gnulib-common.m4: Document the gnulib-internal _GL_ATTRIBUTE_* macros

[Bruno Haible]

When we use the _GL_ATTRIBUTE_* macros, neither the GCC documentation nor
the attribute.h comments directly apply, because of different syntax.

Therefore, this patch adds documentation to these macros.


2021-08-22  Bruno Haible  <bruno@clisp.org>

        gnulib-common.m4: Document the gnulib-internal _GL_ATTRIBUTE_* macros.
        * m4/gnulib-common.m4 (gl_COMMON_BODY): Add comments to the
        _GL_ATTRIBUTE_* macros, based on lib/attribute.h.

diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index c801b3de9..174113b28 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -111,24 +111,39 @@ AC_DEFUN([gl_COMMON_BODY], [
 
 ]dnl There is no _GL_ATTRIBUTE_ALIGNED; use stdalign's _Alignas instead.
 [
+/* _GL_ATTRIBUTE_ALLOC_SIZE ((N)) declares that the Nth argument of the 
function
+   is the size of the returned memory block.
+   _GL_ATTRIBUTE_ALLOC_SIZE ((M, N)) declares that the Mth argument multiplied
+   by the Nth argument of the function is the size of the returned memory 
block.
+ */
+/* Applies to: function, pointer to function, function types.  */
 #if _GL_HAS_ATTRIBUTE (alloc_size)
 # define _GL_ATTRIBUTE_ALLOC_SIZE(args) __attribute__ ((__alloc_size__ args))
 #else
 # define _GL_ATTRIBUTE_ALLOC_SIZE(args)
 #endif
 
+/* _GL_ATTRIBUTE_ALWAYS_INLINE tells that the compiler should always inline the
+   function and report an error if it cannot do so.  */
+/* Applies to: function.  */
 #if _GL_HAS_ATTRIBUTE (always_inline)
 # define _GL_ATTRIBUTE_ALWAYS_INLINE __attribute__ ((__always_inline__))
 #else
 # define _GL_ATTRIBUTE_ALWAYS_INLINE
 #endif
 
+/* _GL_ATTRIBUTE_ARTIFICIAL declares that the function is not important to show
+    in stack traces when debugging.  The compiler should omit the function from
+    stack traces.  */
+/* Applies to: function.  */
 #if _GL_HAS_ATTRIBUTE (artificial)
 # define _GL_ATTRIBUTE_ARTIFICIAL __attribute__ ((__artificial__))
 #else
 # define _GL_ATTRIBUTE_ARTIFICIAL
 #endif
 
+/* _GL_ATTRIBUTE_COLD declares that the function is rarely executed.  */
+/* Applies to: functions.  */
 /* Avoid __attribute__ ((cold)) on MinGW; see thread starting at
    <https://lists.gnu.org/r/emacs-devel/2019-04/msg01152.html>.
    Also, Oracle Studio 12.6 requires 'cold' not '__cold__'.  */
@@ -142,18 +157,25 @@ AC_DEFUN([gl_COMMON_BODY], [
 # define _GL_ATTRIBUTE_COLD
 #endif
 
+/* _GL_ATTRIBUTE_CONST declares that it is OK for a compiler to omit duplicate
+   calls to the function with the same arguments.
+   This attribute is safe for a function that neither depends on nor affects
+   observable state, and always returns exactly once - e.g., does not loop
+   forever, and does not call longjmp.
+   (This attribute is stricter than _GL_ATTRIBUTE_PURE.)  */
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (const)
 # define _GL_ATTRIBUTE_CONST __attribute__ ((__const__))
 #else
 # define _GL_ATTRIBUTE_CONST
 #endif
 
-/* _GL_ATTRIBUTE_DEALLOC (F, I) is for functions returning pointers
+/* _GL_ATTRIBUTE_DEALLOC (F, I) declares that the function returns pointers
    that can be freed by passing them as the Ith argument to the
-   function F.  _GL_ATTRIBUTE_DEALLOC_FREE is for functions that
-   return pointers that can be freed via 'free'; it can be used
-   only after including stdlib.h.  These macros cannot be used on
-   inline functions.  */
+   function F.
+   _GL_ATTRIBUTE_DEALLOC_FREE declares that the function returns pointers that
+   can be freed via 'free'; it can be used only after including <stdlib.h>.  */
+/* Applies to: functions.  Cannot be used on inline functions.  */
 #if _GL_GNUC_PREREQ (11, 0)
 # define _GL_ATTRIBUTE_DEALLOC(f, i) __attribute__ ((__malloc__ (f, i)))
 #else
@@ -161,6 +183,14 @@ AC_DEFUN([gl_COMMON_BODY], [
 #endif
 #define _GL_ATTRIBUTE_DEALLOC_FREE _GL_ATTRIBUTE_DEALLOC (free, 1)
 
+/* _GL_ATTRIBUTE_DEPRECATED: Declares that an entity is deprecated.
+   The compiler may warn if the entity is used.  */
+/* Applies to:
+     - function, variable,
+     - struct, union, struct/union member,
+     - enumeration, enumeration item,
+     - typedef,
+   in C++ also: namespace, class, template specialization.  */
 #if _GL_HAS_C_ATTRIBUTE (deprecated)
 # define _GL_ATTRIBUTE_DEPRECATED [[__deprecated__]]
 #elif _GL_HAS_ATTRIBUTE (deprecated)
@@ -169,6 +199,11 @@ AC_DEFUN([gl_COMMON_BODY], [
 # define _GL_ATTRIBUTE_DEPRECATED
 #endif
 
+/* _GL_ATTRIBUTE_ERROR(msg) requests an error if a function is called and
+   the function call is not optimized away.
+   _GL_ATTRIBUTE_WARNING(msg) requests a warning if a function is called and
+   the function call is not optimized away.  */
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (error)
 # define _GL_ATTRIBUTE_ERROR(msg) __attribute__ ((__error__ (msg)))
 # define _GL_ATTRIBUTE_WARNING(msg) __attribute__ ((__warning__ (msg)))
@@ -180,13 +215,20 @@ AC_DEFUN([gl_COMMON_BODY], [
 # define _GL_ATTRIBUTE_WARNING(msg)
 #endif
 
+/* _GL_ATTRIBUTE_EXTERNALLY_VISIBLE declares that the entity should remain
+   visible to debuggers etc., even with '-fwhole-program'.  */
+/* Applies to: functions, variables.  */
 #if _GL_HAS_ATTRIBUTE (externally_visible)
 # define _GL_ATTRIBUTE_EXTERNALLY_VISIBLE __attribute__ ((externally_visible))
 #else
 # define _GL_ATTRIBUTE_EXTERNALLY_VISIBLE
 #endif
 
-/* FALLTHROUGH is special, because it always expands to something.  */
+/* _GL_ATTRIBUTE_FALLTHROUGH declares that it is not a programming mistake if
+   the control flow falls through to the immediately following 'case' or
+   'default' label.  The compiler should not warn in this case.  */
+/* Applies to: Empty statement (;), inside a 'switch' statement.  */
+/* Always expands to something.  */
 #if _GL_HAS_C_ATTRIBUTE (fallthrough)
 # define _GL_ATTRIBUTE_FALLTHROUGH [[__fallthrough__]]
 #elif _GL_HAS_ATTRIBUTE (fallthrough)
@@ -195,24 +237,47 @@ AC_DEFUN([gl_COMMON_BODY], [
 # define _GL_ATTRIBUTE_FALLTHROUGH ((void) 0)
 #endif
 
+/* _GL_ATTRIBUTE_FORMAT ((ARCHETYPE, STRING-INDEX, FIRST-TO-CHECK))
+   declares that the STRING-INDEXth function argument is a format string of
+   style ARCHETYPE, which is one of:
+     printf, gnu_printf
+     scanf, gnu_scanf,
+     strftime, gnu_strftime,
+     strfmon,
+   or the same thing prefixed and suffixed with '__'.
+   If FIRST-TO-CHECK is not 0, arguments starting at FIRST-TO_CHECK
+   are suitable for the format string.  */
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (format)
 # define _GL_ATTRIBUTE_FORMAT(spec) __attribute__ ((__format__ spec))
 #else
 # define _GL_ATTRIBUTE_FORMAT(spec)
 #endif
 
+/* _GL_ATTRIBUTE_LEAF declares that if the function is called from some other
+   compilation unit, it executes code from that unit only by return or by
+   exception handling.  This declaration lets the compiler optimize that unit
+   more aggressively.  */
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (leaf)
 # define _GL_ATTRIBUTE_LEAF __attribute__ ((__leaf__))
 #else
 # define _GL_ATTRIBUTE_LEAF
 #endif
 
+/* _GL_ATTRIBUTE_MALLOC declares that the function returns a pointer to freshly
+   allocated memory.  */
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (malloc)
 # define _GL_ATTRIBUTE_MALLOC __attribute__ ((__malloc__))
 #else
 # define _GL_ATTRIBUTE_MALLOC
 #endif
 
+/* _GL_ATTRIBUTE_MAY_ALIAS declares that pointers to the type may point to the
+   same storage as pointers to other types.  Thus this declaration disables
+   strict aliasing optimization.  */
+/* Applies to: types.  */
 /* Oracle Studio 12.6 mishandles may_alias despite __has_attribute OK.  */
 #if _GL_HAS_ATTRIBUTE (may_alias) && !defined __SUNPRO_C
 # define _GL_ATTRIBUTE_MAY_ALIAS __attribute__ ((__may_alias__))
@@ -220,6 +285,18 @@ AC_DEFUN([gl_COMMON_BODY], [
 # define _GL_ATTRIBUTE_MAY_ALIAS
 #endif
 
+/* _GL_ATTRIBUTE_MAYBE_UNUSED declares that it is not a programming mistake if
+   the entity is not used.  The compiler should not warn if the entity is not
+   used.  */
+/* Applies to:
+     - function, variable,
+     - struct, union, struct/union member,
+     - enumeration, enumeration item,
+     - typedef,
+   in C++ also: class.  */
+/* In C++ and C2x, this is spelled [[__maybe_unused__]].
+   GCC's syntax is __attribute__ ((__unused__)).
+   clang supports both syntaxes.  */
 #if _GL_HAS_C_ATTRIBUTE (maybe_unused)
 # define _GL_ATTRIBUTE_MAYBE_UNUSED [[__maybe_unused__]]
 #else
@@ -228,6 +305,10 @@ AC_DEFUN([gl_COMMON_BODY], [
 /* Earlier spellings of this macro.  */
 #define _UNUSED_PARAMETER_ _GL_ATTRIBUTE_MAYBE_UNUSED
 
+/* _GL_ATTRIBUTE_NODISCARD declares that the caller of the function should not
+   discard the return value.  The compiler may warn if the caller does not use
+   the return value, unless the caller uses something like ignore_value.  */
+/* Applies to: function, enumeration, class.  */
 #if _GL_HAS_C_ATTRIBUTE (nodiscard)
 # define _GL_ATTRIBUTE_NODISCARD [[__nodiscard__]]
 #elif _GL_HAS_ATTRIBUTE (warn_unused_result)
@@ -236,18 +317,30 @@ AC_DEFUN([gl_COMMON_BODY], [
 # define _GL_ATTRIBUTE_NODISCARD
 #endif
 
+/* _GL_ATTRIBUTE_NOINLINE tells that the compiler should not inline the
+   function.  */
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (noinline)
 # define _GL_ATTRIBUTE_NOINLINE __attribute__ ((__noinline__))
 #else
 # define _GL_ATTRIBUTE_NOINLINE
 #endif
 
+/* _GL_ATTRIBUTE_NONNULL ((N1, N2,...)) declares that the arguments N1, N2,...
+   must not be NULL.
+   _GL_ATTRIBUTE_NONNULL () declares that all pointer arguments must not be
+   null.  */
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (nonnull)
 # define _GL_ATTRIBUTE_NONNULL(args) __attribute__ ((__nonnull__ args))
 #else
 # define _GL_ATTRIBUTE_NONNULL(args)
 #endif
 
+/* _GL_ATTRIBUTE_NONSTRING declares that the contents of a character array is
+   not meant to be NUL-terminated.  */
+/* Applies to: struct/union members and variables that are arrays of element
+   type '[[un]signed] char'.  */
 #if _GL_HAS_ATTRIBUTE (nonstring)
 # define _GL_ATTRIBUTE_NONSTRING __attribute__ ((__nonstring__))
 #else
@@ -256,47 +349,75 @@ AC_DEFUN([gl_COMMON_BODY], [
 
 /* There is no _GL_ATTRIBUTE_NORETURN; use _Noreturn instead.  */
 
+/* _GL_ATTRIBUTE_NOTHROW declares that the function does not throw exceptions.
+ */
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (nothrow) && !defined __cplusplus
 # define _GL_ATTRIBUTE_NOTHROW __attribute__ ((__nothrow__))
 #else
 # define _GL_ATTRIBUTE_NOTHROW
 #endif
 
+/* _GL_ATTRIBUTE_PACKED declares:
+   For struct members: The member has the smallest possible alignment.
+   For struct, union, class: All members have the smallest possible alignment,
+   minimizing the memory required.  */
+/* Applies to: struct members, struct, union,
+   in C++ also: class.  */
 #if _GL_HAS_ATTRIBUTE (packed)
 # define _GL_ATTRIBUTE_PACKED __attribute__ ((__packed__))
 #else
 # define _GL_ATTRIBUTE_PACKED
 #endif
 
+/* _GL_ATTRIBUTE_PURE declares that It is OK for a compiler to omit duplicate
+   calls to the function with the same arguments if observable state is not
+   changed between calls.
+   This attribute is safe for a function that does not affect
+   observable state, and always returns exactly once.
+   (This attribute is looser than _GL_ATTRIBUTE_CONST.)  */
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (pure)
 # define _GL_ATTRIBUTE_PURE __attribute__ ((__pure__))
 #else
 # define _GL_ATTRIBUTE_PURE
 #endif
 
+/* _GL_ATTRIBUTE_RETURNS_NONNULL declares that the function's return value is
+   a non-NULL pointer.  */
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (returns_nonnull)
 # define _GL_ATTRIBUTE_RETURNS_NONNULL __attribute__ ((__returns_nonnull__))
 #else
 # define _GL_ATTRIBUTE_RETURNS_NONNULL
 #endif
 
+/* _GL_ATTRIBUTE_SENTINEL(pos) declares that the variadic function expects a
+   trailing NULL argument.
+   _GL_ATTRIBUTE_SENTINEL () - The last argument is NULL (requires C99).
+   _GL_ATTRIBUTE_SENTINEL ((N)) - The (N+1)st argument from the end is NULL.  
*/
+/* Applies to: functions.  */
 #if _GL_HAS_ATTRIBUTE (sentinel)
 # define _GL_ATTRIBUTE_SENTINEL(pos) __attribute__ ((__sentinel__ pos))
 #else
 # define _GL_ATTRIBUTE_SENTINEL(pos)
 #endif
 
+/* A helper macro.  Don't use it directly.  */
 #if _GL_HAS_ATTRIBUTE (unused)
 # define _GL_ATTRIBUTE_UNUSED __attribute__ ((__unused__))
 #else
 # define _GL_ATTRIBUTE_UNUSED
 #endif
-/* Earlier spellings of this macro.  */
+/* Alternative spelling of this macro, for convenience.  */
 #define _GL_UNUSED _GL_ATTRIBUTE_UNUSED
 
 ]dnl There is no _GL_ATTRIBUTE_VISIBILITY; see m4/visibility.m4 instead.
 [
-/* To support C++ as well as C, use _GL_UNUSED_LABEL with trailing ';'.  */
+/* _GL_UNUSED_LABEL; declares that it is not a programming mistake if the
+   immediately preceding label is not used.  The compiler should not warn
+   if the label is not used.  */
+/* Applies to: label (both in C and C++).  */
 #if !defined __cplusplus || _GL_GNUC_PREREQ (4, 5)
 # define _GL_UNUSED_LABEL _GL_ATTRIBUTE_UNUSED
 #else