Minor hashing-related optimizations.

This commit contains three classes of optimizations:
 - Reducing the number of unnecessary hash copies
 - Improve the performance of the DoubleSha256 function
 - A couple of minor optimizations of the ShaHash functions

The first class is a result of the Bytes function on a ShaHash making a
copy of the bytes before returning them.  It really should have been named
CloneBytes, but that would break the API now.

To address this, a comment has been added to the function which explicitly
calls out the copy behavior.  In addition, all call sites of .Bytes on a
ShaHash in the code base have been updated to simply slice the array when
a copy is not needed.  This saves a significant amount of data copying.

The second optimization modifies the DoubleSha256 function to directly use
fastsha256.Sum256 instead of the hasher interface.  This reduces the
number of allocations needed.  A benchmark for the function has been added
as well.

old: BenchmarkDoubleSha256  500000   3691 ns/op   192 B/op   3 allocs/op
new: BenchmarkDoubleSha256  500000   3081 ns/op    32 B/op   1 allocs/op

The final optimizations are for the ShaHash IsEqual and SetBytes functions
which have been modified to make use of the fact the type is an array and
remove an unneeded subslice.

blockchain/difficulty.go

@@ -57,15 +57,14 @@ var (
 // perform math comparisons.
 func ShaHashToBig(hash *wire.ShaHash) *big.Int {
 	// A ShaHash is in little-endian, but the big package wants the bytes
-	// in big-endian.  Reverse them.  ShaHash.Bytes makes a copy, so it
-	// is safe to modify the returned buffer.
-	buf := hash.Bytes()
+	// in big-endian, so reverse them.
+	buf := *hash
 	blen := len(buf)
 	for i := 0; i < blen/2; i++ {
 		buf[i], buf[blen-1-i] = buf[blen-1-i], buf[i]
 	}
 
-	return new(big.Int).SetBytes(buf)
+	return new(big.Int).SetBytes(buf[:])
 }
 
 // CompactToBig converts a compact representation of a whole number N to an

blockchain/merkle.go

@@ -31,8 +31,8 @@ func nextPowerOfTwo(n int) int {
 func HashMerkleBranches(left *wire.ShaHash, right *wire.ShaHash) *wire.ShaHash {
 	// Concatenate the left and right nodes.
 	var sha [wire.HashSize * 2]byte
-	copy(sha[:wire.HashSize], left.Bytes())
-	copy(sha[wire.HashSize:], right.Bytes())
+	copy(sha[:wire.HashSize], left[:])
+	copy(sha[wire.HashSize:], right[:])
 
 	// Create a new sha hash from the double sha 256.  Ignore the error
 	// here since SetBytes can't fail here due to the fact DoubleSha256

database/ldb/block.go

@@ -134,10 +134,9 @@ func (db *LevelDb) setBlk(sha *wire.ShaHash, blkHeight int64, buf []byte) {
 	shaKey := shaBlkToKey(sha)
 	blkKey := int64ToKey(blkHeight)
 
-	shaB := sha.Bytes()
-	blkVal := make([]byte, len(shaB)+len(buf))
-	copy(blkVal[0:], shaB)
-	copy(blkVal[len(shaB):], buf)
+	blkVal := make([]byte, len(sha)+len(buf))
+	copy(blkVal[0:], sha[:])
+	copy(blkVal[len(sha):], buf)
 
 	db.lBatch().Put(shaKey, lw[:])
 	db.lBatch().Put(blkKey, blkVal)

database/ldb/leveldb.go

@@ -641,8 +641,7 @@ func int64ToKey(keyint int64) []byte {
 }
 
 func shaBlkToKey(sha *wire.ShaHash) []byte {
-	shaB := sha.Bytes()
-	return shaB
+	return sha[:]
 }
 
 // These are used here and in tx.go's deleteOldAddrIndex() to prevent deletion
@@ -651,15 +650,17 @@ var recordSuffixTx = []byte{'t', 'x'}
 var recordSuffixSpentTx = []byte{'s', 'x'}
 
 func shaTxToKey(sha *wire.ShaHash) []byte {
-	shaB := sha.Bytes()
-	shaB = append(shaB, recordSuffixTx...)
-	return shaB
+	key := make([]byte, len(sha)+len(recordSuffixTx))
+	copy(key, sha[:])
+	copy(key[len(sha):], recordSuffixTx)
+	return key
 }
 
 func shaSpentTxToKey(sha *wire.ShaHash) []byte {
-	shaB := sha.Bytes()
-	shaB = append(shaB, recordSuffixSpentTx...)
-	return shaB
+	key := make([]byte, len(sha)+len(recordSuffixSpentTx))
+	copy(key, sha[:])
+	copy(key[len(sha):], recordSuffixSpentTx)
+	return key
 }
 
 func (db *LevelDb) lBatch() *leveldb.Batch {

database/ldb/tx.go

@@ -537,7 +537,7 @@ func (db *LevelDb) UpdateAddrIndexForBlock(blkSha *wire.ShaHash, blkHeight int64
 
 	// Update tip of addrindex.
 	newIndexTip := make([]byte, 40, 40)
-	copy(newIndexTip[0:32], blkSha.Bytes())
+	copy(newIndexTip[0:32], blkSha[:])
 	binary.LittleEndian.PutUint64(newIndexTip[32:40], uint64(blkHeight))
 	batch.Put(addrIndexMetaDataKey, newIndexTip)
 

server.go

@@ -5,7 +5,6 @@
 package main
 
 import (
-	"bytes"
 	"container/list"
 	"crypto/rand"
 	"encoding/binary"
@@ -215,13 +214,15 @@ func (s *server) handleUpdatePeerHeights(state *peerState, umsg updatePeerHeight
 			return
 		}
 
-		latestBlkSha := p.lastAnnouncedBlock.Bytes()
+		// This is a pointer to the underlying memory which doesn't
+		// change.
+		latestBlkSha := p.lastAnnouncedBlock
 		p.StatsMtx.Unlock()
 
 		// If the peer has recently announced a block, and this block
 		// matches our newly accepted block, then update their block
 		// height.
-		if bytes.Equal(latestBlkSha, umsg.newSha.Bytes()) {
+		if *latestBlkSha == *umsg.newSha {
 			p.UpdateLastBlockHeight(umsg.newHeight)
 			p.UpdateLastAnnouncedBlock(nil)
 		}

wire/bench_test.go

@@ -392,3 +392,20 @@ func BenchmarkTxSha(b *testing.B) {
 		genesisCoinbaseTx.TxSha()
 	}
 }
+
+// BenchmarkDoubleSha256 performs a benchmark on how long it takes to perform a
+// double sha 256.
+func BenchmarkDoubleSha256(b *testing.B) {
+	b.StopTimer()
+	var buf bytes.Buffer
+	if err := genesisCoinbaseTx.Serialize(&buf); err != nil {
+		b.Errorf("Serialize: unexpected error: %v", err)
+		return
+	}
+	txBytes := buf.Bytes()
+	b.StartTimer()
+
+	for i := 0; i < b.N; i++ {
+		_ = DoubleSha256(txBytes)
+	}
+}

wire/common.go

@@ -522,10 +522,7 @@ func RandomUint64() (uint64, error) {
 
 // DoubleSha256 calculates sha256(sha256(b)) and returns the resulting bytes.
 func DoubleSha256(b []byte) []byte {
-	hasher := fastsha256.New()
-	hasher.Write(b)
-	sum := hasher.Sum(nil)
-	hasher.Reset()
-	hasher.Write(sum)
-	return hasher.Sum(nil)
+	first := fastsha256.Sum256(b)
+	second := fastsha256.Sum256(first[:])
+	return second[:]
 }

wire/shahash.go

@@ -5,7 +5,6 @@
 package wire
 
 import (
-	"bytes"
 	"encoding/hex"
 	"fmt"
 )
@@ -34,6 +33,10 @@ func (hash ShaHash) String() string {
 }
 
 // Bytes returns the bytes which represent the hash as a byte slice.
+//
+// NOTE: This makes a copy of the bytes and should have probably been named
+// CloneBytes.  It is generally cheaper to just slice the hash directly thereby
+// reusing the same bytes rather than calling this method.
 func (hash *ShaHash) Bytes() []byte {
 	newHash := make([]byte, HashSize)
 	copy(newHash, hash[:])
@@ -49,14 +52,14 @@ func (hash *ShaHash) SetBytes(newHash []byte) error {
 		return fmt.Errorf("invalid sha length of %v, want %v", nhlen,
 			HashSize)
 	}
-	copy(hash[:], newHash[0:HashSize])
+	copy(hash[:], newHash)
 
 	return nil
 }
 
 // IsEqual returns true if target is the same as hash.
 func (hash *ShaHash) IsEqual(target *ShaHash) bool {
-	return bytes.Equal(hash[:], target[:])
+	return *hash == *target
 }
 
 // NewShaHash returns a new ShaHash from a byte slice.  An error is returned if