encoding/json: reduce allocations by Decoder for \uXXXX

Manually convert hex escape sequence to rune instead of calling
strconv.ParseUint.

This inlines the unhex func from docs (and many other packages).

name              old time/op    new time/op    delta
UnicodeDecoder-4     468ns ± 1%     402ns ± 1%  -14.26%  (p=0.000
n=10+10)

name              old speed      new speed      delta
UnicodeDecoder-4  29.9MB/s ± 1%  34.8MB/s ± 1%  +16.59%  (p=0.000
n=10+10)

name              old alloc/op   new alloc/op   delta
UnicodeDecoder-4     44.0B ± 0%     36.0B ± 0%  -18.18%  (p=0.000
n=10+10)

name              old allocs/op  new allocs/op  delta
UnicodeDecoder-4      4.00 ± 0%      2.00 ± 0%  -50.00%  (p=0.000
n=10+10)

Fixes #20567

Change-Id: If350978d5bb98ff517485752184d02249f5d1f3a
Reviewed-on: https://go-review.googlesource.com/44738
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/encoding/json/bench_test.go b/src/encoding/json/bench_test.go
index 85d7ae043b8caa77ca263e37ac11bd91c6b7fff2..42439eb7054e9498df916d8d692032f737e85621 100644 (file)
--- a/src/encoding/json/bench_test.go
+++ b/src/encoding/json/bench_test.go
@@ -133,6 +133,21 @@ func BenchmarkCodeDecoder(b *testing.B) {
        b.SetBytes(int64(len(codeJSON)))
 }
 
+func BenchmarkUnicodeDecoder(b *testing.B) {
+       j := []byte(`"\uD83D\uDE01"`)
+       b.SetBytes(int64(len(j)))
+       r := bytes.NewReader(j)
+       dec := NewDecoder(r)
+       var out string
+       b.ResetTimer()
+       for i := 0; i < b.N; i++ {
+               if err := dec.Decode(&out); err != nil {
+                       b.Fatal("Decode:", err)
+               }
+               r.Seek(0, 0)
+       }
+}
+
 func BenchmarkDecoderStream(b *testing.B) {
        b.StopTimer()
        var buf bytes.Buffer

diff --git a/src/encoding/json/decode.go b/src/encoding/json/decode.go
index 70179e60ac3dd56357d2433c85db43cd8ae44b60..4f98916105b9d9470365e65f8f6ea140f684ca2a 100644 (file)
--- a/src/encoding/json/decode.go
+++ b/src/encoding/json/decode.go
@@ -1148,11 +1148,21 @@ func getu4(s []byte) rune {
        if len(s) < 6 || s[0] != '\\' || s[1] != 'u' {
                return -1
        }
-       r, err := strconv.ParseUint(string(s[2:6]), 16, 64)
-       if err != nil {
-               return -1
+       var r rune
+       for _, c := range s[2:6] {
+               switch {
+               case '0' <= c && c <= '9':
+                       c = c - '0'
+               case 'a' <= c && c <= 'f':
+                       c = c - 'a' + 10
+               case 'A' <= c && c <= 'F':
+                       c = c - 'A' + 10
+               default:
+                       return -1
+               }
+               r = r*16 + rune(c)
        }
-       return rune(r)
+       return r
 }
 
 // unquote converts a quoted JSON string literal s into an actual string t.