vm: don't use Stack for istack

We don't use all of the Stack functionality for it, so drop useless methods
and avoid some interface conversions. It increases single-node TPS by about
0.9%, so nothing really important there, but not a bad change either. Maybe it
can be reworked again with generics though.

pkg/core/interop/contract/call.go

@@ -163,12 +163,12 @@ var ErrNativeCall = errors.New("failed native call")
 
 // CallFromNative performs synchronous call from native contract.
 func CallFromNative(ic *interop.Context, caller util.Uint160, cs *state.Contract, method string, args []stackitem.Item, hasReturn bool) error {
-	startSize := ic.VM.Istack().Len()
+	startSize := len(ic.VM.Istack())
 	if err := callExFromNative(ic, caller, cs, method, args, callflag.All, hasReturn, false, true); err != nil {
 		return err
 	}
 
-	for !ic.VM.HasStopped() && ic.VM.Istack().Len() > startSize {
+	for !ic.VM.HasStopped() && len(ic.VM.Istack()) > startSize {
 		if err := ic.VM.Step(); err != nil {
 			return fmt.Errorf("%w: %v", ErrNativeCall, err)
 		}

pkg/core/interop/runtime/engine.go

@@ -41,7 +41,7 @@ func GetCallingScriptHash(ic *interop.Context) error {
 
 // GetEntryScriptHash returns entry script hash.
 func GetEntryScriptHash(ic *interop.Context) error {
-	return ic.VM.PushContextScriptHash(ic.VM.Istack().Len() - 1)
+	return ic.VM.PushContextScriptHash(len(ic.VM.Istack()) - 1)
 }
 
 // GetScriptContainer returns transaction or block that contains the script

pkg/core/native/oracle.go

@@ -276,7 +276,7 @@ func (o *Oracle) finish(ic *interop.Context, _ []stackitem.Item) stackitem.Item
 
 // FinishInternal processes an oracle response.
 func (o *Oracle) FinishInternal(ic *interop.Context) error {
-	if ic.VM.Istack().Len() != 2 {
+	if len(ic.VM.Istack()) != 2 {
 		return errors.New("Oracle.finish called from non-entry script")
 	}
 	if ic.Invocations[o.Hash] != 1 {

pkg/vm/context.go

@@ -5,7 +5,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"math/big"
 
 	"github.com/nspcc-dev/neo-go/pkg/crypto/hash"
 	"github.com/nspcc-dev/neo-go/pkg/smartcontract/callflag"
@@ -251,42 +250,6 @@ func (c *Context) NumOfReturnVals() int {
 	return c.retCount
 }
 
-// Value implements the stackitem.Item interface.
-func (c *Context) Value() interface{} {
-	return c
-}
-
-// Dup implements the stackitem.Item interface.
-func (c *Context) Dup() stackitem.Item {
-	return c
-}
-
-// TryBool implements the stackitem.Item interface.
-func (c *Context) TryBool() (bool, error) { panic("can't convert Context to Bool") }
-
-// TryBytes implements the stackitem.Item interface.
-func (c *Context) TryBytes() ([]byte, error) {
-	return nil, errors.New("can't convert Context to ByteArray")
-}
-
-// TryInteger implements the stackitem.Item interface.
-func (c *Context) TryInteger() (*big.Int, error) {
-	return nil, errors.New("can't convert Context to Integer")
-}
-
-// Type implements the stackitem.Item interface.
-func (c *Context) Type() stackitem.Type { panic("Context cannot appear on evaluation stack") }
-
-// Convert implements the stackitem.Item interface.
-func (c *Context) Convert(_ stackitem.Type) (stackitem.Item, error) {
-	panic("Context cannot be converted to anything")
-}
-
-// Equals implements the stackitem.Item interface.
-func (c *Context) Equals(s stackitem.Item) bool {
-	return c == s
-}
-
 func (c *Context) atBreakPoint() bool {
 	for _, n := range c.sc.breakPoints {
 		if n == c.nextip {
@@ -296,10 +259,6 @@ func (c *Context) atBreakPoint() bool {
 	return false
 }
 
-func (c *Context) String() string {
-	return "execution context"
-}
-
 // IsDeployed returns whether this context contains a deployed contract.
 func (c *Context) IsDeployed() bool {
 	return c.sc.NEF != nil
@@ -332,13 +291,10 @@ func dumpSlot(s *slot) string {
 // getContextScriptHash returns script hash of the invocation stack element
 // number n.
 func (v *VM) getContextScriptHash(n int) util.Uint160 {
-	istack := v.Istack()
+	if len(v.istack) <= n {
-	if istack.Len() <= n {
 		return util.Uint160{}
 	}
-	element := istack.Peek(n)
+	return v.istack[len(v.istack)-1-n].ScriptHash()
-	ctx := element.value.(*Context)
-	return ctx.ScriptHash()
 }
 
 // IsCalledByEntry checks parent script contexts and return true if the current one

pkg/vm/invocation_tree_test.go

@@ -24,7 +24,7 @@ func TestInvocationTree(t *testing.T) {
 	cnt := 0
 	v := newTestVM()
 	v.SyscallHandler = func(v *VM, _ uint32) error {
-		if v.Istack().Len() > 4 { // top -> call -> syscall -> call -> syscall -> ...
+		if len(v.Istack()) > 4 { // top -> call -> syscall -> call -> syscall -> ...
 			v.Estack().PushVal(1)
 			return nil
 		}

pkg/vm/json_test.go

@@ -166,7 +166,7 @@ func testFile(t *testing.T, filename string) {
 
 					if len(result.InvocationStack) > 0 {
 						for i, s := range result.InvocationStack {
-							ctx := vm.istack.Peek(i).Value().(*Context)
+							ctx := vm.istack[len(vm.istack)-1-i]
 							if ctx.nextip < len(ctx.sc.prog) {
 								require.Equal(t, s.InstructionPointer, ctx.nextip)
 								op, err := opcode.FromString(s.Instruction)

pkg/vm/vm.go

@@ -69,7 +69,7 @@ type VM struct {
 	// callback to get interop price
 	getPrice func(opcode.Opcode, []byte) int64
 
-	istack Stack  // invocation stack.
+	istack []*Context // invocation stack.
 	estack *Stack     // execution stack.
 
 	uncaughtException stackitem.Item // exception being handled
@@ -110,8 +110,7 @@ func NewWithTrigger(t trigger.Type) *VM {
 		trigger: t,
 	}
 
-	initStack(&vm.istack, "invocation", nil)
+	vm.istack = make([]*Context, 0, 8) // Most of invocations use one-two contracts, but they're likely to have internal calls.
-	vm.istack.elems = make([]Element, 0, 8) // Most of invocations use one-two contracts, but they're likely to have internal calls.
 	vm.estack = newStack("evaluation", &vm.refs)
 	return vm
 }
@@ -128,7 +127,7 @@ func (v *VM) SetPriceGetter(f func(opcode.Opcode, []byte) int64) {
 func (v *VM) Reset(t trigger.Type) {
 	v.state = vmstate.None
 	v.getPrice = nil
-	v.istack.elems = v.istack.elems[:0]
+	v.istack = v.istack[:0]
 	v.estack.elems = v.estack.elems[:0]
 	v.uncaughtException = nil
 	v.refs = 0
@@ -157,8 +156,8 @@ func (v *VM) Estack() *Stack {
 }
 
 // Istack returns the invocation stack, so interop hooks can utilize this.
-func (v *VM) Istack() *Stack {
+func (v *VM) Istack() []*Context {
-	return &v.istack
+	return v.istack
 }
 
 // PrintOps prints the opcodes of the current loaded program to stdout.
@@ -284,7 +283,7 @@ func (v *VM) Load(prog []byte) {
 // LoadWithFlags initializes the VM with the program and flags given.
 func (v *VM) LoadWithFlags(prog []byte, f callflag.CallFlag) {
 	// Clear all stacks and state, it could be a reload.
-	v.istack.Clear()
+	v.istack = v.istack[:0]
 	v.estack.Clear()
 	v.state = vmstate.None
 	v.gasConsumed = 0
@@ -359,16 +358,16 @@ func (v *VM) loadScriptWithCallingHash(b []byte, exe *nef.File, caller util.Uint
 		ctx.sc.invTree = newTree
 	}
 	ctx.sc.onUnload = onContextUnload
-	v.istack.PushItem(ctx)
+	v.istack = append(v.istack, ctx)
 }
 
 // Context returns the current executed context. Nil if there is no context,
 // which implies no program is loaded.
 func (v *VM) Context() *Context {
-	if v.istack.Len() == 0 {
+	if len(v.istack) == 0 {
 		return nil
 	}
-	return v.istack.Peek(0).value.(*Context)
+	return v.istack[len(v.istack)-1]
 }
 
 // PopResult is used to pop the first item of the evaluation stack. This allows
@@ -382,7 +381,7 @@ func (v *VM) PopResult() interface{} {
 
 // DumpIStack returns json formatted representation of the invocation stack.
 func (v *VM) DumpIStack() string {
-	b, _ := json.MarshalIndent(v.istack.ToArray(), "", "    ")
+	b, _ := json.MarshalIndent(v.istack, "", "    ")
 	return string(b)
 }
 
@@ -405,7 +404,7 @@ func (v *VM) State() vmstate.State {
 // Ready returns true if the VM is ready to execute the loaded program.
 // It will return false if no program is loaded.
 func (v *VM) Ready() bool {
-	return v.istack.Len() > 0
+	return len(v.istack) > 0
 }
 
 // Run starts execution of the loaded program.
@@ -505,8 +504,8 @@ func (v *VM) StepOut() error {
 		v.state = vmstate.None
 	}
 
-	expSize := v.istack.Len()
+	expSize := len(v.istack)
-	for v.state == vmstate.None && v.istack.Len() >= expSize {
+	for v.state == vmstate.None && len(v.istack) >= expSize {
 		err = v.StepInto()
 	}
 	if v.state == vmstate.None {
@@ -527,10 +526,10 @@ func (v *VM) StepOver() error {
 		v.state = vmstate.None
 	}
 
-	expSize := v.istack.Len()
+	expSize := len(v.istack)
 	for {
 		err = v.StepInto()
-		if !(v.state == vmstate.None && v.istack.Len() > expSize) {
+		if !(v.state == vmstate.None && len(v.istack) > expSize) {
 			break
 		}
 	}
@@ -1467,11 +1466,12 @@ func (v *VM) execute(ctx *Context, op opcode.Opcode, parameter []byte) (err erro
 		}
 
 	case opcode.RET:
-		oldCtx := v.istack.Pop().value.(*Context)
+		oldCtx := v.istack[len(v.istack)-1]
+		v.istack = v.istack[:len(v.istack)-1]
 		oldEstack := v.estack
 
 		v.unloadContext(oldCtx)
-		if v.istack.Len() == 0 {
+		if len(v.istack) == 0 {
 			v.state = vmstate.Halt
 			break
 		}
@@ -1701,7 +1701,7 @@ func (v *VM) call(ctx *Context, offset int) {
 	}
 	// New context -> new exception handlers.
 	newCtx.tryStack.elems = ctx.tryStack.elems[len(ctx.tryStack.elems):]
-	v.istack.PushItem(newCtx)
+	v.istack = append(v.istack, newCtx)
 	newCtx.Jump(offset)
 }
 
@@ -1739,9 +1739,8 @@ func calcJumpOffset(ctx *Context, parameter []byte) (int, int, error) {
 }
 
 func (v *VM) handleException() {
-	for pop := 0; pop < v.istack.Len(); pop++ {
+	for pop := 0; pop < len(v.istack); pop++ {
-		ictxv := v.istack.Peek(pop)
+		ictx := v.istack[len(v.istack)-1-pop]
-		ictx := ictxv.value.(*Context)
 		for j := 0; j < ictx.tryStack.Len(); j++ {
 			e := ictx.tryStack.Peek(j)
 			ectx := e.Value().(*exceptionHandlingContext)
@@ -1751,7 +1750,8 @@ func (v *VM) handleException() {
 				continue
 			}
 			for i := 0; i < pop; i++ {
-				ctx := v.istack.Pop().value.(*Context)
+				ctx := v.istack[len(v.istack)-1]
+				v.istack = v.istack[:len(v.istack)-1]
 				v.unloadContext(ctx)
 			}
 			if ectx.State == eTry && ectx.HasCatch() {
@@ -1937,7 +1937,7 @@ func validateMapKey(key Element) {
 }
 
 func (v *VM) checkInvocationStackSize() {
-	if v.istack.Len() >= MaxInvocationStackSize {
+	if len(v.istack) >= MaxInvocationStackSize {
 		panic("invocation stack is too big")
 	}
 }
@@ -1959,7 +1959,7 @@ func (v *VM) GetCallingScriptHash() util.Uint160 {
 
 // GetEntryScriptHash implements the ScriptHashGetter interface.
 func (v *VM) GetEntryScriptHash() util.Uint160 {
-	return v.getContextScriptHash(v.istack.Len() - 1)
+	return v.getContextScriptHash(len(v.istack) - 1)
 }
 
 // GetCurrentScriptHash implements the ScriptHashGetter interface.

pkg/vm/vm_test.go

@@ -124,7 +124,7 @@ func TestPushBytes1to75(t *testing.T) {
 		errExec := vm.execute(nil, opcode.RET, nil)
 		require.NoError(t, errExec)
 
-		assert.Equal(t, 0, vm.istack.Len())
+		assert.Nil(t, vm.Context())
 		buf.Reset()
 	}
 }