On Sun, 11 Mar 2012 21:49:52 -0500, Mantis <mail.mantis...@gmail.com> wrote:
12.03.2012 4:00, Robert Jacques пишет:
On Sun, 11 Mar 2012 18:15:31 -0500, Timon Gehr <timon.g...@gmx.ch> wrote:
On 03/11/2012 11:58 PM, Robert Jacques wrote:
Manu was arguing that MRV were somehow special and had mystical
optimization potential. That's simply not true.
Not exactly mystical, but it is certainly there.
void main(){
auto a = foo(); // MRV/struct return
bar(&a.x); // defined in a different compilation unit
}
struct return has to write out the whole struct on the stack because of
layout guarantees, probably making the optimized struct return calling
convention somewhat slower for this case. The same does not hold for
MRV.
The layout of the struct only has to exist _when_ the address is
taken. Before that, the compiler/language/optimizer is free to (and
does) do whatever it want. Besides, in your example only the address
of a field is taken, the compiler will optimize away all the other
pieces a (dead variable elimination).
That's the point of discussion. Fields of structure may not be optimized
away, because they are not independent variables. In D you have
unchecked pointer-to-pointer casts, and results of these casts should
depend on target architecture, not on optimizer implementation. At
particular, if such optimizations are allowed, some C API will no longer
be accessible from D.
Unused fields of a structure are optimized away _today_. Unless a piece of code
takes the address of the struct, all of the fields are treated as independent
variables.
