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Kahan summation

A way of adding many floats that keeps a running record of the rounding error and feeds it back in, so the total stays accurate.

Kahan (compensated) summation carries a correction term c = (t - s) - y alongside the sum, bounding the total error independently of the number of terms; reassociation under -ffast-math can optimize the correction away.

See also

-ffast-math reassociation
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