[dmd-internals] non-PODs again

[Maxim Fomin]

2013/3/8 Johannes Pfau <johannespfau at googlemail.com>

>  Am 08.03.2013 05:52, schrieb Maxim Fomin:
>
>
>
> 2013/3/8 Walter Bright <walter at digitalmars.com>
>
>>
>> On 3/7/2013 12:19 PM, Johannes Pfau wrote:
>>
>>> Am 07.03.2013 20:45, schrieb Walter Bright:
>>>
>>>>
>>>> On 3/7/2013 9:36 AM, Johannes Pfau wrote:
>>>>
>>>>> I'm sorry I have to pester you with this again, but I still have some
>>>>> questions regarding POD types and I'd like to fix this in GDC.
>>>>>
>>>>> So from last discussion:
>>>>> >> Wouldn't it be legal to still pass non-PODs in registers when
>>>>> calling functions and only copying them back to
>>>>> >> the stack if the address is needed? As we pass structs by value
>>>>> anyway, how could this be problematic?
>>>>> >
>>>>> > No, not allowed. Consider why there are copy constructors, and what
>>>>> they do.
>>>>>
>>>>> I compiled some test programs with dmd and dmd _does_ pass non-POD
>>>>> values in registers as I suggested above.
>>>>> See this example:
>>>>> https://gist.github.com/jpf91/5064703 (D)
>>>>> https://gist.github.com/jpf91/5064764 (ASM)
>>>>>
>>>>
>>>> That's because objects with constructors are now regarded as POD.
>>>>
>>>
>>> This example uses a postblit to make sure the type is not a POD. It's
>>> obvious in the ASM that the copy ctor is called,
>>>
>>
>>  Oops, I missed that. It's a bug in dmd.
>>
>
> Isn't there another bug with struct parameter which is copied twice - on
> caller and callee side?
>
> function  D main
> Date d = _D1e4Date6__initZ;
> setDate((Date __cpcttmp7 = __cpcttmp7.__cpctor(d); , __cpcttmp7))
>
> function  e.setDate
> x.opAssign((Date __cpcttmp6 = __cpcttmp6.__cpctor(d); , __cpcttmp6))
>
>  setDate assigns d to the global variable x so the second call to the
> cpctor seems to be caused by that and valid.
>
> --
> Johannes Pfau
>
>
> DMD still generates double copy if variable is changed to local

void setDate(Date d)
{
Date x;
x = d;
}

function  e.setDate
Date x = _D1e4Date6__initZ;
x.opAssign((Date __cpcttmp6 = __cpcttmp6.__cpctor(d); , __cpcttmp6))

function  D main
Date d = _D1e4Date6__initZ;
setDate((Date __cpcttmp7 = __cpcttmp7.__cpctor(d); , __cpcttmp7))

Anyway whether variable is tls or not is irrelevant. Compiler should not
make a copy when assigning.
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