Multiple return values...

[Robert Jacques]

On Sun, 11 Mar 2012 12:39:56 -0500, Manu <turkeyman at gmail.com> wrote:
> On 11 March 2012 18:50, Robert Jacques <sandford at jhu.edu> wrote:
>> On Sun, 11 Mar 2012 05:57:05 -0500, Manu <turkeyman at gmail.com> wrote:
>>  On 11 March 2012 04:35, Sean Cavanaugh <WorksOnMyMachine at gmail.com>
>>> wrote:
>>>  On 3/10/2012 8:08 PM, Mantis wrote:

[snip]

>> And Walter has talked about using the XMM registers to return floating
>> point data for exactly this reason. But those optimization apply to all
>> structs and all data types. There's nothing special about MRV. It has to
>> return a set of data in a structured manner; this is identical to the case
>> of returning a struct.
>>
>
> Well you can't build these sorts of tightly packed structs in XMM
> registers... and even for the GPR's, it's very inefficient to do so. I
> haven't done tests, but I suspect this feature is probably a
> de-optimisation in all cases other than an int-pair struct (ranges,
> delegates? I suspect that's why this was invented). Structure
> packing/unpacking code will likely be slower than a store/load.
>
> We just need proper multiple return values, then this can go away. And the
> earlier any ABI breaking changes are implemented, the better, while there
> are still few(/none?) closed source, binary distributed D libraries.

Manu, why are you assuming that the struct is returned in any sort of fixed memory layout? You can _not_ take the memory address, etc of a register so maintaining the layout of a struct is a) impossible and b) not necessary, if you're returning it via registers. The compiler is free to use XMM registers for floats/doubles and GPRs for pointers/ints. And there's no need for the caller to place those struct register values on the stack if they're never used. Furthermore, even on the stack, the struct's memory layout is only ever valid immediately before (and forever after due to pointer escapes) a non-inlined function call or address taking. At a low level, MRV is a structured set of value, ergo it's a struct.