[base] Add atomic CAS operation for 16-bit integers.

The operation will be used for marking of descriptor arrays.

Bug: v8:8486
Change-Id: If73be030614e2c84c77eaeeff419c08ef34a76e9
Reviewed-on: https://chromium-review.googlesource.com/c/1382456Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58322}

src/base/atomicops.h

@@ -66,6 +66,8 @@ typedef intptr_t AtomicWord;
 // Always return the old value of "*ptr"
 //
 // This routine implies no memory barriers.
+Atomic16 Relaxed_CompareAndSwap(volatile Atomic16* ptr, Atomic16 old_value,
+                                Atomic16 new_value);
 Atomic32 Relaxed_CompareAndSwap(volatile Atomic32* ptr, Atomic32 old_value,
                                 Atomic32 new_value);
 

src/base/atomicops_internals_portable.h

@@ -50,6 +50,13 @@ inline void SeqCst_MemoryFence() {
 #endif
 }
 
+inline Atomic16 Relaxed_CompareAndSwap(volatile Atomic16* ptr,
+                                       Atomic16 old_value, Atomic16 new_value) {
+  __atomic_compare_exchange_n(ptr, &old_value, new_value, false,
+                              __ATOMIC_RELAXED, __ATOMIC_RELAXED);
+  return old_value;
+}
+
 inline Atomic32 Relaxed_CompareAndSwap(volatile Atomic32* ptr,
                                        Atomic32 old_value, Atomic32 new_value) {
   __atomic_compare_exchange_n(ptr, &old_value, new_value, false,

src/base/atomicops_internals_std.h

@@ -28,6 +28,14 @@ inline void SeqCst_MemoryFence() {
   std::atomic_thread_fence(std::memory_order_seq_cst);
 }
 
+inline Atomic16 Relaxed_CompareAndSwap(volatile Atomic16* ptr,
+                                       Atomic16 old_value, Atomic16 new_value) {
+  std::atomic_compare_exchange_strong_explicit(
+      helper::to_std_atomic(ptr), &old_value, new_value,
+      std::memory_order_relaxed, std::memory_order_relaxed);
+  return old_value;
+}
+
 inline Atomic32 Relaxed_CompareAndSwap(volatile Atomic32* ptr,
                                        Atomic32 old_value, Atomic32 new_value) {
   std::atomic_compare_exchange_strong_explicit(