Floating Point + Threads?

[JimBob]

"Sean Kelly" <sean at invisibleduck.org> wrote in message 
news:mailman.3597.1303316625.4748.digitalmars-d at puremagic.com...
On Apr 20, 2011, at 5:06 AM, Don wrote:

> Sean Kelly wrote:
>> 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.
>
> ??? Yes there is.
>
> enum PrecisionControl : short {
>    PRECISION80 = 0x300,
>    PRECISION64 = 0x200,
>    PRECISION32 = 0x000
> };
>
>So has Intel deprecated 80-bit FPU support?  Why do the docs for this say 
>that 64-bit
> is the highest precision?  And more importantly, does this mean that we 
> should be setting
> the PC field explicitly instead of relying on fninit?  The docs say that 
> fninit initializes to
> 64-bit precision.  Or is that inaccurate as well?=

You misread the docs, it's talking about precision which is just the size of 
the mantisa, not the actual full size of the floating point data. IE...

80 float = 64 bit precision
64 float = 53 bit precision
32 float = 24 bit precision