deflate: Speed up huffman encoding (#455)

Huffman only:

320.36 -> 414.67MB/s  (github-june-2days-2019.json)
237.41-> 293.24 MB/s (github-ranks-backup.bin)

Level 1:

278.94 -> 308.47 MB/s  (github-june-2days-2019.json)
232.93 -> 261.16MB/s  (github-ranks-backup.bin)

flate/huffman_bit_writer.go

@@ -222,7 +222,7 @@ func (w *huffmanBitWriter) write(b []byte) {
 }
 
 func (w *huffmanBitWriter) writeBits(b int32, nb uint16) {
-	w.bits |= uint64(b) << w.nbits
+	w.bits |= uint64(b) << (w.nbits & 63)
 	w.nbits += nb
 	if w.nbits >= 48 {
 		w.writeOutBits()
@@ -423,7 +423,7 @@ func (w *huffmanBitWriter) storedSize(in []byte) (int, bool) {
 
 func (w *huffmanBitWriter) writeCode(c hcode) {
 	// The function does not get inlined if we "& 63" the shift.
-	w.bits |= uint64(c.code) << w.nbits
+	w.bits |= uint64(c.code) << (w.nbits & 63)
 	w.nbits += c.len
 	if w.nbits >= 48 {
 		w.writeOutBits()
@@ -768,7 +768,7 @@ func (w *huffmanBitWriter) writeTokens(tokens []token, leCodes, oeCodes []hcode)
 		if t < matchType {
 			//w.writeCode(lits[t.literal()])
 			c := lits[t.literal()]
-			bits |= uint64(c.code) << nbits
+			bits |= uint64(c.code) << (nbits & 63)
 			nbits += c.len
 			if nbits >= 48 {
 				binary.LittleEndian.PutUint64(w.bytes[nbytes:], bits)
@@ -796,7 +796,7 @@ func (w *huffmanBitWriter) writeTokens(tokens []token, leCodes, oeCodes []hcode)
 		} else {
 			// inlined
 			c := lengths[lengthCode&31]
-			bits |= uint64(c.code) << nbits
+			bits |= uint64(c.code) << (nbits & 63)
 			nbits += c.len
 			if nbits >= 48 {
 				binary.LittleEndian.PutUint64(w.bytes[nbytes:], bits)
@@ -819,7 +819,7 @@ func (w *huffmanBitWriter) writeTokens(tokens []token, leCodes, oeCodes []hcode)
 		if extraLengthBits > 0 {
 			//w.writeBits(extraLength, extraLengthBits)
 			extraLength := int32(length - lengthBase[lengthCode&31])
-			bits |= uint64(extraLength) << nbits
+			bits |= uint64(extraLength) << (nbits & 63)
 			nbits += extraLengthBits
 			if nbits >= 48 {
 				binary.LittleEndian.PutUint64(w.bytes[nbytes:], bits)
@@ -846,7 +846,7 @@ func (w *huffmanBitWriter) writeTokens(tokens []token, leCodes, oeCodes []hcode)
 		} else {
 			// inlined
 			c := offs[offsetCode]
-			bits |= uint64(c.code) << nbits
+			bits |= uint64(c.code) << (nbits & 63)
 			nbits += c.len
 			if nbits >= 48 {
 				binary.LittleEndian.PutUint64(w.bytes[nbytes:], bits)
@@ -867,7 +867,7 @@ func (w *huffmanBitWriter) writeTokens(tokens []token, leCodes, oeCodes []hcode)
 		offsetComb := offsetCombined[offsetCode]
 		if offsetComb > 1<<16 {
 			//w.writeBits(extraOffset, extraOffsetBits)
-			bits |= uint64(offset&matchOffsetOnlyMask-(offsetComb&0xffff)) << nbits
+			bits |= uint64(offset&matchOffsetOnlyMask-(offsetComb&0xffff)) << (nbits & 63)
 			nbits += uint16(offsetComb >> 16)
 			if nbits >= 48 {
 				binary.LittleEndian.PutUint64(w.bytes[nbytes:], bits)
@@ -996,10 +996,41 @@ func (w *huffmanBitWriter) writeBlockHuff(eof bool, input []byte, sync bool) {
 	encoding := w.literalEncoding.codes[:256]
 	// Go 1.16 LOVES having these on stack. At least 1.5x the speed.
 	bits, nbits, nbytes := w.bits, w.nbits, w.nbytes
+
+	// Unroll, write 3 codes/loop.
+	// Fastest number of unrolls.
+	for len(input) > 3 {
+		// We must have at least 48 bits free.
+		if nbits >= 8 {
+			n := nbits >> 3
+			binary.LittleEndian.PutUint64(w.bytes[nbytes:], bits)
+			bits >>= (n * 8) & 63
+			nbits -= n * 8
+			nbytes += uint8(n)
+		}
+		if nbytes >= bufferFlushSize {
+			if w.err != nil {
+				nbytes = 0
+				return
+			}
+			_, w.err = w.writer.Write(w.bytes[:nbytes])
+			nbytes = 0
+		}
+		a, b := encoding[input[0]], encoding[input[1]]
+		bits |= uint64(a.code) << (nbits & 63)
+		bits |= uint64(b.code) << ((nbits + a.len) & 63)
+		c := encoding[input[2]]
+		nbits += b.len + a.len
+		bits |= uint64(c.code) << (nbits & 63)
+		nbits += c.len
+		input = input[3:]
+	}
+
+	// Remaining...
 	for _, t := range input {
 		// Bitwriting inlined, ~30% speedup
 		c := encoding[t]
-		bits |= uint64(c.code) << nbits
+		bits |= uint64(c.code) << (nbits & 63)
 		nbits += c.len
 		if debugDeflate {
 			count += int(c.len)