go/internal/packed: Small package that provides types to be used in packed structures.

We will need to use BE16 and BE32 in the next patch.

go/internal/packed/packed.go

+// Copyright (C) 2016-2017  Nexedi SA and Contributors.
+//                          Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 3, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// You can also Link and Combine this program with other software covered by
+// the terms of any of the Free Software licenses or any of the Open Source
+// Initiative approved licenses and Convey the resulting work. Corresponding
+// source of such a combination shall include the source code for all other
+// software used.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
+// See https://www.nexedi.com/licensing for rationale and options.
+
+// Package packed provides types to use in packed structures.
+package packed
+
+// uintX has alignment requirement =X; [X]byte has alignment requirement 1.
+// That's why we can use [X]byte and this way keep a struct packed, even if Go
+// does not support packed structs in general.
+//
+// XXX SSA backend does not handle arrays well - it handles structs better -
+// e.g. there are unnecessary clears in array case:
+//	https://github.com/golang/go/issues/15925
+//
+// so in the end we use hand-crafted array-like byte-structs.
+type BE16 struct{ _0, _1 byte }
+type BE32 struct{ _0, _1, _2, _3 byte }
+
+func Ntoh16(v BE16) uint16 {
+	// XXX not as good as BigEndian.Uint16
+	// (unnecessary MOVBLZX AL, AX + shifts not combined into ROLW $8)
+	return uint16(v._1) | uint16(v._0)<<8
+}
+
+func Hton16(v uint16) BE16 {
+	return BE16{byte(v >> 8), byte(v)}
+}
+
+func Ntoh32(v BE32) uint32 {
+	// XXX not as good as BigEndian.Uint32
+	// (unnecessary MOVBLZX AL, AX + shifts not combined into BSWAPL)
+	return uint32(v._3) | uint32(v._2)<<8 | uint32(v._1)<<16 | uint32(v._0)<<24
+}
+
+func Hton32(v uint32) BE32 {
+	return BE32{byte(v >> 24), byte(v >> 16), byte(v >> 8), byte(v)}
+}