Branko Čibej <br...@apache.org> writes: > On 01.06.2012 14:22, Philip Martin wrote: >> 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]); >> ^ > [...] > > >> 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 think it's OK if that macro becomes an alias for memcpy. On modern > systems I'd even expect it to be faster than the current code, since a > properly inlined memcpy will know when to pick word-size over byte-size > operations, since it'll know the alignment of the source and target > (which this macro doesn't).
With Linux/gcc using memcpy causes the generated assembler to be ordered slightly differently but it appears to be the same instructions (to my inexpert eye): 10,19c10,19 < jbe .L281 < .loc 1 971 0 < leaq decimal_table(%rip), %rax < movzwl (%rax,%rsi,4), %eax < .loc 1 972 0 < movb $0, 2(%rdi) < .loc 1 971 0 < movw %ax, (%rdi) < .loc 1 973 0 < movl $2, %eax --- > ja .L271 > .loc 1 965 0 > addl $48, %esi > .loc 1 966 0 > movb $0, 1(%rdi) > .loc 1 967 0 > movl $1, %eax > .loc 1 965 0 > movb %sil, (%rdi) > .loc 1 1019 0 183,192c183,192 < .L281: < .loc 1 965 0 < addl $48, %esi < .loc 1 966 0 < movb $0, 1(%rdi) < .loc 1 967 0 < movl $1, %eax < .loc 1 965 0 < movb %sil, (%rdi) < .loc 1 1019 0 --- > .L271: > .loc 1 971 0 > leaq decimal_table(%rip), %rax > .loc 1 972 0 > movb $0, 2(%rdi) > .loc 1 971 0 > movzwl (%rax,%rsi,4), %eax > movw %ax, (%rdi) > .loc 1 973 0 > movl $2, %eax That's replacing *(apr_uint16_t*)(dest) = *(apr_uint16_t*)(source) with memcpy((dest), (source), 2) -- uberSVN: Apache Subversion Made Easy http://www.uberSVN.com
