encoding/binary: give LittleEndian, BigEndian specific types

Giving them specific types has the benefit that
binary.BigEndian.Uint32(b) is now a direct call, not an
indirect via a mutable interface value, so it can potentially
be inlined.

Recent changes to the spec relaxed the rules for comparison,
so this code is still valid:

	func isLittle(o binary.ByteOrder) { return o == binary.LittleEndian }

The change does break this potential idiom:

	o := binary.BigEndian
	if foo {
		o = binary.LittleEndian
	}

That must rewrite to give o an explicit binary.ByteOrder type.
On balance I think the benefit from the direct call and inlining
outweigh the cost of breaking that idiom.

R=r, r2
CC=golang-dev
https://golang.org/cl/2427042

1 files changed, 5 insertions, 2 deletions

diff --git a/src/pkg/encoding/binary/binary.go b/src/pkg/encoding/binary/binary.go
index 2343e0398b..ebc2ae8b7c 100644
--- a/src/pkg/encoding/binary/binary.go
+++ b/src/pkg/encoding/binary/binary.go
@@ -29,8 +29,11 @@ type ByteOrder interface {
 // allowing, e.g., order == binary.LittleEndian.
 type unused byte
 
-var LittleEndian ByteOrder = littleEndian(0)
-var BigEndian ByteOrder = bigEndian(0)
+// LittleEndian is the little-endian implementation of ByteOrder.
+var LittleEndian littleEndian
+
+// BigEndian is the big-endian implementation of ByteOrder.
+var BigEndian bigEndian
 
 type littleEndian unused