[stdlib] Unchecked subscript on UnsafeBufferPointer

[milseman]

Add a use an unchecked subscript on UnsafeBufferPointer, which skips
debugPrecondition checks (in case we're not inlined) as well as a
force-unwrap check.

[milseman]

@swift-ci please benchmark

[milseman]

@swift-ci please test source compatibility

[milseman]

@swift-ci please test

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
EqualStringSubstring	10	11	+10.0%	0.91x
Improvement
ObjectiveCBridgeStringHash	69	41	-40.6%	1.68x
ObjectiveCBridgeStringCStringUsingEncoding	564	437	-22.5%	1.29x
ObjectiveCBridgeStringGetASCIIContents	295	238	-19.3%	1.24x
CharIteration_korean_unicodeScalars	3409	2984	-12.5%	1.14x
ObjectiveCBridgeStringCompare2	763	679	-11.0%	1.12x
ObjectiveCBridgeStringCompare	809	722	-10.8%	1.12x
ObjectiveCBridgeStringIsEqual	159	144	-9.4%	1.10x
ObjectiveCBridgeStringRangeOfString	773	705	-8.8%	1.10x
StringEqualPointerComparison	589	538	-8.7%	1.09x
CharIteration_chinese_unicodeScalars	2599	2385	-8.2%	1.09x (?)
NormalizedIterator_ascii	176	164	-6.8%	1.07x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
NopDeinit	40207	55250	+37.4%	0.73x
CharIteration_ascii_unicodeScalars	2275	2453	+7.8%	0.93x
Improvement
ObjectiveCBridgeStringHash	69	41	-40.6%	1.68x
ObjectiveCBridgeStringCStringUsingEncoding	549	439	-20.0%	1.25x
ObjectiveCBridgeStringGetASCIIContents	293	238	-18.8%	1.23x
IterateData	1406	1218	-13.4%	1.15x
ObjectiveCBridgeStringIsEqual	160	141	-11.9%	1.13x
ObjectiveCBridgeStringCompare	801	713	-11.0%	1.12x
ObjectiveCBridgeStringCompare2	766	683	-10.8%	1.12x
ObjectiveCBridgeStringRangeOfString	786	707	-10.1%	1.11x
StringEqualPointerComparison	564	512	-9.2%	1.10x
StringComparison_nonBMPSlowestPrenormal	1341	1245	-7.2%	1.08x
NormalizedIterator_ascii	176	164	-6.8%	1.07x

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Improvement
ObjectiveCBridgeStringHash	69	43	-37.7%	1.60x
Histogram	5656	4482	-20.8%	1.26x
ObjectiveCBridgeStringCStringUsingEncoding	552	446	-19.2%	1.24x
ObjectiveCBridgeStringGetASCIIContents	296	244	-17.6%	1.21x
ObjectiveCBridgeStringCompare2	771	680	-11.8%	1.13x
ObjectiveCBridgeStringCompare	810	716	-11.6%	1.13x
ObjectiveCBridgeStringIsEqual	162	144	-11.1%	1.12x
ObjectiveCBridgeStringRangeOfString	774	705	-8.9%	1.10x
StrComplexWalk	7895	7272	-7.9%	1.09x

Code size: -swiftlibs

TEST	OLD	NEW	DELTA	RATIO
Improvement
libswiftSwiftPrivate.dylib	45056	40960	-9.1%	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: 8-Core Intel Xeon E5
  Processor Speed: 3 GHz
  Number of Processors: 1
  Total Number of Cores: 8
  L2 Cache (per Core): 256 KB
  L3 Cache: 25 MB
  Memory: 64 GB

--------------

[milseman]

@swift-ci please smoke test and merge

[milseman]

@swift-ci please smoke test