[stdlib] Speed up _Bitset in size-optimized builds #19664

lorentey · 2018-10-02T14:19:07Z

Due to a possible low-level optimizer issue, a division-by-constant-power-of-two expression gets compiled into an actual division instruction with -Osize. Explicitly changing to unsigned arithmetic ensures they get emitted as right shifts.

There is a similar issue with multiplications, although that one boils down to unnecessary overflow checking. Switching to &* makes sure those get compiled into corresponding left shifts.

lorentey · 2018-10-02T14:19:16Z

@swift-ci please smoke benchmark

lorentey · 2018-10-02T14:19:23Z

@swift-ci please test

lorentey · 2018-10-02T15:12:06Z

The +90% DictionaryKeysContainsNative regression in the -Osize build seems to hint at an issue.

lorentey · 2018-10-02T15:18:55Z

It's also super weird, as this PR doesn't touch those paths.

lorentey · 2018-10-02T15:59:51Z

@swift-ci please smoke benchmark

lorentey · 2018-10-02T17:40:59Z

Wow. Divisions are slow!

airspeedswift · 2018-10-02T17:47:52Z

😲

lorentey · 2018-10-02T19:38:38Z

@swift-ci please smoke benchmark

lorentey · 2018-10-02T19:45:55Z

@swift-ci test

lorentey · 2018-10-02T19:54:02Z

The last commit switches multiplications to use &*, so that they too get compiled down to bit shifts.

lorentey · 2018-10-02T19:54:37Z

@swift-ci please smoke benchmark

lorentey · 2018-10-02T19:55:07Z

@swift-ci please test

lorentey · 2018-10-02T19:55:35Z

@swift-ci please test macOS platform

lorentey · 2018-10-03T10:49:11Z

I'm going to rebase this, revert the initial (mostly unsuccessful) commit and re-run the benchmarks before landing.

…tion/division by powers-of-two The divisions don’t get eliminated in -Osize builds.

However, convert input values to UInts before dividing them.

We know it won’t overflow, because the result will not exceed the capacity, which is guaranteed to fit in an integer.

lorentey · 2018-10-03T11:02:32Z

@swift-ci smoke test

lorentey · 2018-10-03T11:03:14Z

@swift-ci smoke benchmark

swift-ci · 2018-10-03T11:54:39Z

Build comment file:

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
AnyHashableWithAClass	88989	115574	+29.9%	0.77x
Improvement
SetSymmetricDifferenceInt0	426	386	-9.4%	1.10x
SetExclusiveOr	4282	3887	-9.2%	1.10x
CStringLongAscii	3550	3296	-7.2%	1.08x

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
Histogram.o	5004	4604	-8.0%	1.09x
HashQuadratic.o	6776	6328	-6.6%	1.07x
DictionaryCompactMapValues.o	26194	24546	-6.3%	1.07x
DictionaryCopy.o	10662	9992	-6.3%	1.07x
DictionaryRemove.o	18110	17006	-6.1%	1.06x
TwoSum.o	6664	6264	-6.0%	1.06x
DictionarySubscriptDefault.o	32891	31163	-5.3%	1.06x
DictionarySwap.o	31502	29854	-5.2%	1.06x
DictionaryKeysContains.o	19875	18963	-4.6%	1.05x
DictTest2.o	21296	20320	-4.6%	1.05x
DictTest3.o	29840	28528	-4.4%	1.05x
DictTest4Legacy.o	27338	26218	-4.1%	1.04x
RGBHistogram.o	27677	26605	-3.9%	1.04x
DictTest4.o	25884	24908	-3.8%	1.04x
ReversedCollections.o	11755	11371	-3.3%	1.03x
DictionaryGroup.o	18279	17751	-2.9%	1.03x
WordCount.o	67071	65279	-2.7%	1.03x
SetTests.o	61321	59697	-2.6%	1.03x
StringEdits.o	14925	14541	-2.6%	1.03x
CharacterProperties.o	19249	18833	-2.2%	1.02x
DictionaryLiteral.o	1420	1392	-2.0%	1.02x
StringRemoveDupes.o	16229	15909	-2.0%	1.02x
DictTest.o	52863	51887	-1.8%	1.02x
CountAlgo.o	21540	21172	-1.7%	1.02x
DictOfArraysToArrayOfDicts.o	33028	32500	-1.6%	1.02x
ReduceInto.o	24783	24399	-1.5%	1.02x
TestsUtils.o	24325	23957	-1.5%	1.02x
Hash.o	30051	29683	-1.2%	1.01x
ObjectiveCBridging.o	46142	45614	-1.1%	1.01x
DriverUtils.o	169441	167649	-1.1%	1.01x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
AnyHashableWithAClass	109426	139001	+27.0%	0.79x
Improvement
SetSymmetricDifferenceInt0	875	390	-55.4%	2.24x
SetExclusiveOr	8752	3927	-55.1%	2.23x
SetIsSubsetInt25	163	77	-52.8%	2.12x
SetUnionInt0	665	317	-52.3%	2.10x
SetUnion	6659	3182	-52.2%	2.09x
SetIntersectionInt0	127	62	-51.2%	2.05x
SetIntersect	1274	624	-51.0%	2.04x
SetSymmetricDifferenceInt25	469	238	-49.3%	1.97x
SetSubtractingInt0	141	72	-48.9%	1.96x
SetIsSubsetInt50	303	155	-48.8%	1.95x
SetIntersectionInt100	767	393	-48.8%	1.95x
SetIntersectionInt50	443	230	-48.1%	1.93x
SetUnionInt25	270	141	-47.8%	1.91x
SetIntersectionInt25	287	151	-47.4%	1.90x
TwoSum	2343	1244	-46.9%	1.88x
SetSymmetricDifferenceInt100	454	249	-45.2%	1.82x
SetUnionInt50	215	118	-45.1%	1.82x
SetSymmetricDifferenceInt50	456	253	-44.5%	1.80x
DictionarySwap	1956	1109	-43.3%	1.76x
DictionaryRemove	7544	4307	-42.9%	1.75x
SetUnionInt100	124	71	-42.7%	1.75x
DictionaryCopy	131805	76303	-42.1%	1.73x
SetSubtractingInt25	201	118	-41.3%	1.70x
SetIsSubsetInt100	523	308	-41.1%	1.70x
SetSubtractingInt100	337	199	-40.9%	1.69x
RGBHistogram	4044	2408	-40.5%	1.68x
SetSubtractingInt50	261	158	-39.5%	1.65x
SetSymmetricDifferenceBox0	1219	752	-38.3%	1.62x
SetExclusiveOr_OfObjects	12182	7519	-38.3%	1.62x
SetUnionBox0	939	596	-36.5%	1.58x
SetUnion_OfObjects	9374	5975	-36.3%	1.57x
SetIsSubsetInt0	400	256	-36.0%	1.56x
Histogram	962	618	-35.8%	1.56x
DictionarySwapAt	1700	1099	-35.4%	1.55x
DictionaryFilter	108942	71291	-34.6%	1.53x
DictionaryCompactMapValuesOfNilValue	8921	5867	-34.2%	1.52x
SetIntersectionBox0	220	145	-34.1%	1.52x
SetIntersectionBox25	424	283	-33.3%	1.50x
SetSubtractingBox0	230	155	-32.6%	1.48x
SetIsSubsetBox25	271	184	-32.1%	1.47x
Dictionary	597	409	-31.5%	1.46x
DictionarySubscriptDefaultMutation	510	355	-30.4%	1.44x
DictionaryGroup	455	324	-28.8%	1.40x
Dictionary2	867	641	-26.1%	1.35x
SetSymmetricDifferenceBox25	767	568	-25.9%	1.35x
SetUnionBox25	502	376	-25.1%	1.34x
Dictionary3	231	179	-22.5%	1.29x
SetIsSubsetBox0	471	366	-22.3%	1.29x
WordCountUniqueASCII	1985	1555	-21.7%	1.28x
SetSubtractingBox25	371	291	-21.6%	1.27x
DictionaryCompactMapValuesOfCastValue	16246	12765	-21.4%	1.27x
CharacterPropertiesPrecomputed	1924	1514	-21.3%	1.27x
Dictionary4	341	282	-17.3%	1.21x
FrequenciesUsingReduceInto	1442	1194	-17.2%	1.21x
DictionarySubscriptDefaultMutationArray	895	744	-16.9%	1.20x
Prims	1030	872	-15.3%	1.18x
PrimsSplit	1025	872	-14.9%	1.18x
Dictionary4OfObjects	425	363	-14.6%	1.17x
DictionaryRemoveOfObjects	23867	20796	-12.9%	1.15x
StringRemoveDupes	489	433	-11.5%	1.13x
CharacterPropertiesStashedMemo	2288	2057	-10.1%	1.11x
Dictionary4Legacy	715	658	-8.0%	1.09x
DictionaryLiteral	3050	2810	-7.9%	1.09x
CStringLongAscii	3549	3305	-6.9%	1.07x

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
HashQuadratic.o	6184	5832	-5.7%	1.06x
Histogram.o	4672	4480	-4.1%	1.04x
TwoSum.o	6125	5949	-2.9%	1.03x
DictionaryCompactMapValues.o	23578	22970	-2.6%	1.03x
DictionaryCopy.o	9345	9105	-2.6%	1.03x
DictTest2.o	16882	16498	-2.3%	1.02x
DictionaryRemove.o	15163	14827	-2.2%	1.02x
DictTest3.o	23618	23106	-2.2%	1.02x
DictionarySubscriptDefault.o	27939	27347	-2.1%	1.02x
DictionarySwap.o	27331	26771	-2.0%	1.02x
StringRemoveDupes.o	9213	9037	-1.9%	1.02x
DictTest4.o	21307	20939	-1.7%	1.02x
DictionaryKeysContains.o	17147	16859	-1.7%	1.02x
DictTest4Legacy.o	23017	22633	-1.7%	1.02x
StringEdits.o	14382	14174	-1.4%	1.01x
WordCount.o	55520	54720	-1.4%	1.01x
CountAlgo.o	14624	14432	-1.3%	1.01x
RGBHistogram.o	24253	23949	-1.3%	1.01x
ReduceInto.o	16938	16746	-1.1%	1.01x
ReversedCollections.o	11925	11797	-1.1%	1.01x
DictionaryGroup.o	16503	16327	-1.1%	1.01x
DictionaryLiteral.o	1541	1525	-1.0%	1.01x
SetTests.o	53377	52825	-1.0%	1.01x

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Regression
AnyHashableWithAClass	106832	152855	+43.1%	0.70x

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 · 2018-10-03T12:10:38Z

Interesting. The AnyHashableWithAClass regression is new; however, I don't see how it could be caused by this change. I'll look into it later.

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lorentey requested a review from stephentyrone October 2, 2018 19:55

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palimondo mentioned this pull request Oct 3, 2018

[stdlib] Switch Dictionary.subscript(_:default:) to use _modify #19218

Merged

lorentey added 3 commits October 3, 2018 12:01

[stdlib] Bitset: Use explicit left/right shifts instead of multiplica…

a6f7af1

…tion/division by powers-of-two The divisions don’t get eliminated in -Osize builds.

[stdlib] _Bitset: Revert to using division/multiplication

18c5654

However, convert input values to UInts before dividing them.

[stdlib] _Bitset: Use &* for join and capacity

267b80f

We know it won’t overflow, because the result will not exceed the capacity, which is guaranteed to fit in an integer.

lorentey force-pushed the dictionary-subscript-setter-is-slow branch from 6cb27a2 to 267b80f Compare October 3, 2018 11:02

lorentey changed the title ~~[stdlib] Partially force-inline Dictionary’s keyed subscript setter~~ [stdlib] Speed up _Bitset in size-optimized builds Oct 3, 2018

lorentey merged commit da463e0 into swiftlang:master Oct 3, 2018

lorentey deleted the dictionary-subscript-setter-is-slow branch October 3, 2018 12:11

[stdlib] Speed up _Bitset in size-optimized builds #19664

[stdlib] Speed up _Bitset in size-optimized builds #19664

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swift-ci commented Oct 3, 2018

Build comment file:

Performance: -O

Code size: -O

Performance: -Osize

Code size: -Osize

Performance: -Onone

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