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Document undefined case. Clarify test code.

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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2023-04-21 12:54:40 +01:00
parent 678e63007c
commit bbf881053d
2 changed files with 19 additions and 9 deletions
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13
library/bignum_core.c
View file

@ -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)) {

15
tests/suites/test_suite_bignum_core.function
View file

@ -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);
x = ((((mbedtls_mpi_uint) 1) << s) - 1) << tz;
uint32_t s = (sizeof(mbedtls_mpi_uint) * 8 - leading_zeros - trailing_zeros);
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:
;
}