diff --git a/cl/_testrt/slicelen/in.go b/cl/_testrt/slicelen/in.go
new file mode 100644
index 00000000..c77f8ad1
--- /dev/null
+++ b/cl/_testrt/slicelen/in.go
@@ -0,0 +1,16 @@
+package main
+
+import (
+	"unsafe"
+)
+
+func main() {
+	var s *int
+	var lens uint32
+	sl := unsafe.Slice(s, lens)
+	slen := len(sl)
+	println(slen)
+	if slen > 0 {
+		println("len > 0")
+	}
+}
diff --git a/cl/_testrt/slicelen/out.ll b/cl/_testrt/slicelen/out.ll
new file mode 100644
index 00000000..f26787d7
--- /dev/null
+++ b/cl/_testrt/slicelen/out.ll
@@ -0,0 +1,66 @@
+; ModuleID = 'main'
+source_filename = "main"
+
+%"github.com/goplus/llgo/internal/runtime.Slice" = type { ptr, i64, i64 }
+%"github.com/goplus/llgo/internal/runtime.String" = type { ptr, i64 }
+
+@"main.init$guard" = global i1 false, align 1
+@__llgo_argc = global i32 0, align 4
+@__llgo_argv = global ptr null, align 8
+@0 = private unnamed_addr constant [7 x i8] c"len > 0", align 1
+
+define void @main.init() {
+_llgo_0:
+  %0 = load i1, ptr @"main.init$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
+  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()
+  %2 = alloca %"github.com/goplus/llgo/internal/runtime.Slice", align 8
+  %3 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.Slice", ptr %2, i32 0, i32 0
+  store ptr null, ptr %3, align 8
+  %4 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.Slice", ptr %2, i32 0, i32 1
+  store i64 0, ptr %4, align 4
+  %5 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.Slice", ptr %2, i32 0, i32 2
+  store i64 0, ptr %5, align 4
+  %6 = load %"github.com/goplus/llgo/internal/runtime.Slice", ptr %2, align 8
+  %7 = extractvalue %"github.com/goplus/llgo/internal/runtime.Slice" %6, 1
+  call void @"github.com/goplus/llgo/internal/runtime.PrintInt"(i64 %7)
+  call void @"github.com/goplus/llgo/internal/runtime.PrintByte"(i8 10)
+  %8 = icmp sgt i64 %7, 0
+  br i1 %8, label %_llgo_1, label %_llgo_2
+
+_llgo_1:                                          ; preds = %_llgo_0
+  %9 = alloca %"github.com/goplus/llgo/internal/runtime.String", align 8
+  %10 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %9, i32 0, i32 0
+  store ptr @0, ptr %10, align 8
+  %11 = getelementptr inbounds %"github.com/goplus/llgo/internal/runtime.String", ptr %9, i32 0, i32 1
+  store i64 7, ptr %11, align 4
+  %12 = load %"github.com/goplus/llgo/internal/runtime.String", ptr %9, align 8
+  call void @"github.com/goplus/llgo/internal/runtime.PrintString"(%"github.com/goplus/llgo/internal/runtime.String" %12)
+  call void @"github.com/goplus/llgo/internal/runtime.PrintByte"(i8 10)
+  br label %_llgo_2
+
+_llgo_2:                                          ; preds = %_llgo_1, %_llgo_0
+  ret i32 0
+}
+
+declare void @"github.com/goplus/llgo/internal/runtime.init"()
+
+declare void @"github.com/goplus/llgo/internal/runtime.PrintInt"(i64)
+
+declare void @"github.com/goplus/llgo/internal/runtime.PrintByte"(i8)
+
+declare void @"github.com/goplus/llgo/internal/runtime.PrintString"(%"github.com/goplus/llgo/internal/runtime.String")
diff --git a/ssa/expr.go b/ssa/expr.go
index 0e83a8cf..9dda62d2 100644
--- a/ssa/expr.go
+++ b/ssa/expr.go
@@ -1163,8 +1163,8 @@ func (b Builder) BuiltinCall(fn string, args ...Expr) (ret Expr) {
 	case "String": // unsafe.String
 		return b.unsafeString(args[0].impl, args[1].impl)
 	case "Slice": // unsafe.Slice
-		size := args[1].impl
-		return b.unsafeSlice(args[0], size, size)
+		size := b.fitIntSize(args[1])
+		return b.unsafeSlice(args[0], size.impl, size.impl)
 	case "StringData":
 		return b.StringData(args[0]) // TODO(xsw): check return type
 	case "SliceData":