ARM64: FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP (#8331)

Implements the missing parts of the software write watch system. Activates and deactivates the tracking based on the GC state. Code is based on the same function in the runime

Generates an compiler error if FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP is enabled for anything else than ARM64 UNIX as all other assembler helpers are not updated to support it.

src/Native/Runtime/gcrhenv.cpp

@@ -1169,6 +1169,10 @@ void GCToEEInterface::DiagWalkBGCSurvivors(void* gcContext)
 #endif // FEATURE_EVENT_TRACE
 }
 
+#if defined(FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP) && (!defined(TARGET_ARM64) || !defined(TARGET_UNIX))
+#error FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP is only implemented for ARM64 and UNIX
+#endif
+
 void GCToEEInterface::StompWriteBarrier(WriteBarrierParameters* args)
 {
     // CoreRT doesn't patch the write barrier like CoreCLR does, but it
@@ -1198,6 +1202,14 @@ void GCToEEInterface::StompWriteBarrier(WriteBarrierParameters* args)
         g_card_bundle_table = args->card_bundle_table;
 #endif
 
+#ifdef FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
+        if (g_sw_ww_enabled_for_gc_heap && (args->write_watch_table != nullptr))
+        {
+            assert(args->is_runtime_suspended);
+            g_write_watch_table = args->write_watch_table;
+        }
+#endif // FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
+
         // IMPORTANT: managed heap segments may surround unmanaged/stack segments. In such cases adding another managed 
         //     heap segment may put a stack/unmanaged write inside the new heap range. However the old card table would 
         //     not cover it. Therefore we must ensure that the write barriers see the new table before seeing the new bounds.
@@ -1251,14 +1263,35 @@ void GCToEEInterface::StompWriteBarrier(WriteBarrierParameters* args)
         g_card_bundle_table = args->card_bundle_table;
 #endif
 
+#ifdef FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
+        assert(g_write_watch_table == nullptr);
+        g_write_watch_table = args->write_watch_table;
+#endif
+
         g_lowest_address = args->lowest_address;
         g_highest_address = args->highest_address;
         g_ephemeral_low = args->ephemeral_low;
         g_ephemeral_high = args->ephemeral_high;
         return;
     case WriteBarrierOp::SwitchToWriteWatch:
+#ifdef FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
+        assert(args->is_runtime_suspended && "the runtime must be suspended here!");
+        assert(args->write_watch_table != nullptr);
+        g_write_watch_table = args->write_watch_table;
+        g_sw_ww_enabled_for_gc_heap = true;
+#else
+        assert(!"should never be called without FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP");
+#endif // FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
+        break;
+
     case WriteBarrierOp::SwitchToNonWriteWatch:
-        assert(!"CoreRT does not have an implementation of non-OS WriteWatch");
+#ifdef FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
+        assert(args->is_runtime_suspended && "the runtime must be suspended here!");
+        g_write_watch_table = nullptr;
+        g_sw_ww_enabled_for_gc_heap = false;
+#else
+        assert(!"should never be called without FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP");
+#endif // FEATURE_USE_SOFTWARE_WRITE_WATCH_FOR_GC_HEAP
         return;
     default:
         assert(!"Unknokwn WriteBarrierOp enum");