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

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>

diff --git a/manual/math.texi b/manual/math.texi
index c54eaebb65c964bb64aa873b089b7114be057259..79bf3a140155344515b67dbedbf96bcdd5d3f4ad 100644 (file)
--- 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