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manual: floor(log2(fabs(x))) has rounding errors

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Link: <https://inbox.sourceware.org/libc-alpha/20240305150131.GD3653@qaa.vinc17.org/T/#m3ceecda630012995339bcc5448fee451cf277a8b>
Reported-by: Vincent Lefevre <vincent@vinc17.net>
Suggested-by: Vincent Lefevre <vincent@vinc17.net>
Reviewed-by: DJ Delorie <dj@redhat.com>
Cc: Morten Welinder <mwelinder@gmail.com>
Cc: Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>
Cc: Paul Zimmermann <Paul.Zimmermann@inria.fr>
Cc: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
This commit is contained in:
Alejandro Colomar 2024-03-31 22:38:44 +02:00 committed by Adhemerval Zanella
parent b7d15bd1f0
commit 077613291b
1 changed files with 5 additions and 2 deletions
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7
manual/math.texi
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@ -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