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digitalmars.D.ldc - An award is due to LDC for sunning code generation. Null points for

reply Cecil Ward <cecil cecilward.com> writes:
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?
Jul 31 2020
next sibling parent Cecil Ward <cecil cecilward.com> writes:
The two mul instructions will presumably run in parallel due to 
ILP so actually doesn’t mean much just a bit odd.
Jul 31 2020
prev sibling parent Johan <j j.nl> writes:
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
Aug 01 2020