Floating Point + Threads?

[Sean Kelly]

On Apr 20, 2011, at 10:46 AM, JimBob wrote:
> 
> "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

Oops, you're right.  So to summarize: fninit does what we want because it sets 64-bit precision, which is effectively 80-bit mode.  Is this correct?