Multiple return values...

[Andrei Alexandrescu]

On 3/16/12 6:29 AM, Manu wrote:
> On 16 March 2012 04:26, Andrei Alexandrescu
> <SeeWebsiteForEmail at erdani.org <mailto:SeeWebsiteForEmail at erdani.org>>
> wrote:
>
>         A good design should strive to provide general features instead of
>         special cases (E.g. swap is limited to the 2-tuple case). Also, why
>         force an overhead of a function call on such a basic feature as
>         assignment? Is swap usually inlined by the compiler?
>
>
>     Sorry, this is grasping at straws.
>
>     1. swap can be easily generalized to take any number of arguments.
>     I'm very happy that's possible, we've expended great efforts on
>     making variadic functions as powerful as they are. But nobody asked
>     for swap with many arguments until now. Which segues into...
>
>     2. When was the last time you needed to swap arbitrary numbers of
>     elements, and so badly and frequently, you needed a new language
>     feature for that?
>
>
> This is called a swizzle. And constantly comes up when dealing with
> x,y,z,w, or r,g,b,a.
> It could just as easily be expressed this way:
>    a,b = tuple(b,a); // swap
>    r,g,b,a = tuple(a,r,g,b); // swizzle
> At which point this multi assignment boils down to the exact same
> question of syntax as MRV return assignment. I don't really distinguish
> this swap/swizzle from MRV. It all comes back to the return assignment
> syntax.

Actually, as has been mentioned, swizzling can be done very nicely 
inside the language.

>     3. Function overhead is solved by inlining, not by adding new
>     features. That improves all functions, not only swap.
>
> Requiring inlining to make it efficient is not enough. The proposed MRV
> ABI would solve this not only for inlines, but for all multi-assignment
> type constructs, including distant function calls.

If changing the ABI is on the table, I wonder whether we can improve it 
for all structures, not only for multiple return types. Overall I agree 
that defining a specialized ABI for leaving multiple values on the stack 
would be marginally more efficient, but (a) I don't know by how much, 
and (b) I don't know whether it's worth changing the language.


Andrei