Allow querying if the current GC may move objects.

docs/userguide/src/tutorial/code/mygc_semispace/global.rs

@@ -71,6 +71,13 @@ impl<VM: VMBinding> Plan for MyGC<VM> {
     }
     // ANCHOR_END: create_copy_config
 
+    // Modify
+    // ANCHOR: current_gc_may_move_object
+    fn current_gc_may_move_object(&self) -> bool {
+        true
+    }
+    // ANCHOR_END: current_gc_may_move_object
+
     // Modify
     // ANCHOR: schedule_collection
     fn schedule_collection(&'static self, scheduler: &GCWorkScheduler<VM>) {

docs/userguide/src/tutorial/mygc/ss/collection.md

@@ -29,6 +29,13 @@ space here.
 {{#include ../../code/mygc_semispace/global.rs:create_copy_config}}
 ```
 
+Because the semispace GC copies objects in every single GC, we modify the method
+`current_gc_may_move_object()` in `MyGC` so that it always returns `true`.
+
+```rust
+{{#include ../../code/mygc_semispace/global.rs:current_gc_may_move_object}}
+```
+
 ## Introduce collection to MyGC plan
 
 Add a new method to `Plan for MyGC`, `schedule_collection()`. This function 

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

@@ -137,10 +137,7 @@ impl<VM: VMBinding> Plan for GenImmix<VM> {
         let full_heap = !self.gen.is_current_gc_nursery();
         self.gen.release(tls);
         if full_heap {
-            let did_defrag = self.immix_space.release(full_heap);
-            self.last_gc_was_defrag.store(did_defrag, Ordering::Relaxed);
-        } else {
-            self.last_gc_was_defrag.store(false, Ordering::Relaxed);
+            self.immix_space.release(full_heap);
         }
         self.last_gc_was_full_heap
             .store(full_heap, Ordering::Relaxed);
@@ -149,6 +146,17 @@ impl<VM: VMBinding> Plan for GenImmix<VM> {
     fn end_of_gc(&mut self, _tls: VMWorkerThread) {
         self.gen
             .set_next_gc_full_heap(CommonGenPlan::should_next_gc_be_full_heap(self));
+
+        let did_defrag = self.immix_space.end_of_gc();
+        self.last_gc_was_defrag.store(did_defrag, Ordering::Relaxed);
+    }
+
+    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 {

src/plan/global.rs

@@ -299,6 +299,16 @@ 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.
+    ///
+    /// This function is callable during a GC.  From the VM binding's point of view, the information
+    /// of whether the current GC moves object or not is available since `Collection::stop_mutators`
+    /// is called, and remains available until (but not including) `resume_mutators` at which time
+    /// the current GC has just finished.
+    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

@@ -93,8 +93,16 @@ impl<VM: VMBinding> Plan for Immix<VM> {
     fn release(&mut self, tls: VMWorkerThread) {
         self.common.release(tls, true);
         // release the collected region
+        self.immix_space.release(true);
+    }
+
+    fn end_of_gc(&mut self, _tls: VMWorkerThread) {
         self.last_gc_was_defrag
-            .store(self.immix_space.release(true), Ordering::Relaxed);
+            .store(self.immix_space.end_of_gc(), Ordering::Relaxed);
+    }
+
+    fn current_gc_may_move_object(&self) -> bool {
+        self.immix_space.in_defrag()
     }
 
     fn get_collection_reserved_pages(&self) -> usize {

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

@@ -7,6 +7,7 @@ use crate::plan::PlanConstraints;
 use crate::policy::gc_work::TraceKind;
 use crate::policy::gc_work::TRACE_KIND_TRANSITIVE_PIN;
 use crate::policy::immix::ImmixSpace;
+use crate::policy::immix::PREFER_COPY_ON_NURSERY_GC;
 use crate::policy::immix::TRACE_KIND_FAST;
 use crate::policy::sft::SFT;
 use crate::policy::space::Space;
@@ -126,9 +127,7 @@ impl<VM: VMBinding> Plan for StickyImmix<VM> {
 
     fn release(&mut self, tls: crate::util::VMWorkerThread) {
         if self.is_current_gc_nursery() {
-            let was_defrag = self.immix.immix_space.release(false);
-            self.immix
-                .set_last_gc_was_defrag(was_defrag, Ordering::Relaxed);
+            self.immix.immix_space.release(false);
             self.immix.common.los.release(false);
         } else {
             self.immix.release(tls);
@@ -140,6 +139,10 @@ impl<VM: VMBinding> Plan for StickyImmix<VM> {
             crate::plan::generational::global::CommonGenPlan::should_next_gc_be_full_heap(self);
         self.next_gc_full_heap
             .store(next_gc_full_heap, Ordering::Relaxed);
+
+        let was_defrag = self.immix.immix_space.end_of_gc();
+        self.immix
+            .set_last_gc_was_defrag(was_defrag, Ordering::Relaxed);
     }
 
     fn collection_required(&self, space_full: bool, space: Option<SpaceStats<Self::VM>>) -> bool {
@@ -158,6 +161,14 @@ 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() {
+            PREFER_COPY_ON_NURSERY_GC
+        } else {
+            self.get_immix_space().in_defrag()
+        }
+    }
+
     fn get_collection_reserved_pages(&self) -> usize {
         self.immix.get_collection_reserved_pages()
     }

src/policy/immix/defrag.rs

@@ -205,9 +205,9 @@ impl Defrag {
             .store(threshold, Ordering::Release);
     }
 
-    /// Release work. Should be called in ImmixSpace::release.
+    /// Reset the in-defrag state.
     #[allow(clippy::assertions_on_constants)]
-    pub fn release<VM: VMBinding>(&self, _space: &ImmixSpace<VM>) {
+    pub fn reset_in_defrag(&self) {
         debug_assert!(super::DEFRAG);
         self.in_defrag_collection.store(false, Ordering::Release);
     }

src/policy/immix/immixspace.rs

@@ -440,12 +440,10 @@ impl<VM: VMBinding> ImmixSpace<VM> {
         }
     }
 
-    /// Release for the immix space. This is called when a GC finished.
-    /// Return whether this GC was a defrag GC, as a plan may want to know this.
-    pub fn release(&mut self, major_gc: bool) -> bool {
-        let did_defrag = self.defrag.in_defrag();
+    /// Release for the immix space.
+    pub fn release(&mut self, major_gc: bool) {
         if major_gc {
-            // Update line_unavail_state for hole searching afte this GC.
+            // Update line_unavail_state for hole searching after this GC.
             if !super::BLOCK_ONLY {
                 self.line_unavail_state.store(
                     self.line_mark_state.load(Ordering::Acquire),
@@ -460,12 +458,17 @@ impl<VM: VMBinding> ImmixSpace<VM> {
         // Sweep chunks and blocks
         let work_packets = self.generate_sweep_tasks();
         self.scheduler().work_buckets[WorkBucketStage::Release].bulk_add(work_packets);
-        if super::DEFRAG {
-            self.defrag.release(self);
-        }
 
         self.lines_consumed.store(0, Ordering::Relaxed);
+    }
 
+    /// This is called when a GC finished.
+    /// Return whether this GC was a defrag GC, as a plan may want to know this.
+    pub fn end_of_gc(&mut self) -> bool {
+        let did_defrag = self.defrag.in_defrag();
+        if super::DEFRAG {
+            self.defrag.reset_in_defrag();
+        }
         did_defrag
     }