diff --git a/testcrate/build.rs b/testcrate/build.rs
index c18daca..d862e0d 100644
--- a/testcrate/build.rs
+++ b/testcrate/build.rs
@@ -775,14 +775,6 @@ fn main() {
                 (builtins::int::udiv::__udivmodti4(a, b, Some(&mut r)), r)
             }");
     }
-    
-    // count leading zeros
-    gen(|a: MyU64| {
-            Some((a.0 as usize).leading_zeros())
-        },
-        "{
-            builtins::int::__clzsi2(a as usize) as u32
-        }");
 }
 
 macro_rules! gen_float {
diff --git a/testcrate/tests/count_leading_zeros.rs b/testcrate/tests/count_leading_zeros.rs
new file mode 100644
index 0000000..5596501
--- /dev/null
+++ b/testcrate/tests/count_leading_zeros.rs
@@ -0,0 +1,25 @@
+#![feature(compiler_builtins_lib)]
+
+extern crate compiler_builtins;
+
+use compiler_builtins::int::__clzsi2;
+
+#[test]
+fn __clzsi2_test() {
+  let mut i: usize = core::usize::MAX;
+  // Check all values above 0
+  while i > 0 {
+    assert_eq!(__clzsi2(i) as u32, i.leading_zeros());
+    i >>= 1;
+  }
+  // check 0 also
+  i = 0;
+  assert_eq!(__clzsi2(i) as u32, i.leading_zeros());
+  // double check for bit patterns that aren't just solid 1s
+  i = 1;
+  for _ in 0..63 {
+    assert_eq!(__clzsi2(i) as u32, i.leading_zeros());
+    i <<= 2;
+    i += 1;
+  }
+}