👷🏼‍♂️[stdlib] Experiment with alternative ways to do grapheme breaking

[lorentey]

This is a draft attempt at eliminating redundant lookups of Unicode properties during the grapheme breaking process. My hope is that this will (eventually) considerably speed up String's character views, but that will take a bit of work -- String indexing operations currently have no easy way to preserve information about scalars that lie on a Character boundary. (Unfortunately, most scalars tend to lie on a Character boundary...)

When finding grapheme breaks in a string such as ”Cafe\u{301}”, operations such as String.count currently execute the following independent steps:

var state = _GraphemeBreakingState()
state.shouldBreak(“C”, “a”) // 2 lookups, true
state.shouldBreak(“a”, “f”) // 2 lookups, true
state.shouldBreak(“f”, “e”) // 2 lookups, true
state.shouldBreak(“e”, “\u{301}”) // 2 lookups, false
// Total: 8 property lookups

Each shouldBreak(a, b) invocation individually looks up grapheme breaking properties for both scalar values, which includes calling nontrivial functions such as hasBreakWhenPaired and Unicode._GraphemeBreakProperty(from:).

This means that during the course of the string operation, we’re retrieving this information twice for each scalar, so we are duplicating nontrivial work. We should be able to significantly speed up grapheme breaking by caching this metadata directly in the state, and preserving it across character boundaries.

var state = _GraphemeBreakingState()
state.hasBreak(before: “C”) // 1 lookup, true
state.hasBreak(before: “a”) // 1 lookup, true
state.hasBreak(before: “f”) // 1 lookup, true
state.hasBreak(before: “e”) // 1 lookup, true
state.hasBreak(before: “\u{301}”) // 1 lookup, false
// Total: 5 property lookups

To make this work well, I think we need to change String.Index to cache the grapheme breaking property of the addressed scalar, rather than the stride of the addressed Character, as it currently does. This will somewhat slow down code that iterates over strings using indices, but I suspect the improved grapheme breaking throughput will largely make up for this. (If it doesn't, we'll "just" need to also replace the huge switch statements in the current core grapheme breaking algorithms with an artisanal, small batch lookup table.)

Additionally, having indices only store information about their addressed scalar would eliminate a large source of index invalidation surprises: currently range replacements in native Swift strings sometimes invalidate indices way below the slice that they affected, which usually comes as a shock to people.

In case this proves impractical to do in String itself, at minimum we could still use this to speed up _CharacterRecognizer.

rdar://103970243

[lorentey]

I hope to see some improvements in CharacterRecognizer benchmarks at least. (String walks probably won't like the new indices yet, but we'll see...)

@swift-ci benchmark

[lorentey]

@swift-ci benchmark

[lorentey]

@swift-ci benchmark

[lorentey]

Yay, it works:

CharacterRecognizer.mixed                   21.156      13.096     -38.1%   **1.62x**
StringDistance.characters.mixed             3726.0      2568.0     -31.1%   **1.45x**

There are lots of regressions elsewhere, as expected. Substrings and backwards index steps haven't been updated yet.

------- Performance (x86_64): -O -------

REGRESSION                                  OLD         NEW        DELTA    RATIO    
FindString.Loop1.Substring                  300.25      481.4      +60.3%   **0.62x**
FindString.Rec3.Substring                   115.235     179.25     +55.6%   **0.64x**
FindString.Rec3.String                      119.111     181.455    +52.3%   **0.66x**
CSVParsing.Char                             142.563     211.889    +48.6%   **0.67x**
SubstringTrimmingASCIIWhitespace            135.833     199.875    +47.1%   **0.68x**
StringMatch                                 5450.0      7881.25    +44.6%   **0.69x**
StringHasSuffixUnicode                      76111.111   102187.5   +34.3%   **0.74x (?)**
RomanNumbers2                               465.0       624.0      +34.2%   **0.75x**
RemoveWhereQuadraticString                  221.778     274.125    +23.6%   **0.81x**
Calculator                                  137.308     158.417    +15.4%   **0.87x (?)**
CharacterRecognizer.ascii                   63.444      71.3       +12.4%   **0.89x (?)**
LineSink.characters.alpha                   67.714      76.063     +12.3%   **0.89x (?)**
DropLastSequenceLazy                        315.5       351.833    +11.5%   **0.90x (?)**
NormalizedIterator_fastPrenormal            486.17      541.429    +11.4%   **0.90x (?)**
DropLastSequence                            315.0       350.5      +11.3%   **0.90x (?)**
NormalizedIterator_nonBMPSlowestPrenormal   390.469     433.429    +11.0%   **0.90x (?)**
DistinctClassFieldAccesses                  40.371      44.714     +10.8%   **0.90x (?)**
DropLastAnySequence                         326.667     360.667    +10.4%   **0.91x (?)**
LuhnAlgoEager                               183.556     200.625    +9.3%    **0.91x (?)**
Chars2                                      3062.0      3342.857   +9.2%    **0.92x (?)**
OpenClose                                   50.08       54.435     +8.7%    **0.92x (?)**
LuhnAlgoLazy                                184.8       200.667    +8.6%    **0.92x (?)**
NormalizedIterator_slowerPrenormal          271.778     294.545    +8.4%    **0.92x (?)**

IMPROVEMENT                                 OLD         NEW        DELTA    RATIO    
CharacterRecognizer.mixed                   21.156      13.096     -38.1%   **1.62x**
StringDistance.characters.mixed             3726.0      2568.0     -31.1%   **1.45x**
EqualSubstringSubstringGenericEquatable     30.692      24.787     -19.2%   **1.24x (?)**
LessSubstringSubstringGenericComparable     30.719      24.941     -18.8%   **1.23x (?)**
EqualStringSubstring                        30.917      25.267     -18.3%   **1.22x (?)**
EqualSubstringString                        30.923      25.353     -18.0%   **1.22x (?)**
LessSubstringSubstring                      30.813      25.865     -16.1%   **1.19x (?)**
StringComparison_longSharedPrefix           241.2       206.5      -14.4%   **1.17x (?)**
EqualSubstringSubstring                     31.36       26.923     -14.1%   **1.16x (?)**
FlattenListFlatMap                          4520.0      3926.0     -13.1%   **1.15x (?)**
Breadcrumbs.UTF16ToIdxRange.longASCII       15.258      13.536     -11.3%   **1.13x (?)**
StringDistance.characters.ascii             2669.0      2379.0     -10.9%   **1.12x (?)**
Set.isSubset.Int.Empty                      50.08       45.714     -8.7%    **1.10x (?)**
Set.isDisjoint.Box.Empty                    54.465      50.109     -8.0%    **1.09x (?)**
Set.isDisjoint.Seq.Box.Empty                88.222      81.667     -7.4%    **1.08x (?)**
Breadcrumbs.UTF16ToIdx.longASCII            42.491      39.339     -7.4%    **1.08x (?)**
StringUTF16SubstringBuilder                 1506.364    1398.462   -7.2%    **1.08x (?)**
Set.isStrictSubset.Int.Empty                45.731      42.464     -7.1%    **1.08x (?)**

------- Code size: -O -------

IMPROVEMENT        OLD     NEW     DELTA   RATIO  
StringDistance.o   13199   12663   -4.1%   **1.04x**