vm: optimize SWAP instruction, refactor common code

Add `Swap` method to the Stack and use it for both SWAP and XSWAP. Avoid
element popping and pushing (and associated accounting costs).

1.4M->1.5M 100K block import test before:
real    3m51,885s
user    5m54,744s
sys     0m38,444s

After:
real    3m44,292s
user    5m43,494s
sys     0m34,741s

pkg/vm/stack.go

@@ -2,6 +2,7 @@ package vm
 
 import (
 	"encoding/json"
+	"errors"
 	"fmt"
 	"math/big"
 
@@ -371,6 +372,23 @@ func (s *Stack) IterBack(f func(*Element)) {
 	}
 }
 
+// Swap swaps two elements on the stack without popping and pushing them.
+func (s *Stack) Swap(n1, n2 int) error {
+	if n1 < 0 || n2 < 0 {
+		return errors.New("negative index")
+	}
+	if n1 >= s.len || n2 >= s.len {
+		return errors.New("too big index")
+	}
+	if n1 == n2 {
+		return nil
+	}
+	a := s.Peek(n1)
+	b := s.Peek(n2)
+	a.value, b.value = b.value, a.value
+	return nil
+}
+
 // popSigElements pops keys or signatures from the stack as needed for
 // CHECKMULTISIG.
 func (s *Stack) popSigElements() ([][]byte, error) {

pkg/vm/stack_test.go

@@ -226,22 +226,36 @@ func TestSwapElemValues(t *testing.T) {
 	s.PushVal(2)
 	s.PushVal(4)
 
-	a := s.Peek(0)
-	b := s.Peek(1)
-
-	// [ 4 ] -> a
-	// [ 2 ] -> b
-
-	aval := a.value
-	bval := b.value
-	a.value = bval
-	b.value = aval
-
-	// [ 2 ] -> a
-	// [ 4 ] -> b
-
+	assert.NoError(t, s.Swap(0, 1))
 	assert.Equal(t, int64(2), s.Pop().BigInt().Int64())
 	assert.Equal(t, int64(4), s.Pop().BigInt().Int64())
+
+	s.PushVal(1)
+	s.PushVal(2)
+	s.PushVal(3)
+	s.PushVal(4)
+
+	assert.NoError(t, s.Swap(1, 3))
+	assert.Equal(t, int64(4), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(1), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(2), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(3), s.Pop().BigInt().Int64())
+
+	s.PushVal(1)
+	s.PushVal(2)
+	s.PushVal(3)
+	s.PushVal(4)
+
+	assert.Error(t, s.Swap(-1, 0))
+	assert.Error(t, s.Swap(0, -3))
+	assert.Error(t, s.Swap(0, 4))
+	assert.Error(t, s.Swap(5, 0))
+
+	assert.NoError(t, s.Swap(1, 1))
+	assert.Equal(t, int64(4), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(3), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(2), s.Pop().BigInt().Int64())
+	assert.Equal(t, int64(1), s.Pop().BigInt().Int64())
 }
 
 func TestPopSigElements(t *testing.T) {

pkg/vm/vm.go

@@ -510,10 +510,10 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 		v.estack.Push(v.estack.Dup(0))
 
 	case opcode.SWAP:
-		a := v.estack.Pop()
-		b := v.estack.Pop()
-		v.estack.Push(a)
-		v.estack.Push(b)
+		err := v.estack.Swap(1, 0)
+		if err != nil {
+			panic(err.Error())
+		}
 
 	case opcode.TUCK:
 		a := v.estack.Dup(0)
@@ -587,18 +587,9 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 
 	case opcode.XSWAP:
 		n := int(v.estack.Pop().BigInt().Int64())
-		if n < 0 {
-			panic("XSWAP: invalid length")
-		}
-
-		// Swap values of elements instead of reordering stack elements.
-		if n > 0 {
-			a := v.estack.Peek(n)
-			b := v.estack.Peek(0)
-			aval := a.value
-			bval := b.value
-			a.value = bval
-			b.value = aval
+		err := v.estack.Swap(n, 0)
+		if err != nil {
+			panic(err.Error())
 		}
 
 	case opcode.XTUCK:

pkg/vm/vm_test.go

@@ -2409,6 +2409,68 @@ func TestCHECKMULTISIGGood(t *testing.T) {
 	assert.Equal(t, true, vm.estack.Pop().Bool())
 }
 
+func TestSWAPGood(t *testing.T) {
+	prog := makeProgram(opcode.SWAP)
+	vm := load(prog)
+	vm.estack.PushVal(2)
+	vm.estack.PushVal(4)
+	runVM(t, vm)
+	assert.Equal(t, 2, vm.estack.Len())
+	assert.Equal(t, int64(2), vm.estack.Pop().BigInt().Int64())
+	assert.Equal(t, int64(4), vm.estack.Pop().BigInt().Int64())
+}
+
+func TestSWAPBad1(t *testing.T) {
+	prog := makeProgram(opcode.SWAP)
+	vm := load(prog)
+	vm.estack.PushVal(4)
+	checkVMFailed(t, vm)
+}
+
+func TestSWAPBad2(t *testing.T) {
+	prog := makeProgram(opcode.SWAP)
+	vm := load(prog)
+	checkVMFailed(t, vm)
+}
+
+func TestXSWAPGood(t *testing.T) {
+	prog := makeProgram(opcode.XSWAP)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(2)
+	vm.estack.PushVal(3)
+	vm.estack.PushVal(4)
+	vm.estack.PushVal(5)
+	vm.estack.PushVal(3)
+	runVM(t, vm)
+	assert.Equal(t, 5, vm.estack.Len())
+	assert.Equal(t, int64(2), vm.estack.Pop().BigInt().Int64())
+	assert.Equal(t, int64(4), vm.estack.Pop().BigInt().Int64())
+	assert.Equal(t, int64(3), vm.estack.Pop().BigInt().Int64())
+	assert.Equal(t, int64(5), vm.estack.Pop().BigInt().Int64())
+	assert.Equal(t, int64(1), vm.estack.Pop().BigInt().Int64())
+}
+
+func TestXSWAPBad1(t *testing.T) {
+	prog := makeProgram(opcode.XSWAP)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(2)
+	vm.estack.PushVal(-1)
+	checkVMFailed(t, vm)
+}
+
+func TestXSWAPBad2(t *testing.T) {
+	prog := makeProgram(opcode.XSWAP)
+	vm := load(prog)
+	vm.estack.PushVal(1)
+	vm.estack.PushVal(2)
+	vm.estack.PushVal(3)
+	vm.estack.PushVal(4)
+	vm.estack.PushVal(4)
+	checkVMFailed(t, vm)
+}
+
 func makeProgram(opcodes ...opcode.Opcode) []byte {
 	prog := make([]byte, len(opcodes)+1) // RET
 	for i := 0; i < len(opcodes); i++ {