diff --git a/cl/_testdata/print/out.ll b/cl/_testdata/print/out.ll
index af0ea398..2e91fdad 100644
--- a/cl/_testdata/print/out.ll
+++ b/cl/_testdata/print/out.ll
@@ -51,7 +51,7 @@ _llgo_0:
 
 define void @main.gwrite(%"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)
+  %1 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 1
   %2 = icmp eq i64 %1, 0
   br i1 %2, label %_llgo_1, label %_llgo_2
 
@@ -680,7 +680,7 @@ _llgo_2:                                          ; preds = %_llgo_1, %_llgo_0
 
 define void @main.println(%"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)
+  %1 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 1
   br label %_llgo_1
 
 _llgo_1:                                          ; preds = %_llgo_5, %_llgo_0
@@ -690,7 +690,7 @@ _llgo_1:                                          ; preds = %_llgo_5, %_llgo_0
   br i1 %4, label %_llgo_2, label %_llgo_3
 
 _llgo_2:                                          ; preds = %_llgo_1
-  %5 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %0)
+  %5 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 0
   %6 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.iface", ptr %5, i64 %3
   %7 = load %"github.com/goplus/llgo/internal/runtime.iface", ptr %6, align 8
   %8 = icmp ne i64 %3, 0
@@ -783,8 +783,6 @@ _llgo_0:
 
 declare ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64)
 
-declare i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice")
-
 declare i32 @printf(ptr, ...)
 
 declare void @"github.com/goplus/llgo/internal/runtime.init"()
@@ -802,5 +800,3 @@ declare %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llg
 declare { i64, i1 } @"github.com/goplus/llgo/internal/runtime.CheckI2Int"(%"github.com/goplus/llgo/internal/runtime.iface", ptr)
 
 declare { %"github.com/goplus/llgo/internal/runtime.String", i1 } @"github.com/goplus/llgo/internal/runtime.CheckI2String"(%"github.com/goplus/llgo/internal/runtime.iface", ptr)
-
-declare ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice")
diff --git a/cl/_testdata/vargs/out.ll b/cl/_testdata/vargs/out.ll
index 56930b8e..4657338e 100644
--- a/cl/_testdata/vargs/out.ll
+++ b/cl/_testdata/vargs/out.ll
@@ -48,7 +48,7 @@ _llgo_0:
 
 define void @main.test(%"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)
+  %1 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 1
   br label %_llgo_1
 
 _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
@@ -58,7 +58,7 @@ _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
   br i1 %4, label %_llgo_2, label %_llgo_3
 
 _llgo_2:                                          ; preds = %_llgo_1
-  %5 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %0)
+  %5 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 0
   %6 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.iface", ptr %5, i64 %3
   %7 = load %"github.com/goplus/llgo/internal/runtime.iface", ptr %6, align 8
   %8 = call ptr @"github.com/goplus/llgo/internal/runtime.Basic"(i64 2)
@@ -80,10 +80,6 @@ declare %"github.com/goplus/llgo/internal/runtime.iface" @"github.com/goplus/llg
 
 declare %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr, i64, i64, i64, i64, i64)
 
-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 i64 @"github.com/goplus/llgo/internal/runtime.I2Int"(%"github.com/goplus/llgo/internal/runtime.iface", ptr)
 
 declare i32 @printf(ptr, ...)
diff --git a/cl/_testrt/builtin/in.go b/cl/_testrt/builtin/in.go
index a252a6d5..e5416b37 100644
--- a/cl/_testrt/builtin/in.go
+++ b/cl/_testrt/builtin/in.go
@@ -40,6 +40,12 @@ func main() {
 	string_len("hello"[1:])
 	string_len("hello"[1:2])
 	string_len("hello"[5:])
+
+	s = append(s, 5, 6, 7, 8)
+	out(len(s))
+	data := []byte{'a', 'b', 'c'}
+	data = append(data, "def"...)
+	out(len(data))
 }
 
 func string_len(s string) {
diff --git a/cl/_testrt/builtin/out.ll b/cl/_testrt/builtin/out.ll
index 9201f246..e446abe4 100644
--- a/cl/_testrt/builtin/out.ll
+++ b/cl/_testrt/builtin/out.ll
@@ -14,7 +14,8 @@ source_filename = "main"
 @1 = private unnamed_addr constant [6 x i8] c"hello\00", align 1
 @2 = private unnamed_addr constant [6 x i8] c"hello\00", align 1
 @3 = private unnamed_addr constant [6 x i8] c"hello\00", align 1
-@4 = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1
+@4 = private unnamed_addr constant [4 x i8] c"def\00", align 1
+@5 = private unnamed_addr constant [4 x i8] c"%d\0A\00", align 1
 
 define void @main.init() {
 _llgo_0:
@@ -57,7 +58,7 @@ _llgo_0:
   store i64 2, ptr %10, align 4
   store i64 3, ptr %11, align 4
   store i64 4, ptr %12, align 4
-  %13 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
+  %13 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 1
   call void @main.out(i64 %13)
   %14 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64 32)
   %15 = getelementptr inbounds i64, ptr %14, i64 0
@@ -69,64 +70,64 @@ _llgo_0:
   %18 = getelementptr inbounds i64, ptr %14, i64 3
   store i64 4, ptr %18, align 4
   %19 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %14, i64 8, i64 4, i64 0, i64 4, i64 4)
-  %20 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %19)
+  %20 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %19, 1
   call void @main.out(i64 %20)
   call void @main.out(i64 4)
   call void @main.out(i64 4)
   call void @main.out(i64 4)
-  %21 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
+  %21 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 2
   call void @main.out(i64 %21)
   call void @main.out(i64 4)
   call void @main.out(i64 4)
-  %22 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
-  %23 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
-  %24 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
+  %22 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 2
+  %23 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 2
+  %24 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 0
   %25 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %24, i64 8, i64 %22, i64 1, i64 %23, i64 %22)
-  %26 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %25)
+  %26 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %25, 1
   call void @main.out(i64 %26)
-  %27 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
-  %28 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
-  %29 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
+  %27 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 2
+  %28 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 2
+  %29 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 0
   %30 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %29, i64 8, i64 %27, i64 1, i64 %28, i64 %27)
-  %31 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %30)
+  %31 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %30, 2
   call void @main.out(i64 %31)
-  %32 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
-  %33 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
+  %32 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 2
+  %33 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 0
   %34 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %33, i64 8, i64 %32, i64 1, i64 2, i64 %32)
-  %35 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %34)
+  %35 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %34, 1
   call void @main.out(i64 %35)
-  %36 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
-  %37 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
+  %36 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 2
+  %37 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 0
   %38 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %37, i64 8, i64 %36, i64 1, i64 2, i64 %36)
-  %39 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %38)
+  %39 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %38, 2
   call void @main.out(i64 %39)
-  %40 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
-  %41 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
+  %40 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 2
+  %41 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 0
   %42 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %41, i64 8, i64 %40, i64 1, i64 2, i64 2)
-  %43 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %42)
+  %43 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %42, 1
   call void @main.out(i64 %43)
-  %44 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
-  %45 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
+  %44 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 2
+  %45 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %7, 0
   %46 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %45, i64 8, i64 %44, i64 1, i64 2, i64 2)
-  %47 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %46)
+  %47 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %46, 2
   call void @main.out(i64 %47)
   %48 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %8, i64 8, i64 4, i64 1, i64 4, i64 4)
-  %49 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %48)
+  %49 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %48, 1
   call void @main.out(i64 %49)
   %50 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %8, i64 8, i64 4, i64 1, i64 4, i64 4)
-  %51 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %50)
+  %51 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %50, 2
   call void @main.out(i64 %51)
   %52 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %8, i64 8, i64 4, i64 1, i64 2, i64 4)
-  %53 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %52)
+  %53 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %52, 1
   call void @main.out(i64 %53)
   %54 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %8, i64 8, i64 4, i64 1, i64 2, i64 4)
-  %55 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %54)
+  %55 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %54, 2
   call void @main.out(i64 %55)
   %56 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %8, i64 8, i64 4, i64 1, i64 2, i64 2)
-  %57 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %56)
+  %57 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %56, 1
   call void @main.out(i64 %57)
   %58 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %8, i64 8, i64 4, i64 1, i64 2, i64 2)
-  %59 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"github.com/goplus/llgo/internal/runtime.Slice" %58)
+  %59 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %58, 2
   call void @main.out(i64 %59)
   %60 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @0, i64 5)
   call void @main.string_len(%"github.com/goplus/llgo/internal/runtime.String" %60)
@@ -141,12 +142,41 @@ _llgo_0:
   %67 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %66, 1
   %68 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewStringSlice"(%"github.com/goplus/llgo/internal/runtime.String" %66, i64 5, i64 %67)
   call void @main.string_len(%"github.com/goplus/llgo/internal/runtime.String" %68)
+  %69 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64 32)
+  %70 = getelementptr inbounds i64, ptr %69, i64 0
+  store i64 5, ptr %70, align 4
+  %71 = getelementptr inbounds i64, ptr %69, i64 1
+  store i64 6, ptr %71, align 4
+  %72 = getelementptr inbounds i64, ptr %69, i64 2
+  store i64 7, ptr %72, align 4
+  %73 = getelementptr inbounds i64, ptr %69, i64 3
+  store i64 8, ptr %73, align 4
+  %74 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %69, i64 8, i64 4, i64 0, i64 4, i64 4)
+  %75 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %74, 0
+  %76 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %74, 1
+  %77 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.SliceAppend"(%"github.com/goplus/llgo/internal/runtime.Slice" %7, ptr %75, i64 %76, i64 8)
+  %78 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %77, 1
+  call void @main.out(i64 %78)
+  %79 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64 3)
+  %80 = getelementptr inbounds i8, ptr %79, i64 0
+  store i8 97, ptr %80, align 1
+  %81 = getelementptr inbounds i8, ptr %79, i64 1
+  store i8 98, ptr %81, align 1
+  %82 = getelementptr inbounds i8, ptr %79, i64 2
+  store i8 99, ptr %82, align 1
+  %83 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %79, i64 1, i64 3, i64 0, i64 3, i64 3)
+  %84 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @4, i64 3)
+  %85 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %84, 0
+  %86 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %84, 1
+  %87 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.SliceAppend"(%"github.com/goplus/llgo/internal/runtime.Slice" %83, ptr %85, i64 %86, i64 1)
+  %88 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %87, 1
+  call void @main.out(i64 %88)
   ret void
 }
 
 define void @main.out(i64 %0) {
 _llgo_0:
-  %1 = call i32 (ptr, ...) @printf(ptr @4, i64 %0)
+  %1 = call i32 (ptr, ...) @printf(ptr @5, i64 %0)
   ret void
 }
 
@@ -163,14 +193,10 @@ declare ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64)
 
 declare %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr, i64, i64, i64, i64, i64)
 
-declare i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice")
-
-declare i64 @"github.com/goplus/llgo/internal/runtime.SliceCap"(%"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.NewString"(ptr, i64)
 
 declare %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewStringSlice"(%"github.com/goplus/llgo/internal/runtime.String", i64, i64)
 
+declare %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.SliceAppend"(%"github.com/goplus/llgo/internal/runtime.Slice", ptr, i64, i64)
+
 declare i32 @printf(ptr, ...)
diff --git a/cl/_testrt/concat/out.ll b/cl/_testrt/concat/out.ll
index 2ebd426d..af3e1e82 100644
--- a/cl/_testrt/concat/out.ll
+++ b/cl/_testrt/concat/out.ll
@@ -18,7 +18,7 @@ source_filename = "main"
 
 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)
+  %1 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 1
   %2 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @0, i64 0)
   %3 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %2, 0
   %4 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %2, 1
@@ -39,7 +39,7 @@ _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
   br i1 %13, label %_llgo_2, label %_llgo_3
 
 _llgo_2:                                          ; preds = %_llgo_1
-  %14 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %0)
+  %14 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 0
   %15 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %14, i64 %12
   %16 = load %"github.com/goplus/llgo/internal/runtime.String", ptr %15, align 8
   %17 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.StringCat"(%"github.com/goplus/llgo/internal/runtime.String" %11, %"github.com/goplus/llgo/internal/runtime.String" %16)
@@ -100,10 +100,6 @@ _llgo_0:
   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.String", %"github.com/goplus/llgo/internal/runtime.String")
 
 declare %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr, i64)
diff --git a/cl/_testrt/intgen/out.ll b/cl/_testrt/intgen/out.ll
index e4ffe19d..8d91dd12 100644
--- a/cl/_testrt/intgen/out.ll
+++ b/cl/_testrt/intgen/out.ll
@@ -16,7 +16,7 @@ _llgo_0:
   %2 = mul i64 %0, 4
   %3 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64 %2)
   %4 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice"(ptr %3, i64 %0, i64 %0)
-  %5 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %4)
+  %5 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %4, 1
   br label %_llgo_1
 
 _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
@@ -29,7 +29,7 @@ _llgo_2:                                          ; preds = %_llgo_1
   %9 = extractvalue { ptr, ptr } %1, 1
   %10 = extractvalue { ptr, ptr } %1, 0
   %11 = call i32 %10(ptr %9)
-  %12 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %4)
+  %12 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %4, 0
   %13 = getelementptr inbounds i32, ptr %12, i64 %7
   store i32 %11, ptr %13, align 4
   br label %_llgo_1
@@ -76,7 +76,7 @@ _llgo_0:
   store ptr null, ptr %4, align 8
   %5 = load { ptr, ptr }, ptr %2, align 8
   %6 = call %"github.com/goplus/llgo/internal/runtime.Slice" @main.genInts(i64 5, { ptr, ptr } %5)
-  %7 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %6)
+  %7 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %6, 1
   br label %_llgo_1
 
 _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
@@ -86,7 +86,7 @@ _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
   br i1 %10, label %_llgo_2, label %_llgo_3
 
 _llgo_2:                                          ; preds = %_llgo_1
-  %11 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %6)
+  %11 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %6, 0
   %12 = getelementptr inbounds i32, ptr %11, i64 %9
   %13 = load i32, ptr %12, align 4
   %14 = call i32 (ptr, ...) @printf(ptr @0, i32 %13)
@@ -105,7 +105,7 @@ _llgo_3:                                          ; preds = %_llgo_1
   store ptr %16, ptr %20, align 8
   %21 = load { ptr, ptr }, ptr %18, align 8
   %22 = call %"github.com/goplus/llgo/internal/runtime.Slice" @main.genInts(i64 5, { ptr, ptr } %21)
-  %23 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %22)
+  %23 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %22, 1
   br label %_llgo_4
 
 _llgo_4:                                          ; preds = %_llgo_5, %_llgo_3
@@ -115,7 +115,7 @@ _llgo_4:                                          ; preds = %_llgo_5, %_llgo_3
   br i1 %26, label %_llgo_5, label %_llgo_6
 
 _llgo_5:                                          ; preds = %_llgo_4
-  %27 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %22)
+  %27 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %22, 0
   %28 = getelementptr inbounds i32, ptr %27, i64 %25
   %29 = load i32, ptr %28, align 4
   %30 = call i32 (ptr, ...) @printf(ptr @1, i32 %29)
@@ -135,7 +135,7 @@ _llgo_6:                                          ; preds = %_llgo_4
   store ptr %33, ptr %37, align 8
   %38 = load { ptr, ptr }, ptr %35, align 8
   %39 = call %"github.com/goplus/llgo/internal/runtime.Slice" @main.genInts(i64 5, { ptr, ptr } %38)
-  %40 = call i64 @"github.com/goplus/llgo/internal/runtime.SliceLen"(%"github.com/goplus/llgo/internal/runtime.Slice" %39)
+  %40 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %39, 1
   br label %_llgo_7
 
 _llgo_7:                                          ; preds = %_llgo_8, %_llgo_6
@@ -145,7 +145,7 @@ _llgo_7:                                          ; preds = %_llgo_8, %_llgo_6
   br i1 %43, label %_llgo_8, label %_llgo_9
 
 _llgo_8:                                          ; preds = %_llgo_7
-  %44 = call ptr @"github.com/goplus/llgo/internal/runtime.SliceData"(%"github.com/goplus/llgo/internal/runtime.Slice" %39)
+  %44 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %39, 0
   %45 = getelementptr inbounds i32, ptr %44, i64 %42
   %46 = load i32, ptr %45, align 4
   %47 = call i32 (ptr, ...) @printf(ptr @2, i32 %46)
@@ -159,10 +159,6 @@ declare ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(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.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 void @"github.com/goplus/llgo/internal/runtime.init"()
 
 declare i32 @rand()
diff --git a/cl/_testrt/sum/out.ll b/cl/_testrt/sum/out.ll
index ed29adf4..e414fa91 100644
--- a/cl/_testrt/sum/out.ll
+++ b/cl/_testrt/sum/out.ll
@@ -44,7 +44,7 @@ _llgo_0:
 
 define i64 @main.sum(%"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)
+  %1 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 1
   br label %_llgo_1
 
 _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
@@ -55,7 +55,7 @@ _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
   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)
+  %6 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 0
   %7 = getelementptr inbounds i64, ptr %6, i64 %4
   %8 = load i64, ptr %7, align 4
   %9 = add i64 %2, %8
@@ -72,7 +72,3 @@ declare ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64)
 declare %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr, i64, i64, i64, i64, i64)
 
 declare i32 @printf(ptr, ...)
-
-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")
diff --git a/internal/runtime/llgo_autogen.lla b/internal/runtime/llgo_autogen.lla
index 452cc462..fdd1ba93 100644
Binary files a/internal/runtime/llgo_autogen.lla and b/internal/runtime/llgo_autogen.lla differ
diff --git a/internal/runtime/slice.go b/internal/runtime/slice.go
new file mode 100644
index 00000000..d692a710
--- /dev/null
+++ b/internal/runtime/slice.go
@@ -0,0 +1,40 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime
+
+// nextslicecap computes the next appropriate slice length.
+func nextslicecap(newLen, oldCap int) int {
+	newcap := oldCap
+	doublecap := newcap + newcap
+	if newLen > doublecap {
+		return newLen
+	}
+
+	const threshold = 256
+	if oldCap < threshold {
+		return doublecap
+	}
+	for {
+		// Transition from growing 2x for small slices
+		// to growing 1.25x for large slices. This formula
+		// gives a smooth-ish transition between the two.
+		newcap += (newcap + 3*threshold) >> 2
+
+		// We need to check `newcap >= newLen` and whether `newcap` overflowed.
+		// newLen is guaranteed to be larger than zero, hence
+		// when newcap overflows then `uint(newcap) > uint(newLen)`.
+		// This allows to check for both with the same comparison.
+		if uint(newcap) >= uint(newLen) {
+			break
+		}
+	}
+
+	// Set newcap to the requested cap when
+	// the newcap calculation overflowed.
+	if newcap <= 0 {
+		return newLen
+	}
+	return newcap
+}
diff --git a/internal/runtime/z_slice.go b/internal/runtime/z_slice.go
index f20924fb..e6f9845c 100644
--- a/internal/runtime/z_slice.go
+++ b/internal/runtime/z_slice.go
@@ -70,4 +70,25 @@ func SliceData(s Slice) unsafe.Pointer {
 	return s.data
 }
 
+// SliceAppend append elem data and returns a slice.
+func SliceAppend(src Slice, data unsafe.Pointer, num, etSize int) Slice {
+	if etSize == 0 {
+		return src
+	}
+	oldLen := src.len
+	newLen := src.len + num
+	if newLen > src.cap {
+		newCap := nextslicecap(newLen, src.cap)
+		p := AllocZ(uintptr(newCap * etSize))
+		if oldLen != 0 {
+			c.Memcpy(p, src.data, uintptr(oldLen*etSize))
+		}
+		src.data = p
+		src.cap = newCap
+	}
+	src.len = newLen
+	c.Memcpy(c.Advance(src.data, oldLen*etSize), data, uintptr(num*etSize))
+	return src
+}
+
 // -----------------------------------------------------------------------------
diff --git a/ssa/expr.go b/ssa/expr.go
index b532984a..420ceb97 100644
--- a/ssa/expr.go
+++ b/ssa/expr.go
@@ -632,6 +632,36 @@ func (b Builder) StringLen(x Expr) Expr {
 	return Expr{ptr, prog.Int()}
 }
 
+// SliceData returns the data pointer of a slice.
+func (b Builder) SliceData(x Expr) Expr {
+	if debugInstr {
+		log.Printf("SliceData %v\n", x.impl)
+	}
+	prog := b.Prog
+	ptr := llvm.CreateExtractValue(b.impl, x.impl, 0)
+	return Expr{ptr, prog.CStr()}
+}
+
+// SliceLen returns the length of a slice.
+func (b Builder) SliceLen(x Expr) Expr {
+	if debugInstr {
+		log.Printf("SliceLen %v\n", x.impl)
+	}
+	prog := b.Prog
+	ptr := llvm.CreateExtractValue(b.impl, x.impl, 1)
+	return Expr{ptr, prog.Int()}
+}
+
+// SliceCap returns the length of a slice cap.
+func (b Builder) SliceCap(x Expr) Expr {
+	if debugInstr {
+		log.Printf("SliceCap %v\n", x.impl)
+	}
+	prog := b.Prog
+	ptr := llvm.CreateExtractValue(b.impl, x.impl, 2)
+	return Expr{ptr, prog.Int()}
+}
+
 // The IndexAddr instruction yields the address of the element at
 // index `idx` of collection `x`.  `idx` is an integer expression.
 //
@@ -653,8 +683,7 @@ func (b Builder) IndexAddr(x, idx Expr) Expr {
 	pt := prog.Pointer(telem)
 	switch x.raw.Type.Underlying().(type) {
 	case *types.Slice:
-		pkg := b.Func.Pkg
-		ptr := b.InlineCall(pkg.rtFunc("SliceData"), x)
+		ptr := b.SliceData(x)
 		indices := []llvm.Value{idx.impl}
 		return Expr{llvm.CreateInBoundsGEP(b.impl, telem.ll, ptr.impl, indices), pt}
 	}
@@ -757,12 +786,12 @@ func (b Builder) Slice(x, low, high, max Expr) (ret Expr) {
 		return
 	case *types.Slice:
 		nEltSize = b.SizeOf(prog.Index(x.Type))
-		nCap = b.InlineCall(pkg.rtFunc("SliceCap"), x)
+		nCap = b.SliceCap(x)
 		if high.IsNil() {
-			high = b.InlineCall(pkg.rtFunc("SliceLen"), x)
+			high = b.SliceCap(x)
 		}
 		ret.Type = x.Type
-		base = b.InlineCall(pkg.rtFunc("SliceData"), x)
+		base = b.SliceData(x)
 	case *types.Pointer:
 		telem := t.Elem()
 		switch te := telem.Underlying().(type) {
@@ -1297,21 +1326,40 @@ func (b Builder) BuiltinCall(fn string, args ...Expr) (ret Expr) {
 	case "len":
 		if len(args) == 1 {
 			arg := args[0]
-			switch t := arg.raw.Type.Underlying().(type) {
-			case *types.Slice:
-				return b.InlineCall(b.Func.Pkg.rtFunc("SliceLen"), arg)
-			case *types.Basic:
-				if t.Kind() == types.String {
-					return b.StringLen(arg)
-				}
+			switch arg.kind {
+			case vkSlice:
+				return b.SliceLen(arg)
+			case vkString:
+				return b.StringLen(arg)
 			}
 		}
 	case "cap":
 		if len(args) == 1 {
 			arg := args[0]
-			switch arg.raw.Type.Underlying().(type) {
-			case *types.Slice:
-				return b.InlineCall(b.Func.Pkg.rtFunc("SliceCap"), arg)
+			switch arg.kind {
+			case vkSlice:
+				return b.SliceCap(arg)
+			}
+		}
+	case "append":
+		if len(args) == 2 {
+			src := args[0]
+			if src.kind == vkSlice {
+				elem := args[1]
+				switch elem.kind {
+				case vkSlice:
+					etSize := b.Prog.SizeOf(b.Prog.Elem(elem.Type))
+					ret.Type = src.Type
+					ret.impl = b.InlineCall(b.Func.Pkg.rtFunc("SliceAppend"),
+						src, b.SliceData(elem), b.SliceLen(elem), b.Prog.Val(int(etSize))).impl
+					return
+				case vkString:
+					etSize := b.Prog.SizeOf(b.Prog.Type(types.Typ[types.Byte], InGo))
+					ret.Type = src.Type
+					ret.impl = b.InlineCall(b.Func.Pkg.rtFunc("SliceAppend"),
+						src, b.StringData(elem), b.StringLen(elem), b.Prog.Val(int(etSize))).impl
+					return
+				}
 			}
 		}
 	}
diff --git a/ssa/type.go b/ssa/type.go
index 735a3d90..c10e7da6 100644
--- a/ssa/type.go
+++ b/ssa/type.go
@@ -45,6 +45,7 @@ const (
 	vkClosure
 	vkPyFuncRef
 	vkTuple
+	vkSlice
 	vkPhisExpr = -1
 )
 
@@ -103,7 +104,10 @@ func (p Program) Pointer(typ Type) Type {
 }
 
 func (p Program) Elem(typ Type) Type {
-	elem := typ.raw.Type.(*types.Pointer).Elem()
+	elem := typ.raw.Type.(interface {
+		types.Type
+		Elem() types.Type
+	}).Elem()
 	return p.rawType(elem)
 }
 
@@ -237,7 +241,7 @@ func (p Program) toType(raw types.Type) Type {
 	case *types.Interface:
 		return &aType{p.rtIface(), typ, vkInvalid}
 	case *types.Slice:
-		return &aType{p.rtSlice(), typ, vkInvalid}
+		return &aType{p.rtSlice(), typ, vkSlice}
 	case *types.Map:
 		return &aType{p.rtMap(), typ, vkInvalid}
 	case *types.Struct: