diff --git a/library/bignum_core.c b/library/bignum_core.c
index c6d92fb..9b6c414 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -35,6 +35,36 @@
 
 size_t mbedtls_mpi_core_clz(mbedtls_mpi_uint a)
 {
+#if defined(__has_builtin)
+#if __has_builtin(__builtin_clz)
+    if (sizeof(mbedtls_mpi_uint) == sizeof(unsigned int)) {
+        // __builtin_clz is undefined if a == 0
+        if (a == 0) {
+            return sizeof(mbedtls_mpi_uint) * 8;
+        } else {
+            return (size_t) __builtin_clz(a);
+        }
+    }
+#endif
+#if __has_builtin(__builtin_clzl)
+    if (sizeof(mbedtls_mpi_uint) == sizeof(unsigned long)) {
+        if (a == 0) {
+            return sizeof(mbedtls_mpi_uint) * 8;
+        } else {
+            return (size_t) __builtin_clzl(a);
+        }
+    }
+#endif
+#if __has_builtin(__builtin_clzll)
+    if (sizeof(mbedtls_mpi_uint) == sizeof(unsigned long long)) {
+        if (a == 0) {
+            return sizeof(mbedtls_mpi_uint) * 8;
+        } else {
+            return (size_t) __builtin_clzll(a);
+        }
+    }
+#endif
+#endif
     size_t j;
     mbedtls_mpi_uint mask = (mbedtls_mpi_uint) 1 << (biL - 1);
 
diff --git a/tests/suites/test_suite_bignum_core.function b/tests/suites/test_suite_bignum_core.function
index e084b83..d80054c 100644
--- a/tests/suites/test_suite_bignum_core.function
+++ b/tests/suites/test_suite_bignum_core.function
@@ -309,6 +309,43 @@
 }
 /* END_CASE */
 
+
+/* BEGIN_CASE */
+void mpi_core_clz(int lz, int tz)
+{
+    if ((size_t) (lz + tz) >= (sizeof(mbedtls_mpi_uint) * 8)) {
+        // can't fit required number of leading and trailing zeros - skip test
+        goto exit;
+    }
+
+    mbedtls_mpi_uint x;
+    size_t expected;
+
+    // generate x with lz leading zeros and tz trailing zeros, all other bits set.
+    if (lz == -1) {
+        // special case: all zero
+        x = 0;
+        expected = sizeof(mbedtls_mpi_uint) * 8;
+    } else {
+        expected = lz;
+        if ((lz + tz) > 0) {
+            // some zero bits
+            uint32_t s = (sizeof(mbedtls_mpi_uint) * 8 - lz - tz);
+            x = ((1ULL << s) - 1) << tz;
+        } else {
+            // all bits set
+            x = ~0ULL;
+        }
+    }
+
+    size_t n = mbedtls_mpi_core_clz(x);
+    TEST_EQUAL(n, expected);
+exit:
+    ;
+}
+/* END_CASE */
+
+
 /* BEGIN_CASE */
 void mpi_core_lt_ct(char *input_X, char *input_Y, int exp_ret)
 {
diff --git a/tests/suites/test_suite_bignum_core.misc.data b/tests/suites/test_suite_bignum_core.misc.data
index b61d708..c245b02 100644
--- a/tests/suites/test_suite_bignum_core.misc.data
+++ b/tests/suites/test_suite_bignum_core.misc.data
@@ -491,3 +491,38 @@
 Fill random core: 42 bytes, 5 missing limbs
 mpi_core_fill_random:42:0:-5:0:MBEDTLS_ERR_MPI_BAD_INPUT_DATA
 
+CLZ: 0 0: all ones
+mpi_core_clz:0:0
+
+CLZ: -1 -1: all zeros
+mpi_core_clz:-1:-1
+
+CLZ: 1 0
+mpi_core_clz:1:0
+
+CLZ: 1 1
+mpi_core_clz:1:1
+
+CLZ: 4 5
+mpi_core_clz:4:5
+
+CLZ: 8 16
+mpi_core_clz:8:16
+
+CLZ: 31 0
+mpi_core_clz:31:0
+
+CLZ: 32 0
+mpi_core_clz:32:0
+
+CLZ: 33 0
+mpi_core_clz:33:0
+
+CLZ: 63 0
+mpi_core_clz:63:0
+
+CLZ: 64 0
+mpi_core_clz:64:0
+
+CLZ: 100000 0: skip overly long input
+mpi_core_clz:100000:0