libc: Add C23 stdbit.h with optional Kconfig and generic implementation

Documentation/components/libs/libc/index.rst

@@ -51,6 +51,7 @@ functions.  So we have::
   sched     - sched.h
   search    - search.h
   semaphore - semaphore.h
+  stdbit    - stdbit.h (optional C23)
   stdio     - stdio.h
   stdlib    - stdlib.h
   string    - string.h (and legacy strings.h and non-standard nuttx/b2c.h)
@@ -155,5 +156,6 @@ Implementation Details
    :caption: Contents:
 
    search.rst
+   stdbit.rst
    stream.rst
    zoneinfo.rst

Documentation/components/libs/libc/stdbit.rst

@@ -0,0 +1,30 @@
+========
+stdbit.h
+========
+
+The optional C23 ``stdbit.h`` header provides bit manipulation macros
+(endianness, leading/trailing zeros and ones, count, single-bit test,
+bit width, bit floor, bit ceil). NuttX provides this header only when
+explicitly enabled via Kconfig.
+
+Configuration
+=============
+
+- **CONFIG_ARCH_HAVE_STDBIT_H** (bool, selected by arch)
+  Architecture indicates it provides ``arch/<arch>/include/stdbit.h``.
+
+- **CONFIG_ARCH_STDBIT_H** (bool "stdbit.h", depends on ARCH_HAVE_STDBIT_H)
+  Use the redirecting header. The build copies
+  ``include/nuttx/lib/stdbit.h`` to ``include/stdbit.h``; that header
+  then includes ``<arch/stdbit.h>`` when this option is set.
+
+- **CONFIG_LIBC_STDBIT_GENERIC** (bool "stdbit.h (generic C23)")
+  Use the generic C23 implementation. The same redirecting file
+  ``include/nuttx/lib/stdbit.h`` is copied to ``include/stdbit.h``,
+  and the generic implementation is used (no arch header). Requires
+  compiler builtins (e.g. ``__builtin_clz``, ``__builtin_ctz``,
+  ``__builtin_popcount``); see ``CONFIG_HAVE_BUILTIN_*`` in
+  ``nuttx/compiler.h``.
+
+Either **CONFIG_ARCH_STDBIT_H** or **CONFIG_LIBC_STDBIT_GENERIC** may be
+enabled so that ``#include <stdbit.h>`` is available.

Kconfig

@@ -625,6 +625,31 @@ config ARCH_STDARG_H
 		ARCH_STDARG_H=y and providing. If ARCH_STDARG_H, is not defined, then
 		the stdarg.h header file will stay out-of-the-way in include/nuttx/.
 
+config ARCH_HAVE_STDBIT_H
+	bool
+	default n
+	---help---
+		Selected by architecture specific logic if the architecture
+		provides a stdbit.h header file.
+
+config ARCH_STDBIT_H
+	bool "stdbit.h"
+	depends on ARCH_HAVE_STDBIT_H
+	default n
+	---help---
+		Redirecting stdbit.h lives at include/nuttx/lib/stdbit.h. When
+		this is set, the build copies it to include/stdbit.h and it
+		includes arch/stdbit.h if the architecture provides one.
+
+config LIBC_STDBIT_GENERIC
+	bool "stdbit.h (generic C23)"
+	default n
+	---help---
+		Use generic C23 stdbit implementation. When set, copy
+		include/nuttx/lib/stdbit.h to include/stdbit.h. No arch
+		header is used; builtins (e.g. __builtin_clz) must be
+		available (see CONFIG_HAVE_BUILTIN_* in compiler.h).
+
 config ARCH_HAVE_SETJMP
 	bool
 	default n

cmake/nuttx_generate_headers.cmake

@@ -137,6 +137,17 @@ else()
   file(REMOVE ${CMAKE_BINARY_DIR}/include/setjmp.h)
 endif()
 
+# Target used to copy include/nuttx/lib/stdbit.h.  If CONFIG_ARCH_STDBIT_H or
+# CONFIG_LIBC_STDBIT_GENERIC is set, copy stdbit.h to include/ for C23 bit
+# utilities.
+
+if(CONFIG_ARCH_STDBIT_H OR CONFIG_LIBC_STDBIT_GENERIC)
+  nuttx_create_symlink(${NUTTX_DIR}/include/nuttx/lib/stdbit.h
+                       ${CMAKE_BINARY_DIR}/include/stdbit.h)
+else()
+  file(REMOVE ${CMAKE_BINARY_DIR}/include/stdbit.h)
+endif()
+
 # Add final context target that ties together all of the above The context
 # target is invoked on each target build to assure that NuttX is properly
 # configured.  The basic configuration steps include creation of the the
@@ -151,7 +162,10 @@ add_custom_target(
     $<$<BOOL:${CONFIG_ARCH_STDARG_H}>:${CMAKE_BINARY_DIR}/include/stdarg.h>
     $<$<BOOL:${NEED_MATH_H}>:${CMAKE_BINARY_DIR}/include/math.h>
     $<$<BOOL:${CONFIG_ARCH_FLOAT_H}>:${CMAKE_BINARY_DIR}/include/float.h>
-    $<$<BOOL:${CONFIG_ARCH_SETJMP_H}>:${CMAKE_BINARY_DIR}/include/setjmp.h>)
+    $<$<BOOL:${CONFIG_ARCH_SETJMP_H}>:${CMAKE_BINARY_DIR}/include/setjmp.h>
+    $<$<BOOL:${CONFIG_ARCH_STDBIT_H}>:${CMAKE_BINARY_DIR}/include/stdbit.h>
+    $<$<BOOL:${CONFIG_LIBC_STDBIT_GENERIC}>:${CMAKE_BINARY_DIR}/include/stdbit.h>
+)
 
 # apps_context is a PHONY target used as an intermediate process to control the
 # time order of context preparation actions of app building