opIndexMember

[claptrap]

On Sunday, 27 December 2020 at 15:02:16 UTC, Dennis wrote:
> On Sunday, 27 December 2020 at 14:46:43 UTC, claptrap wrote:
>> So you can do stuff like this....
>
> What's wrong with this?
> ```
> struct VecArray
> {
>      float[] _x;
>      float[] _y;
>      float[] _z;
>
>     Vector opIndex(size_t idx) {
>         return Vector(_x[idx], _y[idx], _z[idx]);
>     }
> }
> ```
>
> Performance in debug builds?

It wont work with ref returns, and performance in release builds.

I had some instances where the return type was being fully 
constructed even though only one member was accessed, but I dont 
remember exactly what code caused that.

godbolt with LDC -03...

===============================================

import std;

struct Vector { float x,y,z; }

struct VecArray
{
      float[] _x;
      float[] _y;
      float[] _z;

     Vector opIndex(size_t idx) {
         return Vector(_x[idx], _y[idx], _z[idx]);
     }
}

float doSomething1(ref VecArray array)
{
     return array[0].x * 2.0;
}

float doSomething2(ref VecArray array)
{
     return array._x[0] * 2.0;
}

==============================================

doSomthing1:

float example.doSomething1(ref example.VecArray):
         push    rax
         cmp     qword ptr [rdi], 0
         je      .LBB6_4
         cmp     qword ptr [rdi + 16], 0
         je      .LBB6_4
         cmp     qword ptr [rdi + 32], 0
         je      .LBB6_4
         mov     rax, qword ptr [rdi + 8]
         movss   xmm0, dword ptr [rax]
         addss   xmm0, xmm0
         pop     rax
         ret
.LBB6_4:
         lea     rsi, [rip + .L.str.1]
         mov     edi, 11
         mov     edx, 12
         call    _d_arraybounds at PLT

===============================================

doSomthing2:

float example.doSomething2(ref example.VecArray):
         push    rax
         cmp     qword ptr [rdi], 0
         je      .LBB7_2
         mov     rax, qword ptr [rdi + 8]
         movss   xmm0, dword ptr [rax]
         addss   xmm0, xmm0
         pop     rax
         ret
.LBB7_2:
         lea     rsi, [rip + .L.str.1]
         mov     edi, 11
         mov     edx, 23
         call    _d_arraybounds at PLT