at: (self scanFor: key))
> - 		ifNil:
> - 			["it just has been reclaimed"
> - 			false]
> - 		ifNotNil: [:element | element ~~ vacuum]!
>
> Item was changed:
>  ----- Method: WeakSet>>includes: (in category 'testing') -----
>  includes: anObject
>
>  	(array at: (self scanFor: anObject))
>  		ifNil: [ ^false ]
> + 		ifNotNil: [ :object |
> + 			object == flag
> + 				ifTrue: [ ^false ]
> + 				ifFalse: [ ^true ] ]!
> - 		ifNotNil: [ :object | ^object ~~ flag ]!