From 077613291b271b64fa60b8a22c3b39c9db697b65 Mon Sep 17 00:00:00 2001 From: Alejandro Colomar Date: Sun, 31 Mar 2024 22:38:44 +0200 Subject: manual: floor(log2(fabs(x))) has rounding errors Link: Reported-by: Vincent Lefevre Suggested-by: Vincent Lefevre Reviewed-by: DJ Delorie Cc: Morten Welinder Cc: Adhemerval Zanella Netto Cc: Paul Zimmermann Cc: Andreas Schwab Signed-off-by: Alejandro Colomar --- manual/math.texi | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/manual/math.texi b/manual/math.texi index c54eaebb65..79bf3a1401 100644 --- a/manual/math.texi +++ b/manual/math.texi @@ -560,8 +560,11 @@ These functions return the base-2 logarithm of @var{x}. @standardsx{logbfNx, TS 18661-3:2015, math.h} @safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}} These functions extract the exponent of @var{x} and return it as a -floating-point value. If @code{FLT_RADIX} is two, @code{logb} is equal -to @code{floor (log2 (fabs (x)))}, except it's probably faster. +floating-point value. +If @code{FLT_RADIX} is two, +@code{logb (x)} is similar to @code{floor (log2 (fabs (x)))}, +except that the latter may give an incorrect integer +due to intermediate rounding. If @var{x} is de-normalized, @code{logb} returns the exponent @var{x} would have if it were normalized. If @var{x} is infinity (positive or -- cgit 1.4.1