zstd: Minor decoder improvements (#467)

Around 1%. Parallel results are outliers.
```
benchmark                                                    old ns/op     new ns/op     delta
BenchmarkDecoder_DecoderSmall/kppkn.gtb.zst-32               4569931       4482917       -1.90%
BenchmarkDecoder_DecoderSmall/geo.protodata.zst-32           1041234       1035972       -0.51%
BenchmarkDecoder_DecoderSmall/plrabn12.txt.zst-32            14863278      14590060      -1.84%
BenchmarkDecoder_DecoderSmall/lcet10.txt.zst-32              10867406      10873836      +0.06%
BenchmarkDecoder_DecoderSmall/asyoulik.txt.zst-32            3639723       3623579       -0.44%
BenchmarkDecoder_DecoderSmall/alice29.txt.zst-32             4795746       4712242       -1.74%
BenchmarkDecoder_DecoderSmall/html_x_4.zst-32                1998022       2011177       +0.66%
BenchmarkDecoder_DecoderSmall/paper-100k.pdf.zst-32          195653        192640        -1.54%
BenchmarkDecoder_DecoderSmall/fireworks.jpeg.zst-32          79823         79958         +0.17%
BenchmarkDecoder_DecoderSmall/urls.10K.zst-32                12293289      12420972      +1.04%
BenchmarkDecoder_DecoderSmall/html.zst-32                    1115542       1120073       +0.41%
BenchmarkDecoder_DecoderSmall/comp-data.bin.zst-32           87392         86033         -1.56%
BenchmarkDecoder_DecodeAll/kppkn.gtb.zst-32                  565993        561070        -0.87%
BenchmarkDecoder_DecodeAll/geo.protodata.zst-32              129586        128363        -0.94%
BenchmarkDecoder_DecodeAll/plrabn12.txt.zst-32               1793372       1780863       -0.70%
BenchmarkDecoder_DecodeAll/lcet10.txt.zst-32                 1332514       1317041       -1.16%
BenchmarkDecoder_DecodeAll/asyoulik.txt.zst-32               453067        450499        -0.57%
BenchmarkDecoder_DecodeAll/alice29.txt.zst-32                589049        587888        -0.20%
BenchmarkDecoder_DecodeAll/html_x_4.zst-32                   246588        245277        -0.53%
BenchmarkDecoder_DecodeAll/paper-100k.pdf.zst-32             24020         23410         -2.54%
BenchmarkDecoder_DecodeAll/fireworks.jpeg.zst-32             9355          9349          -0.06%
BenchmarkDecoder_DecodeAll/urls.10K.zst-32                   1505506       1488845       -1.11%
BenchmarkDecoder_DecodeAll/html.zst-32                       139603        137762        -1.32%
BenchmarkDecoder_DecodeAll/comp-data.bin.zst-32              10905         10724         -1.66%
BenchmarkDecoder_DecodeAllParallel/kppkn.gtb.zst-32          29132         28871         -0.90%
BenchmarkDecoder_DecodeAllParallel/geo.protodata.zst-32      6839          6824          -0.22%
BenchmarkDecoder_DecodeAllParallel/plrabn12.txt.zst-32       94506         93872         -0.67%
BenchmarkDecoder_DecodeAllParallel/lcet10.txt.zst-32         70281         69755         -0.75%
BenchmarkDecoder_DecodeAllParallel/asyoulik.txt.zst-32       24104         23808         -1.23%
BenchmarkDecoder_DecodeAllParallel/alice29.txt.zst-32        31807         31375         -1.36%
BenchmarkDecoder_DecodeAllParallel/html_x_4.zst-32           13238         13140         -0.74%
BenchmarkDecoder_DecodeAllParallel/paper-100k.pdf.zst-32     1347          1327          -1.48%
BenchmarkDecoder_DecodeAllParallel/fireworks.jpeg.zst-32     2011          1709          -15.02%
BenchmarkDecoder_DecodeAllParallel/urls.10K.zst-32           76267         76475         +0.27%
BenchmarkDecoder_DecodeAllParallel/html.zst-32               7523          7567          +0.58%
BenchmarkDecoder_DecodeAllParallel/comp-data.bin.zst-32      936           784           -16.21%
```

zstd/bitreader.go

@@ -50,16 +50,23 @@ func (b *bitReader) getBits(n uint8) int {
 	if n == 0 /*|| b.bitsRead >= 64 */ {
 		return 0
 	}
-	return b.getBitsFast(n)
+	return int(b.get32BitsFast(n))
 }
 
-// getBitsFast requires that at least one bit is requested every time.
+// get32BitsFast requires that at least one bit is requested every time.
 // There are no checks if the buffer is filled.
-func (b *bitReader) getBitsFast(n uint8) int {
+func (b *bitReader) get32BitsFast(n uint8) uint32 {
 	const regMask = 64 - 1
 	v := uint32((b.value << (b.bitsRead & regMask)) >> ((regMask + 1 - n) & regMask))
 	b.bitsRead += n
-	return int(v)
+	return v
+}
+
+func (b *bitReader) get16BitsFast(n uint8) uint16 {
+	const regMask = 64 - 1
+	v := uint16((b.value << (b.bitsRead & regMask)) >> ((regMask + 1 - n) & regMask))
+	b.bitsRead += n
+	return v
 }
 
 // fillFast() will make sure at least 32 bits are available.

zstd/fse_decoder.go

@@ -379,7 +379,7 @@ func (s decSymbol) final() (int, uint8) {
 // This can only be used if no symbols are 0 bits.
 // At least tablelog bits must be available in the bit reader.
 func (s *fseState) nextFast(br *bitReader) (uint32, uint8) {
-	lowBits := uint16(br.getBitsFast(s.state.nbBits()))
+	lowBits := br.get16BitsFast(s.state.nbBits())
 	s.state = s.dt[s.state.newState()+lowBits]
 	return s.state.baseline(), s.state.addBits()
 }

zstd/seqdec.go

@@ -278,7 +278,7 @@ func (s *sequenceDecs) decode(seqs int, br *bitReader, hist []byte) error {
 			mlState = mlTable[mlState.newState()&maxTableMask]
 			ofState = ofTable[ofState.newState()&maxTableMask]
 		} else {
-			bits := br.getBitsFast(nBits)
+			bits := br.get32BitsFast(nBits)
 			lowBits := uint16(bits >> ((ofState.nbBits() + mlState.nbBits()) & 31))
 			llState = llTable[(llState.newState()+lowBits)&maxTableMask]
 
@@ -326,7 +326,7 @@ func (s *sequenceDecs) updateAlt(br *bitReader) {
 		s.offsets.state.state = s.offsets.state.dt[c.newState()]
 		return
 	}
-	bits := br.getBitsFast(nBits)
+	bits := br.get32BitsFast(nBits)
 	lowBits := uint16(bits >> ((c.nbBits() + b.nbBits()) & 31))
 	s.litLengths.state.state = s.litLengths.state.dt[a.newState()+lowBits]