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This conversion will not lose precision!

With a 64-bit mantissa, ULP(x) is ≤ 1 for x in [-2e64, 2e64]. If you have the time you can count up to 2^64 by repeatedly incrementing a float80(0.0) by 1.

Addition, subtraction and multiplication on float80s are perfectly stable provided the results stays within [-2e64, 2e64].

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There is no guarantee on stability of those ops. Intel and IEEE guarantees 1 ULP. Even if you use the 80 bit type. The error is not guaranteed to not be additive over multiple operations either. And for multiplication and division? Let's not even go there.

This will mess up all kinds of sharp comparisons on the result of casted operations.

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