[nly]

> However, I've heard that in HFT, they don't do that because the overhead itself is too costly.

I've worked at several HFTs and 2 had independently settled on 64bit signed fixed point with a implied 10^9 scaling factor between internal systems.

However in-process one just used 'double' for all FX conversions, scaling etc. With 15 digits of precision and careful rounding choices it's fine.

Dealing with the outside world you'd obviously just convert as cheaply as possible - and there are some crazy fast algorithms doing fast binary float -> correctly rounded decimal conversions these days