Array init

[Don]

Max Samukha wrote:
> Steven Schveighoffer wrote:
> 
>>> That's an interesting question. The compiler does set void-initialized
>>> member variables to zeros:
>>>
>>> struct S { double x = void, y = void; }
>>> void main() {
>>>     S s; // x and y are initialized to 0
>>> }
>>>
>>> I think this is a bug. Such members should be left uninitialized or
>>> initialized to the default initializer for the type (NaN in this case) or
>>> void initialization should be disallowed for member variables.
>> An initialized double is NaN, so I don't think the compiler is
>> initializing those variables.
>>
>> Your test is not valid, you need to fill the memory with something to see
>> if the code doesn't change it.
> 
> I should have posted the asm listing of the test case. The compiler does 
> generate initialization code that sets the memory allocated for the 
> structure to zeros, which defeats the purpose of void initializers.
> 
> 
> ...
>         dd      offset FLAT:_D4Test1S6__initZ at SYM32                       
>         db      000h,000h,000h,000h,000h,000h,000h,000h                   
>         db      000h,000h,000h,000h,000h,000h,000h,000h
> ...
> _Dmain:                                                                   
>                 push    EBP                                               
>                 mov     EBP,ESP                                           
>                 sub     ESP,010h                                          
>                 push    ESI                                               
>                 push    EDI                                               
>                 mov     ESI,offset FLAT:_D4Test1S6__initZ at SYM32           
>                 lea     EDI,-010h[EBP]                                    
>                 movsd                                                     
>                 movsd                                                     
>                 movsd                                                     
>                 movsd                                                     
>                 xor     EAX,EAX
>                 pop     EDI
>                 pop     ESI
>                 leave
>                 ret
> .text._Dmain    ends
> 
> I guess the rationale behind this behavior is that filling void-initialized 
> fields with zeros may be as fast or faster than skipping them while 
> selectively initializing other fields.

Yes, it would definitely be faster if everything else is initialized to 
zero.

> IMO, the initialization code shouldn't be generated for aggregates that have 
> all members void-initialized, and consequently for arrays of such 
> aggregates.

Agreed. (But it's just a performance issue, it's not incorrect to 
initialize them to zero).