Make empty dictionary and set literals use the singleton

[Catfish-Man]

rdar://problem/34740754 let foo: [Int: String] = [:] allocates storage for empty dictionary

[Catfish-Man]

@swift-ci please benchmark

[Catfish-Man]

@swift-ci please benchmark

[Catfish-Man]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 2a9aa16

[lorentey]

Extremely approved!

[Catfish-Man]

@swift-ci please benchmark

[Catfish-Man]

@swift-ci please benchmark

[Catfish-Man]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - bf09c28

[airspeedswift]

there should be a facepalm GitHub reaction...

[Catfish-Man]

@swift-ci please benchmark

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 2a9aa16

[swift-ci]

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
NSDictionaryCastToSwift	4080	2310	-43.4%	1.77x
DataAppendDataLargeToLarge	50800	37200	-26.8%	1.37x (?)

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
DictionaryKeysContains.o	9903	10751	+8.6%	0.92x
DictionaryLiteral.o	1360	1392	+2.4%	0.98x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
NSDictionaryCastToSwift	4070	2310	-43.2%	1.76x
ObjectiveCBridgeStubFromNSDateRef	4480	4080	-8.9%	1.10x (?)

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
DictionaryKeysContains.o	7655	8103	+5.9%	0.94x

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Improvement
NSDictionaryCastToSwift	5320	3400	-36.1%	1.56x
Dictionary3	707	614	-13.2%	1.15x
CharIndexing_punctuatedJapanese_unicodeScalars	113200	99200	-12.4%	1.14x (?)
CharIndexing_russian_unicodeScalars	542400	477320	-12.0%	1.14x (?)
CharIndexing_tweet_unicodeScalars	1278200	1129480	-11.6%	1.13x (?)
CharIndexing_japanese_unicodeScalars	777520	688520	-11.4%	1.13x (?)
Histogram	9413	8414	-10.6%	1.12x (?)
DataAppendArray	5700	5100	-10.5%	1.12x (?)
CharIndexing_korean_unicodeScalars	622080	558320	-10.2%	1.11x (?)
CharIndexing_ascii_unicodeScalars	628840	566440	-9.9%	1.11x (?)
CharIndexing_chinese_unicodeScalars	476400	436640	-8.3%	1.09x (?)
CharIndexing_punctuated_unicodeScalars	136560	125200	-8.3%	1.09x (?)
CharIndexing_utf16_unicodeScalars	555440	515880	-7.1%	1.08x (?)
NormalizedIterator_fastPrenormal	1630	1520	-6.7%	1.07x (?)

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

[Catfish-Man]

@swift-ci please smoke test and merge

[Catfish-Man]

@swift-ci please smoke test and merge

[Catfish-Man]

@swift-ci please smoke test and merge

[Catfish-Man]

@swift-ci please smoke test and merge