perf: Add 8-bit lookup table to Huffman decoder

[orisano]

Improved Huffman decoding performance by introducing a lookup table for the first 8 bits of each code, reducing bit-by-bit processing in the common case. This optimization achieves around 30% speedup.

Optimization details:

• Added 256-entry lookup table for first 8 bits
• Fast path for codes ≤ 8 bits
• Fallback to bit-by-bit decoding for longer codes

Benchmark results:
Before:

goos: darwin
goarch: arm64
pkg: github.com/cosnicolaou/pbzip2/internal/bzip2
cpu: Apple M2 Max
BenchmarkDecodeDigits-12    	     363	   3222846 ns/op	  31.03 MB/s	 3613296 B/op	      51 allocs/op
BenchmarkDecodeDigits-12    	     366	   3483579 ns/op	  28.71 MB/s	 3613312 B/op	      51 allocs/op
BenchmarkDecodeDigits-12    	     361	   3212324 ns/op	  31.13 MB/s	 3613296 B/op	      51 allocs/op
BenchmarkDecodeNewton-12    	      92	  13106987 ns/op	  43.27 MB/s	 3631140 B/op	      51 allocs/op
BenchmarkDecodeNewton-12    	      94	  12964625 ns/op	  43.75 MB/s	 3631149 B/op	      51 allocs/op
BenchmarkDecodeNewton-12    	      92	  13332504 ns/op	  42.54 MB/s	 3631141 B/op	      51 allocs/op
BenchmarkDecodeRand-12      	     950	   1178515 ns/op	  13.90 MB/s	 3644460 B/op	      51 allocs/op
BenchmarkDecodeRand-12      	    1022	   1180043 ns/op	  13.88 MB/s	 3644463 B/op	      51 allocs/op
BenchmarkDecodeRand-12      	    1014	   1174522 ns/op	  13.95 MB/s	 3644462 B/op	      51 allocs/op
BenchmarkWiktionary-12      	       1	160628872250 ns/op	  65.36 MB/s	367217144 B/op	  542483 allocs/op
BenchmarkWiktionary-12      	       1	165487979792 ns/op	  63.44 MB/s	367217160 B/op	  542483 allocs/op
BenchmarkWiktionary-12      	       1	163573653500 ns/op	  64.19 MB/s	367217160 B/op	  542483 allocs/op

After:

goos: darwin
goarch: arm64
pkg: github.com/cosnicolaou/pbzip2/internal/bzip2
cpu: Apple M2 Max
BenchmarkDecodeDigits-12    	     586	   1983453 ns/op	  50.42 MB/s	 3616560 B/op	      51 allocs/op
BenchmarkDecodeDigits-12    	     600	   2009215 ns/op	  49.77 MB/s	 3616572 B/op	      51 allocs/op
BenchmarkDecodeDigits-12    	     602	   1990462 ns/op	  50.24 MB/s	 3616565 B/op	      51 allocs/op
BenchmarkDecodeNewton-12    	     134	   8641542 ns/op	  65.64 MB/s	 3634410 B/op	      51 allocs/op
BenchmarkDecodeNewton-12    	     136	   8632566 ns/op	  65.70 MB/s	 3634412 B/op	      51 allocs/op
BenchmarkDecodeNewton-12    	     138	   8686795 ns/op	  65.29 MB/s	 3634412 B/op	      51 allocs/op
BenchmarkDecodeRand-12      	    2049	    580980 ns/op	  28.20 MB/s	 3647723 B/op	      51 allocs/op
BenchmarkDecodeRand-12      	    2020	    578564 ns/op	  28.32 MB/s	 3647725 B/op	      51 allocs/op
BenchmarkDecodeRand-12      	    2050	    581352 ns/op	  28.18 MB/s	 3647725 B/op	      51 allocs/op
BenchmarkWiktionary-12      	       1	116550048542 ns/op	  90.08 MB/s	404888416 B/op	  542481 allocs/op
BenchmarkWiktionary-12      	       1	118146102375 ns/op	  88.87 MB/s	404888432 B/op	  542481 allocs/op
BenchmarkWiktionary-12      	       1	117903373792 ns/op	  89.05 MB/s	404888432 B/op	  542481 allocs/op

[klauspost]

Very nice. I don't see any improvements in the hot code.

[orisano]

Looking forward to your new version release once this PR gets merged.

[orisano]

@cosnicolaou Are you planning to release the next version?

[cosnicolaou]

I can, it wasn't clear to me from the last PR if you had more PRs to come?

[orisano]

All the PRs I was planning to send have been sent and merged.

[cosnicolaou]

done! Thanks for all of the perf improvements! Cheers, Cos.

…

On Thu, Nov 7, 2024 at 7:59 AM Nao Yonashiro ***@***.***> wrote: All the PRs I was planning to send have been sent and merged. — Reply to this email directly, view it on GitHub <#45 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ACCMUYE433KK6LEAQAFNVTLZ7OE5BAVCNFSM6AAAAABRDA5UDWVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDINRSGYYDIOJTGU> . You are receiving this because you were mentioned.Message ID: ***@***.***>