[m-rev.] for review: use __sync_synchronize() as a fallback memory barrier with clang

[Julien Fischer]

For review by anyone.

This should be enough get the hlc.par.gc grade working on M1 based Macs.
A better solution would be to use the ARM specific barrier instructions,
but that's a separate change.

------------------

Use __sync_synchronize() as a fallback memory barrier with clang.

runtime/mercury_atomic_ops.h:
     clang provides the GCC builtin function __sync_synchronize; use that
     as a fallback memory barrier.

     Avoid unreadable #error directive output from clang.

     Add an XXX about C11 atomic ops.

Julien.

diff --git a/runtime/mercury_atomic_ops.h b/runtime/mercury_atomic_ops.h
index f9806eb7b..1ce95f3d8 100644
--- a/runtime/mercury_atomic_ops.h
+++ b/runtime/mercury_atomic_ops.h
@@ -6,6 +6,9 @@

  // mercury_atomic.h - defines atomic operations and other primitives used by
  // the parallel runtime.
+//
+// XXX we should have a version of this module that uses C11 atomics
+// where possible.

  #ifndef MERCURY_ATOMIC_OPS_H
  #define MERCURY_ATOMIC_OPS_H
@@ -566,7 +569,7 @@ MR_EXTERN_INLINE MR_bool    MR_atomic_dec_and_is_zero_uint(
              __asm__ __volatile__("mfence");                                 \
          } while (0)

-#elif MR_GNUC > 4 || (MR_GNUC == 4 && __GNUC_MINOR__ >= 1)
+#elif defined(MR_CLANG) || MR_GNUC > 4 || (MR_GNUC == 4 && __GNUC_MINOR__ >= 1)

      // Our memory fences are better than GCC's. GCC only implements a full
      // fence.
@@ -580,8 +583,9 @@ MR_EXTERN_INLINE MR_bool    MR_atomic_dec_and_is_zero_uint(

  #else

-    #error "Please implement memory fence operations " \
-        "for this compiler/architecture"
+    // Do not break this string literal; doing so produces an unreadable
+    // mess with clang.
+    #error "Please implement memory fence operations for this compiler/architecture"

  #endif