> On Jan 8, 2016, at 6:32 AM, Jakub Jelinek <ja...@redhat.com> wrote: > > On Fri, Jan 08, 2016 at 12:24:49PM +0100, Eric Botcazou wrote: >>> See some existing PR. The GCC middle-end cannot assume that pointers >>> are aligned according to their type (while at least the C language would >>> support that notion). >> >> Only on x86. It could (and used to) do it on strict-alignment architectures. > > I think we only assume it if the pointer is actually dereferenced, otherwise > it just breaks too much code in the wild. And while memcpy dereferences, it > dereferences it through a char * cast, and thus only the minimum alignment > is assumed.
Is the char* cast actually part of the definition of memcpy? The fact that memcpy doesn't make use of the known alignment is a rather unfortunate issue (I'd call it a defect); it results in clearly inferior code on strict-alignment machines when the alignment is in fact clearly known. If it really is necessary to say that memcpy is defined to behave as if it had a cast to char* in its implementation, it would still be useful -- and obviously valid -- to rely on the greater alignment when there is other code in the block that uses it. And it would be helpful to have a way in the source program to say "I want you to rely on the type alignment". paul