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, 7 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 2234021..84efa46 100644
--- a/NEWS
+++ b/NEWS
@@ -21,6 +21,13 @@ Major new features:
   by _GNU_SOURCE, or by compiling with the GCC options -std=c23,
   -std=gnu23, -std=c2x or -std=gnu2x.
 
+* The following ISO C23 function families (introduced in TS
+  18661-4:2015) are now supported in <math.h>.  Each family includes
+  functions for float, double, long double, _FloatN and _FloatNx, and a
+  type-generic macro in <tgmath.h>.
+
+  - Logarithmic functions: log2p1.
+
 * A new tunable, glibc.rtld.enable_secure, used to run a program
   as if it were a setuid process. This is currently a testing tool to allow
   more extensive verification tests for AT_SECURE programs and not meant to