Revert "Do not allow naked pointers (remove configure option)"

This reverts commit 4984208.

configure.ac

@@ -357,6 +357,8 @@ AC_SUBST([mkexe_ldflags_exp])
 AC_SUBST([PACKLD])
 AC_SUBST([build_libraries_manpages])
 AC_SUBST([compute_deps])
+AC_SUBST([naked_pointers])
+AC_SUBST([naked_pointers_checker])
 AC_SUBST([intel_jcc_bug_cflags])
 AC_SUBST([stack_allocation])
 AC_SUBST([poll_insertion])
@@ -583,10 +585,9 @@ AC_ARG_ENABLE([cpp-mangling],
   [AS_HELP_STRING([--enable-cpp-mangling],
     [use cpp mangling for exported symbols])])
 
-AC_ARG_ENABLE([naked-pointers], [],
-  [AS_IF([test "x$enableval" != 'xno'],
-    [AC_MSG_ERROR([Naked pointers were prohibited in OCaml 5.0.])])],
-  [])
+AC_ARG_ENABLE([naked-pointers],
+  [AS_HELP_STRING([--disable-naked-pointers],
+    [do not allow naked pointers])])
 
 AC_ARG_ENABLE([naked-pointers-checker], [],
   [AS_IF([test "x$enableval" != 'xno'],
@@ -2639,6 +2640,30 @@ AS_IF([test x"$enable_cpp_mangling" = "xyes"],
     AC_DEFINE([WITH_CPP_MANGLING])],
   [cpp_mangling=false])
 
+## No naked pointers
+
+AS_IF([test x"$enable_naked_pointers" = "xno" ],
+  [naked_pointers=false
+   AC_DEFINE([NO_NAKED_POINTERS])],
+  [naked_pointers=true])
+
+AS_IF([test x"$enable_naked_pointers_checker" = "xyes" ],
+  [AS_IF([test x"$enable_naked_pointers" = "xno" ],
+         [AC_MSG_ERROR(m4_normalize([
+               --enable-naked-pointers-checker and --disable-naked-pointers
+               are incompatible]))])
+   AS_CASE(["$arch","$system"],
+    [amd64,linux|amd64,macosx \
+    |amd64,openbsd|amd64,win64 \
+    |amd64,freebsd|amd64,solaris \
+    |arm64,linux|arm64,macosx],
+      [naked_pointers_checker=true
+      AC_DEFINE([NAKED_POINTERS_CHECKER])],
+    [*],
+      [AC_MSG_ERROR([naked pointers checker not supported on this platform])]
+  )],
+  [naked_pointers_checker=false])
+
 ## Check for mmap support for huge pages and contiguous heap
 OCAML_MMAP_SUPPORTS_HUGE_PAGES
 

ocamltest/ocamltest_config.ml.in

@@ -104,6 +104,6 @@ let stack_allocation = @stack_allocation@
 
 let poll_insertion = @poll_insertion@
 
-let naked_pointers = false
+let naked_pointers = @naked_pointers@
 
 let tsan = @tsan@

runtime4/caml/m.h.in

@@ -84,7 +84,7 @@
 
 #undef WITH_FRAME_POINTERS
 
-#define NO_NAKED_POINTERS 1
+#undef NO_NAKED_POINTERS
 
 #undef NAKED_POINTERS_CHECKER
 

utils/config.common.ml.in

@@ -46,7 +46,6 @@ and cfg_magic_number = {magic|@CFG_MAGIC_NUMBER@|magic}
 
 let safe_string = true
 let default_safe_string = true
-let naked_pointers = false
 let flambda_backend = true
 
 let interface_suffix = ref ".mli"

utils/config.generated.ml.in

@@ -97,6 +97,7 @@ let flexdll_dirs = [@flexdll_dir@]
 
 let ar_supports_response_files = @ar_supports_response_files@
 
+let naked_pointers = "@naked_pointers@" = "true"
 let runtime5 = "@enable_runtime5@" = "yes"
 
 let reserved_header_bits =