(j3.2006) Liaison to IFIP WG 2.5

Van Snyder van.snyder
Tue Aug 21 20:30:15 EDT 2007

On Tue, 2007-08-21 at 17:32 -0600, keith bierman wrote:
> That's doing things naively. If you are clever you can do rafts of  
> computation with one rounding mode, and then change the mode and do  
> the next set. All the moreso with enough cores to be working in  
> parallel. Of course, then it is intractable if it's just modules,  
> hand code, and an optimizer stuck trying to paste it all back
> together.

This is pretty simple for add and subtract, but for multiply there are
nine cases, and for divide when the divisor doesn't include zero there
are six.  When the divisor includes zero you just produce NaN (or should
it be [-Inf,+Inf]?).  So for multiply you'd set the rounding mode down
and then execute a 9-way where/elsewhere block, then set it up and
execute another where/elsewhere block.  Maybe speculative execution has
gotten good enough that the branches don't kill the pipeline.
Van Snyder                    |  What fraction of Americans believe 
Van.Snyder at jpl.nasa.gov       |  Wrestling is real and NASA is fake?
Any alleged opinions are my own and have not been approved or
disapproved by JPL, CalTech, NASA, the President, or anybody else.

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