WIP vectorization for UTF16->UTF8

[Catfish-Man]

No description provided.

[Catfish-Man]

We should do an x86 version of this, but it was giving me weird errors and I wanted to get the core thing working before dealing with them. TBH it's probably still faster on x86 than it was even this way though.

[Catfish-Man]

Making a temporary buffer here is sort of awful and I want to improve it at some point, but it's also not really hurting anything and simplifies the rest of the code a lot

[Catfish-Man]

@swift-ci please test

[Catfish-Man]

@swift-ci please benchmark

[Catfish-Man]

@swift-ci please Apple Silicon benchmark

[Catfish-Man]

------- Performance (arm64): -Osize -------
REGRESSION                               OLD        NEW        DELTA       RATIO    
NSString.bridged.byteCount.ascii.utf8    0.0        0.339      +33900.0%   **0.00x (?)**
Calculator                               115.5      128.056    +10.9%      **0.90x**
Chars2                                   2731.25    2982.432   +9.2%       **0.92x**

IMPROVEMENT                              OLD        NEW        DELTA       RATIO    
UTF16Decode.initDecoding                 69.2       4.077      -94.1%      **16.97x**
UTF16Decode.initFromCustom.cont          251.375    21.8       -91.3%      **11.53x**
ArrayAppendGenericStructs                1134.545   870.0      -23.3%      **1.30x (?)**
Array.removeAll.keepingCapacity.Object   2.522      2.238      -11.3%      **1.13x (?)**
InsertCharacterEndIndex                  58.865     54.923     -6.7%       **1.07x**

I'll take that. (The NSString.bridged.byteCount.ascii.utf8 result is noise due to it running too fast after earlier speedups)

[Catfish-Man]

Some of those failures do look real, so this'll stay as a draft for now

[Catfish-Man]

Somehow the x86 results look better despite not using the hand vectorized path? I guess I should try using the fallback path on arm64 and see if it does ok there 😂

 IMPROVEMENT                                   OLD         NEW         DELTA    RATIO    
17:07:59  UTF16Decode.initDecoding                      176.167     6.55        -96.3%   **26.89x**
17:07:59  UTF16Decode.initFromCustom.cont               475.5       37.615      -92.1%   **12.64x**
17:07:59  Breadcrumbs.CopyAllUTF16CodeUnits.longMixed   223.364     160.133     -28.3%   **1.39x**
17:07:59  Breadcrumbs.CopyAllUTF16CodeUnits.Mixed       226.2       162.733     -28.1%   **1.39x**
17:07:59  Breadcrumbs.CopyUTF16CodeUnits.longMixed      229.6       165.643     -27.9%   **1.39x**

[Catfish-Man]

@swift-ci please test

[Catfish-Man]

@swift-ci please Apple Silicon benchmark

[Catfish-Man]

@swift-ci please benchmark

[Catfish-Man]

Turns out not accidentally processing twice as much data improves the speedup!

------- Performance (arm64): -Osize -------

REGRESSION                        OLD       NEW       DELTA    RATIO    
MapReduceClass2                   59.048    64.658    +9.5%    **0.91x**
MapReduceClassShort2              91.654    100.0     +9.1%    **0.92x (?)**

IMPROVEMENT                       OLD       NEW       DELTA    RATIO    
UTF16Decode.initDecoding          72.619    2.239     -96.9%   **32.42x**
UTF16Decode.initFromCustom.cont   252.375   22.0      -91.3%   **11.47x**
BufferFillFromSlice               11.326    10.068    -11.1%   **1.12x (?)**
ArrayAppendToGeneric              179.5     165.496   -7.8%    **1.08x (?)**
String.replaceSubrange.String     6.076     5.615     -7.6%    **1.08x**
InsertCharacterEndIndex           60.472    56.405    -6.7%    **1.07x**

[Catfish-Man]

@swift-ci please Apple Silicon benchmark

[Catfish-Man]

@swift-ci please benchmark

[Catfish-Man]

@swift-ci please Apple Silicon benchmark

[Catfish-Man]

IMPROVEMENT                                  OLD         NEW         DELTA       RATIO    
UTF16Decode.initDecoding                     69.524      2.135       -96.9%      **32.55x**
UTF16Decode.initFromCustom.cont              252.0       21.648      -91.4%      **11.64x**
Calculator                                   128.056     115.55      -9.8%       **1.11x**
StringHasPrefixUnicode                       24014.085   21890.411   -8.8%       **1.10x**

Just as good as before, so I think that means I get to delete all the architecture-specific bits of the patch :)

[Catfish-Man]

@swift-ci please benchmark

[Catfish-Man]

@swift-ci please Apple Silicon benchmark