Re: [PATCH] Classify non-POD struct types more or less correctly on AMD64

Mark Kettenis
2004-01-10T18:59:13Z

Date: Sat, 10 Jan 2004 13:36:22 -0500
From: Daniel Jacobowitz <drow@mvista.com>

On Sat, Jan 10, 2004 at 07:00:35PM +0100, Mark Kettenis wrote:
> This (together with the previous patch) fixes the problems I saw with
> gdb.cp/bs15503.exp. The check for non-POD-ness isn't complete though.
> I hope to revisit that later, after someone tells me how to properly
> determine non-POD-ness.
>
> Mark
>
> P.S. The amd64_non_pod_p function should probably be moved to the
> generic cod, but we can do that later.

Does the x86-64 ABI really pass non-POD and POD types of the same size
differently? If so, I hope the ABI defines non-POD rather than relying
on the C++ definition, since we do not generally have enough
information in the debug info to determine whether a type is POD.

Oh joy, the ABI doesn't define POD-ness. There's just a footnote
which says:

The term POD is from the ANSI/ISO C++ Standard, and stands for Plain
Old Data. Although the exact definition is technical, a POD is
essentially a structure or union that could have been written in C;
there cannot be any member functions, or base classes, or similar
C++ extensions.

And yes, GCC really does pass them differently for aggregates up to 16
bytes in size.

> + /* ??? A class with a base class certainly isn't POD, but does this
> + catch all non-POD structure types? */
> + if (TYPE_CODE (type) == TYPE_CODE_STRUCT && TYPE_N_BASECLASSES (type) > 0)
> + return 1;

No, at least any type with explicitly declared methods is non-POD. For
DWARF you can probably get this right by checking for a non-artificial
method but for stabs you're SOL.

Tried that, and a lot of testcases started FAILing. Apparently GCC
thinks differently. In userdef.cc, we have a "class A1" with
explicitly declared methods, but GCC returns instances of "class A1"
in registers nevertheless.

Looks like we can't do very much about it. Fortunately most real-life
classes will be larger than 16 bytes.

Mark