CTFE and Static If Question

[ag0aep6g]

On 07.05.20 17:00, jmh530 wrote:
> Does foo!y0(rt) generate the same code as foo(rt, y0)?
> 
> How is the code generated by foo(rt, x0) different from foo(rt,y0)?
> 
> auto foo(bool rtct)(int rt) {
>      static if (rtct)
>          return rt + 1;
>      else
>          return rt;
> }
> 
> auto foo(int rt, bool rtct) {
>      if (rtct == true)
>          return rt + 1;
>      else
>          return rt;
> }
> 
> void main() {
>      int rt = 3;
>      bool x0 = true;
>      bool x1 = false;
>      assert(foo(rt, x0) == 4);
>      assert(foo(rt, x1) == 3);
> 
>      enum y0 = true;
>      enum y1 = false;
>      assert(foo!y0(rt) == 4);
>      assert(foo!y1(rt) == 3);
>      assert(foo(rt, y0) == 4);
>      assert(foo(rt, y1) == 3);
> }

The `static if` is guaranteed to be evaluated during compilation. That 
means, `foo!y0` effectively becomes this:

     auto foo(int rt) { return rt + 1; }

There is no such guarantee for `foo(rt, y0)`. It doesn't matter that y0 
is an enum.

But a half-decent optimizer will have no problem replacing all your 
calls with their results. Compared with LDC and GDC, DMD has a poor 
optimizer, but even DMD turns this:

     int main() {
         int rt = 3;
         bool x0 = true;
         bool x1 = false;
         enum y0 = true;
         enum y1 = false;
         return
             foo(rt, x0) +
             foo(rt, x1) +
             foo!y0(rt) +
             foo!y1(rt) +
             foo(rt, y0) +
             foo(rt, y1);
     }

into this:

     int main() { return 21; }