Size/perf optimisation for mbedtls_mpi_core_clz

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>

tests/suites/test_suite_bignum_core.function

@@ -309,6 +309,43 @@ exit:
 }
 /* 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)
 {