On Wed, Aug 8, 2012 at 3:29 AM, Miles Bader <mi...@gnu.org> wrote:
> Richard Guenther <richard.guent...@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
