fp64lib is incompatible with multi-threaded code

[sprintersb]

There are modules like power series (p64_powserx.S) that are using static storage (STS/LDS) to temporarily store state.

This means that fp64lib cannot be used in multi-threaded programs, because one thread may override data of some other thread. There are more functions that are not re-entrant like in: fp64_asinx.S, fp64_sinx.S, fp64_tostring.S, fp64_strtod.S.

Modules fp64_stackB.S and fp64_stackC.S are using the same anti-pattern and are used by many other modules.

[fp64lib]

The code being not threadsafe was a deliberately taken design decision, see https://fp64lib.org/documentation/fp64lib-design-principles/, topic #7 and #9. The reason was to have the smallest possible footprint, so that the complete library can be used on an Arduino Uno leaving enough memory for a typical application like a calculator or a GPS tracker.