Multiple return values...

[Robert Jacques]

On Sat, 10 Mar 2012 19:27:05 -0600, Manu <turkeyman at gmail.com> wrote:
> On 11 March 2012 00:25, Sean Cavanaugh <WorksOnMyMachine at gmail.com> wrote:
>
>> On 3/10/2012 4:37 AM, Manu wrote:
>>
>>>
>>> If I pass a structure TO a function by value, I know what happens, a
>>> copy is written to the stack which the function expects to find there.
>>>
>>
>> This is only true if the compiler is forced to use the ABI, when inlining
>> is impossible, or the type being passed is too complex. Structs of pods,
>> most compilers do magical things to provided you don't actively work
>> against the code gen (virtual methods, dllexports etc), too many separate
>> .obj units in C++ etc.
>>
>
> Talking about the typical case here, obviously not inlined, calling through
> the ABI, struct may be simple, just 2-3 values, can you show me a case
> where C++ is able to to anything particularly fancy?
>
> I've never seen the compiler do wildly unexpected things unless whole
> program optimisation is enabled, which I don't imagine D will be able to
> support any time real soon?
>
> ...and even then, relying on WPO for the language to generate good code in
> certain circumstances is a really really bad idea. This makes the task of
> implementing an efficient compiler for the language extremely difficult.
> In most cases, making concise expression of the operation you want to
> perform possible in the language will generate better results anyway,
> without depending on an aggressive optimiser. It will also make the code
> more explicit and readable.
>

Manu, please go read the D ABI (http://dlang.org/abi.html). Remember, your example of returning two values using Tuple vs 'real' MRV? The D ABI states that those values will be returned via registers. Returning something larger? Then the NVRO kicks in which gives you a zero copy approach. On x86-64 these limits are different, since you have more registers to play with, but the concept is the same. In fact, returning arguments has always been more efficient than passing arguments.