Implement fake atomics in runtime4

These are necessary for the atomic externals in stdlib to be usable with runtime4.
They are copies of the versions in runtime/, but assume Domain_alone = true.

Once we implement flambda2 support for the atomic primitives, the externals should
be updated to use them, and these functions may be deleted.

Author: TheNumbat

ocaml/runtime4/misc.c

@@ -28,6 +28,7 @@ __declspec(noreturn) void __cdecl abort(void);
 #include <stdio.h>
 #include <string.h>
 #include <stdarg.h>
+#include "caml/alloc.h"
 #include "caml/config.h"
 #include "caml/misc.h"
 #include "caml/memory.h"
@@ -225,3 +226,30 @@ void caml_flambda2_invalid (value message)
   fprintf (stderr, "Consider using [Sys.opaque_identity].\n");
   abort ();
 }
+
+/* Fake atomic operations - runtime4 is single threaded, but we need to
+   provide these symbols for compatibility with the 5 Stdlib updates. */
+
+CAMLprim value caml_atomic_make(value v)
+{
+  CAMLparam1(v);
+  value ref = caml_alloc_small(1, 0);
+  Field(ref, 0) = v;
+  CAMLreturn(ref);
+}
+
+CAMLprim value caml_atomic_load(value ref)
+{
+  return Field(ref, 0);
+}
+
+CAMLprim value caml_atomic_cas (value ref, value oldv, value newv)
+{
+  value* p = Op_val(ref);
+  if (*p == oldv) {
+    caml_modify(p, newv);
+    return Val_int(1);
+  } else {
+    return Val_int(0);
+  }
+}