Allow VM to query if the current GC moves objects.

Author: wks

src/plan/generational/copying/global.rs

@@ -127,6 +127,10 @@ impl<VM: VMBinding> Plan for GenCopy<VM> {
         self.gen.get_used_pages() + self.tospace().reserved_pages()
     }
 
+    fn current_gc_may_move_object(&self) -> bool {
+        true
+    }
+
     /// Return the number of pages available for allocation. Assuming all future allocations goes to nursery.
     fn get_available_pages(&self) -> usize {
         // super.get_available_pages() / 2 to reserve pages for copying

src/plan/generational/immix/global.rs

@@ -151,6 +151,14 @@ impl<VM: VMBinding> Plan for GenImmix<VM> {
             .set_next_gc_full_heap(CommonGenPlan::should_next_gc_be_full_heap(self));
     }
 
+    fn current_gc_may_move_object(&self) -> bool {
+        if self.is_current_gc_nursery() {
+            true
+        } else {
+            self.immix_space.in_defrag()
+        }
+    }
+
     fn get_collection_reserved_pages(&self) -> usize {
         self.gen.get_collection_reserved_pages() + self.immix_space.defrag_headroom_pages()
     }

src/plan/global.rs

@@ -299,6 +299,11 @@ pub trait Plan: 'static + HasSpaces + Sync + Downcast {
         true
     }
 
+    /// Return whether the current GC may move any object.  The VM binding can make use of this
+    /// information and choose to or not to update some data structures that record the addresses
+    /// of objects.
+    fn current_gc_may_move_object(&self) -> bool;
+
     /// An object is firstly reached by a sanity GC. So the object is reachable
     /// in the current GC, and all the GC work has been done for the object (such as
     /// tracing and releasing). A plan can implement this to

src/plan/immix/global.rs

@@ -97,6 +97,10 @@ impl<VM: VMBinding> Plan for Immix<VM> {
             .store(self.immix_space.release(true), Ordering::Relaxed);
     }
 
+    fn current_gc_may_move_object(&self) -> bool {
+        self.immix_space.in_defrag()
+    }
+
     fn get_collection_reserved_pages(&self) -> usize {
         self.immix_space.defrag_headroom_pages()
     }

src/plan/markcompact/global.rs

@@ -171,6 +171,10 @@ impl<VM: VMBinding> Plan for MarkCompact<VM> {
     fn get_collection_reserved_pages(&self) -> usize {
         0
     }
+
+    fn current_gc_may_move_object(&self) -> bool {
+        true
+    }
 }
 
 impl<VM: VMBinding> MarkCompact<VM> {

src/plan/marksweep/global.rs

@@ -73,6 +73,10 @@ impl<VM: VMBinding> Plan for MarkSweep<VM> {
         self.base().collection_required(self, space_full)
     }
 
+    fn current_gc_may_move_object(&self) -> bool {
+        false
+    }
+
     fn get_used_pages(&self) -> usize {
         self.common.get_used_pages() + self.ms.reserved_pages()
     }

src/plan/nogc/global.rs

@@ -74,6 +74,10 @@ impl<VM: VMBinding> Plan for NoGC<VM> {
         unreachable!("GC triggered in nogc")
     }
 
+    fn current_gc_may_move_object(&self) -> bool {
+        false
+    }
+
     fn get_used_pages(&self) -> usize {
         self.nogc_space.reserved_pages()
             + self.immortal.reserved_pages()

src/plan/pageprotect/global.rs

@@ -62,6 +62,10 @@ impl<VM: VMBinding> Plan for PageProtect<VM> {
         self.base().collection_required(self, space_full)
     }
 
+    fn current_gc_may_move_object(&self) -> bool {
+        false
+    }
+
     fn get_used_pages(&self) -> usize {
         self.space.reserved_pages() + self.common.get_used_pages()
     }

src/plan/semispace/global.rs

@@ -100,6 +100,10 @@ impl<VM: VMBinding> Plan for SemiSpace<VM> {
         self.base().collection_required(self, space_full)
     }
 
+    fn current_gc_may_move_object(&self) -> bool {
+        true
+    }
+
     fn get_collection_reserved_pages(&self) -> usize {
         self.tospace().reserved_pages()
     }

src/plan/sticky/immix/global.rs

@@ -158,6 +158,14 @@ impl<VM: VMBinding> Plan for StickyImmix<VM> {
         self.gc_full_heap.load(Ordering::Relaxed) && self.immix.last_collection_was_exhaustive()
     }
 
+    fn current_gc_may_move_object(&self) -> bool {
+        if self.is_current_gc_nursery() {
+            !cfg!(feature = "sticky_immix_non_moving_nursery")
+        } else {
+            return self.get_immix_space().in_defrag();
+        }
+    }
+
     fn get_collection_reserved_pages(&self) -> usize {
         self.immix.get_collection_reserved_pages()
     }