Fix RangeIteration and StringWalk benchmarks' use of globals

[atrick]

StringWalk benchmark: remove global variable access from the loop.

Global variables are being used as if they are "free black
holes". They might be even more expensive than actually calling
blackHole. But the right thing to do is use local variables and pass
them into a black hole at the end of computation.

RangeIteration benchmark: remove global variable access from the critical path.

This benchmark was "accidentally" accessing a global variable in the
main iteration loop, which is in some situations very expensive and
has nothing to do with what the benchmark is trying to test.

[atrick]

@swift-ci benchmark

[atrick]

@swift-ci smoke test

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
RangeIterationSigned	87	108	+24.1%	0.81x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	5587	4206	-24.7%	1.33x (?)

Code size: -O

Improvement	OLD	NEW	DELTA	RATIO
RangeIteration.o	1748	1684	-3.7%	1.04x

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
RangeIterationSigned	87	108	+24.1%	0.81x

Code size: -Osize

Improvement	OLD	NEW	DELTA	RATIO
RangeIteration.o	1738	1674	-3.7%	1.04x

Performance: -Onone

Improvement	OLD	NEW	DELTA	RATIO
RangeIterationSigned	9913	8177	-17.5%	1.21x

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

[lorentey]

Interesting; this was a weird pattern.

[atrick]

@swift-ci smoke test

[milseman]

LGTM

[atrick]

@swift-ci smoke test

[atrick]

@swift-ci smoke test