encoding/base64: Optimize DecodeString

Optimize DecodeString for the common case where most of the input isn't
a newline or a padding character.
Also add some testcases found when fuzzing this implementation against
upstream.
Change Decode benchmark to run with different input sizes.

name                 old time/op    new time/op    delta
DecodeString/2-4       71.5ns ± 4%    70.0ns ± 6%     ~     (p=0.246 n=5+5)
DecodeString/4-4        112ns ±25%      91ns ± 2%     ~     (p=0.056 n=5+5)
DecodeString/8-4        136ns ± 5%     126ns ± 5%   -7.33%  (p=0.016 n=5+5)
DecodeString/64-4       872ns ±29%     652ns ±21%  -25.23%  (p=0.032 n=5+5)
DecodeString/8192-4    90.9µs ±21%    61.0µs ±13%  -32.87%  (p=0.008 n=5+5)

name                 old speed      new speed      delta
DecodeString/2-4     56.0MB/s ± 4%  57.2MB/s ± 6%     ~     (p=0.310 n=5+5)
DecodeString/4-4     73.4MB/s ±23%  87.7MB/s ± 2%     ~     (p=0.056 n=5+5)
DecodeString/8-4     87.8MB/s ± 5%  94.8MB/s ± 5%   +7.98%  (p=0.016 n=5+5)
DecodeString/64-4     103MB/s ±24%   136MB/s ±19%  +32.63%  (p=0.032 n=5+5)
DecodeString/8192-4   122MB/s ±19%   180MB/s ±11%  +47.75%  (p=0.008 n=5+5)

Improves #19636

Change-Id: I39667f4fb682a12b3137946d017ad999553c5780
Reviewed-on: https://go-review.googlesource.com/34950Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/base64/base64.go

@@ -273,44 +273,50 @@ func (e CorruptInputError) Error() string {
 // indicates if end-of-message padding or a partial quantum was encountered
 // and thus any additional data is an error.
 func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
-	var inIdx int
 	si := 0
 
-	// skip over newlines
-	for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
-		si++
-	}
-
 	for si < len(src) && !end {
 		// Decode quantum using the base64 alphabet
 		var dbuf [4]byte
 		dinc, dlen := 3, 4
 
-		for j := range dbuf {
+		for j := 0; j < len(dbuf); j++ {
 			if len(src) == si {
-				if enc.padChar != NoPadding || j < 2 {
+				switch {
+				case j == 0:
+					return n, false, nil
+				case j == 1, enc.padChar != NoPadding:
 					return n, false, CorruptInputError(si - j)
 				}
 				dinc, dlen, end = j-1, j, true
 				break
 			}
 			in := src[si]
-			inIdx = si
 
 			si++
-			// skip over newlines
-			for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
-				si++
+
+			out := enc.decodeMap[in]
+			if out != 0xFF {
+				dbuf[j] = out
+				continue
 			}
 
+			if in == '\n' || in == '\r' {
+				j--
+				continue
+			}
 			if rune(in) == enc.padChar {
 				// We've reached the end and there's padding
 				switch j {
 				case 0, 1:
 					// incorrect padding
-					return n, false, CorruptInputError(inIdx)
+					return n, false, CorruptInputError(si - 1)
 				case 2:
 					// "==" is expected, the first "=" is already consumed.
+					// skip over newlines
+					for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
+						si++
+					}
 					if si == len(src) {
 						// not enough padding
 						return n, false, CorruptInputError(len(src))
@@ -321,11 +327,11 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 					}
 
 					si++
+				}
 				// skip over newlines
 				for si < len(src) && (src[si] == '\n' || src[si] == '\r') {
 					si++
 				}
-				}
 				if si < len(src) {
 					// trailing garbage
 					err = CorruptInputError(si)
@@ -333,10 +339,7 @@ func (enc *Encoding) decode(dst, src []byte) (n int, end bool, err error) {
 				dinc, dlen, end = 3, j, true
 				break
 			}
-			dbuf[j] = enc.decodeMap[in]
-			if dbuf[j] == 0xFF {
-				return n, false, CorruptInputError(inIdx)
-			}
+			return n, false, CorruptInputError(si - 1)
 		}
 
 		// Convert 4x 6bit source bytes into 3 bytes

src/encoding/base64/base64_test.go

@@ -7,6 +7,7 @@ package base64
 import (
 	"bytes"
 	"errors"
+	"fmt"
 	"io"
 	"io/ioutil"
 	"reflect"
@@ -202,6 +203,9 @@ func TestDecodeCorrupt(t *testing.T) {
 		offset int // -1 means no corruption.
 	}{
 		{"", -1},
+		{"\n", -1},
+		{"AAA=\n", -1},
+		{"AAAA\n", -1},
 		{"!!!!", 0},
 		{"====", 0},
 		{"x===", 1},
@@ -468,11 +472,20 @@ func BenchmarkEncodeToString(b *testing.B) {
 }
 
 func BenchmarkDecodeString(b *testing.B) {
-	data := StdEncoding.EncodeToString(make([]byte, 8192))
+	sizes := []int{2, 4, 8, 64, 8192}
+	benchFunc := func(b *testing.B, benchSize int) {
+		data := StdEncoding.EncodeToString(make([]byte, benchSize))
 		b.SetBytes(int64(len(data)))
+		b.ResetTimer()
 		for i := 0; i < b.N; i++ {
 			StdEncoding.DecodeString(data)
 		}
+	}
+	for _, size := range sizes {
+		b.Run(fmt.Sprintf("%d", size), func(b *testing.B) {
+			benchFunc(b, size)
+		})
+	}
 }
 
 func TestDecoderRaw(t *testing.T) {