Array init

[Max Samukha]

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.

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