Multiple return values...

[Robert Jacques]

On Mon, 12 Mar 2012 02:15:55 -0500, Timon Gehr <timon.gehr at gmx.ch> wrote:

> On 03/12/2012 05:01 AM, Robert Jacques wrote:
>> On Sun, 11 Mar 2012 21:49:52 -0500, Mantis <mail.mantis.88 at gmail.com>
>> wrote:
>>> 12.03.2012 4:00, Robert Jacques пишет:
>>>> On Sun, 11 Mar 2012 18:15:31 -0500, Timon Gehr <timon.gehr at 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.
>
> The only point I was trying to make is that the 'unless' part does not
> apply to MRV.
>

True. But the principal argument is that MRV implemented via structs is somehow less efficient than 'real' MRV. MRV structs would never have their address taken and thus the 'unless' clause never happens.