encoding/binary: cache struct sizes to speed up Read and Write

A majority of work is spent in dataSize when en/decoding the same
struct over and over again. This wastes a lot of work, since
the result doesn't change for a given reflect.Value.

Cache the result of the function for structs, so that subsequent
calls to dataSize can avoid doing work.

    name         old time/op    new time/op     delta
    ReadStruct     1.00µs ± 1%     0.37µs ± 1%   -62.99%  (p=0.029 n=4+4)
    WriteStruct    1.00µs ± 3%     0.37µs ± 1%   -62.69%  (p=0.008 n=5+5)

    name         old speed      new speed       delta
    ReadStruct   75.1MB/s ± 1%  202.9MB/s ± 1%  +170.16%  (p=0.029 n=4+4)
    WriteStruct  74.8MB/s ± 3%  200.4MB/s ± 1%  +167.96%  (p=0.008 n=5+5)

Fixes #34471

Change-Id: Ic5d987ca95f1197415ef93643a0af6fc1224fdf0
Reviewed-on: https://go-review.googlesource.com/c/go/+/199539Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/binary/binary.go

@@ -26,6 +26,7 @@ import (
 	"io"
 	"math"
 	"reflect"
+	"sync"
 )
 
 // A ByteOrder specifies how to convert byte sequences into
@@ -363,18 +364,32 @@ func Size(v interface{}) int {
 	return dataSize(reflect.Indirect(reflect.ValueOf(v)))
 }
 
+var structSize sync.Map // map[reflect.Type]int
+
 // dataSize returns the number of bytes the actual data represented by v occupies in memory.
 // For compound structures, it sums the sizes of the elements. Thus, for instance, for a slice
 // it returns the length of the slice times the element size and does not count the memory
 // occupied by the header. If the type of v is not acceptable, dataSize returns -1.
 func dataSize(v reflect.Value) int {
-	if v.Kind() == reflect.Slice {
+	switch v.Kind() {
+	case reflect.Slice:
 		if s := sizeof(v.Type().Elem()); s >= 0 {
 			return s * v.Len()
 		}
 		return -1
+
+	case reflect.Struct:
+		t := v.Type()
+		if size, ok := structSize.Load(t); ok {
+			return size.(int)
+		}
+		size := sizeof(t)
+		structSize.Store(t, size)
+		return size
+
+	default:
+		return sizeof(v.Type())
 	}
-	return sizeof(v.Type())
 }
 
 // sizeof returns the size >= 0 of variables for the given type or -1 if the type is not acceptable.

src/encoding/binary/binary_test.go

@@ -7,9 +7,11 @@ package binary
 import (
 	"bytes"
 	"io"
+	"io/ioutil"
 	"math"
 	"reflect"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -296,6 +298,58 @@ func TestBlankFields(t *testing.T) {
 	}
 }
 
+func TestSizeStructCache(t *testing.T) {
+	// Reset the cache, otherwise multiple test runs fail.
+	structSize = sync.Map{}
+
+	count := func() int {
+		var i int
+		structSize.Range(func(_, _ interface{}) bool {
+			i++
+			return true
+		})
+		return i
+	}
+
+	var total int
+	added := func() int {
+		delta := count() - total
+		total += delta
+		return delta
+	}
+
+	type foo struct {
+		A uint32
+	}
+
+	type bar struct {
+		A Struct
+		B foo
+		C Struct
+	}
+
+	testcases := []struct {
+		val  interface{}
+		want int
+	}{
+		{new(foo), 1},
+		{new(bar), 1},
+		{new(bar), 0},
+		{new(struct{ A Struct }), 1},
+		{new(struct{ A Struct }), 0},
+	}
+
+	for _, tc := range testcases {
+		if Size(tc.val) == -1 {
+			t.Fatalf("Can't get the size of %T", tc.val)
+		}
+
+		if n := added(); n != tc.want {
+			t.Errorf("Sizing %T added %d entries to the cache, want %d", tc.val, n, tc.want)
+		}
+	}
+}
+
 // An attempt to read into a struct with an unexported field will
 // panic. This is probably not the best choice, but at this point
 // anything else would be an API change.
@@ -436,6 +490,14 @@ func BenchmarkReadStruct(b *testing.B) {
 	}
 }
 
+func BenchmarkWriteStruct(b *testing.B) {
+	b.SetBytes(int64(Size(&s)))
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Write(ioutil.Discard, BigEndian, &s)
+	}
+}
+
 func BenchmarkReadInts(b *testing.B) {
 	var ls Struct
 	bsr := &byteSliceReader{}