[stdlib] _HashTable.copyContents: Fix out of bounds access

[lorentey]

There is a distressing typo in _HashTable that causes bitmap copy operations to copy up to 64 times more memory than necessary. This sometimes leads to the copy operation overrunning the tail-allocated buffer.

https://bugs.swift.org/browse/SR-9348

[lorentey]

@swift-ci please test

[aschwaighofer]

LGTM

[lorentey]

I suspect we don't have CoW benchmarks, but if we do, they should show up as improvements here.

@swift-ci benchmark

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
StringEqualPointerComparison	600	657	+9.5%	0.91x (?)

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
StringEqualPointerComparison	571	628	+10.0%	0.91x (?)

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Regression
ArrayOfPOD	778	856	+10.0%	0.91x (?)

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

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