
MessageID: <20190223203030.GX21289@port70.net> Date: Sat, 23 Feb 2019 21:30:31 +0100 From: Szabolcs Nagy <nsz@...t70.net> To: musl@...ts.openwall.com Cc: Shane Seelig <stseelig@...l.com> Subject: Re: x87 asin and acos * Shane Seelig <stseelig@...l.com> [20190223 09:21:08 0500]: > Currently 'asin' uses the algorithm: > arcsin(x) == arctan(x/(sqrt((1x)(1+x)))) > If the following algorithm were to be used instead, an 'fadd' could be > removed. > arcsin(x) == arctan(x/(sqrt(1x**2))) that change seems valid as far as the result is concerned. (the worst case rounding error of the sqrt argument should be around LDBL_EPS in both cases) but the fenv behaviour is not valid: for tiny x, x*x raises spurious underflow exception for large x, x*x raises spurious overflow exception (1x)(1+x) avoids these issues. note that there is not much performance difference between the two expressions: 1x and 1+x are independent computations so the latency is 1 add + 1 mul in both cases, and the entire function is likely dominated by fpsqrt followed by fpatan both of which have huge latency compared to add or mul.
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