Implement C23 log10p1

C23 adds various <math.h> function families originally defined in TS
18661-4.  Add the log10p1 functions (log10(1+x): like log1p, but for
base-10 logarithms).

This is directly analogous to the log2p1 implementation (except that
whereas log2p1 has a smaller underflow range than log1p, log10p1 has a
larger underflow range).  The test inputs are copied from those for
log1p and log2p1, plus a few more inputs in that wider underflow
range.

Tested for x86_64 and x86, and with build-many-glibcs.py.

1 files changed, 14 insertions, 0 deletions

diff --git a/manual/math.texi b/manual/math.texi
index 56f2295..18b7173 100644
--- a/manual/math.texi
+++ b/manual/math.texi
@@ -780,6 +780,20 @@ near zero.
 The @code{log2p1} functions are from TS 18661-4:2015.
 @end deftypefun
 
+@deftypefun double log10p1 (double @var{x})
+@deftypefunx float log10p1f (float @var{x})
+@deftypefunx {long double} log10p1l (long double @var{x})
+@deftypefunx _FloatN log10p1fN (_Float@var{N} @var{x})
+@deftypefunx _FloatNx log10p1fNx (_Float@var{N}x @var{x})
+@standards{TS 18661-4:2015, math.h}
+@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
+These functions return a value equivalent to @w{@code{log10 (1 + @var{x})}}.
+They are computed in a way that is accurate even if @var{x} is
+near zero.
+
+The @code{log10p1} functions are from TS 18661-4:2015.
+@end deftypefun
+
 @cindex complex exponentiation functions
 @cindex complex logarithm functions