Immix: Clear alloc bits during sweeping (#748)

No longer clear alloc bit when an object is being moved. This fixes an
assertion error when trace_object is called on the same object twice
during defrag GC.

Clear alloc bits eagerly during sweeping instead of during allocation.
This ensures an address has the alloc bit (i.e. valid-object bit) if and
only if that address corresponds to a valid object reference during
mutator execution.

Author: wks

src/policy/immix/block.rs

@@ -241,6 +241,9 @@ impl Block {
                         holes += 1;
                     }
 
+                    #[cfg(feature = "global_alloc_bit")]
+                    crate::util::alloc_bit::bzero_alloc_bit(line.start(), Line::BYTES);
+
                     #[cfg(feature = "immix_zero_on_release")]
                     crate::util::memory::zero(line.start(), Line::BYTES);
 

src/policy/immix/immixspace.rs

@@ -510,8 +510,6 @@ impl<VM: VMBinding> ImmixSpace<VM> {
                 Block::containing::<VM>(object).set_state(BlockState::Marked);
                 object
             } else {
-                #[cfg(feature = "global_alloc_bit")]
-                crate::util::alloc_bit::unset_alloc_bit::<VM>(object);
                 ForwardingWord::forward_object::<VM>(object, semantics, copy_context)
             };
             debug_assert_eq!(

src/util/alloc/immix_allocator.rs

@@ -257,8 +257,6 @@ impl<VM: VMBinding> ImmixAllocator<VM> {
                     end_line,
                     self.tls
                 );
-                #[cfg(feature = "global_alloc_bit")]
-                crate::util::alloc_bit::bzero_alloc_bit(self.cursor, self.limit - self.cursor);
                 crate::util::memory::zero(self.cursor, self.limit - self.cursor);
                 debug_assert!(
                     align_allocation_no_fill::<VM>(self.cursor, align, offset) + size <= self.limit