null Vs [] return arrays

[Steven Schveighoffer]

On Sun, 27 Mar 2011 09:37:47 -0400, bearophile <bearophileHUGS at lycos.com>  
wrote:

> I have compiled this little D2 program:
>
>
> int[] foo() {
>     return [];
> }
> int[] bar() {
>     return null;
> }
> void main() {}
>
>
>
> Using DMD 2.052,  dmd -O -release -inline test2.d
>
> This is the asm of the two functions:
>
> _D5test23fooFZAi    comdat
> L0:     push    EAX
>         mov EAX,offset FLAT:_D11TypeInfo_Ai6__initZ
>         push    0
>         push    EAX
>         call    near ptr __d_arrayliteralT
>         mov EDX,EAX
>         add ESP,8
>         pop ECX
>         xor EAX,EAX
>         ret
>
> _D5test23barFZAi    comdat
>         xor EAX,EAX
>         xor EDX,EDX
>         ret
>
> Is this expected and desired? Isn't it better to compile the foo() as  
> bar()?

Probably.  The runtime that allocates an array looks like this (irrelevant  
parts collapsed):


extern (C) void* _d_arrayliteralT(TypeInfo ti, size_t length, ...)
{
     auto sizeelem = ti.next.tsize();            // array element size
     void* result;

     ...
     if (length == 0 || sizeelem == 0)
         result = null;
     else
     {
        ...
     }
     return result;
}

So essentially, you are getting the same thing, but using [] is slower.

-Steve