Clear all side forwarding bits (#974)

This reverts an optimization which was intended to reduce the number of
side forwarding bits cleared for ImmixSpace by clearing the forwarding
bits metadata of defrag sources, only. This approach does not work for
StickyImmix because it may move young objects, which is not "defrag".

Now ImmixSpace clears the on-the-side forwarding bits for all chunks
just like how it clears the on-the-side marking bits.

This PR prefers correctness over performance, and may degrade the
performance of VMs that use on-the-side forwarding bits. Currently it
only affects the Ruby binding, but we plan to let the Ruby binding use
in-header forwarding states, too. The performance of VMs that use
in-header forwarding bits is not affected by this PR.

Fixes: #972

Author: wks

src/policy/immix/immixspace.rs

@@ -86,7 +86,8 @@ impl<VM: VMBinding> SFT for ImmixSpace<VM> {
     }
 
     fn get_forwarded_object(&self, object: ObjectReference) -> Option<ObjectReference> {
-        if !Block::containing::<VM>(object).is_defrag_source() {
+        // If we never move objects, look no further.
+        if super::NEVER_MOVE_OBJECTS {
             return None;
         }
 
@@ -108,17 +109,6 @@ impl<VM: VMBinding> SFT for ImmixSpace<VM> {
             return false;
         }
 
-        // If the forwarding bits are on the side,
-        if VM::VMObjectModel::LOCAL_FORWARDING_BITS_SPEC.is_on_side() {
-            // we need to ensure `object` is in a defrag source
-            // because `PrepareBlockState` does not clear forwarding bits
-            // for non-defrag-source blocks.
-            if !Block::containing::<VM>(object).is_defrag_source() {
-                // Objects not in defrag sources cannot be forwarded.
-                return false;
-            }
-        }
-
         // If the object is forwarded, it is live, too.
         ForwardingWord::is_forwarded::<VM>(object)
     }
@@ -843,6 +833,10 @@ impl<VM: VMBinding> PrepareBlockState<VM> {
                 unimplemented!("We cannot bulk zero unlogged bit.")
             }
         }
+        // If the forwarding bits are on the side, we need to clear them, too.
+        if let MetadataSpec::OnSide(side) = *VM::VMObjectModel::LOCAL_FORWARDING_BITS_SPEC {
+            side.bzero_metadata(self.chunk.start(), Chunk::BYTES);
+        }
     }
 }
 
@@ -876,19 +870,6 @@ impl<VM: VMBinding> GCWork<VM> for PrepareBlockState<VM> {
             block.set_state(BlockState::Unmarked);
             debug_assert!(!block.get_state().is_reusable());
             debug_assert_ne!(block.get_state(), BlockState::Marked);
-            // Clear forwarding bits if necessary.
-            if is_defrag_source {
-                // Note that `ImmixSpace::is_live` depends on the fact that we only clear side
-                // forwarding bits for defrag sources.  If we change the code here, we need to
-                // make sure `ImmixSpace::is_live` is fixed, too.
-                if let MetadataSpec::OnSide(side) = *VM::VMObjectModel::LOCAL_FORWARDING_BITS_SPEC {
-                    // Clear on-the-side forwarding bits.
-                    side.bzero_metadata(block.start(), Block::BYTES);
-                }
-            }
-            // NOTE: We don't need to reset the forwarding pointer metadata because it is meaningless
-            // until the forwarding bits are also set, at which time we also write the forwarding
-            // pointer.
         }
     }
 }