fix a bug in dead code elimination (#168)

* fix a bug in dead code elimination

* add some tests

compiler/compiler.go

@@ -6,7 +6,6 @@ import (
 	"io/ioutil"
 	"path/filepath"
 	"reflect"
-	"sort"
 	"strings"
 
 	"github.com/d5/tengo"
@@ -722,16 +721,15 @@ func (c *Compiler) optimizeFunc(node ast.Node) {
 	// are considered as unreachable.
 
 	// pass 1. identify all jump destinations
-	var dsts []int
+	dsts := make(map[int]bool)
 	iterateInstructions(c.scopes[c.scopeIndex].instructions, func(pos int, opcode Opcode, operands []int) bool {
 		switch opcode {
 		case OpJump, OpJumpFalsy, OpAndJump, OpOrJump:
-			dsts = append(dsts, operands[0])
+			dsts[operands[0]] = true
 		}
 
 		return true
 	})
-	sort.Ints(dsts) // sort jump positions
 
 	var newInsts []byte
 
@@ -746,7 +744,7 @@ func (c *Compiler) optimizeFunc(node ast.Node) {
 				return true
 			}
 			deadCode = true
-		case dstIdx < len(dsts) && pos == dsts[dstIdx]:
+		case dsts[pos]:
 			dstIdx++
 			deadCode = false
 		case deadCode:
@@ -761,7 +759,7 @@ func (c *Compiler) optimizeFunc(node ast.Node) {
 	// pass 3. update jump positions
 	var lastOp Opcode
 	var appendReturn bool
-	endPos := len(newInsts)
+	endPos := len(c.scopes[c.scopeIndex].instructions)
 	iterateInstructions(newInsts, func(pos int, opcode Opcode, operands []int) bool {
 		switch opcode {
 		case OpJump, OpJumpFalsy, OpAndJump, OpOrJump:

runtime/vm_if_test.go

@@ -65,4 +65,39 @@ func() {
 	// expression statement in init (should not leave objects on stack)
 	expect(t, `a := 1; if a; a { out = a }`, nil, 1)
 	expect(t, `a := 1; if a + 4; a { out = a }`, nil, 1)
+
+	// dead code elimination
+	expect(t, `
+out = func() {
+	if false { return 1 }
+
+	a := undefined
+
+	a = 2
+	if !a {
+		b := func() {
+			return is_callable(a) ? a(8) : a
+		}()
+		if is_error(b) { 
+			return b 
+		} else if !is_undefined(b) { 
+			return immutable(b)
+		}
+	}
+	
+	a = 3
+	if a {
+		b := func() {
+			return is_callable(a) ? a(9) : a
+		}()
+		if is_error(b) { 
+			return b 
+		} else if !is_undefined(b) { 
+			return immutable(b)
+		}
+	}
+
+	return a
+}()
+`, nil, 3)
 }