[cxx-abi-dev] Passing non-trivial types through ...

[Jason Merrill]

On 08/13/2014 08:04 PM, Dennis Handly wrote:
>> From: Jason Merrill <jason at redhat.com>
>> so my proposal would be incompatible with the current aCC behavior.
>
> Yes.  But I don't remember why we allowed it and what the user was doing on
> the callee side, since we didn't provide any help.

G++ 3.1 and earlier did the same bitwise copy, which makes sense for a C 
legacy definition of va_arg like GCC had before 3.0:

#define va_arg(AP, TYPE)                                              \
  (AP = (__gnuc_va_list) ((char *) (AP) + __va_rounded_size (TYPE)),   \
   *((TYPE *) (void *) ((char *) (AP) - __va_rounded_size (TYPE))))

>> From: John McCall <rjmccall at apple.com>
>> This is clearly the right way for any vendor who wants to accept non-POD
>> variadic arguments to do it:  no solution involving memcpy can be
>> correct for all types

Yep.  The only question is whether it's better, for code that is in a 
gray area of the standard, to stick with the broken historical practice 
or do something more correct.

> Yes, except will the callee side be allowed to modify the object?

Whether the result of va_arg is modifiable (or even an lvalue) is 
unspecified.  In this case it would be.

Jason