Assign instead of append to keys slice (#2932)

Co-authored-by: Stephen Buttolph <[email protected]>

x/merkledb/codec.go

@@ -90,12 +90,14 @@ func encodeDBNode(n *dbNode) []byte {
 		return w.b
 	}
 
-	// By allocating BranchFactorLargest rather than len(n.children), this slice
+	// By allocating BranchFactorLargest rather than [numChildren], this slice
 	// is allocated on the stack rather than the heap. BranchFactorLargest is
-	// at least len(n.children) which avoids memory allocations.
-	keys := make([]byte, 0, BranchFactorLargest)
+	// at least [numChildren] which avoids memory allocations.
+	keys := make([]byte, numChildren, BranchFactorLargest)
+	i := 0
 	for k := range n.children {
-		keys = append(keys, k)
+		keys[i] = k
+		i++
 	}
 
 	// Ensure that the order of entries is correct.

x/merkledb/hashing.go

@@ -51,13 +51,15 @@ func (*sha256Hasher) HashNode(n *node) ids.ID {
 
 	// Avoid allocating keys entirely if the node doesn't have any children.
 	if numChildren != 0 {
-		// By allocating BranchFactorLargest rather than len(n.children), this
+		// By allocating BranchFactorLargest rather than [numChildren], this
 		// slice is allocated on the stack rather than the heap.
-		// BranchFactorLargest is at least len(n.children) which avoids memory
+		// BranchFactorLargest is at least [numChildren] which avoids memory
 		// allocations.
-		keys := make([]byte, 0, BranchFactorLargest)
+		keys := make([]byte, numChildren, BranchFactorLargest)
+		i := 0
 		for k := range n.children {
-			keys = append(keys, k)
+			keys[i] = k
+			i++
 		}
 
 		// Ensure that the order of entries is correct.