Floating Point + Threads?

[Timon Gehr]

> Could be something somewhere is getting truncated from real to double, which
> would mean 12 fewer bits of mantisa. Maybe the FPU is set to lower precision
> in one of the threads?

Yes indeed, this is a _Windows_ "bug".
I have experienced this in Windows before, the main thread's FPU state register is
initialized to lower FPU-Precision (64bits) by default by the OS, presumably to
make FP calculations faster. However, when you start a new thread, the FPU will
use the whole 80 bits for computation because, curiously, the FPU is not
reconfigured for those.
Suggested fix: Add

asm{fninit};

to the beginning of your main function, and the difference between the two will be
gone.

This would be a compatibility issue DMD/windows which disables the "real" data
type. You might want to file a bug report for druntime if my suggested fix works.
(This would imply that the real type was basically identical to the double type in
Windows all along!)