An 8 bit positional population count implementation for Go using AVX-2.

The algorithm gathers corresponding bits of a the 32 bytes in an AVX
register using vpmovmskb and then performs scalar popcnt instructions
to compute the populations.  The 8 counters are kept in 8 general
purpose registers for the duration of the algorithm.  This algorithm
achieves up to 11 GB/s on Haswell and Skylake.

An enhanced implementation performs one CSA step to reduce 96 bytes into
64 before adding the results to counters with interleaved dependency
chains.  This one achieves up to 16 GB/s.

This code is the result of a Stack Overflow question: https://stackoverflow.com/q/63248047/417501

This code is an experiment and should not be integrated into third party
programs.  Use http://github.com/fuzxxl/pospop instead.

Benchmark results on a Intel(R) Xeon(R) W-2133 CPU @ 3.60GHz processor.

goos: linux
goarch: amd64
BenchmarkReference/10-12	 	14835181	        80.6 ns/op	 124.12 MB/s
BenchmarkReference/32-12 		 4425584	       258 ns/op	 124.24 MB/s
BenchmarkReference/1000-12         	  126886	      7919 ns/op	 126.28 MB/s
BenchmarkReference/2000-12         	   69042	     15809 ns/op	 126.51 MB/s
BenchmarkReference/4000-12         	   36360	     31707 ns/op	 126.16 MB/s
BenchmarkReference/10000-12        	   15001	     78828 ns/op	 126.86 MB/s
BenchmarkReference/100000-12       	    1291	    795464 ns/op	 125.71 MB/s
BenchmarkReference/10000000-12     	      14	  78161869 ns/op	 127.94 MB/s
BenchmarkReference/1000000000-12   	       1	7779490681 ns/op	 128.54 MB/s
BenchmarkScalarReg/10-12           	49062314	        23.5 ns/op	 425.79 MB/s
BenchmarkScalarReg/32-12           	16471662	        72.1 ns/op	 443.77 MB/s
BenchmarkScalarReg/1000-12         	  530245	      2242 ns/op	 445.94 MB/s
BenchmarkScalarReg/2000-12         	  267555	      4471 ns/op	 447.28 MB/s
BenchmarkScalarReg/4000-12         	  133797	      8931 ns/op	 447.90 MB/s
BenchmarkScalarReg/10000-12        	   50118	     22347 ns/op	 447.49 MB/s
BenchmarkScalarReg/100000-12       	    5326	    222991 ns/op	 448.45 MB/s
BenchmarkScalarReg/10000000-12     	      51	  21961648 ns/op	 455.34 MB/s
BenchmarkScalarReg/1000000000-12   	       1	2198404416 ns/op	 454.88 MB/s
BenchmarkScalarMem/10-12           	33757770	        35.9 ns/op	 278.25 MB/s
BenchmarkScalarMem/32-12           	10092933	       104 ns/op	 306.44 MB/s
BenchmarkScalarMem/1000-12         	  375811	      3195 ns/op	 312.95 MB/s
BenchmarkScalarMem/2000-12         	  188787	      6351 ns/op	 314.93 MB/s
BenchmarkScalarMem/4000-12         	   82556	     12662 ns/op	 315.90 MB/s
BenchmarkScalarMem/10000-12        	   36529	     31582 ns/op	 316.63 MB/s
BenchmarkScalarMem/100000-12       	    3664	    317268 ns/op	 315.19 MB/s
BenchmarkScalarMem/10000000-12     	      37	  31625600 ns/op	 316.20 MB/s
BenchmarkScalarMem/1000000000-12   	       1	3160044812 ns/op	 316.45 MB/s
BenchmarkReg/10-12                 	48229350	        24.0 ns/op	 417.40 MB/s
BenchmarkReg/32-12                 	185803699	         6.14 ns/op	5212.18 MB/s
BenchmarkReg/1000-12               	 9914378	       108 ns/op	9299.97 MB/s
BenchmarkReg/2000-12               	 5322217	       211 ns/op	9469.33 MB/s
BenchmarkReg/4000-12               	 3261402	       356 ns/op	11235.73 MB/s
BenchmarkReg/10000-12              	 1305704	       888 ns/op	11256.88 MB/s
BenchmarkReg/100000-12             	  116280	      8679 ns/op	11522.49 MB/s
BenchmarkReg/10000000-12           	    1310	    899563 ns/op	11116.51 MB/s
BenchmarkReg/1000000000-12         	      13	  90355989 ns/op	11067.33 MB/s
BenchmarkRegCSA3/10-12             	48295509	        24.1 ns/op	 415.67 MB/s
BenchmarkRegCSA3/32-12             	192285118	         6.23 ns/op	5133.08 MB/s
BenchmarkRegCSA3/1000-12           	14713641	        79.8 ns/op	12530.09 MB/s
BenchmarkRegCSA3/2000-12           	 6915092	       160 ns/op	12474.22 MB/s
BenchmarkRegCSA3/4000-12           	 4544103	       249 ns/op	16076.66 MB/s
BenchmarkRegCSA3/10000-12          	 1852636	       643 ns/op	15544.93 MB/s
BenchmarkRegCSA3/100000-12         	  197012	      6080 ns/op	16447.29 MB/s
BenchmarkRegCSA3/10000000-12       	    1712	    677164 ns/op	14767.46 MB/s
BenchmarkRegCSA3/1000000000-12     	      16	  69779618 ns/op	14330.83 MB/s
BenchmarkRegCSA7/10-12             	47901508	        23.9 ns/op	 418.06 MB/s
BenchmarkRegCSA7/32-12             	186971718	         6.26 ns/op	5113.49 MB/s
BenchmarkRegCSA7/1000-12           	16944604	        70.8 ns/op	14120.96 MB/s
BenchmarkRegCSA7/2000-12           	 7842502	       140 ns/op	14311.54 MB/s
BenchmarkRegCSA7/4000-12           	 5453896	       202 ns/op	19824.29 MB/s
BenchmarkRegCSA7/10000-12          	 2285653	       512 ns/op	19520.84 MB/s
BenchmarkRegCSA7/100000-12         	  250798	      4740 ns/op	21095.40 MB/s
BenchmarkRegCSA7/10000000-12       	    2007	    584689 ns/op	17103.11 MB/s
BenchmarkRegCSA7/1000000000-12     	      18	  66597449 ns/op	15015.59 MB/s
BenchmarkMem/10-12                 	32649992	        35.7 ns/op	 280.11 MB/s
BenchmarkMem/32-12                 	191536897	         6.00 ns/op	5336.23 MB/s
BenchmarkMem/1000-12               	 7814206	       138 ns/op	7243.51 MB/s
BenchmarkMem/2000-12               	 4420798	       259 ns/op	7710.20 MB/s
BenchmarkMem/4000-12               	 2801730	       409 ns/op	9790.36 MB/s
BenchmarkMem/10000-12              	 1090711	      1075 ns/op	9304.78 MB/s
BenchmarkMem/100000-12             	  100048	     10439 ns/op	9579.30 MB/s
BenchmarkMem/10000000-12           	    1106	   1078156 ns/op	9275.10 MB/s
BenchmarkMem/1000000000-12         	      10	 111021710 ns/op	9007.25 MB/s
PASS
ok  	pospopcnt	125.448s