[C++ PATCH] Don't create an INTEGER_CST for aggregates (empty structs)

[Paolo Carlini]

Gabriel Dos Reis wrote:

>There is no way (C++-wise and ABI-wise) you tell when a value of empty
>class has been loaded in a register or not.  An empty class does not
>contain any data:  Everything is in the type -- which is a purely
>compile-time construct.
>  
>
Thanks Gaby (and Andrew) for your clarifications about the C++ 
front-end: I'm a novice, indeed, and appreciate that.

What I'm missing, now, but I don't think it's a purely front-end issue 
(I'm also implicitly taking into account you message to the library 
reflector) is whether optimizers downstream, in the middle-end, can 
factor out the ABI prescriptions about argument passing. In other terms: 
if something can figure that there is *nothing* to pass, actually, 
neither in registers nor in stack! Is middle-end/20408 really fixable on 
x86 or not?

If it's really impossible to optimize out the difference, maybe we have 
to live with it and go ahead anyway with the clean-ups that started the 
issue (in that case, the negative impact is small). If, on the other 
hand, it's only a matter of waiting for some optimizations to be 
implemented we have to *seriously* take care of that, making sure that 
the libstdc++-v3 development in that specific area doesn't proceed "too 
fast", assuming unreasonable optimizations.

Third possibility: a new attribute of sort (you suggested that, I 
think), but I'm not sure whether it's really doable: even an extension 
cannot be against the ABI, if the ABI is the real issue.

Yes, I'm confused: anyone willing to clarify??

Thanks in advance,
Paolo.