stdlib: two small Array improvements

[eeckstein]

• make the non-native Array.count smaller more efficient

Use _CocoaArrayWrapper.endIndex which returns the NSArray.count.
In the old version, "count" translated to RandomAccessCollection.count, which ended up in multiple calls to endIndex.

• Add custom .first to Array (copied from @airspeedswift's [stdlib] Add custom .first to Array #28349)

Without this, Array.first compiled down to RandomAccessCollection.first, which ended up in pretty unefficient code.

rdar://problem/46291397

[eeckstein]

@swift-ci test

[eeckstein]

@swift-ci benchmark

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
NSStringConversion.Long	492	556	+13.0%	0.88x (?)
StringBuilderLong	840	930	+10.7%	0.90x (?)
Set.isSuperset.Seq.Empty.Int	48	52	+8.3%	0.92x (?)
MapReduceLazyCollectionShort	26	28	+7.7%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
ArrayAppendGenericStructs	910	610	-33.0%	1.49x (?)
EqualSubstringSubstring	27	21	-22.2%	1.29x
LessSubstringSubstring	27	21	-22.2%	1.29x (?)
EqualSubstringSubstringGenericEquatable	27	21	-22.2%	1.29x
LessSubstringSubstringGenericComparable	27	21	-22.2%	1.29x
EqualStringSubstring	28	22	-21.4%	1.27x (?)
Dictionary4	195	157	-19.5%	1.24x
EqualSubstringString	27	22	-18.5%	1.23x
StringComparison_longSharedPrefix	356	319	-10.4%	1.12x (?)
LazilyFilteredArrayContains	18500	16700	-9.7%	1.11x
ArraySetElement	284	263	-7.4%	1.08x (?)
PrefixWhileAnySeqCntRange	203	189	-6.9%	1.07x (?)
ObjectiveCBridgeStubFromNSDate	2810	2620	-6.8%	1.07x (?)

Code size: -O

Improvement	OLD	NEW	DELTA	RATIO
ClassArrayGetter.o	5528	5019	-9.2%	1.10x
NopDeinit.o	5297	5044	-4.8%	1.05x
ArrayOfGenericRef.o	11422	10910	-4.5%	1.05x
ArrayOfRef.o	11539	11043	-4.3%	1.04x
COWTree.o	12236	11892	-2.8%	1.03x
DictionaryGroup.o	17573	17224	-2.0%	1.02x

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
NSStringConversion.Long	484	545	+12.6%	0.89x (?)
NSStringConversion.Medium	234	256	+9.4%	0.91x
Improvement	OLD	NEW	DELTA	RATIO
LessSubstringSubstring	27	21	-22.2%	1.29x
EqualSubstringString	27	21	-22.2%	1.29x (?)
LessSubstringSubstringGenericComparable	27	21	-22.2%	1.29x
EqualSubstringSubstring	28	22	-21.4%	1.27x (?)
EqualStringSubstring	28	22	-21.4%	1.27x (?)
EqualSubstringSubstringGenericEquatable	28	22	-21.4%	1.27x (?)
Set.subtracting.Empty.Box	8	7	-12.5%	1.14x (?)
RemoveWhereMoveInts	19	17	-10.5%	1.12x (?)
Array2D	4160	3728	-10.4%	1.12x (?)
StringComparison_longSharedPrefix	356	320	-10.1%	1.11x (?)
SortIntPyramid	530	480	-9.4%	1.10x
DataCreateEmptyArray	1650	1500	-9.1%	1.10x (?)
ArraySetElement	284	264	-7.0%	1.08x (?)

Code size: -Osize

Improvement	OLD	NEW	DELTA	RATIO
ClassArrayGetter.o	5619	5125	-8.8%	1.10x
NopDeinit.o	6257	5838	-6.7%	1.07x
ArrayOfRef.o	12635	12091	-4.3%	1.04x
ArrayOfGenericRef.o	12598	12070	-4.2%	1.04x
COWTree.o	13020	12508	-3.9%	1.04x
DictionaryGroup.o	13684	13396	-2.1%	1.02x
DictionarySubscriptDefault.o	17559	17335	-1.3%	1.01x
MapReduce.o	25459	25203	-1.0%	1.01x

Performance: -Onone

Improvement	OLD	NEW	DELTA	RATIO
LazilyFilteredArrays2	82900	48900	-41.0%	1.70x
BucketSort	8231	6176	-25.0%	1.33x
EqualSubstringSubstring	32	26	-18.7%	1.23x (?)
EqualSubstringSubstringGenericEquatable	31	26	-16.1%	1.19x (?)
LessSubstringSubstringGenericComparable	31	26	-16.1%	1.19x (?)
LessSubstringSubstring	32	27	-15.6%	1.19x (?)
EqualStringSubstring	32	27	-15.6%	1.19x
EqualSubstringString	32	27	-15.6%	1.19x
ArrayOfPOD	722	648	-10.2%	1.11x (?)

Code size: -swiftlibs

Improvement	OLD	NEW	DELTA	RATIO
libswiftStdlibUnittest.dylib	360448	356352	-1.1%	1.01x

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the
regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false
alarms. Unexpected regressions which are marked with '(?)' are probably noise.
If you see regressions which you cannot explain you can try to run the
benchmarks again. If regressions still show up, please consult with the
performance team (@eeckstein).

Hardware Overview

  Model Name: Mac mini
  Model Identifier: Macmini8,1
  Processor Name: Intel Core i7
  Processor Speed: 3.2 GHz
  Number of Processors: 1
  Total Number of Cores: 6
  L2 Cache (per Core): 256 KB
  L3 Cache: 12 MB
  Memory: 64 GB

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - ed69ab5456ae082a309c19d4825404f5dfe39663

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - ed69ab5456ae082a309c19d4825404f5dfe39663

[eeckstein]

@swift-ci test

[eeckstein]

@swift-ci test

[airspeedswift]

thanks!