fix a bug in bytecode optimization code (#292)

* fix a bug in bytecode optimization code

* add a test

bytecode.go

@@ -97,6 +97,7 @@ func (b *Bytecode) RemoveDuplicates() {
 	var deduped []Object
 
 	indexMap := make(map[int]int) // mapping from old constant index to new index
+	fns := make(map[*CompiledFunction]int)
 	ints := make(map[int64]int)
 	strings := make(map[string]int)
 	floats := make(map[float64]int)
@@ -106,9 +107,14 @@ func (b *Bytecode) RemoveDuplicates() {
 	for curIdx, c := range b.Constants {
 		switch c := c.(type) {
 		case *CompiledFunction:
-			// add to deduped list
-			indexMap[curIdx] = len(deduped)
+			if newIdx, ok := fns[c]; ok {
+				indexMap[curIdx] = newIdx
+			} else {
+				newIdx = len(deduped)
+				fns[c] = newIdx
+				indexMap[curIdx] = newIdx
 				deduped = append(deduped, c)
+			}
 		case *ImmutableMap:
 			modName := inferModuleName(c)
 			newIdx, ok := immutableMaps[modName]

vm_test.go

@@ -2654,6 +2654,25 @@ export func(a) {
 	expectRun(t, `out = import("mod0")`,
 		Opts().Module("mod0", `for v:=0;;v++ { if v == 3 { break } } }`),
 		tengo.UndefinedValue)
+
+	// duplicate compiled functions
+	// NOTE: module "mod" has a function with some local variable, and it's
+	//  imported twice by the main script. That causes the same CompiledFunction
+	//  put in constants twice and the Bytecode optimization (removing duplicate
+	//  constants) should still work correctly.
+	expectRun(t, `
+m1 := import("mod")
+m2 := import("mod")
+out = m1.x
+	`,
+		Opts().Module("mod", `
+f1 := func(a, b) {
+	c := a + b + 1
+	return a + b + 1
+}
+export { x: 1 }
+`),
+		1)
 }
 
 func TestModuleBlockScopes(t *testing.T) {