diff --git a/gnovm/pkg/gnolang/alloc.go b/gnovm/pkg/gnolang/alloc.go
index 1d46ec47da3..75f76f3d913 100644
--- a/gnovm/pkg/gnolang/alloc.go
+++ b/gnovm/pkg/gnolang/alloc.go
@@ -253,9 +253,12 @@ func (alloc *Allocator) Allocate(size int64) {
 		return
 	}
 
+	// if alloc on throwaway still exceeds memory,
+	// means GC does not work, panic.
 	if alloc.throwAway {
 		if alloc.bytes > alloc.maxBytes {
 			debug2.Println2("---exceed memory size............")
+			panic("exceed memory size")
 		}
 	}
 
diff --git a/gnovm/pkg/gnolang/op_assign.go b/gnovm/pkg/gnolang/op_assign.go
index 9828f2b3ed4..512667ca5d2 100644
--- a/gnovm/pkg/gnolang/op_assign.go
+++ b/gnovm/pkg/gnolang/op_assign.go
@@ -54,6 +54,7 @@ func (m *Machine) doOpAssign() {
 
 func (m *Machine) doOpAddAssign() {
 	s := m.PopStmt().(*AssignStmt)
+	debug2.Println2("doOpAddAssign, s: ", s)
 	rv := m.PopValue() // only one.
 	lv := m.PopAsPointer(s.Lhs[0])
 	if debug {
diff --git a/gnovm/pkg/gnolang/op_binary.go b/gnovm/pkg/gnolang/op_binary.go
index 6d26fa7ce54..14a50bfe988 100644
--- a/gnovm/pkg/gnolang/op_binary.go
+++ b/gnovm/pkg/gnolang/op_binary.go
@@ -685,11 +685,13 @@ func isGeq(lv, rv *TypedValue) bool {
 
 // for doOpAdd and doOpAddAssign.
 func addAssign(alloc *Allocator, lv, rv *TypedValue) {
+	debug2.Println2("addAssign, lv: ", lv)
 	// set the result in lv.
 	// NOTE this block is replicated in op_assign.go
 	switch baseOf(lv.T) {
 	case StringType, UntypedStringType:
 		lv.V = alloc.NewString(lv.GetString() + rv.GetString())
+		lv.SetNeedsValueAllocation(true)
 	case IntType:
 		lv.SetInt(lv.GetInt() + rv.GetInt())
 	case Int8Type:
diff --git a/gnovm/pkg/gnolang/op_exec.go b/gnovm/pkg/gnolang/op_exec.go
index be046cf8eac..eadaa5d2b49 100644
--- a/gnovm/pkg/gnolang/op_exec.go
+++ b/gnovm/pkg/gnolang/op_exec.go
@@ -433,7 +433,7 @@ EXEC_SWITCH:
 	if debug {
 		debug.Printf("EXEC: %v\n", s)
 	}
-	debug2.Printf2("EXEC: %v\n", s)
+	debug2.Printf2("EXEC: %v, type of s: %v \n", s, reflect.TypeOf(s))
 	switch cs := s.(type) {
 	case *AssignStmt:
 		switch cs.Op {
diff --git a/gnovm/pkg/gnolang/preprocess.go b/gnovm/pkg/gnolang/preprocess.go
index 3996112c63a..db98d5726ad 100644
--- a/gnovm/pkg/gnolang/preprocess.go
+++ b/gnovm/pkg/gnolang/preprocess.go
@@ -436,7 +436,7 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 		findGotoLoopDefines(ctx, bn)
 		findLoopUses1(ctx, bn)
 		findLoopUses2(ctx, bn)
-		findBlockAlloc(store, ctx, bn)
+		findBlockAllocation(store, ctx, bn)
 	}
 	return n
 }
@@ -1019,8 +1019,8 @@ func preprocess1(store Store, ctx BlockNode, n Node) Node {
 						cx := evalConst(store, last, n)
 						// built-in functions must be called.
 						if !cx.IsUndefined() &&
-							cx.T.Kind() == FuncKind &&
-							ftype != TRANS_CALL_FUNC {
+								cx.T.Kind() == FuncKind &&
+								ftype != TRANS_CALL_FUNC {
 							panic(fmt.Sprintf(
 								"use of builtin %s not in function call",
 								n.Name))
@@ -1837,8 +1837,8 @@ func preprocess1(store Store, ctx BlockNode, n Node) Node {
 					// Case 1: If receiver is pointer type but n.X is
 					// not:
 					if rcvr != nil &&
-						rcvr.Kind() == PointerKind &&
-						nxt2.Kind() != PointerKind {
+							rcvr.Kind() == PointerKind &&
+							nxt2.Kind() != PointerKind {
 						// Go spec: "If x is addressable and &x's
 						// method set contains m, x.m() is shorthand
 						// for (&x).m()"
@@ -1870,8 +1870,8 @@ func preprocess1(store Store, ctx BlockNode, n Node) Node {
 							))
 						}
 					} else if len(tr) > 0 &&
-						tr[len(tr)-1].IsDerefType() &&
-						nxt2.Kind() != PointerKind {
+							tr[len(tr)-1].IsDerefType() &&
+							nxt2.Kind() != PointerKind {
 						// Case 2: If tr[0] is deref type, but xt
 						// is not pointer type, replace n.X with
 						// &RefExpr{X: n.X}.
@@ -2385,13 +2385,13 @@ func preprocess1(store Store, ctx BlockNode, n Node) Node {
 
 // defineOrDecl merges the code logic from op define (:=) and declare (var/const).
 func defineOrDecl(
-	store Store,
-	bn BlockNode,
-	n Node,
-	isConst bool,
-	nameExprs []NameExpr,
-	typeExpr Expr,
-	valueExprs []Expr,
+		store Store,
+		bn BlockNode,
+		n Node,
+		isConst bool,
+		nameExprs []NameExpr,
+		typeExpr Expr,
+		valueExprs []Expr,
 ) {
 	numNames := len(nameExprs)
 	numVals := len(valueExprs)
@@ -2425,15 +2425,15 @@ func defineOrDecl(
 // parseAssignFromExprList parses assignment to multiple variables from a list of expressions.
 // This function will alter the value of sts, tvs.
 func parseAssignFromExprList(
-	store Store,
-	bn BlockNode,
-	n Node,
-	sts []Type,
-	tvs []TypedValue,
-	isConst bool,
-	nameExprs []NameExpr,
-	typeExpr Expr,
-	valueExprs []Expr,
+		store Store,
+		bn BlockNode,
+		n Node,
+		sts []Type,
+		tvs []TypedValue,
+		isConst bool,
+		nameExprs []NameExpr,
+		typeExpr Expr,
+		valueExprs []Expr,
 ) {
 	numNames := len(nameExprs)
 
@@ -2484,7 +2484,7 @@ func parseAssignFromExprList(
 		if len(valueExprs) > 0 {
 			vx := valueExprs[i]
 			if cx, ok := vx.(*ConstExpr); ok &&
-				!cx.TypedValue.IsUndefined() {
+					!cx.TypedValue.IsUndefined() {
 				if isConst {
 					// const _ = <const_expr>: static block should contain value
 					tvs[i] = cx.TypedValue
@@ -2512,14 +2512,14 @@ func parseAssignFromExprList(
 // - a, b := n.(T)
 // - a, b := n[i], where n is a map
 func parseMultipleAssignFromOneExpr(
-	store Store,
-	bn BlockNode,
-	n Node,
-	sts []Type,
-	tvs []TypedValue,
-	nameExprs []NameExpr,
-	typeExpr Expr,
-	valueExpr Expr,
+		store Store,
+		bn BlockNode,
+		n Node,
+		sts []Type,
+		tvs []TypedValue,
+		nameExprs []NameExpr,
+		typeExpr Expr,
+		valueExpr Expr,
 ) {
 	var tuple *tupleType
 	numNames := len(nameExprs)
@@ -2853,7 +2853,7 @@ func findLoopUses1(ctx BlockNode, bn BlockNode) {
 	})
 }
 
-func findBlockAlloc(store Store, ctx BlockNode, bn BlockNode) {
+func findBlockAllocation(store Store, ctx BlockNode, bn BlockNode) {
 	// create stack of BlockNodes.
 	var stack []BlockNode = make([]BlockNode, 0, 32)
 	var last BlockNode = ctx
@@ -2867,6 +2867,9 @@ func findBlockAlloc(store Store, ctx BlockNode, bn BlockNode) {
 			debug.Printf("findBlockAlloc %s (%v) stage:%v\n", n.String(), reflect.TypeOf(n), stage)
 		}
 		debug2.Printf2("findBlockAlloc %s (%v) stage:%v\n", n.String(), reflect.TypeOf(n), stage)
+		debug2.Println2("last: ", last)
+		debug2.Println2("last...BlockNames: ", last.GetBlockNames())
+		debug2.Println2("last...Externs: ", last.GetExternNames())
 
 		switch stage {
 		// ----------------------------------------
@@ -2889,7 +2892,7 @@ func findBlockAlloc(store Store, ctx BlockNode, bn BlockNode) {
 						// set alloc annotation
 						for i, rx := range n.Rhs {
 							debug2.Printf2("n.Rhs[%d] is %v, type of rx: %v \n", i, rx, reflect.TypeOf(rx))
-							switch rx.(type) {
+							switch rxx := rx.(type) {
 							case *NameExpr:
 								rt := evalStaticTypeOf(store, last, nx)
 								debug2.Println2("===rt: ", rt, reflect.TypeOf(rt))
@@ -2898,6 +2901,21 @@ func findBlockAlloc(store Store, ctx BlockNode, bn BlockNode) {
 									// value copy, alloc
 									nx.Alloc = true
 								}
+							case *ConstExpr:
+								if bx, ok := rxx.Source.(*BinaryExpr); ok {
+									debug2.Println2("BinaryExpr: ", bx)
+									if bx.Op == ADD {
+										lt := evalStaticTypeOf(store, last, bx.Left)
+										rt := evalStaticTypeOf(store, last, bx.Right)
+										debug2.Printf2("lt: %v, rt: %v: \n", lt, rt)
+										_, ok1 := lt.(PrimitiveType)
+										_, ok2 := rt.(PrimitiveType)
+										// 	s := "hello" + "world"
+										if ok1 && lt.Kind() == StringKind && ok2 && rt.Kind() == StringKind {
+											nx.Alloc = true
+										}
+									}
+								}
 							case *CompositeLitExpr, *FuncLitExpr: // TODO: more ...
 								nx.Alloc = true
 							}
@@ -3449,7 +3467,7 @@ func findContinuableNode(last BlockNode, store Store) {
 }
 
 func findBranchLabel(last BlockNode, label Name) (
-	bn BlockNode, depth uint8, bodyIdx int,
+		bn BlockNode, depth uint8, bodyIdx int,
 ) {
 	for {
 		switch cbn := last.(type) {
@@ -3489,7 +3507,7 @@ func findBranchLabel(last BlockNode, label Name) (
 }
 
 func findGotoLabel(last BlockNode, label Name) (
-	bn BlockNode, depth uint8, bodyIdx int,
+		bn BlockNode, depth uint8, bodyIdx int,
 ) {
 	for {
 		switch cbn := last.(type) {
@@ -3733,7 +3751,7 @@ func isNamedConversion(xt, t Type) bool {
 		// covert right to the type of left if one side is unnamed type and the other side is not
 
 		if t.IsNamed() && !xt.IsNamed() ||
-			!t.IsNamed() && xt.IsNamed() {
+				!t.IsNamed() && xt.IsNamed() {
 			return true
 		}
 	}
@@ -4851,6 +4869,7 @@ func isLocallyDefined(bn BlockNode, n Name) bool {
 func isLocallyDefined2(bn BlockNode, n Name) bool {
 	debug2.Println2("isLocallyDefined2, n: ", n)
 	_, isLocal := bn.GetLocalIndex(n)
+	debug2.Println2("isLocal: ", isLocal)
 	return isLocal
 }
 
diff --git a/gnovm/tests/files/alloc1.gno b/gnovm/tests/files/alloc1.gno
index 2b0ec1fa9ca..a45c3e4ee60 100644
--- a/gnovm/tests/files/alloc1.gno
+++ b/gnovm/tests/files/alloc1.gno
@@ -60,13 +60,13 @@ func main() {
 }
 
 // Output:
-// MemStats 1:  Allocator{maxBytes:10000000000, bytes:1944}
+// MemStats 1:  Allocator{maxBytes:9223372036854775807, bytes:1944}
 // &(struct{("foo" string)} main.Foo)
-// MemStats 2:  Allocator{maxBytes:10000000000, bytes:6019}
-// MemStats before GC1:  Allocator{maxBytes:10000000000, bytes:7518}
-// MemStats after GC1:  Allocator{maxBytes:10000000000, bytes:2424}
+// MemStats 2:  Allocator{maxBytes:9223372036854775807, bytes:6027}
+// MemStats before GC1:  Allocator{maxBytes:9223372036854775807, bytes:7534}
+// MemStats after GC1:  Allocator{maxBytes:9223372036854775807, bytes:2720}
 // func lit
-// MemStats before GC2:  Allocator{maxBytes:10000000000, bytes:5443}
-// MemStats after GC2:  Allocator{maxBytes:10000000000, bytes:2424}
-// MemStats before GC3:  Allocator{maxBytes:10000000000, bytes:3923}
-// MemStats after GC3:  Allocator{maxBytes:10000000000, bytes:2424}
+// MemStats before GC2:  Allocator{maxBytes:9223372036854775807, bytes:5747}
+// MemStats after GC2:  Allocator{maxBytes:9223372036854775807, bytes:2888}
+// MemStats before GC3:  Allocator{maxBytes:9223372036854775807, bytes:4395}
+// MemStats after GC3:  Allocator{maxBytes:9223372036854775807, bytes:2888}
diff --git a/gnovm/tests/files/alloc_amino.gno b/gnovm/tests/files/alloc_amino.gno
index fd0fa5bd9c0..21f43b49abb 100644
--- a/gnovm/tests/files/alloc_amino.gno
+++ b/gnovm/tests/files/alloc_amino.gno
@@ -9,8 +9,9 @@ import "runtime"
 type Foo struct{ name string }
 
 var f = Foo{name: "foo"}
+var m = map[string]Foo{"a": f}
 
 func main() {
-	println(f) // here should alloc amino for f
+	println(f, m) // here should alloc amino for f
 	runtime.GC()
 }
diff --git a/gnovm/tests/files/alloc_string.gno b/gnovm/tests/files/alloc_string.gno
new file mode 100644
index 00000000000..b555bdfb729
--- /dev/null
+++ b/gnovm/tests/files/alloc_string.gno
@@ -0,0 +1,14 @@
+// MAXALLOC: 25000
+// max total allocation of 100 mb.
+
+package main
+
+import "runtime"
+
+var s = "hello"
+
+func main() {
+	s += "world" // new string
+	s += "!!!"   // new string
+	runtime.GC()
+}
diff --git a/gnovm/tests/files/alloc_string2.gno b/gnovm/tests/files/alloc_string2.gno
new file mode 100644
index 00000000000..1e5b1f07923
--- /dev/null
+++ b/gnovm/tests/files/alloc_string2.gno
@@ -0,0 +1,12 @@
+// MAXALLOC: 25000
+// max total allocation of 100 mb.
+
+package main
+
+import "runtime"
+
+func main() {
+	var s = "hello"
+	s += "world"
+	runtime.GC()
+}
diff --git a/gnovm/tests/files/alloc_string3.gno b/gnovm/tests/files/alloc_string3.gno
new file mode 100644
index 00000000000..a594c4dc73e
--- /dev/null
+++ b/gnovm/tests/files/alloc_string3.gno
@@ -0,0 +1,11 @@
+// MAXALLOC: 25000
+// max total allocation of 100 mb.
+
+package main
+
+import "runtime"
+
+func main() {
+	s := "hello" + "world" // const
+	runtime.GC()
+}
diff --git a/gnovm/tests/files/alloc_string4.gno b/gnovm/tests/files/alloc_string4.gno
new file mode 100644
index 00000000000..4ce8f55c9c1
--- /dev/null
+++ b/gnovm/tests/files/alloc_string4.gno
@@ -0,0 +1,11 @@
+// MAXALLOC: 25000
+// max total allocation of 100 mb.
+
+package main
+
+import "runtime"
+
+func main() {
+	s := "hello"
+	runtime.GC()
+}