diff --git a/cl/_testdata/print/out.ll b/cl/_testdata/print/out.ll
index 2fd4a841..dd88c08e 100644
--- a/cl/_testdata/print/out.ll
+++ b/cl/_testdata/print/out.ll
@@ -555,64 +555,66 @@ _llgo_24:                                         ; preds = %_llgo_22
   br label %_llgo_12
 
 _llgo_25:                                         ; preds = %_llgo_27
-  %43 = fptosi double %59 to i64
-  %44 = add i64 %60, 2
+  %43 = fptosi double %60 to i64
+  %44 = add i64 %61, 2
   %45 = add i64 %43, 48
   %46 = trunc i64 %45 to i8
-  %47 = getelementptr inbounds i8, ptr %11, i64 %44
-  store i8 %46, ptr %47, align 1
-  %48 = sitofp i64 %43 to double
-  %49 = fsub double %59, %48
-  %50 = fmul double %49, 1.000000e+01
-  %51 = add i64 %60, 1
+  %47 = icmp slt i64 %44, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %47)
+  %48 = getelementptr inbounds i8, ptr %11, i64 %44
+  store i8 %46, ptr %48, align 1
+  %49 = sitofp i64 %43 to double
+  %50 = fsub double %60, %49
+  %51 = fmul double %50, 1.000000e+01
+  %52 = add i64 %61, 1
   br label %_llgo_27
 
 _llgo_26:                                         ; preds = %_llgo_27
-  %52 = getelementptr inbounds i8, ptr %11, i64 2
-  %53 = load i8, ptr %52, align 1
-  %54 = getelementptr inbounds i8, ptr %11, i64 1
-  store i8 %53, ptr %54, align 1
-  %55 = getelementptr inbounds i8, ptr %11, i64 2
-  store i8 46, ptr %55, align 1
-  %56 = getelementptr inbounds i8, ptr %11, i64 9
-  store i8 101, ptr %56, align 1
-  %57 = getelementptr inbounds i8, ptr %11, i64 10
-  store i8 43, ptr %57, align 1
-  %58 = icmp slt i64 %19, 0
-  br i1 %58, label %_llgo_28, label %_llgo_29
+  %53 = getelementptr inbounds i8, ptr %11, i64 2
+  %54 = load i8, ptr %53, align 1
+  %55 = getelementptr inbounds i8, ptr %11, i64 1
+  store i8 %54, ptr %55, align 1
+  %56 = getelementptr inbounds i8, ptr %11, i64 2
+  store i8 46, ptr %56, align 1
+  %57 = getelementptr inbounds i8, ptr %11, i64 9
+  store i8 101, ptr %57, align 1
+  %58 = getelementptr inbounds i8, ptr %11, i64 10
+  store i8 43, ptr %58, align 1
+  %59 = icmp slt i64 %19, 0
+  br i1 %59, label %_llgo_28, label %_llgo_29
 
 _llgo_27:                                         ; preds = %_llgo_25, %_llgo_12
-  %59 = phi double [ %18, %_llgo_12 ], [ %50, %_llgo_25 ]
-  %60 = phi i64 [ 0, %_llgo_12 ], [ %51, %_llgo_25 ]
-  %61 = icmp slt i64 %60, 7
-  br i1 %61, label %_llgo_25, label %_llgo_26
+  %60 = phi double [ %18, %_llgo_12 ], [ %51, %_llgo_25 ]
+  %61 = phi i64 [ 0, %_llgo_12 ], [ %52, %_llgo_25 ]
+  %62 = icmp slt i64 %61, 7
+  br i1 %62, label %_llgo_25, label %_llgo_26
 
 _llgo_28:                                         ; preds = %_llgo_26
-  %62 = sub i64 0, %19
-  %63 = getelementptr inbounds i8, ptr %11, i64 10
-  store i8 45, ptr %63, align 1
+  %63 = sub i64 0, %19
+  %64 = getelementptr inbounds i8, ptr %11, i64 10
+  store i8 45, ptr %64, align 1
   br label %_llgo_29
 
 _llgo_29:                                         ; preds = %_llgo_28, %_llgo_26
-  %64 = phi i64 [ %19, %_llgo_26 ], [ %62, %_llgo_28 ]
-  %65 = sdiv i64 %64, 100
-  %66 = trunc i64 %65 to i8
-  %67 = add i8 %66, 48
-  %68 = getelementptr inbounds i8, ptr %11, i64 11
-  store i8 %67, ptr %68, align 1
-  %69 = sdiv i64 %64, 10
-  %70 = trunc i64 %69 to i8
-  %71 = urem i8 %70, 10
-  %72 = add i8 %71, 48
-  %73 = getelementptr inbounds i8, ptr %11, i64 12
-  store i8 %72, ptr %73, align 1
-  %74 = srem i64 %64, 10
-  %75 = trunc i64 %74 to i8
-  %76 = add i8 %75, 48
-  %77 = getelementptr inbounds i8, ptr %11, i64 13
-  store i8 %76, ptr %77, align 1
-  %78 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %11, i64 1, i64 14, i64 0, i64 14, i64 14)
-  call void @main.gwrite(%"github.com/goplus/llgo/internal/runtime.Slice" %78)
+  %65 = phi i64 [ %19, %_llgo_26 ], [ %63, %_llgo_28 ]
+  %66 = sdiv i64 %65, 100
+  %67 = trunc i64 %66 to i8
+  %68 = add i8 %67, 48
+  %69 = getelementptr inbounds i8, ptr %11, i64 11
+  store i8 %68, ptr %69, align 1
+  %70 = sdiv i64 %65, 10
+  %71 = trunc i64 %70 to i8
+  %72 = urem i8 %71, 10
+  %73 = add i8 %72, 48
+  %74 = getelementptr inbounds i8, ptr %11, i64 12
+  store i8 %73, ptr %74, align 1
+  %75 = srem i64 %65, 10
+  %76 = trunc i64 %75 to i8
+  %77 = add i8 %76, 48
+  %78 = getelementptr inbounds i8, ptr %11, i64 13
+  store i8 %77, ptr %78, align 1
+  %79 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %11, i64 1, i64 14, i64 0, i64 14, i64 14)
+  call void @main.gwrite(%"github.com/goplus/llgo/internal/runtime.Slice" %79)
   ret void
 }
 
@@ -622,43 +624,49 @@ _llgo_0:
   br label %_llgo_3
 
 _llgo_1:                                          ; preds = %_llgo_3
-  %2 = urem i64 %14, 16
+  %2 = urem i64 %17, 16
   %3 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @10, i64 16)
   %4 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %3, 0
   %5 = getelementptr inbounds i8, ptr %4, i64 %2
   %6 = load i8, ptr %5, align 1
-  %7 = getelementptr inbounds i8, ptr %1, i64 %15
-  store i8 %6, ptr %7, align 1
-  %8 = icmp ult i64 %14, 16
-  br i1 %8, label %_llgo_5, label %_llgo_4
+  %7 = icmp slt i64 %18, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %7)
+  %8 = getelementptr inbounds i8, ptr %1, i64 %18
+  store i8 %6, ptr %8, align 1
+  %9 = icmp ult i64 %17, 16
+  br i1 %9, label %_llgo_5, label %_llgo_4
 
 _llgo_2:                                          ; preds = %_llgo_5, %_llgo_3
-  %9 = sub i64 %15, 1
-  %10 = getelementptr inbounds i8, ptr %1, i64 %9
-  store i8 120, ptr %10, align 1
-  %11 = sub i64 %9, 1
-  %12 = getelementptr inbounds i8, ptr %1, i64 %11
-  store i8 48, ptr %12, align 1
-  %13 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %1, i64 1, i64 100, i64 %11, i64 100, i64 100)
-  call void @main.gwrite(%"github.com/goplus/llgo/internal/runtime.Slice" %13)
+  %10 = sub i64 %18, 1
+  %11 = icmp slt i64 %10, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %11)
+  %12 = getelementptr inbounds i8, ptr %1, i64 %10
+  store i8 120, ptr %12, align 1
+  %13 = sub i64 %10, 1
+  %14 = icmp slt i64 %13, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %14)
+  %15 = getelementptr inbounds i8, ptr %1, i64 %13
+  store i8 48, ptr %15, align 1
+  %16 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %1, i64 1, i64 100, i64 %13, i64 100, i64 100)
+  call void @main.gwrite(%"github.com/goplus/llgo/internal/runtime.Slice" %16)
   ret void
 
 _llgo_3:                                          ; preds = %_llgo_4, %_llgo_0
-  %14 = phi i64 [ %0, %_llgo_0 ], [ %17, %_llgo_4 ]
-  %15 = phi i64 [ 99, %_llgo_0 ], [ %18, %_llgo_4 ]
-  %16 = icmp sgt i64 %15, 0
-  br i1 %16, label %_llgo_1, label %_llgo_2
+  %17 = phi i64 [ %0, %_llgo_0 ], [ %20, %_llgo_4 ]
+  %18 = phi i64 [ 99, %_llgo_0 ], [ %21, %_llgo_4 ]
+  %19 = icmp sgt i64 %18, 0
+  br i1 %19, label %_llgo_1, label %_llgo_2
 
 _llgo_4:                                          ; preds = %_llgo_5, %_llgo_1
-  %17 = udiv i64 %14, 16
-  %18 = sub i64 %15, 1
+  %20 = udiv i64 %17, 16
+  %21 = sub i64 %18, 1
   br label %_llgo_3
 
 _llgo_5:                                          ; preds = %_llgo_1
-  %19 = sub i64 100, %15
-  %20 = load i64, ptr @main.minhexdigits, align 4
-  %21 = icmp sge i64 %19, %20
-  br i1 %21, label %_llgo_2, label %_llgo_4
+  %22 = sub i64 100, %18
+  %23 = load i64, ptr @main.minhexdigits, align 4
+  %24 = icmp sge i64 %22, %23
+  br i1 %24, label %_llgo_2, label %_llgo_4
 }
 
 define void @main.printint(i64 %0) {
@@ -690,23 +698,25 @@ _llgo_1:                                          ; preds = %_llgo_5, %_llgo_0
   br i1 %4, label %_llgo_2, label %_llgo_3
 
 _llgo_2:                                          ; preds = %_llgo_1
-  %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
-  br i1 %8, label %_llgo_4, label %_llgo_5
+  %5 = icmp slt i64 %3, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %5)
+  %6 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 0
+  %7 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.iface", ptr %6, i64 %3
+  %8 = load %"github.com/goplus/llgo/internal/runtime.iface", ptr %7, align 8
+  %9 = icmp ne i64 %3, 0
+  br i1 %9, label %_llgo_4, label %_llgo_5
 
 _llgo_3:                                          ; preds = %_llgo_1
   call void @main.printnl()
   ret void
 
 _llgo_4:                                          ; preds = %_llgo_2
-  %9 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @12, i64 1)
-  call void @main.printstring(%"github.com/goplus/llgo/internal/runtime.String" %9)
+  %10 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @12, i64 1)
+  call void @main.printstring(%"github.com/goplus/llgo/internal/runtime.String" %10)
   br label %_llgo_5
 
 _llgo_5:                                          ; preds = %_llgo_4, %_llgo_2
-  call void @main.printany(%"github.com/goplus/llgo/internal/runtime.iface" %7)
+  call void @main.printany(%"github.com/goplus/llgo/internal/runtime.iface" %8)
   br label %_llgo_1
 }
 
@@ -737,28 +747,30 @@ _llgo_0:
   br label %_llgo_3
 
 _llgo_1:                                          ; preds = %_llgo_3
-  %2 = urem i64 %8, 10
+  %2 = urem i64 %9, 10
   %3 = add i64 %2, 48
   %4 = trunc i64 %3 to i8
-  %5 = getelementptr inbounds i8, ptr %1, i64 %9
-  store i8 %4, ptr %5, align 1
-  %6 = icmp ult i64 %8, 10
-  br i1 %6, label %_llgo_2, label %_llgo_4
+  %5 = icmp slt i64 %10, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %5)
+  %6 = getelementptr inbounds i8, ptr %1, i64 %10
+  store i8 %4, ptr %6, align 1
+  %7 = icmp ult i64 %9, 10
+  br i1 %7, label %_llgo_2, label %_llgo_4
 
 _llgo_2:                                          ; preds = %_llgo_3, %_llgo_1
-  %7 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %1, i64 1, i64 100, i64 %9, i64 100, i64 100)
-  call void @main.gwrite(%"github.com/goplus/llgo/internal/runtime.Slice" %7)
+  %8 = call %"github.com/goplus/llgo/internal/runtime.Slice" @"github.com/goplus/llgo/internal/runtime.NewSlice3"(ptr %1, i64 1, i64 100, i64 %10, i64 100, i64 100)
+  call void @main.gwrite(%"github.com/goplus/llgo/internal/runtime.Slice" %8)
   ret void
 
 _llgo_3:                                          ; preds = %_llgo_4, %_llgo_0
-  %8 = phi i64 [ %0, %_llgo_0 ], [ %11, %_llgo_4 ]
-  %9 = phi i64 [ 99, %_llgo_0 ], [ %12, %_llgo_4 ]
-  %10 = icmp sgt i64 %9, 0
-  br i1 %10, label %_llgo_1, label %_llgo_2
+  %9 = phi i64 [ %0, %_llgo_0 ], [ %12, %_llgo_4 ]
+  %10 = phi i64 [ 99, %_llgo_0 ], [ %13, %_llgo_4 ]
+  %11 = icmp sgt i64 %10, 0
+  br i1 %11, label %_llgo_1, label %_llgo_2
 
 _llgo_4:                                          ; preds = %_llgo_1
-  %11 = udiv i64 %8, 10
-  %12 = sub i64 %9, 1
+  %12 = udiv i64 %9, 10
+  %13 = sub i64 %10, 1
   br label %_llgo_3
 }
 
@@ -800,3 +812,5 @@ 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 void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1)
diff --git a/cl/_testdata/utf8/in.go b/cl/_testdata/utf8/in.go
new file mode 100644
index 00000000..3edfa053
--- /dev/null
+++ b/cl/_testdata/utf8/in.go
@@ -0,0 +1,23 @@
+package main
+
+import (
+	"unicode/utf8"
+)
+
+func main() {
+	var str = "中abcd"
+	for i := 0; i < len(str); {
+		r, n := utf8.DecodeRuneInString(str[i:])
+		i += n
+		println(r)
+	}
+	println(index(2) == 3)
+}
+
+var array = [...]uint8{
+	1, 2, 3, 4, 5, 6, 7, 8,
+}
+
+func index(n int8) uint8 {
+	return array[n]
+}
diff --git a/cl/_testdata/utf8/out.ll b/cl/_testdata/utf8/out.ll
new file mode 100644
index 00000000..c9aebbfd
--- /dev/null
+++ b/cl/_testdata/utf8/out.ll
@@ -0,0 +1,101 @@
+; ModuleID = 'main'
+source_filename = "main"
+
+%"github.com/goplus/llgo/internal/runtime.String" = type { ptr, i64 }
+
+@main.array = global [8 x i8] undef
+@"main.init$guard" = global ptr null
+@__llgo_argc = global ptr null
+@__llgo_argv = global ptr null
+@0 = private unnamed_addr constant [8 x i8] c"\E4\B8\ADabcd\00", align 1
+@1 = private unnamed_addr constant [2 x i8] c"\0A\00", align 1
+@2 = private unnamed_addr constant [2 x i8] c"\0A\00", align 1
+@3 = private unnamed_addr constant [8 x i8] c"\E4\B8\ADabcd\00", align 1
+
+define i8 @main.index(i8 %0) {
+_llgo_0:
+  %1 = icmp slt i8 %0, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %1)
+  %2 = zext i8 %0 to i64
+  %3 = getelementptr inbounds i8, ptr @main.array, i64 %2
+  %4 = load i8, ptr %3, align 1
+  ret i8 %4
+}
+
+define void @main.init() {
+_llgo_0:
+  %0 = load i1, ptr @"main.init$guard", align 1
+  br i1 %0, label %_llgo_2, label %_llgo_1
+
+_llgo_1:                                          ; preds = %_llgo_0
+  store i1 true, ptr @"main.init$guard", align 1
+  call void @"unicode/utf8.init"()
+  store i8 1, ptr @main.array, align 1
+  store i8 2, ptr getelementptr inbounds (i8, ptr @main.array, i64 1), align 1
+  store i8 3, ptr getelementptr inbounds (i8, ptr @main.array, i64 2), align 1
+  store i8 4, ptr getelementptr inbounds (i8, ptr @main.array, i64 3), align 1
+  store i8 5, ptr getelementptr inbounds (i8, ptr @main.array, i64 4), align 1
+  store i8 6, ptr getelementptr inbounds (i8, ptr @main.array, i64 5), align 1
+  store i8 7, ptr getelementptr inbounds (i8, ptr @main.array, i64 6), align 1
+  store i8 8, ptr getelementptr inbounds (i8, ptr @main.array, i64 7), align 1
+  br label %_llgo_2
+
+_llgo_2:                                          ; preds = %_llgo_1, %_llgo_0
+  ret void
+}
+
+define i32 @main(i32 %0, ptr %1) {
+_llgo_0:
+  store i32 %0, ptr @__llgo_argc, align 4
+  store ptr %1, ptr @__llgo_argv, align 8
+  call void @"github.com/goplus/llgo/internal/runtime.init"()
+  call void @main.init()
+  br label %_llgo_3
+
+_llgo_1:                                          ; preds = %_llgo_3
+  %2 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @0, i64 7)
+  %3 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %2, 1
+  %4 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewStringSlice"(%"github.com/goplus/llgo/internal/runtime.String" %2, i64 %14, i64 %3)
+  %5 = call { i32, i64 } @"unicode/utf8.DecodeRuneInString"(%"github.com/goplus/llgo/internal/runtime.String" %4)
+  %6 = extractvalue { i32, i64 } %5, 0
+  %7 = extractvalue { i32, i64 } %5, 1
+  %8 = add i64 %14, %7
+  %9 = sext i32 %6 to i64
+  call void @"github.com/goplus/llgo/internal/runtime.PrintInt"(i64 %9)
+  %10 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @1, i64 1)
+  call void @"github.com/goplus/llgo/internal/runtime.PrintString"(%"github.com/goplus/llgo/internal/runtime.String" %10)
+  br label %_llgo_3
+
+_llgo_2:                                          ; preds = %_llgo_3
+  %11 = call i8 @main.index(i8 2)
+  %12 = icmp eq i8 %11, 3
+  call void @"github.com/goplus/llgo/internal/runtime.PrintBool"(i1 %12)
+  %13 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @2, i64 1)
+  call void @"github.com/goplus/llgo/internal/runtime.PrintString"(%"github.com/goplus/llgo/internal/runtime.String" %13)
+  ret i32 0
+
+_llgo_3:                                          ; preds = %_llgo_1, %_llgo_0
+  %14 = phi i64 [ 0, %_llgo_0 ], [ %8, %_llgo_1 ]
+  %15 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @3, i64 7)
+  %16 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %15, 1
+  %17 = icmp slt i64 %14, %16
+  br i1 %17, label %_llgo_1, label %_llgo_2
+}
+
+declare void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1)
+
+declare void @"unicode/utf8.init"()
+
+declare void @"github.com/goplus/llgo/internal/runtime.init"()
+
+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 { i32, i64 } @"unicode/utf8.DecodeRuneInString"(%"github.com/goplus/llgo/internal/runtime.String")
+
+declare void @"github.com/goplus/llgo/internal/runtime.PrintInt"(i64)
+
+declare void @"github.com/goplus/llgo/internal/runtime.PrintString"(%"github.com/goplus/llgo/internal/runtime.String")
+
+declare void @"github.com/goplus/llgo/internal/runtime.PrintBool"(i1)
diff --git a/cl/_testdata/vargs/out.ll b/cl/_testdata/vargs/out.ll
index 7c75776b..def89a93 100644
--- a/cl/_testdata/vargs/out.ll
+++ b/cl/_testdata/vargs/out.ll
@@ -58,12 +58,14 @@ _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
   br i1 %4, label %_llgo_2, label %_llgo_3
 
 _llgo_2:                                          ; preds = %_llgo_1
-  %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)
-  %9 = call i64 @"github.com/goplus/llgo/internal/runtime.I2Int"(%"github.com/goplus/llgo/internal/runtime.iface" %7, ptr %8)
-  %10 = call i32 (ptr, ...) @printf(ptr @0, i64 %9)
+  %5 = icmp slt i64 %3, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %5)
+  %6 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 0
+  %7 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.iface", ptr %6, i64 %3
+  %8 = load %"github.com/goplus/llgo/internal/runtime.iface", ptr %7, align 8
+  %9 = call ptr @"github.com/goplus/llgo/internal/runtime.Basic"(i64 2)
+  %10 = call i64 @"github.com/goplus/llgo/internal/runtime.I2Int"(%"github.com/goplus/llgo/internal/runtime.iface" %8, ptr %9)
+  %11 = call i32 (ptr, ...) @printf(ptr @0, i64 %10)
   br label %_llgo_1
 
 _llgo_3:                                          ; preds = %_llgo_1
@@ -80,6 +82,8 @@ 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 void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1)
+
 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/concat/out.ll b/cl/_testrt/concat/out.ll
index 00c7d18a..19f0abd8 100644
--- a/cl/_testrt/concat/out.ll
+++ b/cl/_testrt/concat/out.ll
@@ -25,8 +25,8 @@ _llgo_0:
   br label %_llgo_1
 
 _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
-  %5 = phi ptr [ %3, %_llgo_0 ], [ %18, %_llgo_2 ]
-  %6 = phi i64 [ %4, %_llgo_0 ], [ %19, %_llgo_2 ]
+  %5 = phi ptr [ %3, %_llgo_0 ], [ %19, %_llgo_2 ]
+  %6 = phi i64 [ %4, %_llgo_0 ], [ %20, %_llgo_2 ]
   %7 = phi i64 [ -1, %_llgo_0 ], [ %12, %_llgo_2 ]
   %8 = alloca %"github.com/goplus/llgo/internal/runtime.String", align 8
   %9 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %8, i32 0, i32 0
@@ -39,12 +39,14 @@ _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
   br i1 %13, label %_llgo_2, label %_llgo_3
 
 _llgo_2:                                          ; preds = %_llgo_1
-  %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)
-  %18 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %17, 0
-  %19 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %17, 1
+  %14 = icmp slt i64 %12, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %14)
+  %15 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 0
+  %16 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %15, i64 %12
+  %17 = load %"github.com/goplus/llgo/internal/runtime.String", ptr %16, align 8
+  %18 = 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" %17)
+  %19 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %18, 0
+  %20 = extractvalue %"github.com/goplus/llgo/internal/runtime.String" %18, 1
   br label %_llgo_1
 
 _llgo_3:                                          ; preds = %_llgo_1
@@ -100,6 +102,8 @@ _llgo_0:
   ret i32 0
 }
 
+declare void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1)
+
 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 a344eb5f..1fe32d61 100644
--- a/cl/_testrt/intgen/out.ll
+++ b/cl/_testrt/intgen/out.ll
@@ -29,9 +29,11 @@ _llgo_2:                                          ; preds = %_llgo_1
   %9 = extractvalue { ptr, ptr } %1, 1
   %10 = extractvalue { ptr, ptr } %1, 0
   %11 = call i32 %10(ptr %9)
-  %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
+  %12 = icmp slt i64 %7, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %12)
+  %13 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %4, 0
+  %14 = getelementptr inbounds i32, ptr %13, i64 %7
+  store i32 %11, ptr %14, align 4
   br label %_llgo_1
 
 _llgo_3:                                          ; preds = %_llgo_1
@@ -86,69 +88,75 @@ _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
   br i1 %10, label %_llgo_2, label %_llgo_3
 
 _llgo_2:                                          ; preds = %_llgo_1
-  %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)
+  %11 = icmp slt i64 %9, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %11)
+  %12 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %6, 0
+  %13 = getelementptr inbounds i32, ptr %12, i64 %9
+  %14 = load i32, ptr %13, align 4
+  %15 = call i32 (ptr, ...) @printf(ptr @0, i32 %14)
   br label %_llgo_1
 
 _llgo_3:                                          ; preds = %_llgo_1
-  %15 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64 4)
-  store i32 1, ptr %15, align 4
-  %16 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocU"(i64 8)
-  %17 = getelementptr inbounds { ptr }, ptr %16, i32 0, i32 0
-  store ptr %15, ptr %17, align 8
-  %18 = alloca { ptr, ptr }, align 8
-  %19 = getelementptr inbounds { ptr, ptr }, ptr %18, i32 0, i32 0
-  store ptr @"main.main$1", ptr %19, align 8
-  %20 = getelementptr inbounds { ptr, ptr }, ptr %18, i32 0, i32 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 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %22, 1
+  %16 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64 4)
+  store i32 1, ptr %16, align 4
+  %17 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocU"(i64 8)
+  %18 = getelementptr inbounds { ptr }, ptr %17, i32 0, i32 0
+  store ptr %16, ptr %18, align 8
+  %19 = alloca { ptr, ptr }, align 8
+  %20 = getelementptr inbounds { ptr, ptr }, ptr %19, i32 0, i32 0
+  store ptr @"main.main$1", ptr %20, align 8
+  %21 = getelementptr inbounds { ptr, ptr }, ptr %19, i32 0, i32 1
+  store ptr %17, ptr %21, align 8
+  %22 = load { ptr, ptr }, ptr %19, align 8
+  %23 = call %"github.com/goplus/llgo/internal/runtime.Slice" @main.genInts(i64 5, { ptr, ptr } %22)
+  %24 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %23, 1
   br label %_llgo_4
 
 _llgo_4:                                          ; preds = %_llgo_5, %_llgo_3
-  %24 = phi i64 [ -1, %_llgo_3 ], [ %25, %_llgo_5 ]
-  %25 = add i64 %24, 1
-  %26 = icmp slt i64 %25, %23
-  br i1 %26, label %_llgo_5, label %_llgo_6
+  %25 = phi i64 [ -1, %_llgo_3 ], [ %26, %_llgo_5 ]
+  %26 = add i64 %25, 1
+  %27 = icmp slt i64 %26, %24
+  br i1 %27, label %_llgo_5, label %_llgo_6
 
 _llgo_5:                                          ; preds = %_llgo_4
-  %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)
+  %28 = icmp slt i64 %26, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %28)
+  %29 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %23, 0
+  %30 = getelementptr inbounds i32, ptr %29, i64 %26
+  %31 = load i32, ptr %30, align 4
+  %32 = call i32 (ptr, ...) @printf(ptr @1, i32 %31)
   br label %_llgo_4
 
 _llgo_6:                                          ; preds = %_llgo_4
-  %31 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64 4)
-  %32 = getelementptr inbounds %main.generator, ptr %31, i32 0, i32 0
-  store i32 1, ptr %32, align 4
-  %33 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocU"(i64 8)
-  %34 = getelementptr inbounds { ptr }, ptr %33, i32 0, i32 0
-  store ptr %31, ptr %34, align 8
-  %35 = alloca { ptr, ptr }, align 8
-  %36 = getelementptr inbounds { ptr, ptr }, ptr %35, i32 0, i32 0
-  store ptr @"main.next$bound", ptr %36, align 8
-  %37 = getelementptr inbounds { ptr, ptr }, ptr %35, i32 0, i32 1
-  store ptr %33, ptr %37, align 8
-  %38 = load { ptr, ptr }, ptr %35, align 8
-  %39 = call %"github.com/goplus/llgo/internal/runtime.Slice" @main.genInts(i64 5, { ptr, ptr } %38)
-  %40 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %39, 1
+  %33 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocZ"(i64 4)
+  %34 = getelementptr inbounds %main.generator, ptr %33, i32 0, i32 0
+  store i32 1, ptr %34, align 4
+  %35 = call ptr @"github.com/goplus/llgo/internal/runtime.AllocU"(i64 8)
+  %36 = getelementptr inbounds { ptr }, ptr %35, i32 0, i32 0
+  store ptr %33, ptr %36, align 8
+  %37 = alloca { ptr, ptr }, align 8
+  %38 = getelementptr inbounds { ptr, ptr }, ptr %37, i32 0, i32 0
+  store ptr @"main.next$bound", ptr %38, align 8
+  %39 = getelementptr inbounds { ptr, ptr }, ptr %37, i32 0, i32 1
+  store ptr %35, ptr %39, align 8
+  %40 = load { ptr, ptr }, ptr %37, align 8
+  %41 = call %"github.com/goplus/llgo/internal/runtime.Slice" @main.genInts(i64 5, { ptr, ptr } %40)
+  %42 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %41, 1
   br label %_llgo_7
 
 _llgo_7:                                          ; preds = %_llgo_8, %_llgo_6
-  %41 = phi i64 [ -1, %_llgo_6 ], [ %42, %_llgo_8 ]
-  %42 = add i64 %41, 1
-  %43 = icmp slt i64 %42, %40
-  br i1 %43, label %_llgo_8, label %_llgo_9
+  %43 = phi i64 [ -1, %_llgo_6 ], [ %44, %_llgo_8 ]
+  %44 = add i64 %43, 1
+  %45 = icmp slt i64 %44, %42
+  br i1 %45, label %_llgo_8, label %_llgo_9
 
 _llgo_8:                                          ; preds = %_llgo_7
-  %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)
+  %46 = icmp slt i64 %44, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %46)
+  %47 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %41, 0
+  %48 = getelementptr inbounds i32, ptr %47, i64 %44
+  %49 = load i32, ptr %48, align 4
+  %50 = call i32 (ptr, ...) @printf(ptr @2, i32 %49)
   br label %_llgo_7
 
 _llgo_9:                                          ; preds = %_llgo_7
@@ -159,6 +167,8 @@ 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 void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1)
+
 declare void @"github.com/goplus/llgo/internal/runtime.init"()
 
 declare i32 @rand()
diff --git a/cl/_testrt/mask/out.ll b/cl/_testrt/mask/out.ll
index 6ebc1e08..68d454fe 100644
--- a/cl/_testrt/mask/out.ll
+++ b/cl/_testrt/mask/out.ll
@@ -16,12 +16,6 @@ source_filename = "main"
 @7 = private unnamed_addr constant [2 x i8] c"\0A\00", align 1
 @8 = private unnamed_addr constant [2 x i8] c"\0A\00", align 1
 @9 = private unnamed_addr constant [2 x i8] c"\0A\00", align 1
-@10 = private unnamed_addr constant [22 x i8] c"negative shift amount\00", align 1
-@11 = private unnamed_addr constant [22 x i8] c"negative shift amount\00", align 1
-@12 = private unnamed_addr constant [22 x i8] c"negative shift amount\00", align 1
-@13 = private unnamed_addr constant [22 x i8] c"negative shift amount\00", align 1
-@14 = private unnamed_addr constant [22 x i8] c"negative shift amount\00", align 1
-@15 = private unnamed_addr constant [22 x i8] c"negative shift amount\00", align 1
 
 define void @main.init() {
 _llgo_0:
@@ -105,71 +99,65 @@ _llgo_0:
 define i64 @main.mask_shl(i64 %0, i64 %1) {
 _llgo_0:
   %2 = icmp slt i64 %1, 0
-  %3 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @10, i64 21)
-  call void @"github.com/goplus/llgo/internal/runtime.CheckRuntimeError"(i1 %2, %"github.com/goplus/llgo/internal/runtime.String" %3)
-  %4 = icmp uge i64 %1, 64
-  %5 = shl i64 %0, %1
-  %6 = select i1 %4, i64 0, i64 %5
-  ret i64 %6
+  call void @"github.com/goplus/llgo/internal/runtime.AssertNegativeShift"(i1 %2)
+  %3 = icmp uge i64 %1, 64
+  %4 = shl i64 %0, %1
+  %5 = select i1 %3, i64 0, i64 %4
+  ret i64 %5
 }
 
 define i8 @main.mask_shl8(i8 %0, i64 %1) {
 _llgo_0:
   %2 = icmp slt i64 %1, 0
-  %3 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @11, i64 21)
-  call void @"github.com/goplus/llgo/internal/runtime.CheckRuntimeError"(i1 %2, %"github.com/goplus/llgo/internal/runtime.String" %3)
-  %4 = trunc i64 %1 to i8
-  %5 = icmp uge i8 %4, 8
-  %6 = shl i8 %0, %4
-  %7 = select i1 %5, i8 0, i8 %6
-  ret i8 %7
+  call void @"github.com/goplus/llgo/internal/runtime.AssertNegativeShift"(i1 %2)
+  %3 = trunc i64 %1 to i8
+  %4 = icmp uge i8 %3, 8
+  %5 = shl i8 %0, %3
+  %6 = select i1 %4, i8 0, i8 %5
+  ret i8 %6
 }
 
 define i8 @main.mask_shl8u(i8 %0, i64 %1) {
 _llgo_0:
   %2 = icmp slt i64 %1, 0
-  %3 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @12, i64 21)
-  call void @"github.com/goplus/llgo/internal/runtime.CheckRuntimeError"(i1 %2, %"github.com/goplus/llgo/internal/runtime.String" %3)
-  %4 = trunc i64 %1 to i8
-  %5 = icmp uge i8 %4, 8
-  %6 = shl i8 %0, %4
-  %7 = select i1 %5, i8 0, i8 %6
-  ret i8 %7
+  call void @"github.com/goplus/llgo/internal/runtime.AssertNegativeShift"(i1 %2)
+  %3 = trunc i64 %1 to i8
+  %4 = icmp uge i8 %3, 8
+  %5 = shl i8 %0, %3
+  %6 = select i1 %4, i8 0, i8 %5
+  ret i8 %6
 }
 
 define i64 @main.mask_shr(i64 %0, i64 %1) {
 _llgo_0:
   %2 = icmp slt i64 %1, 0
-  %3 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @13, i64 21)
-  call void @"github.com/goplus/llgo/internal/runtime.CheckRuntimeError"(i1 %2, %"github.com/goplus/llgo/internal/runtime.String" %3)
-  %4 = icmp uge i64 %1, 64
-  %5 = select i1 %4, i64 63, i64 %1
-  %6 = ashr i64 %0, %5
-  ret i64 %6
+  call void @"github.com/goplus/llgo/internal/runtime.AssertNegativeShift"(i1 %2)
+  %3 = icmp uge i64 %1, 64
+  %4 = select i1 %3, i64 63, i64 %1
+  %5 = ashr i64 %0, %4
+  ret i64 %5
 }
 
 define i8 @main.mask_shr8(i8 %0, i64 %1) {
 _llgo_0:
   %2 = icmp slt i64 %1, 0
-  %3 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @14, i64 21)
-  call void @"github.com/goplus/llgo/internal/runtime.CheckRuntimeError"(i1 %2, %"github.com/goplus/llgo/internal/runtime.String" %3)
-  %4 = trunc i64 %1 to i8
-  %5 = icmp uge i8 %4, 8
-  %6 = select i1 %5, i8 7, i8 %4
-  %7 = ashr i8 %0, %6
-  ret i8 %7
+  call void @"github.com/goplus/llgo/internal/runtime.AssertNegativeShift"(i1 %2)
+  %3 = trunc i64 %1 to i8
+  %4 = icmp uge i8 %3, 8
+  %5 = select i1 %4, i8 7, i8 %3
+  %6 = ashr i8 %0, %5
+  ret i8 %6
 }
 
 define i8 @main.mask_shr8u(i8 %0, i64 %1) {
 _llgo_0:
   %2 = icmp slt i64 %1, 0
-  %3 = call %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/llgo/internal/runtime.NewString"(ptr @15, i64 21)
-  call void @"github.com/goplus/llgo/internal/runtime.CheckRuntimeError"(i1 %2, %"github.com/goplus/llgo/internal/runtime.String" %3)
-  %4 = trunc i64 %1 to i8
-  %5 = icmp uge i8 %4, 8
-  %6 = lshr i8 %0, %4
-  %7 = select i1 %5, i8 0, i8 %6
-  ret i8 %7
+  call void @"github.com/goplus/llgo/internal/runtime.AssertNegativeShift"(i1 %2)
+  %3 = trunc i64 %1 to i8
+  %4 = icmp uge i8 %3, 8
+  %5 = lshr i8 %0, %3
+  %6 = select i1 %4, i8 0, i8 %5
+  ret i8 %6
 }
 
 declare void @"github.com/goplus/llgo/internal/runtime.init"()
@@ -182,4 +170,4 @@ declare %"github.com/goplus/llgo/internal/runtime.String" @"github.com/goplus/ll
 
 declare void @"github.com/goplus/llgo/internal/runtime.PrintUint"(i64)
 
-declare void @"github.com/goplus/llgo/internal/runtime.CheckRuntimeError"(i1, %"github.com/goplus/llgo/internal/runtime.String")
+declare void @"github.com/goplus/llgo/internal/runtime.AssertNegativeShift"(i1)
diff --git a/cl/_testrt/qsort/out.ll b/cl/_testrt/qsort/out.ll
index 2d203ee1..916dd34a 100644
--- a/cl/_testrt/qsort/out.ll
+++ b/cl/_testrt/qsort/out.ll
@@ -48,9 +48,11 @@ _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
   br i1 %12, label %_llgo_2, label %_llgo_3
 
 _llgo_2:                                          ; preds = %_llgo_1
-  %13 = getelementptr inbounds i64, ptr %2, i64 %11
-  %14 = load i64, ptr %13, align 4
-  %15 = call i32 (ptr, ...) @printf(ptr @0, i64 %14)
+  %13 = icmp slt i64 %11, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %13)
+  %14 = getelementptr inbounds i64, ptr %2, i64 %11
+  %15 = load i64, ptr %14, align 4
+  %16 = call i32 (ptr, ...) @printf(ptr @0, i64 %15)
   br label %_llgo_1
 
 _llgo_3:                                          ; preds = %_llgo_1
@@ -72,4 +74,6 @@ _llgo_0:
   ret i32 %5
 }
 
+declare void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1)
+
 declare i32 @printf(ptr, ...)
diff --git a/cl/_testrt/sum/out.ll b/cl/_testrt/sum/out.ll
index 70c4be76..e13b24a2 100644
--- a/cl/_testrt/sum/out.ll
+++ b/cl/_testrt/sum/out.ll
@@ -48,17 +48,19 @@ _llgo_0:
   br label %_llgo_1
 
 _llgo_1:                                          ; preds = %_llgo_2, %_llgo_0
-  %2 = phi i64 [ 0, %_llgo_0 ], [ %9, %_llgo_2 ]
+  %2 = phi i64 [ 0, %_llgo_0 ], [ %10, %_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 = 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
+  %6 = icmp slt i64 %4, 0
+  call void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1 %6)
+  %7 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %0, 0
+  %8 = getelementptr inbounds i64, ptr %7, i64 %4
+  %9 = load i64, ptr %8, align 4
+  %10 = add i64 %2, %9
   br label %_llgo_1
 
 _llgo_3:                                          ; preds = %_llgo_1
@@ -72,3 +74,5 @@ 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 void @"github.com/goplus/llgo/internal/runtime.AssertIndexRange"(i1)
diff --git a/internal/abi/llgo_autogen.lla b/internal/abi/llgo_autogen.lla
index be9b522a..cccf2f81 100644
Binary files a/internal/abi/llgo_autogen.lla and b/internal/abi/llgo_autogen.lla differ
diff --git a/internal/runtime/llgo_autogen.lla b/internal/runtime/llgo_autogen.lla
index 4f9b4461..fe0aad96 100644
Binary files a/internal/runtime/llgo_autogen.lla and b/internal/runtime/llgo_autogen.lla differ
diff --git a/ssa/expr.go b/ssa/expr.go
index 49e353d2..8132b989 100644
--- a/ssa/expr.go
+++ b/ssa/expr.go
@@ -324,9 +324,9 @@ func (b Builder) BinOp(op token.Token, x, y Expr) Expr {
 		case token.AND_NOT:
 			return Expr{b.impl.CreateAnd(x.impl, b.impl.CreateNot(y.impl, ""), ""), x.Type}
 		case token.SHL, token.SHR:
-			if y.kind == vkSigned {
+			if needsNegativeCheck(y) {
 				check := Expr{b.impl.CreateICmp(llvm.IntSLT, y.impl, llvm.ConstInt(y.ll, 0, false), ""), b.Prog.Bool()}
-				b.InlineCall(b.Func.Pkg.rtFunc("CheckRuntimeError"), check, b.Str("negative shift amount"))
+				b.InlineCall(b.Func.Pkg.rtFunc("AssertNegativeShift"), check)
 			}
 			xsize, ysize := b.Prog.SizeOf(x.Type), b.Prog.SizeOf(y.Type)
 			if xsize != ysize {
@@ -723,6 +723,7 @@ func (b Builder) IndexAddr(x, idx Expr) Expr {
 	if debugInstr {
 		log.Printf("IndexAddr %v, %v\n", x.impl, idx.impl)
 	}
+	idx = b.checkIndex(idx)
 	prog := b.Prog
 	telem := prog.Index(x.Type)
 	pt := prog.Pointer(telem)
@@ -737,6 +738,30 @@ func (b Builder) IndexAddr(x, idx Expr) Expr {
 	return Expr{llvm.CreateInBoundsGEP(b.impl, telem.ll, x.impl, indices), pt}
 }
 
+func needsNegativeCheck(x Expr) bool {
+	if x.kind == vkSigned {
+		if rv := x.impl.IsAConstantInt(); !rv.IsNil() && rv.SExtValue() >= 0 {
+			return false
+		}
+		return true
+	}
+	return false
+}
+
+// check index >= 0 and size to uint
+func (b Builder) checkIndex(idx Expr) Expr {
+	if needsNegativeCheck(idx) {
+		check := Expr{b.impl.CreateICmp(llvm.IntSLT, idx.impl, llvm.ConstInt(idx.ll, 0, false), ""), b.Prog.Bool()}
+		b.InlineCall(b.Func.Pkg.rtFunc("AssertIndexRange"), check)
+	}
+	typ := b.Prog.Uint()
+	if b.Prog.SizeOf(idx.Type) < b.Prog.SizeOf(typ) {
+		idx.Type = typ
+		idx.impl = castUintptr(b, idx.impl, typ)
+	}
+	return idx
+}
+
 // The Index instruction yields element Index of collection X, an array,
 // string or type parameter containing an array, a string, a pointer to an,
 // array or a slice.
@@ -768,6 +793,7 @@ func (b Builder) Index(x, idx Expr, addr func(Expr) Expr) Expr {
 			b.Store(ptr, x)
 		}
 	}
+	idx = b.checkIndex(idx)
 	pt := prog.Pointer(telem)
 	indices := []llvm.Value{idx.impl}
 	buf := Expr{llvm.CreateInBoundsGEP(b.impl, telem.ll, ptr.impl, indices), pt}