Removes redundant buffer zeroing in StringGuts.swift by using `init(unsafeUninitializedCapacity:initializingWith:)

[valeriyvan]

Removes redundant buffer zeroing by using init(unsafeUninitializedCapacity:initializingWith:) instead of init(repeating:count:)

[milseman]

@swift-ci please benchmark

[milseman]

@swift-ci please test

[milseman]

This looks great! Let me see if we have benchmark coverage.

[swift-ci]

Performance: -O

Code size: -O

Performance: -Osize

Improvement	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	3133	2853	-8.9%	1.10x (?)

Code size: -Osize

Performance: -Onone

Improvement	OLD	NEW	DELTA	RATIO
DataAppendDataSmallToSmall	3180	2820	-11.3%	1.13x (?)

Code size: -swiftlibs

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

[milseman]

Oh, this is only in the corelibs-foundation SPI, so it wouldn't have benchmark coverage. This function isn't called anymore (hence the deprecated availability). You might have greater effect targeting the version of this that's in corelibs-foundation here: https://github.com/apple/swift-corelibs-foundation/blob/master/Sources/Foundation/NSStringAPI.swift#L72

[valeriyvan]

swiftlang/swift-corelibs-foundation#2701