Floating Point + Threads?

[dsimcha]

On 4/16/2011 2:15 PM, Timon Gehr wrote:
>
>> 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!)

Close:  If I add this instruction to the function for the new thread, 
the difference goes away.  The relevant statement is:

     auto t = new Thread( {
         asm { fninit; }
         res2 = sumRange(terms);
     } );

At any rate, this is a **huge** WTF that should probably be fixed in 
druntime.  Once I understand it a little better, I'll file a bug report.