Re: null Vs [] return arrays

[Steven Schveighoffer]

On Sun, 27 Mar 2011 09:37:47 -0400, bearophile <bearophileh...@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