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Re: Irix6 long doubles implemented wrong? (27_io/ostream_inserter_arith)
On Jan 28, 2003, Alexandre Oliva <aoliva@redhat.com> wrote:
> To invert: ??? this is the missing bit :-(
> To divide, invert the divisor and multiply.
Just figured out how to do division, using addition, subtraction and
multiplication:
- obtain an initial estimate of the result by dividing the high
parts
- compute a residue by subtracting from the dividend the result of
multiplying the result estimate and the divisor
- divide the high part of the residue by the high part of the divisor,
and add it to the result
- repeat the last two steps until the result stops changing
I think it doesn't take more than 2 iterations for the result to
stabilize. Here's the pseudo code:
long double divide(long double in1, long double in2) {
long double result, prev;
result.high = in1.high / in2.high;
if (isinf (result.high) || isnan (result.high)) {
result.low = result.high;
return result;
}
result.low = 0.0;
do {
long double partial;
prev = result;
partial.high = (in1 - result * in2).high / in2.high;
partial.low = 0.0;
result += partial;
} while (result != prev);
return result;
}
Other operations, such as extending from double, compares, etc, are
pretty obvious, so I'll refrain from posting them here. Conversion
to/from int can be done with extension to/truncation from double, and
conversion to/from long long takes a bit more work to use all 64 bits,
but that's it. Wheee!
Any candidates to implement say gcc/config/tp-ibmext-bit.c using these
ideas, even though we have no immediate use for them?
--
Alexandre Oliva Enjoy Guarana', see http://www.ic.unicamp.br/~oliva/
Red Hat GCC Developer aoliva@{redhat.com, gcc.gnu.org}
CS PhD student at IC-Unicamp oliva@{lsd.ic.unicamp.br, gnu.org}
Free Software Evangelist Professional serial bug killer