Multiple return values...

[Iain Buclaw]

On 16 March 2012 16:37, Andrei Alexandrescu
<SeeWebsiteForEmail at erdani.org> wrote:
> 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


If you were to forget all about MRV for a brief moment, the change
request being proposed here is to return *all* structures (including
delegates, complex types and vectors) in registers if at all possible
even if the underlying ABI default is to return it in memory.


-- 
Iain Buclaw

*(p < e ? p++ : p) = (c & 0x0f) + '0';