fix(vm): Release VM properly (gnolang#1116)

Properly clean slices using make (the internal logic is expecting slices
to not be nil).

It closes gnolang#1033

- [X] Added new tests, or not needed, or not feasible
- [X] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [X] Updated the official documentation or not needed
- [X] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [X] Added references to related issues and PRs
- [X] Provided any useful hints for running manual tests
- [X] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).

---------

Signed-off-by: Antonio Navarro Perez <antnavper@gmail.com>
Co-authored-by: Manfred Touron <94029+moul@users.noreply.github.com>

gnovm/pkg/gnolang/machine.go

@@ -81,8 +81,8 @@ type MachineOptions struct {
 var machinePool = sync.Pool{
 	New: func() interface{} {
 		return &Machine{
-			Ops:    make([]Op, OpSize),
-			Values: make([]TypedValue, ValueSize),
+			Ops:    make([]Op, VMSliceSize),
+			Values: make([]TypedValue, VMSliceSize),
 		}
 	},
 }
@@ -117,20 +117,14 @@ func NewMachineWithOptions(opts MachineOptions) *Machine {
 	}
 	context := opts.Context
 	mm := machinePool.Get().(*Machine)
-	*mm = Machine{
-		Ops:        mm.Ops,
-		NumOps:     0,
-		Values:     mm.Values,
-		NumValues:  0,
-		Package:    pv,
-		Alloc:      alloc,
-		CheckTypes: checkTypes,
-		ReadOnly:   readOnly,
-		MaxCycles:  maxCycles,
-		Output:     output,
-		Store:      store,
-		Context:    context,
-	}
+	mm.Package = pv
+	mm.Alloc = alloc
+	mm.CheckTypes = checkTypes
+	mm.ReadOnly = readOnly
+	mm.MaxCycles = maxCycles
+	mm.Output = output
+	mm.Store = store
+	mm.Context = context
 
 	if pv != nil {
 		mm.SetActivePackage(pv)
@@ -139,13 +133,12 @@ func NewMachineWithOptions(opts MachineOptions) *Machine {
 }
 
 const (
-	OpSize    = 1024
-	ValueSize = 1024
+	VMSliceSize = 1024
 )
 
 var (
-	opZeroed    [OpSize]Op
-	valueZeroed [ValueSize]TypedValue
+	opZeroed    [VMSliceSize]Op
+	valueZeroed [VMSliceSize]TypedValue
 )
 
 // m should not be used after this call
@@ -154,13 +147,14 @@ var (
 // and prevent objects that were not taken from
 // the pool, to call Release
 func (m *Machine) Release() {
-	// copy()
 	// here we zero in the values for the next user
 	m.NumOps = 0
 	m.NumValues = 0
-	// this is the fastest way to zero-in a slice in Go
-	copy(m.Ops, opZeroed[:])
-	copy(m.Values, valueZeroed[:])
+
+	ops, values := m.Ops[:VMSliceSize:VMSliceSize], m.Values[:VMSliceSize:VMSliceSize]
+	copy(ops, opZeroed[:])
+	copy(values, valueZeroed[:])
+	*m = Machine{Ops: ops, Values: values}
 
 	machinePool.Put(m)
 }

gnovm/pkg/gnolang/machine_test.go

@@ -0,0 +1,10 @@
+package gnolang
+
+import "testing"
+
+func BenchmarkCreateNewMachine(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		m := NewMachineWithOptions(MachineOptions{})
+		m.Release()
+	}
+}