[stdlib][SR-9438] Re-implement integer-to-string conversion (redux) #85180

xwu · 2025-10-28T17:50:34Z

Inspired by #84826, I've dusted off and completely reworked a native implementation of integer-to-string conversion.

Besides existing tests in this repository, the core of the implementation has been comprehensively tested in a separate package for all bases between 2–36 to demonstrate identical output for all 8-bit and 16-bit values, and for randomly generated 32-bit, 64-bit, and 128-bit values.

Resolves #51902.

This implication largely ports the C++ implementation with the following differences: * Checks preconditions for radix and buffer size * Declines to return "-0" * Uses base-10 fast path that looks up two digits at a time

…to string conversion

xwu · 2025-10-28T17:56:35Z

@swift-ci test

xwu · 2025-10-28T18:04:25Z

@swift-ci benchmark

@swift-ci

A companion to #85180.

xwu · 2025-11-03T16:06:24Z

@swift-ci benchmark

xwu · 2025-11-03T19:40:19Z

Performance (x86_64): -O

Regression	OLD	NEW	DELTA	RATIO
NSString.bridged.byteCount.ascii.utf16	0.0	0.535	+53500.0%	0.00x (?)
LessSubstringSubstringGenericComparable	24.208	32.138	+32.8%	0.75x (?)
EqualSubstringString	24.341	32.155	+32.1%	0.76x
LessSubstringSubstring	24.302	30.732	+26.5%	0.79x
EqualStringSubstring	25.755	32.146	+24.8%	0.80x
SubstringEqualString	145.8	181.75	+24.7%	0.80x
EqualSubstringSubstringGenericEquatable	26.485	32.16	+21.4%	0.82x
UTF8Decode_InitFromCustom_contiguous	146.313	177.308	+21.2%	0.83x (?)
UTF8Decode_InitFromData	150.364	181.7	+20.8%	0.83x (?)
EqualSubstringSubstring	26.674	32.155	+20.5%	0.83x
UTF8Decode_InitFromCustom_noncontiguous	175.143	210.333	+20.1%	0.83x (?)
NormalizedIterator_ascii	69.903	83.5	+19.5%	0.84x (?)
StringComparison_longSharedPrefix	212.455	252.222	+18.7%	0.84x (?)
UTF8Decode_InitDecoding	151.231	177.182	+17.2%	0.85x (?)
DataToStringEmpty	95.5	111.84	+17.1%	0.85x (?)
UTF8Decode_InitFromBytes	173.0	201.909	+16.7%	0.86x (?)
SequenceAlgosAnySequence	7984.615	9250.0	+15.8%	0.86x (?)
SubstringEquatable	293.125	332.714	+13.5%	0.88x
StringComparison_latin1	308.769	344.429	+11.5%	0.90x (?)
SortStringsUnicode	2177.0	2427.5	+11.5%	0.90x (?)
SuffixAnySeqCntRangeLazy	13700.0	15125.0	+10.4%	0.91x (?)
StringComparison_fastPrenormal	431.667	476.346	+10.4%	0.91x (?)
String.replaceSubrange.Substring	10.718	11.761	+9.7%	0.91x (?)

Improvement	OLD	NEW	DELTA	RATIO
IntegerToString.UInt128.decimal	418.5	155.556	-62.8%	2.69x
IntegerToString.UInt64.hex	289.75	121.133	-58.2%	2.39x
IntegerToString.UInt128.hex	434.2	196.3	-54.8%	2.21x
IntegerToString.Int.hex	168.429	97.957	-41.8%	1.72x
Breadcrumbs.CopyUTF16CodeUnits.longMixed	219.6	162.0	-26.2%	1.36x
Breadcrumbs.CopyAllUTF16CodeUnits.longMixed	213.909	159.667	-25.4%	1.34x
Breadcrumbs.CopyAllUTF16CodeUnits.Mixed	216.636	161.714	-25.4%	1.34x
IntegerToString.UInt64.decimal	195.6	148.7	-24.0%	1.32x
Dictionary2	503.864	427.143	-15.2%	1.18x (?)
StringFromLongWholeSubstring	3.118	2.733	-12.3%	1.14x (?)
StringComparison_ascii	241.6	212.818	-11.9%	1.14x (?)
StringSwitch	325.714	289.857	-11.0%	1.12x (?)
Calculator	173.286	154.333	-10.9%	1.12x (?)
Dictionary2OfObjects	972.5	867.5	-10.8%	1.12x (?)
NSString.bridged.byteCount.utf8.utf16	1.51	1.392	-7.8%	1.08x (?)
AngryPhonebook.Strasse.Small	156.8	144.6	-7.8%	1.08x (?)
StringUTF16Builder	377.903	348.657	-7.7%	1.08x (?)
StringBuilder	344.0	318.0	-7.6%	1.08x (?)
BridgeString.find.native.longNonASCII	402.5	373.0	-7.3%	1.08x (?)
Chars2	3420.833	3181.081	-7.0%	1.08x (?)

Code size: -O

Performance (x86_64): -Osize

Regression	OLD	NEW	DELTA	RATIO
EqualSubstringSubstringGenericEquatable	23.221	31.213	+34.4%	0.74x
EqualStringSubstring	24.081	32.263	+34.0%	0.75x
EqualSubstringSubstring	23.303	31.219	+34.0%	0.75x (?)
EqualSubstringString	24.212	32.154	+32.8%	0.75x
SubstringEqualString	135.188	178.385	+32.0%	0.76x (?)
LessSubstringSubstringGenericComparable	25.0	32.397	+29.6%	0.77x (?)
LessSubstringSubstring	25.0	32.385	+29.5%	0.77x
SubstringEquatable	282.0	348.667	+23.6%	0.81x (?)
UTF8Decode_InitFromCustom_noncontiguous	234.3	276.0	+17.8%	0.85x
UTF8Decode_InitFromCustom_contiguous	152.333	178.846	+17.4%	0.85x
UTF8Decode_InitFromData	157.2	181.5	+15.5%	0.87x
UTF8Decode_InitDecoding	151.417	173.778	+14.8%	0.87x (?)
NormalizedIterator_ascii	73.219	83.759	+14.4%	0.87x (?)
UTF8Decode_InitFromBytes	176.75	201.727	+14.1%	0.88x
SubstringFromLongStringGeneric2	26.449	29.733	+12.4%	0.89x (?)
StringComparison_longSharedPrefix	218.545	244.111	+11.7%	0.90x (?)
StringComparison_fastPrenormal	442.182	492.857	+11.5%	0.90x (?)
NormalizedIterator_emoji	328.286	362.815	+10.5%	0.90x (?)
StringComparison_latin1	320.286	351.538	+9.8%	0.91x (?)
String.replaceSubrange.Substring	10.831	11.764	+8.6%	0.92x (?)
DropLastAnySeqCntRangeLazy	13818.0	14976.0	+8.4%	0.92x (?)
CStringLongNonAscii	147.063	159.333	+8.3%	0.92x (?)
SequenceAlgosAnySequence	8546.154	9252.0	+8.3%	0.92x (?)
DropLastAnySeqCRangeIterLazy	14140.0	15270.0	+8.0%	0.93x (?)
SortStringsUnicode	2259.0	2437.5	+7.9%	0.93x (?)

Improvement	OLD	NEW	DELTA	RATIO
IntegerToString.UInt128.decimal	393.0	155.4	-60.5%	2.53x
IntegerToString.UInt64.hex	278.75	125.684	-54.9%	2.22x
IntegerToString.UInt128.hex	425.4	199.364	-53.1%	2.13x
IntegerToString.Int.hex	157.9	98.529	-37.6%	1.60x
Breadcrumbs.CopyAllUTF16CodeUnits.longMixed	213.818	159.667	-25.3%	1.34x
Breadcrumbs.CopyAllUTF16CodeUnits.Mixed	216.727	162.214	-25.2%	1.34x
Breadcrumbs.CopyUTF16CodeUnits.longMixed	219.5	165.643	-24.5%	1.33x
IntegerToString.UInt64.decimal	192.0	153.929	-19.8%	1.25x (?)
StringFromLongWholeSubstring	3.61	2.904	-19.6%	1.24x
StringSwitch	320.571	273.0	-14.8%	1.17x (?)
Dictionary2	546.875	484.25	-11.5%	1.13x (?)
ConvertFloatingPoint.MockFloat64ToInt64	618.667	550.5	-11.0%	1.12x (?)
StringInterpolationSmall	992.083	882.778	-11.0%	1.12x (?)
CharacterRecognizer.mixed	17.618	16.087	-8.7%	1.10x (?)
AngryPhonebook.Strasse.Small	156.714	143.267	-8.6%	1.09x (?)
StringAdder	341.857	313.571	-8.3%	1.09x (?)

Code size: -Osize

Performance (x86_64): -Onone

Regression	OLD	NEW	DELTA	RATIO
LessSubstringSubstring	27.4	36.017	+31.4%	0.76x
EqualStringSubstring	29.0	37.269	+28.5%	0.78x
LessSubstringSubstringGenericComparable	27.3	34.552	+26.6%	0.79x (?)
EqualSubstringSubstringGenericEquatable	27.735	34.09	+22.9%	0.81x
EqualSubstringSubstring	27.625	33.424	+21.0%	0.83x
EqualSubstringString	29.133	35.092	+20.5%	0.83x
UTF8Decode_InitFromData	154.5	182.625	+18.2%	0.85x (?)
UTF8Decode_InitDecoding	158.75	185.25	+16.7%	0.86x (?)
UTF8Decode_InitFromCustom_contiguous	161.769	188.182	+16.3%	0.86x
UTF8Decode_InitFromBytes	177.667	201.6	+13.5%	0.88x (?)
SuffixAnySeqCntRange	15606.0	17590.0	+12.7%	0.89x (?)
DropFirstAnySeqCRangeIterLazy	14728.0	16600.0	+12.7%	0.89x (?)
SequenceAlgosAnySequence	8722.222	9791.304	+12.3%	0.89x (?)
Set.subtracting.Seq.Int.Empty	549.75	617.0	+12.2%	0.89x (?)
DropFirstAnySeqCRangeIter	14806.0	16604.0	+12.1%	0.89x (?)
LazilyFilteredRange	409980.0	454590.0	+10.9%	0.90x (?)
SuffixAnyCollection	3550.0	3926.0	+10.6%	0.90x (?)
Data.init.Sequence.809B.Count.RE	17904.0	19761.0	+10.4%	0.91x (?)
SubstringEquatable	3262.0	3589.0	+10.0%	0.91x (?)
PrefixWhileAnySeqCntRangeLazy	14970.0	16392.0	+9.5%	0.91x (?)
SuffixAnySeqCRangeIter	16307.0	17770.0	+9.0%	0.92x (?)
Data.init.Sequence.64kB.Count.RE.I	14642.0	15903.0	+8.6%	0.92x (?)
PrefixAnySeqCntRange	12593.0	13670.0	+8.6%	0.92x (?)
SuffixAnySeqCRangeIterLazy	16343.0	17732.0	+8.5%	0.92x (?)
Data.append.Sequence.809B.Count0.RE.I	18080.0	19585.0	+8.3%	0.92x (?)
FatCompactMap	265910.0	287730.0	+8.2%	0.92x (?)
DropWhileAnySeqCRangeIterLazy	18134.0	19603.0	+8.1%	0.93x (?)
DropLastAnySeqCRangeIter	18935.0	20399.0	+7.7%	0.93x (?)
SortStringsUnicode	2972.5	3200.0	+7.7%	0.93x (?)
StrComplexWalk	4645.0	5000.0	+7.6%	0.93x (?)
Data.append.Sequence.64kB.Count.RE	15195.0	16352.0	+7.6%	0.93x (?)
Data.init.Sequence.809B.Count0.RE	18573.0	19986.0	+7.6%	0.93x (?)

Improvement	OLD	NEW	DELTA	RATIO
IntegerToString.UInt128.decimal	436.0	169.444	-61.1%	2.57x
IntegerToString.UInt128.hex	455.0	213.636	-53.0%	2.13x
IntegerToString.UInt64.hex	535.25	354.429	-33.8%	1.51x
Breadcrumbs.CopyUTF16CodeUnits.longMixed	224.9	168.0	-25.3%	1.34x
Breadcrumbs.CopyAllUTF16CodeUnits.longMixed	211.091	160.2	-24.1%	1.32x
Breadcrumbs.CopyAllUTF16CodeUnits.Mixed	232.889	178.3	-23.4%	1.31x
IntegerToString.Int.hex	406.0	331.571	-18.3%	1.22x (?)
IntegerToString.UInt64.decimal	429.6	383.8	-10.7%	1.12x (?)
SubstringRemoveFirst1	0.222	0.2	-9.9%	1.11x (?)
CharIndexing_korean_unicodeScalars	232720.0	211720.0	-9.0%	1.10x (?)
CharIndexing_chinese_unicodeScalars_Backwards	183840.0	168240.0	-8.5%	1.09x (?)
BridgeString.find.native.longNonASCII	397.0	365.0	-8.1%	1.09x (?)
FloatingPointPrinting_Double_interpolated	33466.667	30925.0	-7.6%	1.08x (?)
ObjectiveCBridgeStringHash	61.536	57.031	-7.3%	1.08x (?)
FloatingPointPrinting_Float_interpolated	29492.308	27450.0	-6.9%	1.07x (?)

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 · 2025-11-03T19:42:43Z

(I have seen these string benchmarks fluctuate spuriously before with no change in code generation. Otherwise looks like it's a pretty targeted improvement—lack of change for Int decimal description benchmark is likely because the test fixture contains only values that fit in 32-bit Int which would benefit less from the two-digits-at-a-time optimization.)

@swift-ci

A companion to swiftlang#85180.

@swift-ci

…wiftlang#85180) Inspired by swiftlang#84826, I've dusted off and completely reworked a native implementation of integer-to-string conversion. Besides existing tests in this repository, the core of the implementation has been comprehensively tested in a separate package for all bases between 2–36 to demonstrate identical output for all 8-bit and 16-bit values, and for randomly generated 32-bit, 64-bit, and 128-bit values. Resolves swiftlang#51902.

xwu added 8 commits October 26, 2025 21:11

Add _UInt64ToASCII implementation in native Swift

51d9585

This implication largely ports the C++ implementation with the following differences: * Checks preconditions for radix and buffer size * Declines to return "-0" * Uses base-10 fast path that looks up two digits at a time

Invoke _UInt64ToASCII in BinaryInteger.description

ad93577

Refactor legacy integer to string conversion ABIs

3dbbaeb

Remove C++ uint64ToString and related implementations

3adad52

Note ABI stability of integer-to-string reimplementation in Swift

2c2e397

Add base-8 fast path and make minor optimizations for native integer …

a0536ae

…to string conversion

[stdlib] Further rework integer-to-string conversion.

b51843e

[stdlib] ABI-related fixups for legacy integer-to-string entrypoints.

782f47b

xwu requested a review from a team as a code owner October 28, 2025 17:50

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xwu mentioned this pull request Oct 28, 2025

[stdlib][SR-9438] Re-implement integer-to-string conversion #36636

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xwu changed the title ~~[stdlib][SR-9438] Re-implement integer-to-string conversion in native Swift (redux)~~ [stdlib][SR-9438] Re-implement integer-to-string conversion (redux) Oct 28, 2025

xwu mentioned this pull request Oct 30, 2025

[benchmark] Add integer-to-string benchmark #85209

Merged

stephentyrone approved these changes Nov 4, 2025

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xwu merged commit 591fec2 into swiftlang:main Nov 4, 2025
6 checks passed

xwu deleted the sr-9438-redux branch November 4, 2025 02:23

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[stdlib][SR-9438] Re-implement integer-to-string conversion (redux) #85180

[stdlib][SR-9438] Re-implement integer-to-string conversion (redux) #85180

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Performance (x86_64): -O

Code size: -O

Performance (x86_64): -Osize

Code size: -Osize

Performance (x86_64): -Onone

Code size: -swiftlibs

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