util/sha256x: new package (#5337)

The hash.Hash provided by sha256.New is much more efficient
if we always provide it with data a multiple of the block size.
This avoids double-copying of data into the internal block
of sha256.digest.x. Effectively, we are managing a block ourselves
to ensure we only ever call hash.Hash.Write with full blocks.

Performance:

	name    old time/op    new time/op    delta
	Hash    33.5µs ± 1%    20.6µs ± 1%  -38.40%  (p=0.000 n=10+9)

The logic has gone through CPU-hours of fuzzing.

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

util/sha256x/sha256.go

@@ -0,0 +1,149 @@
+// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package sha256x is like crypto/sha256 with extra methods.
+// It exports a concrete Hash type
+// rather than only returning an interface implementation.
+package sha256x
+
+import (
+	"crypto/sha256"
+	"encoding/binary"
+	"hash"
+)
+
+var _ hash.Hash = (*Hash)(nil)
+
+// Hash is a hash.Hash for SHA-256,
+// but has efficient methods for hashing fixed-width integers.
+type Hash struct {
+	// The optimization is to maintain our own block and
+	// only call h.Write with entire blocks.
+	// This avoids double-copying of buffers within sha256.digest itself.
+	// However, it does mean that sha256.digest.x goes unused,
+	// which is a waste of 64B.
+
+	h  hash.Hash              // always *sha256.digest
+	x  [sha256.BlockSize]byte // equivalent to sha256.digest.x
+	nx int                    // equivalent to sha256.digest.nx
+}
+
+func New() *Hash {
+	return &Hash{h: sha256.New()}
+}
+
+func (h *Hash) Write(b []byte) (int, error) {
+	h.HashBytes(b)
+	return len(b), nil
+}
+
+func (h *Hash) Sum(b []byte) []byte {
+	if h.nx > 0 {
+		// This causes block mis-alignment. Future operations will be correct,
+		// but are less efficient until Reset is called.
+		h.h.Write(h.x[:h.nx])
+		h.nx = 0
+	}
+	return h.h.Sum(b)
+}
+
+func (h *Hash) Reset() {
+	h.h.Reset()
+	h.nx = 0
+}
+
+func (h *Hash) Size() int {
+	return h.h.Size()
+}
+
+func (h *Hash) BlockSize() int {
+	return h.h.BlockSize()
+}
+
+func (h *Hash) HashUint8(n uint8) {
+	// NOTE: This method is carefully written to be inlineable.
+	if h.nx <= len(h.x)-1 {
+		h.x[h.nx] = n
+		h.nx += 1
+	} else {
+		h.hashUint8Slow(n) // mark "noinline" to keep this within inline budget
+	}
+}
+
+//go:noinline
+func (h *Hash) hashUint8Slow(n uint8) { h.hashUint(uint64(n), 1) }
+
+func (h *Hash) HashUint16(n uint16) {
+	// NOTE: This method is carefully written to be inlineable.
+	if h.nx <= len(h.x)-2 {
+		binary.LittleEndian.PutUint16(h.x[h.nx:], n)
+		h.nx += 2
+	} else {
+		h.hashUint16Slow(n) // mark "noinline" to keep this within inline budget
+	}
+}
+
+//go:noinline
+func (h *Hash) hashUint16Slow(n uint16) { h.hashUint(uint64(n), 2) }
+
+func (h *Hash) HashUint32(n uint32) {
+	// NOTE: This method is carefully written to be inlineable.
+	if h.nx <= len(h.x)-4 {
+		binary.LittleEndian.PutUint32(h.x[h.nx:], n)
+		h.nx += 4
+	} else {
+		h.hashUint32Slow(n) // mark "noinline" to keep this within inline budget
+	}
+}
+
+//go:noinline
+func (h *Hash) hashUint32Slow(n uint32) { h.hashUint(uint64(n), 4) }
+
+func (h *Hash) HashUint64(n uint64) {
+	// NOTE: This method is carefully written to be inlineable.
+	if h.nx <= len(h.x)-8 {
+		binary.LittleEndian.PutUint64(h.x[h.nx:], n)
+		h.nx += 8
+	} else {
+		h.hashUint64Slow(n) // mark "noinline" to keep this within inline budget
+	}
+}
+
+//go:noinline
+func (h *Hash) hashUint64Slow(n uint64) { h.hashUint(uint64(n), 8) }
+
+func (h *Hash) hashUint(n uint64, i int) {
+	for ; i > 0; i-- {
+		if h.nx == len(h.x) {
+			h.h.Write(h.x[:])
+			h.nx = 0
+		}
+		h.x[h.nx] = byte(n)
+		h.nx += 1
+		n >>= 8
+	}
+}
+
+func (h *Hash) HashBytes(b []byte) {
+	// Nearly identical to sha256.digest.Write.
+	if h.nx > 0 {
+		n := copy(h.x[h.nx:], b)
+		h.nx += n
+		if h.nx == len(h.x) {
+			h.h.Write(h.x[:])
+			h.nx = 0
+		}
+		b = b[n:]
+	}
+	if len(b) >= len(h.x) {
+		n := len(b) &^ (len(h.x) - 1) // n is a multiple of len(h.x)
+		h.h.Write(b[:n])
+		b = b[n:]
+	}
+	if len(b) > 0 {
+		h.nx = copy(h.x[:], b)
+	}
+}
+
+// TODO: Add Hash.MarshalBinary and Hash.UnmarshalBinary?