By ref and by pointer kills performance.

[Walter Bright]

On 2/12/2024 7:31 PM, Richard (Rikki) Andrew Cattermole wrote:
> dmd having bad codegen here isn't a surprise, that is to be expected.

I'm used to people saying that DMD doesn't do data flow analysis. It does. In 
fact, it is based on my OptimumC, which was the first C compiler on DOS to do 
DFA back in the 1980s.

This isn't a case of buggy DFA. It's a case of doing DFA correctly.

The issue is pointer aliasing. A pointer can point to anything, including const 
data. Therefore, storing through a pointer can alter any value that is reachable 
via a pointer. Therefore, storing through a pointer invalidates any cached value 
already read.

This is what you're seeing.

C99 tried to address this with __restrict, but few people use it or understand 
it. D didn't bother with it because people will inevitably misuse __restrict and 
get their data mysteriously corrupted.