[5.0 backport] conform Unsafe(Mutable)RawBufferPointer conform to _HasContiguousBytes

[weissi]

(cherry picked from commit f87ffd2)

backport of #21611 to swift-5.0-branch

[weissi]

@swift-ci please test

[milseman]

LGTM

[weissi]

@milseman thanks! @airspeedswift / @milseman there's special authorisation required to merge to the 5.0 branch, should I ping anyone or can either of you do that?

[airspeedswift]

I can approve it. However, should we have added benchmarks showing the perf improvement?

[airspeedswift]

Also tests of this new path, probably.

[weissi]

@swift-ci Please benchmark

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
DictionaryOfAnyHashableStrings_lookup	4225	4665	+10.4%	0.91x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
Hanoi	4031	3690	-8.5%	1.09x

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Improvement
PrefixWhileArray	35038	31329	-10.6%	1.12x

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

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

[weissi]

@airspeedswift does #21706 look good for the benchmarks? One you're happy with this and it's merged to master, I'll make a separate PR targeting swift-5.0-branch. Then we can merge that and see the results of this very PR.

[weissi]

@airspeedswift I assume you're happy with tests instead of benchmarks too (#21743)?

[airspeedswift]

@weissi yep