compiler: drop unused call results

Close #683.

pkg/compiler/codegen.go

@@ -795,6 +795,20 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 			emit.Call(c.prog.BinWriter, opcode.CALLL, f.label)
 		}
 
+		if c.scope != nil && c.scope.voidCalls[n] {
+			var sz int
+			if f != nil {
+				sz = f.decl.Type.Results.NumFields()
+			} else if !isBuiltin {
+				// lambda invocation
+				f := c.typeOf(n.Fun).Underlying().(*types.Signature)
+				sz = f.Results().Len()
+			}
+			for i := 0; i < sz; i++ {
+				emit.Opcode(c.prog.BinWriter, opcode.DROP)
+			}
+		}
+
 		return nil
 
 	case *ast.SelectorExpr:

pkg/compiler/func_scope.go

@@ -90,8 +90,24 @@ func (c *funcScope) analyzeVoidCalls(node ast.Node) bool {
 		}
 	case *ast.BinaryExpr:
 		return false
+	case *ast.IfStmt:
+		// we can't just return `false`, because we still need to process body
+		ce, ok := n.Cond.(*ast.CallExpr)
+		if ok {
+			c.voidCalls[ce] = false
+		}
+	case *ast.CaseClause:
+		for _, e := range n.List {
+			ce, ok := e.(*ast.CallExpr)
+			if ok {
+				c.voidCalls[ce] = false
+			}
+		}
 	case *ast.CallExpr:
-		c.voidCalls[n] = true
+		_, ok := c.voidCalls[n]
+		if !ok {
+			c.voidCalls[n] = true
+		}
 		return false
 	}
 	return true

pkg/compiler/function_call_test.go

@@ -24,8 +24,8 @@ func TestSimpleFunctionCall(t *testing.T) {
 }
 
 func TestNotAssignedFunctionCall(t *testing.T) {
-	src := `
-		package testcase
+	t.Run("Simple", func(t *testing.T) {
+		src := `package testcase
 		func Main() int {
 			getSomeInteger()
 			getSomeInteger()
@@ -34,12 +34,53 @@ func TestNotAssignedFunctionCall(t *testing.T) {
 
 		func getSomeInteger() int {
 			return 0
-		}
-	`
-	// disable stack checks because it is hard right now
-	// to distinguish between simple function call traversal
-	// and the same traversal inside an assignment.
-	evalWithoutStackChecks(t, src, big.NewInt(0))
+		}`
+		eval(t, src, big.NewInt(0))
+	})
+	t.Run("If", func(t *testing.T) {
+		src := `package testcase
+		func f() bool { return true }
+		func Main() int {
+			if f() {
+				return 42
+			}
+			return 0
+		}`
+		eval(t, src, big.NewInt(42))
+	})
+	t.Run("Switch", func(t *testing.T) {
+		src := `package testcase
+		func f() bool { return true }
+		func Main() int {
+			switch true {
+			case f():
+				return 42
+			default:
+				return 0
+			}
+		}`
+		eval(t, src, big.NewInt(42))
+	})
+	t.Run("Builtin", func(t *testing.T) {
+		src := `package foo
+		import "github.com/nspcc-dev/neo-go/pkg/interop/util"
+		func Main() int {
+			util.FromAddress("NPAsqZkx9WhNd4P72uhZxBhLinSuNkxfB8")
+			util.FromAddress("NPAsqZkx9WhNd4P72uhZxBhLinSuNkxfB8")
+			return 1
+		}`
+		eval(t, src, big.NewInt(1))
+	})
+	t.Run("Lambda", func(t *testing.T) {
+		src := `package foo
+		func Main() int {
+			f := func() (int, int) { return 1, 2 }
+			f()
+			f()
+			return 42
+		}`
+		eval(t, src, big.NewInt(42))
+	})
 }
 
 func TestMultipleFunctionCalls(t *testing.T) {

pkg/compiler/vm_test.go

@@ -32,12 +32,6 @@ func runTestCases(t *testing.T, tcases []testCase) {
 	}
 }
 
-func evalWithoutStackChecks(t *testing.T, src string, result interface{}) {
-	v := vmAndCompile(t, src)
-	require.NoError(t, v.Run())
-	assertResult(t, v, result)
-}
-
 func eval(t *testing.T, src string, result interface{}) {
 	vm := vmAndCompile(t, src)
 	err := vm.Run()