diff --git a/cl/_testrt/concat/in.go b/cl/_testrt/concat/in.go
index 5328026a..811a0c8d 100644
--- a/cl/_testrt/concat/in.go
+++ b/cl/_testrt/concat/in.go
@@ -4,6 +4,14 @@ import (
 	"github.com/goplus/llgo/internal/runtime/c"
 )
 
-func main() {
-	c.Fprintf(c.Stderr, c.Str("Hello %d\n"), 100)
+func concat(args ...string) (ret string) {
+	for _, v := range args {
+		ret += v
+	}
+	return
+}
+
+func main() {
+	result := concat("Hello", " ", "World")
+	c.Fprintf(c.Stderr, c.Str("Hello %s\n"), c.AllocaCStr(result))
 }
diff --git a/cl/_testrt/concat/out.ll b/cl/_testrt/concat/out.ll
index ee0cd9d7..06e49967 100644
--- a/cl/_testrt/concat/out.ll
+++ b/cl/_testrt/concat/out.ll
@@ -1,9 +1,40 @@
 ; ModuleID = 'main'
 source_filename = "main"
 
+%"github.com/goplus/llgo/internal/runtime.String" = type { ptr, i64 }
+%"github.com/goplus/llgo/internal/runtime.Slice" = type { ptr, i64, i64 }
+
 @"main.init$guard" = global ptr null
+@0 = private unnamed_addr constant [1 x i8] zeroinitializer, align 1
+@1 = private unnamed_addr constant [6 x i8] c"Hello\00", align 1
+@2 = private unnamed_addr constant [2 x i8] c" \00", align 1
+@3 = private unnamed_addr constant [6 x i8] c"World\00", align 1
 @__stderrp = external global ptr
-@0 = private unnamed_addr constant [10 x i8] c"Hello %d\0A\00", align 1
+@4 = private unnamed_addr constant [10 x i8] c"Hello %s\0A\00", align 1
+
+define %"github.com/goplus/llgo/internal/runtime.String" @main.concat(%"github.com/goplus/llgo/internal/runtime.Slice" %0) {
+_llgo_0:
+  %1 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %0)
+  br label %_llgo_1
+
+_llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
+  %2 = phi %"github.com/goplus/llgo/internal/runtime.String" [ %10, %_llgo_0 ], [ %9, %_llgo_2 ]
+  %3 = phi i64 [ -1, %_llgo_0 ], [ %4, %_llgo_2 ]
+  %4 = add i64 %3, 1
+  %5 = icmp slt i64 %4, %1
+  br i1 %5, label %_llgo_2, label %_llgo_3
+
+_llgo_2:                                          ; preds = %_llgo_1
+  %6 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %0)
+  %7 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %6, i64 %4
+  %8 = load %"github.com/goplus/llgo/internal/runtime.String", ptr %7, align 8
+  %9 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.StringCat"(%"github.com/goplus/llgo/internal/runtime.String" %2, %"github.com/goplus/llgo/internal/runtime.String" %8)
+  br label %_llgo_1
+
+_llgo_3:                                          ; preds = %_llgo_1
+  ret %"github.com/goplus/llgo/internal/runtime.String" %2
+  %10 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @0, i64 0)
+}
 
 define void @main.init() {
 _llgo_0:
@@ -22,11 +53,43 @@ define void @main() {
 _llgo_0:
   call void @"github.com/goplus/llgo/internal/runtime.init"()
   call void @main.init()
-  %0 = load ptr, ptr @__stderrp, align 8
-  %1 = call i32 (ptr, ptr, ...) @fprintf(ptr %0, ptr @0, i64 100)
+  %0 = call ptr @"github.com/goplus/llgo/internal/runtime.Alloc"(i64 48)
+  %1 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %0, i64 0
+  %2 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @1, i64 5)
+  store %"github.com/goplus/llgo/internal/runtime.String" %2, ptr %1, align 8
+  %3 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %0, i64 1
+  %4 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @2, i64 1)
+  store %"github.com/goplus/llgo/internal/runtime.String" %4, ptr %3, align 8
+  %5 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %0, i64 2
+  %6 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @3, i64 5)
+  store %"github.com/goplus/llgo/internal/runtime.String" %6, ptr %5, align 8
+  %7 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice"(ptr %0, i64 3, i64 3)
+  %8 = call %"github.com/goplus/llgo/internal/runtime.String" @main.concat(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
+  %9 = load ptr, ptr @__stderrp, align 8
+  %10 = call i64 @"github.com/goplus/llgo/internal/runtime.StringLen"(%"github.com/goplus/llgo/internal/runtime.String" %8)
+  %11 = add i64 %10, 1
+  %12 = alloca i8, i64 %11, align 1
+  %13 = call ptr @"github.com/goplus/llgo/internal/runtime.CStrCopy"(ptr %12, %"github.com/goplus/llgo/internal/runtime.String" %8)
+  %14 = call i32 (ptr, ptr, ...) @fprintf(ptr %9, ptr @4, ptr %13)
   ret void
 }
 
+declare i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice")
+
+declare ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice")
+
+declare %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.StringCat"(%"github.com/goplus/llgo/internal/runtime.Slice")
+
+declare %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr, i64)
+
 declare void @"github.com/goplus/llgo/internal/runtime.init"()
 
+declare ptr @"github.com/goplus/llgo/internal/runtime.Alloc"(i64)
+
+declare %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice"(ptr, i64, i64)
+
+declare i64 @"github.com/goplus/llgo/internal/runtime.StringLen"(%"github.com/goplus/llgo/internal/runtime.Slice")
+
+declare ptr @"github.com/goplus/llgo/internal/runtime.CStrCopy"(ptr, %"github.com/goplus/llgo/internal/runtime.String")
+
 declare i32 @fprintf(ptr, ptr, ...)
diff --git a/cl/builtin_test.go b/cl/builtin_test.go
index 66d0e839..da94ef47 100644
--- a/cl/builtin_test.go
+++ b/cl/builtin_test.go
@@ -25,14 +25,24 @@ import (
 	"golang.org/x/tools/go/ssa"
 )
 
+func TestErrAdvance(t *testing.T) {
+	defer func() {
+		if r := recover(); r == nil {
+			t.Fatal("advance: no error?")
+		}
+	}()
+	var ctx context
+	ctx.advance(nil, nil)
+}
+
 func TestErrAlloca(t *testing.T) {
 	defer func() {
 		if r := recover(); r == nil {
 			t.Fatal("alloca: no error?")
 		}
 	}()
-	var ctz context
-	ctz.alloca(nil, nil)
+	var ctx context
+	ctx.alloca(nil, nil)
 }
 
 func TestCStrNoArgs(t *testing.T) {
diff --git a/cl/compile.go b/cl/compile.go
index c374a5d5..1d09b2fc 100644
--- a/cl/compile.go
+++ b/cl/compile.go
@@ -300,6 +300,15 @@ func cstr(b llssa.Builder, args []ssa.Value) (ret llssa.Expr) {
 	panic("cstr(<string-literal>): invalid arguments")
 }
 
+func (p *context) advance(b llssa.Builder, args []ssa.Value) (ret llssa.Expr) {
+	if len(args) == 2 {
+		ptr := p.compileValue(b, args[0])
+		offset := p.compileValue(b, args[1])
+		return b.Advance(ptr, offset)
+	}
+	panic("advance(p ptr, offset int): invalid arguments")
+}
+
 // func alloca(size uintptr) unsafe.Pointer
 func (p *context) alloca(b llssa.Builder, args []ssa.Value) (ret llssa.Expr) {
 	if len(args) == 1 {
@@ -355,6 +364,8 @@ func (p *context) compileInstrOrValue(b llssa.Builder, iv instrOrValue, asValue
 				ret = b.Call(fn.Expr, args...)
 			case llgoCstr:
 				ret = cstr(b, call.Args)
+			case llgoAdvance:
+				ret = p.advance(b, call.Args)
 			case llgoAlloca:
 				ret = p.alloca(b, call.Args)
 			case llgoAllocaCStr:
diff --git a/cl/import.go b/cl/import.go
index bdfa1050..e02e5d34 100644
--- a/cl/import.go
+++ b/cl/import.go
@@ -200,6 +200,7 @@ const (
 	llgoCstr        = llgoInstrBase + 1
 	llgoAlloca      = llgoInstrBase + 2
 	llgoAllocaCStr  = llgoInstrBase + 3
+	llgoAdvance     = llgoInstrBase + 4
 )
 
 func (p *context) funcName(pkg *types.Package, fn *ssa.Function, ignore bool) (string, int) {
@@ -237,6 +238,8 @@ func (p *context) funcOf(fn *ssa.Function) (ret llssa.Function, ftype int) {
 		switch name {
 		case "cstr":
 			ftype = llgoCstr
+		case "advance":
+			ftype = llgoAdvance
 		case "alloca":
 			ftype = llgoAlloca
 		case "allocaCStr":
diff --git a/internal/runtime/c/c.go b/internal/runtime/c/c.go
index 05bd8ec6..ca111468 100644
--- a/internal/runtime/c/c.go
+++ b/internal/runtime/c/c.go
@@ -42,6 +42,9 @@ var Stderr FilePtr
 //go:linkname Str llgo.cstr
 func Str(string) *Char
 
+//go:linkname Advance llgo.advance
+func Advance(ptr Pointer, offset int) Pointer
+
 //go:linkname Alloca llgo.alloca
 func Alloca(size uintptr) Pointer
 
diff --git a/internal/runtime/llgo_autogen.ll b/internal/runtime/llgo_autogen.ll
index a51d9cbe..cdb2cbde 100644
--- a/internal/runtime/llgo_autogen.ll
+++ b/internal/runtime/llgo_autogen.ll
@@ -265,6 +265,67 @@ _llgo_0:
   ret i64 %3
 }
 
+define %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.StringCat"(%"github.com/goplus/llgo/internal/runtime.Slice" %0) {
+_llgo_0:
+  %1 = alloca %"github.com/goplus/llgo/internal/runtime.String", align 8
+  %2 = alloca %"github.com/goplus/llgo/internal/runtime.String", align 8
+  %3 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %0)
+  br label %_llgo_1
+
+_llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
+  %4 = phi i64 [ 0, %_llgo_0 ], [ %13, %_llgo_2 ]
+  %5 = phi i64 [ -1, %_llgo_0 ], [ %6, %_llgo_2 ]
+  %6 = add i64 %5, 1
+  %7 = icmp slt i64 %6, %3
+  br i1 %7, label %_llgo_2, label %_llgo_3
+
+_llgo_2:                                          ; preds = %_llgo_1
+  %8 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %0)
+  %9 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %8, i64 %6
+  %10 = load %"github.com/goplus/llgo/internal/runtime.String", ptr %9, align 8
+  store %"github.com/goplus/llgo/internal/runtime.String" %10, ptr %2, align 8
+  %11 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %2, i32 0, i32 1
+  %12 = load i64, ptr %11, align 4
+  %13 = add i64 %4, %12
+  br label %_llgo_1
+
+_llgo_3:                                          ; preds = %_llgo_1
+  %14 = call ptr @"github.com/goplus/llgo/internal/runtime.Alloc"(i64 %4)
+  %15 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %1, i32 0, i32 0
+  store ptr %14, ptr %15, align 8
+  %16 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %1, i32 0, i32 1
+  store i64 %4, ptr %16, align 4
+  %17 = alloca %"github.com/goplus/llgo/internal/runtime.String", align 8
+  %18 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %0)
+  br label %_llgo_4
+
+_llgo_4:                                          ; preds = %_llgo_5, %_llgo_3
+  %19 = phi ptr [ %14, %_llgo_3 ], [ %33, %_llgo_5 ]
+  %20 = phi i64 [ -1, %_llgo_3 ], [ %21, %_llgo_5 ]
+  %21 = add i64 %20, 1
+  %22 = icmp slt i64 %21, %18
+  br i1 %22, label %_llgo_5, label %_llgo_6
+
+_llgo_5:                                          ; preds = %_llgo_4
+  %23 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %0)
+  %24 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %23, i64 %21
+  %25 = load %"github.com/goplus/llgo/internal/runtime.String", ptr %24, align 8
+  store %"github.com/goplus/llgo/internal/runtime.String" %25, ptr %17, align 8
+  %26 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %17, i32 0, i32 0
+  %27 = load ptr, ptr %26, align 8
+  %28 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %17, i32 0, i32 1
+  %29 = load i64, ptr %28, align 4
+  %30 = call ptr @memcpy(ptr %19, ptr %27, i64 %29)
+  %31 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %17, i32 0, i32 1
+  %32 = load i64, ptr %31, align 4
+  %33 = getelementptr i8, ptr %19, i64 %32
+  br label %_llgo_4
+
+_llgo_6:                                          ; preds = %_llgo_4
+  %34 = load %"github.com/goplus/llgo/internal/runtime.String", ptr %1, align 8
+  ret %"github.com/goplus/llgo/internal/runtime.String" %34
+}
+
 define ptr @"github.com/goplus/llgo/internal/runtime.StringData"(%"github.com/goplus/llgo/internal/runtime.String" %0) {
 _llgo_0:
   %1 = alloca %"github.com/goplus/llgo/internal/runtime.String", align 8
diff --git a/internal/runtime/z_string.go b/internal/runtime/z_string.go
index 1ee8109a..e362da23 100644
--- a/internal/runtime/z_string.go
+++ b/internal/runtime/z_string.go
@@ -55,6 +55,22 @@ func StringData(s String) unsafe.Pointer {
 	return s.data
 }
 
+// StringCat concatenates strings.
+func StringCat(args ...String) (ret String) {
+	n := 0
+	for _, v := range args {
+		n += v.len
+	}
+	dest := Alloc(uintptr(n))
+	ret.data = dest
+	ret.len = n
+	for _, v := range args {
+		c.Memcpy(dest, v.data, uintptr(v.len))
+		dest = c.Advance(dest, v.len)
+	}
+	return
+}
+
 // -----------------------------------------------------------------------------
 
 // CStrCopy copies a Go string to a C string buffer and returns it.
diff --git a/ssa/expr.go b/ssa/expr.go
index c19ee41d..e19c7a12 100644
--- a/ssa/expr.go
+++ b/ssa/expr.go
@@ -268,12 +268,17 @@ func (b Builder) BinOp(op token.Token, x, y Expr) Expr {
 	case isMathOp(op): // op: + - * / %
 		kind := x.kind
 		switch kind {
-		case vkString, vkComplex:
-			panic("todo")
-		}
-		idx := mathOpIdx(op, kind)
-		if llop := mathOpToLLVM[idx]; llop != 0 {
-			return Expr{llvm.CreateBinOp(b.impl, llop, x.impl, y.impl), x.Type}
+		case vkString:
+			if op == token.ADD {
+				pkg := b.fn.pkg
+				return b.InlineCall(pkg.rtFunc("StringCat"), x, y)
+			}
+		case vkComplex:
+		default:
+			idx := mathOpIdx(op, kind)
+			if llop := mathOpToLLVM[idx]; llop != 0 {
+				return Expr{llvm.CreateBinOp(b.impl, llop, x.impl, y.impl), x.Type}
+			}
 		}
 	case isLogicOp(op): // op: & | ^ << >> &^
 		if op == token.AND_NOT {
@@ -359,6 +364,15 @@ func (b Builder) Phi(t Type) Phi {
 
 // -----------------------------------------------------------------------------
 
+// Advance returns the pointer ptr advanced by offset bytes.
+func (b Builder) Advance(ptr Expr, offset Expr) Expr {
+	if debugInstr {
+		log.Printf("Advance %v, %v\n", ptr.impl, offset.impl)
+	}
+	ret := llvm.CreateGEP(b.impl, b.Prog.tyInt8(), ptr.impl, []llvm.Value{offset.impl})
+	return Expr{ret, ptr.Type}
+}
+
 // Load returns the value at the pointer ptr.
 func (b Builder) Load(ptr Expr) Expr {
 	if debugInstr {