[stdlib] Minor clean-up and new fast path for larger (e.g. 128-bit) integer printing

[xwu]

A follow-up to #85180.

First, make the guarantee that our Swift-native implementation of integer-to-ASCII conversion always fills the suffix of the given mutable span.

(Some minor swift-format corrections interspersed.)

Then, use that guarantee to work in 64-bit chunks for larger integers—currently limited to decimal printing with a hardcoded divisor for chunking.

[xwu]

@swift-ci please benchmark

[xwu]

4-5x improvement in UInt128 decimal printing benchmark

Performance (x86_64): -O

Regression	OLD	NEW	DELTA	RATIO
EqualSubstringString	22.0	30.051	+36.6%	0.73x (?)
LessSubstringSubstring	23.686	30.156	+27.3%	0.79x (?)
LessSubstringSubstringGenericComparable	23.688	30.154	+27.3%	0.79x (?)
EqualStringSubstring	23.633	30.04	+27.1%	0.79x (?)
EqualSubstringSubstringGenericEquatable	24.586	30.36	+23.5%	0.81x (?)
EqualSubstringSubstring	24.594	30.36	+23.4%	0.81x (?)
UTF8Decode_InitDecoding	135.769	167.0	+23.0%	0.81x (?)
Data.hash.Medium	30.433	37.227	+22.3%	0.82x (?)
SubstringEqualString	130.929	159.5	+21.8%	0.82x (?)
UTF8Decode_InitFromCustom_contiguous	137.769	167.364	+21.5%	0.82x (?)
UTF8Decode_InitFromData	141.615	171.182	+20.9%	0.83x (?)
UTF8Decode_InitFromBytes	158.182	190.1	+20.2%	0.83x (?)
UTF8Decode_InitFromCustom_noncontiguous	165.273	196.4	+18.8%	0.84x (?)
StringComparison_longSharedPrefix	208.3	238.2	+14.4%	0.87x (?)
SubstringEquatable	274.857	311.0	+13.1%	0.88x (?)
NormalizedIterator_fastPrenormal	489.4	551.154	+12.6%	0.89x (?)
StringDistance.characters.ascii	914.0	1017.5	+11.3%	0.90x (?)
FindString.Loop1.Substring	213.545	235.0	+10.0%	0.91x (?)
StringHasSuffixUnicode	47954.545	51956.522	+8.3%	0.92x (?)
SortStringsUnicode	2100.0	2272.5	+8.2%	0.92x (?)
Improvement	OLD	NEW	DELTA	RATIO
NSString.bridged.byteCount.ascii.utf16	0.5	0.0	-99.8%	501.00x (?)
IntegerToString.UInt128.decimal	152.778	29.545	-80.7%	5.17x
Breadcrumbs.CopyAllUTF16CodeUnits.longMixed	200.5	149.813	-25.3%	1.34x (?)
Breadcrumbs.CopyAllUTF16CodeUnits.Mixed	202.1	151.8	-24.9%	1.33x (?)
Breadcrumbs.CopyUTF16CodeUnits.longMixed	205.727	155.6	-24.4%	1.32x (?)
Calculator	169.273	148.455	-12.3%	1.14x (?)
StringComparison_ascii	224.556	198.7	-11.5%	1.13x (?)
StringSwitch	304.0	270.833	-10.9%	1.12x (?)
NormalizedIterator_ascii	77.5	70.222	-9.4%	1.10x (?)
StringHashing_nonBMPSlowestPrenormal	335.849	305.814	-8.9%	1.10x (?)
SortStrings	522.25	476.25	-8.8%	1.10x (?)
StringHashing_slowerPrenormal	243.226	221.846	-8.8%	1.10x (?)
Chars2	3347.619	3056.522	-8.7%	1.10x (?)
NSString.bridged.byteCount.utf8.utf16	1.393	1.273	-8.6%	1.09x (?)
ObjectiveCBridgeStringHash	62.333	57.324	-8.0%	1.09x (?)
String.replaceSubrange.Substring	11.104	10.245	-7.7%	1.08x (?)
StringHashing_zalgo	2316.667	2138.235	-7.7%	1.08x (?)
StringHashing_emoji	259.467	239.5	-7.7%	1.08x (?)
StringWordBuilder	1247.857	1152.143	-7.7%	1.08x (?)
StringComparison_zalgo	40100.0	37375.0	-6.8%	1.07x (?)

Code size: -O

Performance (x86_64): -Osize

Regression	OLD	NEW	DELTA	RATIO
EqualSubstringSubstringGenericEquatable	23.44	30.527	+30.2%	0.77x (?)
EqualSubstringSubstring	23.438	30.516	+30.2%	0.77x
EqualStringSubstring	23.733	30.152	+27.0%	0.79x (?)
EqualSubstringString	23.727	30.038	+26.6%	0.79x (?)
Data.hash.Medium	30.448	37.885	+24.4%	0.80x (?)
UTF8Decode_InitFromCustom_contiguous	135.538	167.909	+23.9%	0.81x (?)
UTF8Decode_InitFromBytes	156.538	193.333	+23.5%	0.81x (?)
UTF8Decode_InitFromData	138.538	171.0	+23.4%	0.81x (?)
LessSubstringSubstringGenericComparable	24.971	30.703	+23.0%	0.81x (?)
LessSubstringSubstring	24.97	30.7	+22.9%	0.81x (?)
SubstringEqualString	131.071	159.917	+22.0%	0.82x
UTF8Decode_InitDecoding	137.462	166.9	+21.4%	0.82x (?)
UTF8Decode_InitFromCustom_noncontiguous	217.0	259.5	+19.6%	0.84x (?)
SubstringEquatable	263.857	300.143	+13.8%	0.88x (?)
StringComparison_longSharedPrefix	216.364	245.4	+13.4%	0.88x (?)
FindString.Loop1.Substring	204.875	228.6	+11.6%	0.90x (?)
StringDistance.characters.ascii	913.0	1012.5	+10.9%	0.90x (?)
StringComparison_latin1	304.625	332.429	+9.1%	0.92x (?)
StringHasSuffixUnicode	47833.333	52041.667	+8.8%	0.92x (?)
SortStringsUnicode	2120.0	2280.0	+7.5%	0.93x (?)
CStringLongNonAscii	137.846	148.231	+7.5%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
NSString.bridged.byteCount.ascii.utf16	0.544	0.0	-99.8%	545.00x (?)
IntegerToString.UInt128.decimal	152.556	29.588	-80.6%	5.16x
Breadcrumbs.CopyAllUTF16CodeUnits.longMixed	200.545	150.938	-24.7%	1.33x
Breadcrumbs.CopyAllUTF16CodeUnits.Mixed	202.167	152.714	-24.5%	1.32x (?)
Breadcrumbs.CopyUTF16CodeUnits.longMixed	205.818	156.0	-24.2%	1.32x
StringHashing_nonBMPSlowestPrenormal	336.415	306.364	-8.9%	1.10x (?)
SortStrings	516.0	470.0	-8.9%	1.10x (?)
StringHashing_slowerPrenormal	243.478	222.245	-8.7%	1.10x (?)
String.replaceSubrange.Substring	11.11	10.235	-7.9%	1.09x (?)
StringHashing_emoji	260.138	240.25	-7.6%	1.08x (?)
StringHashing_zalgo	2314.286	2145.0	-7.3%	1.08x (?)
StringWordBuilder	1243.571	1152.667	-7.3%	1.08x (?)
StringSwitch	293.857	272.571	-7.2%	1.08x (?)
Chars2	4023.684	3733.333	-7.2%	1.08x (?)
StringWordBuilderReservingCapacity	1195.0	1116.0	-6.6%	1.07x (?)
CSVParsing.UTF16	48.0	44.848	-6.6%	1.07x (?)

Code size: -Osize

Performance (x86_64): -Onone

Regression	OLD	NEW	DELTA	RATIO
LessSubstringSubstring	25.594	34.188	+33.6%	0.75x (?)
EqualSubstringSubstring	25.375	33.458	+31.9%	0.76x (?)
LessSubstringSubstringGenericComparable	25.067	33.049	+31.8%	0.76x (?)
EqualSubstringSubstringGenericEquatable	25.706	33.548	+30.5%	0.77x (?)
EqualStringSubstring	26.783	34.5	+28.8%	0.78x (?)
EqualSubstringString	27.4	34.731	+26.8%	0.79x (?)
UTF8Decode_InitFromCustom_contiguous	146.25	181.6	+24.2%	0.81x (?)
UTF8Decode_InitFromData	142.667	172.444	+20.9%	0.83x (?)
UTF8Decode_InitDecoding	148.5	179.0	+20.5%	0.83x (?)
UTF8Decode_InitFromBytes	161.818	193.5	+19.6%	0.84x (?)
Data.hash.Medium	34.607	41.0	+18.5%	0.84x (?)
StringHasSuffixUnicode	49869.565	54500.0	+9.3%	0.92x (?)
CharacterRecognizer.ascii	171.167	184.455	+7.8%	0.93x (?)
CharIteration_japanese_unicodeScalars_Backwards	175720.0	189040.0	+7.6%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
IntegerToString.UInt128.decimal	164.444	41.563	-74.7%	3.96x
Breadcrumbs.CopyAllUTF16CodeUnits.longMixed	201.083	150.313	-25.2%	1.34x (?)
Breadcrumbs.CopyUTF16CodeUnits.longMixed	214.818	164.071	-23.6%	1.31x (?)
Breadcrumbs.CopyAllUTF16CodeUnits.Mixed	218.1	168.0	-23.0%	1.30x (?)
StringWithCString2	0.007	0.006	-12.5%	1.14x (?)
String.replaceSubrange.Substring	11.977	11.0	-8.2%	1.09x (?)
ObjectiveCBridgeStringHash	62.483	57.438	-8.1%	1.09x (?)

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: 32 GB

[xwu]

@swift-ci smoke test