Implement C23 log2p1

C23 adds various <math.h> function families originally defined in TS
18661-4.  Add the log2p1 functions (log2(1+x): like log1p, but for
base-2 logarithms).

This illustrates the intended structure of implementations of all
these function families: define them initially with a type-generic
template implementation.  If someone wishes to add type-specific
implementations, it is likely such implementations can be both faster
and more accurate than the type-generic one and can then override it
for types for which they are implemented (adding benchmarks would be
desirable in such cases to demonstrate that a new implementation is
indeed faster).

The test inputs are copied from those for log1p.  Note that these
changes make gen-auto-libm-tests depend on MPFR 4.2 (or later).

The bulk of the changes are fairly generic for any such new function.
(sysdeps/powerpc/nofpu/Makefile only needs changing for those
type-generic templates that use fabs.)

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 dfb685a..c99501c 100644
--- a/manual/math.texi
+++ b/manual/math.texi
@@ -754,6 +754,20 @@ They are computed in a way that is accurate even if @var{x} is
 near zero.
 @end deftypefun
 
+@deftypefun double log2p1 (double @var{x})
+@deftypefunx float log2p1f (float @var{x})
+@deftypefunx {long double} log2p1l (long double @var{x})
+@deftypefunx _FloatN log2p1fN (_Float@var{N} @var{x})
+@deftypefunx _FloatNx log2p1fNx (_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{log2 (1 + @var{x})}}.
+They are computed in a way that is accurate even if @var{x} is
+near zero.
+
+The @code{log2p1} functions are from TS 18661-4:2015.
+@end deftypefun
+
 @cindex complex exponentiation functions
 @cindex complex logarithm functions