soft-fp: support after-rounding tininess detection

[Richard Henderson]

On 02/11/2014 02:28 PM, Joseph S. Myers wrote:
> On Tue, 11 Feb 2014, Richard Henderson wrote:
> 
>> Am I missing something, why the SLL is needed for this case?
>> And if it isn't, then the temporary ought not be needed.
> 
> After-rounding means after rounding to normal precision, as if the 
> exponent range were unbounded - not after rounding to a representable 
> floating point value taking account of subnormals having lesser precision.  
> For semiraw values N bits below the normal range, just rounding without a 
> shift would round to N bits less than the normal precision.  (Shifting 1 
> bit instead of N is OK because if N > 1 then the result after rounding 
> will always still be subnormal, whatever precision it's rounded to, and 
> not have the overflow bit set if the shift was only 1 bit.)  See IEEE 
> 754-2008 subclause 7.5.

Unfortunately I don't have access to the standard to look at that.

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.


r~