Strange counter-performance in an alternative `decimalLength9` function

[Johan]

On Wednesday, 26 February 2020 at 00:50:35 UTC, Basile B. wrote:
> So after reading the translation of RYU I was interested too 
> see if the decimalLength() function can be written to be 
> faster, as it cascades up to 8 CMP.
>
> ...
>
> Then bad surprise. Even with ldmd (so ldc2 basically) feeded 
> with the args from the script line. Maybe the fdecimalLength9 
> version is slightly faster. Only *slightly*. Good news, I've 
> lost my time. So I try an alternative version that uses a table 
> of delegates instead of a switch (ffdecimalLength9) and 
> surprise, "tada", it is like **100x** slower then the two 
> others.
>
> How is that possible ?

Hi Basile,
   I recently saw this presentation: 
https://www.youtube.com/watch?v=Czr5dBfs72U
It has some ideas that may help you make sure your measurements 
are good and may give you ideas to find the performance 
bottleneck or where to optimize.
llvm-mca is featured on godbolt.org: 
https://mca.godbolt.org/z/YWp3yv

cheers,
   Johan