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compress/bzip2: remove bit-tricks

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Since compiler is now able to generate conditional moves, we can replace
bit-tricks with simple if/else. This even results in slightly better performance:

name            old time/op    new time/op    delta
DecodeDigits-6    13.4ms ± 4%    13.0ms ± 2%  -2.63%  (p=0.003 n=10+10)
DecodeTwain-6     37.5ms ± 1%    36.3ms ± 1%  -3.03%  (p=0.000 n=10+9)
DecodeRand-6      4.23ms ± 1%    4.07ms ± 1%  -3.67%  (p=0.000 n=10+9)

name            old speed      new speed      delta
DecodeDigits-6  7.47MB/s ± 4%  7.67MB/s ± 2%  +2.69%  (p=0.002 n=10+10)
DecodeTwain-6   10.4MB/s ± 1%  10.7MB/s ± 1%  +3.25%  (p=0.000 n=10+8)
DecodeRand-6    3.87MB/s ± 1%  4.03MB/s ± 2%  +4.08%  (p=0.000 n=10+10)
diff --git a/src/compress/bzip2/huffman.go b/src/compress/bzip2/huffman.go

Change-Id: Ie96ef1a9e07013b07e78f22cdccd531f3341caca
Reviewed-on: https://go-review.googlesource.com/102015
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
Reviewed-by: Joe Tsai <joetsai@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Ilya Tocar 2018-03-21 16:42:02 -05:00
parent 88129f0cb2
commit 9eb219480e
1 changed files with 18 additions and 14 deletions
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32
src/compress/bzip2/huffman.go
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@ -43,30 +43,34 @@ func (t *huffmanTree) Decode(br *bitReader) (v uint16) {
if br.bits > 0 {
// Get next bit - fast path.
br.bits--
bit = 0 - (uint16(br.n>>br.bits) & 1)
bit = uint16(br.n>>(br.bits&63)) & 1
} else {
// Get next bit - slow path.
// Use ReadBits to retrieve a single bit
// from the underling io.ByteReader.
bit = 0 - uint16(br.ReadBits(1))
bit = uint16(br.ReadBits(1))
}
// now
// bit = 0xffff if the next bit was 1
// bit = 0x0000 if the next bit was 0
// 1 means left, 0 means right.
//
// if bit == 0xffff {
// nodeIndex = node.left
// } else {
// nodeIndex = node.right
// }
nodeIndex = (bit & node.left) | (^bit & node.right)
// Trick a compiler into generating conditional move instead of branch,
// by making both loads unconditional.
l, r := node.left, node.right
if bit == 1 {
nodeIndex = l
} else {
nodeIndex = r
}
if nodeIndex == invalidNodeValue {
// We found a leaf. Use the value of bit to decide
// whether is a left or a right value.
return (bit & node.leftValue) | (^bit & node.rightValue)
l, r := node.leftValue, node.rightValue
if bit == 1 {
v = l
} else {
v = r
}
return
}
}
}