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Re: Question on valid transformations for cbrt

From: Segher Boessenkool <segher at kernel dot crashing dot org>
To: Roger Sayle <roger at eyesopen dot com>
Cc: gcc at gcc dot gnu dot org, "Kaveh R. Ghazi" <ghazi at caip dot rutgers dot edu>
Date: Wed, 10 Mar 2004 16:31:37 +0100
Subject: Re: Question on valid transformations for cbrt
References: <Pine.LNX.4.44.0403070640560.29012-100000@www.eyesopen.com>

The problem with cbrt, unlike sqrt, is that its is more expressive than
pow.  i.e. cbrt(-29.7) = -3.1 but pow(-29.7,1.0/3.0) => EDOM.  Hence
we can't normalize cbrt(x) as pow(x,1.0/3.0),

You can "normalize" cbrt(x) as sign(x) * pow(abs(x), 1.0/3) -- maybe that helps some further optimizations?

but we can safely go the
other way pow(x,1.0/3.0) => cbrt(x).

Segher

Follow-Ups:
- Re: Question on valid transformations for cbrt
  - From: Robert Dewar

References:
- Re: Question on valid transformations for cbrt
  - From: Roger Sayle

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