Remove multiplication from conditional assignments
Multiplication is not constant flow on any CPU we are generally
targetting, so replace this with bit twiddling.
@@ -71,22 +71,44 @@ static const unsigned char base64_dec_map[128] =
static void mbedtls_base64_cond_assign_uchar(unsigned char * dest, const unsigned char * const src,
unsigned char condition)
{
- /* make sure assign is 0 or 1 in a time-constant manner */
- condition = (condition | (unsigned char)-condition) >> 7;
+ /* MSVC has a warning about unary minus on unsigned integer types,
+ * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
- *dest = ( *dest ) * ( 1 - condition ) + ( *src ) * condition;
+ /* Generate bitmask from condition, mask will either be 0xFFFFFFFF or 0 */
+ unsigned char mask = -( unsigned char )( ( condition | -condition ) >> 7 );
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
+
+ *dest = ( ( *src ) & mask ) | ( ( *dest ) & ~mask );
}
/*
* Constant flow conditional assignment to uint_32
*/
static void mbedtls_base64_cond_assign_uint32(uint32_t * dest, const uint32_t src,
- unsigned char condition)
+ uint32_t condition)
{
- /* make sure assign is 0 or 1 in a time-constant manner */
- condition = (condition | (unsigned char)-condition) >> 7;
+ /* MSVC has a warning about unary minus on unsigned integer types,
+ * but this is well-defined and precisely what we want to do here. */
+#if defined(_MSC_VER)
+#pragma warning( push )
+#pragma warning( disable : 4146 )
+#endif
+
+ /* Generate bitmask from condition, mask will either be 0xFFFFFFFF or 0 */
+ uint32_t mask = -( uint32_t )( ( condition | -condition ) >> 31 );
+
+#if defined(_MSC_VER)
+#pragma warning( pop )
+#endif
- *dest = ( *dest ) * ( 1 - condition ) + ( src ) * condition;
+ *dest = ( src & mask ) | ( ( *dest ) & ~mask );
}
