Re: [f-cpu] IEEE FP exceptions

[devik <devik@cdi.cz>]

> There is, however, a big problem with FP status:  Since instructions
> complete out-of-order, it's undefined which instruction the status
> comes from.  You can only query the status of a series of FP instructions
> collectively, unless you serialize the instruction stream after every
> FP operation (which is going to be slooooooooow).

from what I've understood from the paper, the use is to:

double compute_krakovian(matrix *x)
{
	clear_fpuflag()
	... some heavy numerical math ...
        if (test_flag_for_error)
		return compute_krakovian_precise(x);
	return y;
}

so that to do block of computation, serialize and check flag. The
flag could be also "hidden" part of our 96bit FPU word and thus
it could be copied/preserved during dataflow - one then could
also check for errors or imprecisions in single result. That
would allow very nice handling of precision, no extra context,
no problem with OOC and compiler could simulate IEEE behaviour
of global status flag by ORing status words obtained from
all dataflow registers in cross-section at the place of
"check()" call (yes gcc can handle it easily).

Yet another "in-pipeline" exception would be over ;-)

devik

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