Re: [cxx-conversion] Make double_int a class with methods and operators. (issue6443093)

[Gabriel Dos Reis <gdr at integrable-solutions dot net>]

On Wed, Aug 8, 2012 at 3:29 AM, Miles Bader <miles@gnu.org> wrote:
> Richard Guenther <richard.guenther@gmail.com> writes:
>>>>> Constructors are allowed, but PODs are often passed more efficiently.
>>>>> That property seemed particularly important for double_int.
>>>>
>>>> Show us the difference in timing.  Show us the generated code.  I
>>>> can't imagine that it could ever matter.
>>>
>>> I'm also curious about that statement...  PODs don't really seem to
>>> offer much advantage with modern compilers, except in a few very
>>> specific cases (of which this doesn't seem to be one), e.g. in unions.
>>
>> They make a difference for the by-value passing ABI.  double-ints can
>> be passed in two registers on most platforms.
>
> Sure, but that doesn't seem to depend on PODness -- non-PODs can be
> passed in two registers as well, AFAICS...
>
> E.g., in the following:
>
>    typedef long valtype;
>
>    struct X { valtype x, y; };
>    struct Y { Y (valtype a, valtype b) : x (a), y (b) { } valtype x, y; };
>
>    extern void fx (X x);
>    void test_x () {X x = { 1, 2 }; fx (x); }
>
>    extern void fy (Y y);
>    void test_y () {Y y (1, 2); fy (y); }
>
> test_x and test_y use exactly the same calling sequence (and contain
> exactly the same assembly code)...  [on x86-64]

It is not PODness in the standard sense that matters.
It is podness from the ABI perspecitve: Y does not
have a user-defined copy-constructor nor a desctructor;
it is not a polymorphic type, so it is OK to pass in registers
just like X.

-- Gaby