Re: C++PATCH: va_arg & aggr type

[Jason Merrill <jason at cygnus dot com>]

>>>>> Nathan Sidwell <nathan@acm.org> writes:

 > Ok, I was vague in what I meant, and used a scalar example, when I
 > should've used an aggr. What I meant to convey was that a tree
 > representing
 >         const struct X *xp = &va_arg (args, struct X);
 > would be better than one representing
 >         struct X _tmp = va_arg (args, struct X);
 >         const struct X *xp = &_tmp;
 > which has the copy

Note that the semantics are different for these two; the second one
guarantees that *xp will be valid while xp is in scope, while the first one
points xp to a temporary which goes away at the end of the statement.  I
think there's probably a better way to avoid the copy, but I don't know the
new va_arg code at all; maybe return a TARGET_EXPR for aggregate values?

 >> > *************** lvalue_p_1 (ref, treat_class_rvalues_as_
 >> > *** 142,147 ****
 >> > --- 142,148 ----
 >> >                      treat_class_rvalues_as_lvalues);
  
 >> >       case TARGET_EXPR:
 >> > +     case VA_ARG_EXPR:
 >> >         return treat_class_rvalues_as_lvalues ? clk_class : clk_none;

 >> We can get VA_ARG_EXPRs for non-class types, can't we?
 > Yes, but we only need to treat aggr things as lvalues so that we can
 > copy them. We don't want va_arg to give an lvalue do we? But, in that
 > case we need to check that TREE_TYPE (ref) is an aggr type, so I attach
 > the an updated patch. (This does mean that I don't understand why
 > TARGET_EXPR doesn't do such a check.)

Probably an oversight; feel free to change it, too.

 > This one ok?

Yes.

Jason