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encoding/base32: simplify and speed up decoder

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First, we can lift the enc.decodeMap nil check out of the loop.

Second, we can make it clear to the compiler that 'in := src[0]' doesn't
need a bounds check, by making len(src)==0 a single if check that always
stops the loop. This is by far the largest speed-up.

Third, we can use a dst slice index instead of reslicing dst, which
removes work from the loop body.

While at it, we can merge the two 'switch dlen' pieces of code, which
simplifies the code and doesn't affect performance.

name            old time/op    new time/op    delta
DecodeString-8    80.2µs ± 0%    67.5µs ± 0%  -15.81%  (p=0.002 n=6+6)

name            old speed      new speed      delta
DecodeString-8   163MB/s ± 0%   194MB/s ± 0%  +18.78%  (p=0.002 n=6+6)

Change-Id: Iefeaae94c03453f8760452b1da706a77b3522718
Reviewed-on: https://go-review.googlesource.com/c/154422
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
This commit is contained in:
Daniel Martí 2018-12-16 19:11:28 +01:00
parent da50e10c2e
commit e90e7a59dc
1 changed files with 22 additions and 30 deletions
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52
src/encoding/base32/base32.go
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@ -284,7 +284,12 @@ func (e CorruptInputError) Error() string {
// additional data is an error. This method assumes that src has been
// stripped of all supported whitespace ('\r' and '\n').
func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
// Lift the nil check outside of the loop.
_ = enc.decodeMap
dsti := 0
olen := len(src)
for len(src) > 0 && !end {
// Decode quantum using the base32 alphabet
var dbuf [8]byte
@ -292,17 +297,15 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
for j := 0; j < 8; {
// We have reached the end and are missing padding
if len(src) == 0 && enc.padChar != NoPadding {
return n, false, CorruptInputError(olen - len(src) - j)
}
// We have reached the end and are not expecing any padding
if len(src) == 0 && enc.padChar == NoPadding {
if len(src) == 0 {
if enc.padChar != NoPadding {
// We have reached the end and are missing padding
return n, false, CorruptInputError(olen - len(src) - j)
}
// We have reached the end and are not expecing any padding
dlen, end = j, true
break
}
in := src[0]
src = src[1:]
if in == byte(enc.padChar) && j >= 2 && len(src) < 8 {
@ -339,37 +342,26 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
// quantum
switch dlen {
case 8:
dst[4] = dbuf[6]<<5 | dbuf[7]
dst[dsti+4] = dbuf[6]<<5 | dbuf[7]
n++
fallthrough
case 7:
dst[3] = dbuf[4]<<7 | dbuf[5]<<2 | dbuf[6]>>3
dst[dsti+3] = dbuf[4]<<7 | dbuf[5]<<2 | dbuf[6]>>3
n++
fallthrough
case 5:
dst[2] = dbuf[3]<<4 | dbuf[4]>>1
dst[dsti+2] = dbuf[3]<<4 | dbuf[4]>>1
n++
fallthrough
case 4:
dst[1] = dbuf[1]<<6 | dbuf[2]<<1 | dbuf[3]>>4
dst[dsti+1] = dbuf[1]<<6 | dbuf[2]<<1 | dbuf[3]>>4
n++
fallthrough
case 2:
dst[0] = dbuf[0]<<3 | dbuf[1]>>2
}
if !end {
dst = dst[5:]
}
switch dlen {
case 2:
n += 1
case 4:
n += 2
case 5:
n += 3
case 7:
n += 4
case 8:
n += 5
dst[dsti+0] = dbuf[0]<<3 | dbuf[1]>>2
n++
}
dsti += 5
}
return n, end, nil
}