[stdlib] Dictionary.updateValue(_:,forKey:): Don’t overwrite the existing key

[lorentey]

updateValue(_:, forKey:) currently replaces the old key, which is undocumented, unnecessary and somewhat surprising. The previous incarnation of Dictionary implemented updates this way because of a technical detail; this wrinkle is now eliminated.

rdar://problem/32144087

This change is detectable for types where equality is not the same as identity, so this is technically source breaking. Therefore at minimum, this requires a discussion on the fora before landing.

I know of no use cases where the old behavior was actually desirable; most/all practical code that would care about this seem to prefer to leave the old key in place.

[lorentey]

@swift-ci please test

[airspeedswift]

I don't think we'd classify this as falling under the source-breaking umbrella so pretty sure you're safe.

[lorentey]

This may speed up the Dictionary subscript setter a bit.

@swift-ci please smoke benchmark

[swift-ci]

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
IterateData	1563	1732	+10.8%	0.90x (?)
Improvement
Dictionary4OfObjects	377	348	-7.7%	1.08x

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
DictionaryKeysContains.o	20535	20183	-1.7%	1.02x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
IterateData	1649	1834	+11.2%	0.90x (?)
Improvement
Dictionary4OfObjects	469	413	-11.9%	1.14x

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
DictionaryKeysContains.o	18247	17895	-1.9%	1.02x

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

---

[lorentey]

Yeah, the speedup is mostly below the reporting threshold.