Floating Point + Threads?

[Sean Kelly]

On Apr 16, 2011, at 1:02 PM, Robert Jacques wrote:

> On Sat, 16 Apr 2011 15:32:12 -0400, Walter Bright <newshound2 at digitalmars.com> wrote:
>> 
>> 
>> The dmd startup code (actually the C startup code) does an fninit. I never thought about new thread starts. So, yeah, druntime should do an fninit on thread creation.
> 
> The documentation I've found on fninit seems to indicate it defaults to 64-bit precision, which means that by default we aren't seeing the benefit of D's reals. I'd much prefer 80-bit precision by default.

There is no option to set "80-bit precision" via the FPU control word.  Section 8.1.5.2 of the Intel 64 SDM says the following:

"The precision-control (PC) field (bits 8 and 9 of the x87 FPU control word) determines the precision (64, 53, or 24 bits) of floating-point calculations made by the x87 FPU (see Table 8-2). The default precision is double extended precision, which uses the full 64-bit significand available with the double extended-precision floating-point format of the x87 FPU data registers. This setting is best suited for most applications, because it allows applications to take full advantage of the maximum precision available with the x87 FPU data registers."

So it sounds like finit/fninit does what we want.