A Friendly Challenge for D

[Jabari Zakiya]

On Tuesday, 16 October 2018 at 16:57:12 UTC, welkam wrote:
> So I run profiler and 97% of time is spent in void twinsSieve 
> function and hotspots are seg[k] = seg[k] |  1; lines. Since 
> seg[k] can only be 1 or 0 I removed that or operation. And the 
> results are. Queue the drum-roll... 5% slower.
>
> I thought that all of my studying was getting somewhere. That I 
> beginning to understand things but no. Performing OR operation 
> and then storing data is faster than just storing data. 
> [sarcasm] Of course it makes sense [/sarcasm]
>
> I looked at assembler and nothing changed except
>
> orb    $0x1,(%rbx,%rdi,1)
>
> is changed to
>
> movb   $0x1,(%rbx,%rdi,1)
>
> I`m completely lost.

This is the exact same behavior I found with the Nim compiler too.

seg[k] = 1  is slower than seg[k] = seg[k] or 1, which is why 
it's coded like that.

Thanks for finding the exact instruction difference.

How many other unknown gotchas are in the compilers? :-(