Am 01.06.2012 14:22, schrieb Philip Martin:
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.
I just verified that memcpy will indeed be properly inlined. Switched to memcpy as per your suggestion in r1345883. Thanks, Stefan^2.
