[stdlib] Make _convertConstStringToUTF8PointerArgument transparent

[airspeedswift]

This function not being inlinable prevents optimizing it away, and it doesn't actually access any string internals.

[airspeedswift]

@swift-ci please test

[airspeedswift]

@swift-ci please benchmark

[lorentey]

We'll definitely want to change this implementation, but making this @_transparent doesn't prevent that. (And we can also remove the attribute if it turns out we want this to call internal things later.)

I think ideally this intrinsic would be eventually replaced by something that follows a withCString-style, closure based approach.

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
FlattenListLoop	956	1385	+44.9%	0.69x (?)
DropFirstArrayLazy	13	14	+7.7%	0.93x (?)

Code size: -O

Improvement	OLD	NEW	DELTA	RATIO
CString.o	6743	6375	-5.5%	1.06x
NSStringConversion.o	12108	11762	-2.9%	1.03x
ObjectiveCNoBridgingStubs.o	8364	8136	-2.7%	1.03x
ObjectiveCBridging.o	61999	61061	-1.5%	1.02x

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
FlattenListLoop	991	1416	+42.9%	0.70x (?)
IterateData	866	937	+8.2%	0.92x (?)
StringWalk	1520	1640	+7.9%	0.93x (?)
FlattenListFlatMap	2847	3063	+7.6%	0.93x (?)

Code size: -Osize

Improvement	OLD	NEW	DELTA	RATIO
CString.o	6433	6149	-4.4%	1.05x
NSStringConversion.o	11542	11232	-2.7%	1.03x
ObjectiveCNoBridgingStubs.o	8045	7847	-2.5%	1.03x
ObjectiveCBridging.o	58410	57408	-1.7%	1.02x

Performance: -Onone

Improvement	OLD	NEW	DELTA	RATIO
DataCountMedium	55	50	-9.1%	1.10x (?)

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: 6-Core 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

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 7adafca

[airspeedswift]

@swift-ci please test macOS platform

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 7adafca

[airspeedswift]

@swift-ci please test linux platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 7adafca

[airspeedswift]

@swift-ci please smoke test and merge