From: Hannes Frederic Sowa > Sent: 15 December 2016 14:57 > On 15.12.2016 14:56, David Laight wrote: > > From: Hannes Frederic Sowa > >> Sent: 15 December 2016 12:50 > >> On 15.12.2016 13:28, David Laight wrote: > >>> From: Hannes Frederic Sowa > >>>> Sent: 15 December 2016 12:23 > >>> ... > >>>> Hmm? Even the Intel ABI expects alignment of unsigned long long to be 8 > >>>> bytes on 32 bit. Do you question that? > >>> > >>> Yes. > >>> > >>> The linux ABI for x86 (32 bit) only requires 32bit alignment for u64 (etc). > >> > >> Hmm, u64 on 32 bit is unsigned long long and not unsigned long. Thus I > >> am actually not sure if the ABI would say anything about that (sorry > >> also for my wrong statement above). > >> > >> Alignment requirement of unsigned long long on gcc with -m32 actually > >> seem to be 8. > > > > It depends on the architecture. > > For x86 it is definitely 4. > > May I ask for a reference? Ask anyone who has had to do compatibility layers to support 32bit binaries on 64bit systems. > I couldn't see unsigned long long being > mentioned in the ia32 abi spec that I found. I agree that those accesses > might be synthetically assembled by gcc and for me the alignment of 4 > would have seemed natural. But my gcc at least in 32 bit mode disagrees > with that. Try (retyped): echo 'struct { long a; long long b; } s; int bar { return sizeof s; }' >foo.c gcc [-m32] -O2 -S foo.c; cat foo.s And look at what is generated. > Right now ipv6 addresses have an alignment of 4. So we couldn't even > naturally pass them to siphash but would need to copy them around, which > I feel like a source of bugs. That is more of a problem on systems that don't support misaligned accesses. Reading the 64bit values with two explicit 32bit reads would work. I think you can get gcc to do that by adding an aligned(4) attribute to the structure member. David ��.n��������+%������w��{.n�����{���{ay�ʇڙ���f���h������_�(�階�ݢj"��������G����?���&��