[stdlib] Dictionary.Keys, .Values: Implement custom iterators

[lorentey]

Implement custom iterators for Dictionary's Keys and Values views.

IndexingIterator is pretty slow, especially for Cocoa indices, and especially with #19599. Defining these trivial iterators should be a nice speed boost for any code that walks over these views.

[lorentey]

@swift-ci please test

[lorentey]

@swift-ci please smoke benchmark

[lorentey]

@swift-ci please test source compatibility

[lorentey]

cc @airspeedswift

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
StringComparison_fastPrenormal	840	919	+9.4%	0.91x
FloatingPointPrinting_Float_description_uniform	5182	5585	+7.8%	0.93x
Improvement
IterateData	1738	1564	-10.0%	1.11x (?)
CStringLongAscii	3538	3293	-6.9%	1.07x
UTF8Decode_InitFromData	1391	1295	-6.9%	1.07x (?)

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
DictionarySwap.o	30403	33467	+10.1%	0.91x
Improvement
DictionaryKeysContains.o	20343	19395	-4.7%	1.05x
DictionaryBridgeToObjC.o	7543	7263	-3.7%	1.04x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
FloatingPointPrinting_Float_description_uniform	5193	5836	+12.4%	0.89x
Improvement
IterateData	1830	1626	-11.1%	1.13x
CStringLongAscii	3547	3295	-7.1%	1.08x

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
DictionarySwap.o	26827	29091	+8.4%	0.92x
Improvement
DictionaryKeysContains.o	17735	17099	-3.6%	1.04x
DictionaryBridgeToObjC.o	7029	6917	-1.6%	1.02x

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Regression
StrToInt	82601	103552	+25.4%	0.80x
Improvement
DictionaryBridgeToObjC_Access	1859	1260	-32.2%	1.48x
CharIndexing_punctuated_unicodeScalars_Backwards	92817	76466	-17.6%	1.21x
CharIndexing_japanese_unicodeScalars	450228	376360	-16.4%	1.20x
CharIndexing_punctuated_unicodeScalars	83261	70008	-15.9%	1.19x
CharIndexing_tweet_unicodeScalars	738707	621306	-15.9%	1.19x
CharIndexing_tweet_unicodeScalars_Backwards	850535	717851	-15.6%	1.18x
CharIndexing_korean_unicodeScalars	359722	304015	-15.5%	1.18x
CharIndexing_ascii_unicodeScalars	370974	313740	-15.4%	1.18x
CharIndexing_chinese_unicodeScalars	279992	237081	-15.3%	1.18x
DropFirstArrayLazy	34391	31161	-9.4%	1.10x
PrefixArrayLazy	34214	31078	-9.2%	1.10x

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 Pro
  Model Identifier: MacPro6,1
  Processor Name: 12-Core Intel Xeon E5
  Processor Speed: 2.7 GHz
  Number of Processors: 1
  Total Number of Cores: 12
  L2 Cache (per Core): 256 KB
  L3 Cache: 30 MB
  Memory: 64 GB

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[lorentey]

Source compat failure (fluent) is unrelated; see #19594.