Document undefined case. Clarify test code.

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

library/bignum_core.c

@@ -33,11 +33,18 @@
 #include "bn_mul.h"
 #include "constant_time_internal.h"
 
+/**
+ * \brief          Count leading zeros
+ *
+ * \warning        The result is undefined if \p a == 0
+ *
+ * \param a        The value to operate on
+ *
+ * \return         The number of leading zeros, if \p a != 0. If \p a == 0, the result
+ *                 is undefined.
+ */
 inline size_t mbedtls_mpi_core_clz(mbedtls_mpi_uint a)
 {
-    /* Note: the result is undefined for a == 0
-     * (because this is the behaviour of __builtin_clz).
-     */
 #if defined(__has_builtin)
 #if __has_builtin(__builtin_clz)
     if (sizeof(mbedtls_mpi_uint) == sizeof(unsigned int)) {

tests/suites/test_suite_bignum_core.function

@@ -311,25 +311,28 @@ exit:
 
 
 /* BEGIN_CASE */
-void mpi_core_clz(int lz, int tz)
+void mpi_core_clz(int leading_zeros, int trailing_zeros)
 {
-    if ((size_t) (lz + tz) >= (sizeof(mbedtls_mpi_uint) * 8)) {
+    if ((size_t) (leading_zeros + trailing_zeros) >= (sizeof(mbedtls_mpi_uint) * 8)) {
         // can't fit required number of leading and trailing zeros - skip test
         goto exit;
     }
 
+    // Construct a test input value where the count of leading zeros and
+    // trailing zeros is given in the test case, and we add ones to fill
+    // the gap.
     mbedtls_mpi_uint x;
-    if ((lz + tz) > 0) {
+    if ((leading_zeros + trailing_zeros) > 0) {
         // some zero bits
-        uint32_t s = (sizeof(mbedtls_mpi_uint) * 8 - lz - tz);
+        uint32_t s = (sizeof(mbedtls_mpi_uint) * 8 - leading_zeros - trailing_zeros);
-        x = ((((mbedtls_mpi_uint) 1) << s) - 1) << tz;
+        x = ((((mbedtls_mpi_uint) 1) << s) - 1) << trailing_zeros;
     } else {
         // all bits set
         x = ~((mbedtls_mpi_uint) 0);
     }
 
     size_t n = mbedtls_mpi_core_clz(x);
-    TEST_EQUAL(n, lz);
+    TEST_EQUAL(n, leading_zeros);
 exit:
     ;
 }