encoding/binary: remove bound checks from conversions.

* This the simplest solution I could came up with
that doesn't required changing the compiler.
* The bound checks become constants now
so they are removed during opt phase.

Updates #14808

Change-Id: If32c33d7ec08bb400321b465015d152f0a5d3001
Reviewed-on: https://go-review.googlesource.com/20654
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/binary/binary.go

@@ -48,30 +48,38 @@ var BigEndian bigEndian
 
 type littleEndian struct{}
 
-func (littleEndian) Uint16(b []byte) uint16 { return uint16(b[0]) | uint16(b[1])<<8 }
+func (littleEndian) Uint16(b []byte) uint16 {
+	b = b[:2] // bounds check hint to compiler; see golang.org/issue/14808
+	return uint16(b[0]) | uint16(b[1])<<8
+}
 
 func (littleEndian) PutUint16(b []byte, v uint16) {
+	b = b[:2] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v)
 	b[1] = byte(v >> 8)
 }
 
 func (littleEndian) Uint32(b []byte) uint32 {
+	b = b[:4] // bounds check hint to compiler; see golang.org/issue/14808
 	return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24
 }
 
 func (littleEndian) PutUint32(b []byte, v uint32) {
+	b = b[:4] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v)
 	b[1] = byte(v >> 8)
 	b[2] = byte(v >> 16)
 	b[3] = byte(v >> 24)
 }
 
 func (littleEndian) Uint64(b []byte) uint64 {
+	b = b[:8] // bounds check hint to compiler; see golang.org/issue/14808
 	return uint64(b[0]) | uint64(b[1])<<8 | uint64(b[2])<<16 | uint64(b[3])<<24 |
 		uint64(b[4])<<32 | uint64(b[5])<<40 | uint64(b[6])<<48 | uint64(b[7])<<56
 }
 
 func (littleEndian) PutUint64(b []byte, v uint64) {
+	b = b[:8] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v)
 	b[1] = byte(v >> 8)
 	b[2] = byte(v >> 16)
@@ -88,30 +96,38 @@ func (littleEndian) GoString() string { return "binary.LittleEndian" }
 
 type bigEndian struct{}
 
-func (bigEndian) Uint16(b []byte) uint16 { return uint16(b[1]) | uint16(b[0])<<8 }
+func (bigEndian) Uint16(b []byte) uint16 {
+	b = b[:2] // bounds check hint to compiler; see golang.org/issue/14808
+	return uint16(b[1]) | uint16(b[0])<<8
+}
 
 func (bigEndian) PutUint16(b []byte, v uint16) {
+	b = b[:2] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v >> 8)
 	b[1] = byte(v)
 }
 
 func (bigEndian) Uint32(b []byte) uint32 {
+	b = b[:4] // bounds check hint to compiler; see golang.org/issue/14808
 	return uint32(b[3]) | uint32(b[2])<<8 | uint32(b[1])<<16 | uint32(b[0])<<24
 }
 
 func (bigEndian) PutUint32(b []byte, v uint32) {
+	b = b[:4] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v >> 24)
 	b[1] = byte(v >> 16)
 	b[2] = byte(v >> 8)
 	b[3] = byte(v)
 }
 
 func (bigEndian) Uint64(b []byte) uint64 {
+	b = b[:8] // bounds check hint to compiler; see golang.org/issue/14808
 	return uint64(b[7]) | uint64(b[6])<<8 | uint64(b[5])<<16 | uint64(b[4])<<24 |
 		uint64(b[3])<<32 | uint64(b[2])<<40 | uint64(b[1])<<48 | uint64(b[0])<<56
 }
 
 func (bigEndian) PutUint64(b []byte, v uint64) {
+	b = b[:8] // early bounds check to guarantee safety of writes below
 	b[0] = byte(v >> 56)
 	b[1] = byte(v >> 48)
 	b[2] = byte(v >> 40)