soft-fp: support after-rounding tininess detection

[Richard Henderson]

On 02/12/2014 07:27 AM, Joseph S. Myers wrote:
> On Wed, 12 Feb 2014, Richard Henderson wrote:
> 
>> But it still makes no sense to me.  E.g. rounding up,
>>
>>    X_f = 111...1100
>>
>> with the shift sees no carry into OVERFLOW, but then the subsequent
>> "real" rounding does see carry into (OVERFLOW>>1).  So now we have
>> underflow signaled for a result that's not subnormal.
> 
> Yes, that's correct: for both before-rounding and after-rounding tininess 
> detection you get underflow signaled for some cases where the final result 
> is not subnormal or zero.
...
> In the x86 manual I have to hand (Intel 64 and 
> IA-32 Architectures Software Developer's Manual Combined Volumes: 1, 2A, 
> 2B, 2C, 3A, 3B and 3C Order Number: 325462-047US June 2013), this is 
> described in "4.9.1.5 Numeric Underflow Exception (#U)" (presumably also 
> at similar locations in other versions of the manuals).
> 

Ok then.


r~