Multiple return values...

[Robert Jacques]

On Sun, 11 Mar 2012 17:48:53 -0500, Timon Gehr <timon.gehr at gmx.ch> wrote:

> On 03/11/2012 10:57 PM, Robert Jacques wrote:
>> On Sun, 11 Mar 2012 15:38:21 -0500, Timon Gehr <timon.gehr at gmx.ch> wrote:
>>
>>> On 03/11/2012 09:30 PM, Robert Jacques wrote:
>>>> Manu, why are you assuming that the struct is returned in any sort of
>>>> fixed memory layout?
>>>
>>> Because that is what the ABI says.
>>>
>>> * 1, 2 and 4 byte structs are returned in EAX.
>>> * 8 byte structs are returned in EDX,EAX, where EDX gets the most
>>> significant half.
>>> * For other struct sizes, the return value is stored through a hidden
>>> pointer passed as an argument to the function.
>>
>> You made my point. Manu is arguing that structs small enough to be worth
>> passing in registers must maintain some sort of fixed memory layout;
>
> They do.
>
>> i.e. they can't broken up across a desperate set of registers.
>>
>
> That was not his point. If you have:
>
> struct S{
>      float a;
>      float b;
> }
>
> S foo();
>
> Then EDX and EAX contain float data upon function return. That is not
> something you want.

That's an argument for using the right register for the job. And we can / will be doing this on x86-64, as other compilers have already done. Manu was arguing that MRV were somehow special and had mystical optimization potential. That's simply not true.