GCC gives a compiler warning where the COPY_TWO_BYTES macro is used. A
typical warning is:
../src/subversion/libsvn_subr/string.c:971:11: warning: dereferencing
type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
COPY_TWO_BYTES(dest, decimal_table[(apr_size_t)number]);
^
The code is:
/* Copy the two bytes at SOURCE[0] and SOURCE[1] to DEST[0] and DEST[1] */
#if SVN_UNALIGNED_ACCESS_IS_OK
# define COPY_TWO_BYTES(dest,source)\
*(apr_uint16_t*)(dest) = *(apr_uint16_t*)(source);
#else
# define COPY_TWO_BYTES(dest,source) \
do { \
(dest)[0] = (source)[0]; \
(dest)[1] = (source)[1]; \
} while (0)
#endif
apr_size_t
svn__ui64toa(char * dest, apr_uint64_t number)
{
char buffer[SVN_INT64_BUFFER_SIZE];
apr_uint32_t reduced; /* used for 32 bit DIV */
char* target;
/* Small numbers are by far the most common case.
* Therefore, we use special code.
*/
if (number < 100)
{
if (number < 10)
{
dest[0] = (char)('0' + number);
dest[1] = 0;
return 1;
}
else
{
COPY_TWO_BYTES(dest, decimal_table[(apr_size_t)number]);
dest[2] = 0;
return 2;
}
}
Is the warning something we can ignore? My understanding of C aliasing
is that writing to memory is only supposed to happen as the declared
type of that memory.
Is COPY_TWO_BYTES a significant optimisation? On Linux we can avoid the
warning by simply using
memcpy(dest, source, 2)
since that memcpy call will be inlined these days.
--
uberSVN: Apache Subversion Made Easy
http://www.uberSVN.com
