An award is due to LDC for sunning code generation. Null points for GCC in comparison.

[Johan]

On Saturday, 1 August 2020 at 02:48:20 UTC, Cecil Ward wrote:
> An award is due to LDC.
>
> LDC deserves some kind of gold medal for optimising this to 
> just a couple of mul instructions and the trimmings and no loop 
> at all! Incredible job from LDC. with -O3
>
> uint foo( in uint x ) { if (x ==0) return 0 else return x + 
> foo( x - 1 );}
>
>  Since the answer is n(n+1)/2 I’m not sure why there are two 
> mul instructions.
>
> Golden onion for GDC, does tail-end recursion but then with -O3 
> -march=native -frelease it generations an evil-looking brew of 
> SIMD and a loop with some unrolling. GDC just doesn’t grok the 
> algorithmic intent like LDC does. I’m assuming that induction 
> variable analysis is the key that gives LDC the superb win?

Cheers, but it's LLVM that is doing the optimization, not LDC.
Maybe this gives some insight: 
https://kristerw.blogspot.com/2019/04/how-llvm-optimizes-geometric-sums.html

- Johan