[libre-riscv-dev] [Bug 208] implement CORDIC in a general way sufficient to do transcendentals - bugzilla-daemon

From: bugzilla-daemon@libre-soc.org
To: libre-riscv-dev@lists.libre-riscv.org
Subject: [libre-riscv-dev] [Bug 208] implement CORDIC in a general way sufficient to do transcendentals
Date: Wed, 15 Apr 2020 19:16:26 +0000
Message-ID: <bug-208-13-lqitKE5vs4@https.bugs.libre-soc.org/> (raw)
In-Reply-To: <bug-208-13@https.bugs.libre-soc.org/>

https://bugs.libre-soc.org/show_bug.cgi?id=208

--- Comment #8 from Luke Kenneth Casson Leighton <lkcl@lkcl.net> ---

ah.

on other thing.

i don't really trust the standard c-library atan to be accurate enough.
i think we should use python bigfloat, here.  it can produce arbitrary
precision floating-point.

that way we can ensure that for FP64 CORDIC (64-bit sin/cos), by using
128-bit (or greater) atan2 tables, we get the full accuracy needed to
get the right answers for IEEE754 sin/cos FP64.

if we try to use the "standard" math atan2 library i guarantee we will
fail to get sufficient accuracy at the limits of the range.

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