Optimize setting the ModuleInstance.DataInstances (#852)

Signed-off-by: Clifton Kaznocha <ckaznocha@users.noreply.github.com>

internal/wasm/store.go

@@ -181,13 +181,6 @@ func (m *ModuleInstance) addSections(module *Module, importedFunctions, function
 	}
 
 	m.BuildExports(module.ExportSection)
-	m.buildDataInstances(module.DataSection)
-}
-
-func (m *ModuleInstance) buildDataInstances(segments []*DataSegment) {
-	for _, d := range segments {
-		m.DataInstances = append(m.DataInstances, d.Init)
-	}
 }
 
 func (m *ModuleInstance) buildElementInstances(elements []*ElementSegment) {
@@ -237,11 +230,13 @@ func (m *ModuleInstance) validateData(data []*DataSegment) (err error) {
 	return
 }
 
-// applyData uses the given data segments and mutate the memory according to the initial contents on it.
+// applyData uses the given data segments and mutate the memory according to the initial contents on it
-// This is called after all the validation phase passes and out of bounds memory access error here is
+// and populate the `DataInstances`. This is called after all the validation phase passes and out of
-// not a validation error, but rather a runtime error.
+// bounds memory access error here is not a validation error, but rather a runtime error.
 func (m *ModuleInstance) applyData(data []*DataSegment) error {
+	m.DataInstances = make([][]byte, len(data))
 	for i, d := range data {
+		m.DataInstances[i] = d.Init
 		if !d.IsPassive() {
 			offset := executeConstExpression(m.Globals, d.OffsetExpression).(int32)
 			if offset < 0 || int(offset)+len(d.Init) > len(m.Memory.Buffer) {

internal/wasm/store_test.go

@@ -788,6 +788,7 @@ func TestModuleInstance_applyData(t *testing.T) {
 		})
 		require.NoError(t, err)
 		require.Equal(t, []byte{0xa, 0xf, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1, 0x5}, m.Memory.Buffer)
+		require.Equal(t, [][]byte{{0xa, 0xf}, {0x1, 0x5}}, m.DataInstances)
 	})
 	t.Run("error", func(t *testing.T) {
 		m := &ModuleInstance{Memory: &MemoryInstance{Buffer: make([]byte, 5)}}