unroll nextMany scalar fill loop by 4

[ahxxm]

Description

Seems to be a hot path, unrolling the loop to amortize branching makes BenchmarkNextsRLE 33% faster on my CPU(WSL2, 6800h), 17-32% improvement on run-heavy real datasets, no regressions on others.

Type of Change

• Performance improvement

Benchmarks

BenchmarkNextsRLE baseline

goos: linux
goarch: amd64
pkg: github.com/RoaringBitmap/roaring/v2
cpu: AMD Ryzen 7 6800H with Radeon Graphics
BenchmarkNextsRLE/next-16         	     178	   7062349 ns/op	     112 B/op	       1 allocs/op
BenchmarkNextsRLE/next-16         	     172	   7306398 ns/op	     112 B/op	       1 allocs/op
BenchmarkNextsRLE/next-16         	     169	   7223803 ns/op	     112 B/op	       1 allocs/op
BenchmarkNextsRLE/next-16         	     169	   7055468 ns/op	     112 B/op	       1 allocs/op
BenchmarkNextsRLE/next-16         	     162	   6897329 ns/op	     112 B/op	       1 allocs/op
BenchmarkNextsRLE/nextmany-16     	    1131	   1157612 ns/op	    8320 B/op	       2 allocs/op
BenchmarkNextsRLE/nextmany-16     	    1078	   1164884 ns/op	    8320 B/op	       2 allocs/op
BenchmarkNextsRLE/nextmany-16     	    1088	   1180406 ns/op	    8320 B/op	       2 allocs/op
BenchmarkNextsRLE/nextmany-16     	    1028	   1145430 ns/op	    8320 B/op	       2 allocs/op
BenchmarkNextsRLE/nextmany-16     	    1112	   1146625 ns/op	    8320 B/op	       2 allocs/op
PASS
ok  	github.com/RoaringBitmap/roaring/v2	17.469s

BenchmarkNextsRLE after

goos: linux
goarch: amd64
pkg: github.com/RoaringBitmap/roaring/v2
cpu: AMD Ryzen 7 6800H with Radeon Graphics
BenchmarkNextsRLE/next-16         	     181	   7009443 ns/op	     112 B/op	       1 allocs/op
BenchmarkNextsRLE/next-16         	     165	   6994907 ns/op	     112 B/op	       1 allocs/op
BenchmarkNextsRLE/next-16         	     170	   7212088 ns/op	     112 B/op	       1 allocs/op
BenchmarkNextsRLE/next-16         	     178	   7269497 ns/op	     112 B/op	       1 allocs/op
BenchmarkNextsRLE/next-16         	     170	   6876003 ns/op	     112 B/op	       1 allocs/op
BenchmarkNextsRLE/nextmany-16     	    1497	    808012 ns/op	    8320 B/op	       2 allocs/op
BenchmarkNextsRLE/nextmany-16     	    1596	    774690 ns/op	    8320 B/op	       2 allocs/op
BenchmarkNextsRLE/nextmany-16     	    1573	    780475 ns/op	    8320 B/op	       2 allocs/op
BenchmarkNextsRLE/nextmany-16     	    1338	    769944 ns/op	    8320 B/op	       2 allocs/op
BenchmarkNextsRLE/nextmany-16     	    1418	    766276 ns/op	    8320 B/op	       2 allocs/op
PASS
ok  	github.com/RoaringBitmap/roaring/v2	16.812s

Real benchmarks and details

Dataset	Baseline (ns/op)	After (ns/op)
census-income_srt	8,077,007	5,687,244
census-income	14,479,076	15,422,757
census1881_srt	1,236,144	1,019,923
census1881	1,556,954	1,544,594
dimension_003	6,917,545	5,758,556
dimension_008	4,133,790	2,999,982
dimension_033	4,798,393	3,269,909
uscensus2000	61,391	62,133
weather_sept_85_srt	21,223,028	15,980,821
weather_sept_85	30,582,768	29,055,214
wikileaks-noquotes_srt	485,889	419,402
wikileaks-noquotes	924,636	951,535

Baseline

goos: linux
goarch: amd64
pkg: github.com/RoaringBitmap/roaring/v2
cpu: AMD Ryzen 7 6800H with Radeon Graphics
BenchmarkRealDataNextMany/census-income_srt-16     	     148	   8077007 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/census-income-16         	      84	  14479076 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/census1881_srt-16        	     944	   1236144 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/census1881-16            	     734	   1556954 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/dimension_003-16         	     182	   6917545 ns/op	 1998119 B/op	   15483 allocs/op
BenchmarkRealDataNextMany/dimension_008-16         	     297	   4133790 ns/op	  687114 B/op	    5241 allocs/op
BenchmarkRealDataNextMany/dimension_033-16         	     266	   4798393 ns/op	   38528 B/op	     174 allocs/op
BenchmarkRealDataNextMany/uscensus2000-16          	   18696	     61391 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/weather_sept_85_srt-16   	      61	  21223028 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/weather_sept_85-16       	      40	  30582768 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/wikileaks-noquotes_srt-16         	    2812	    485889 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/wikileaks-noquotes-16             	    1308	    924636 ns/op	   41984 B/op	     201 allocs/op
PASS
ok  	github.com/RoaringBitmap/roaring/v2	176.780s

After

goos: linux
goarch: amd64
pkg: github.com/RoaringBitmap/roaring/v2
cpu: AMD Ryzen 7 6800H with Radeon Graphics
BenchmarkRealDataNextMany/census-income_srt-16     	     195	   5687244 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/census-income-16         	      88	  15422757 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/census1881_srt-16        	    1209	   1019923 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/census1881-16            	     762	   1544594 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/dimension_003-16         	     207	   5758556 ns/op	 1998120 B/op	   15483 allocs/op
BenchmarkRealDataNextMany/dimension_008-16         	     376	   2999982 ns/op	  687114 B/op	    5241 allocs/op
BenchmarkRealDataNextMany/dimension_033-16         	     339	   3269909 ns/op	   38528 B/op	     174 allocs/op
BenchmarkRealDataNextMany/uscensus2000-16          	   18458	     62133 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/weather_sept_85_srt-16   	      79	  15980821 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/weather_sept_85-16       	      42	  29055214 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/wikileaks-noquotes_srt-16         	    3120	    419402 ns/op	   41984 B/op	     201 allocs/op
BenchmarkRealDataNextMany/wikileaks-noquotes-16             	    1236	    951535 ns/op	   41984 B/op	     201 allocs/op
PASS
ok  	github.com/RoaringBitmap/roaring/v2	174.272s

[lemire]

Running tests.