On Thu, Apr 14, 2016 at 11:36 AM, Jakub Jelinek <ja...@redhat.com> wrote: > On Thu, Apr 14, 2016 at 11:25:07AM +0100, Ramana Radhakrishnan wrote: >> > I see the test failing on aarch64-none-linux-gnu (native) >> > with no output, just: >> > spawn [open ...] >> > FAIL: g++.dg/abi/empty13.C -std=gnu++98 execution test >> > >> > And I see it passing on aarch64-none-elf. >>
I don't understand why this passes on aarch64-none-elf btw. >> IIRC on AArch64 we document that empty classes and structs take up a >> slot as per the PCS . >> >> http://infocenter.arm.com/help/topic/com.arm.doc.ihi0059b/IHI0059B_cppabi64.pdf >> >> See section 3.1 >> >> For the purposes of parameter passing in >> [ >> AAPCS64 >> ], a parameter whose type is an empty class shall be treated as if its >> type were an aggregate with a >> single >> member of type unsigned byte. > > One thing is passing such arguments in registers, another is how they are > passed on the stack (e.g. if there are several other arguments that are > passed in registers). It would just be treated like a structure with an unsigned byte and treated the same way for parameter passing on the stack - i.e. IIRC take a slot of 8 bytes. > Also, note that the (new) definition of C++ empty class I believe includes > even empty classes that are normally very large, i.e. don't pretend they > are just single byte, but can be e.g. 1024 bytes or longer. > Do you want to still pass them in registers and/or on the stack as if > it is 1 byte, 0 bytes or more bytes? I don't know off-hand. Is this a valid example for what you have in mind ? struct baz { char a[1024]; }; struct foo : baz { }; int bar (struct foo b, int x) if so, that would get treated as though it is just a 1 byte quantity for C++. as per current behaviour by both gcc and llvm for aarch64. > How do you pass empty C structures? I don't think we do anything special for empty C structures which IIRC means they don't take a slot. > > E.g. looking at sparc-solaris2.12 cross, C for empty structures passes > both in registers and on the stack as if there is one argument (and C++ did > too for the empty classes that pretended to have size 1), so appart from the > ICE in emit_move_insn there is no ABI change, only perhaps for the larger > structs. > > Guess we need to figure out what do the various targets do for C empty > structures and decide what we do want to do with C++ empty classes. > Perhaps the C++ FE should just set some flag on the empty class parameters, > and let the backends decide what to do with those. Ramana > > Jakub
