Clear pin bits (#1166)

If an object dies, we need to make sure his pin bit gets cleared. This
PR enables clearing the pin bits when sweeping - clearing the whole
block when `super::BLOCK_ONLY` is true, or at line granularity when a
line is not marked.
I've only added it to the immix policy, since for everything else
pinning is either unsupported or it's a NOP.

Author: udesou

src/policy/immix/block.rs

@@ -8,6 +8,8 @@ use crate::util::linear_scan::{Region, RegionIterator};
 use crate::util::metadata::side_metadata::{MetadataByteArrayRef, SideMetadataSpec};
 #[cfg(feature = "vo_bit")]
 use crate::util::metadata::vo_bit;
+#[cfg(feature = "object_pinning")]
+use crate::util::metadata::MetadataSpec;
 use crate::util::Address;
 use crate::vm::*;
 use std::sync::atomic::Ordering;
@@ -201,6 +203,15 @@ impl Block {
                     #[cfg(feature = "vo_bit")]
                     vo_bit::helper::on_region_swept::<VM, _>(self, false);
 
+                    // If the pin bit is not on the side, we cannot bulk zero.
+                    // We shouldn't need to clear it here in that case, since the pin bit
+                    // should be overwritten at each object allocation. The same applies below
+                    // when we are sweeping on a line granularity.
+                    #[cfg(feature = "object_pinning")]
+                    if let MetadataSpec::OnSide(side) = *VM::VMObjectModel::LOCAL_PINNING_BIT_SPEC {
+                        side.bzero_metadata(self.start(), Block::BYTES);
+                    }
+
                     // Release the block if it is allocated but not marked by the current GC.
                     space.release_block(*self);
                     true
@@ -233,6 +244,12 @@ impl Block {
                     #[cfg(feature = "immix_zero_on_release")]
                     crate::util::memory::zero(line.start(), Line::BYTES);
 
+                    // We need to clear the pin bit if it is on the side, as this line can be reused
+                    #[cfg(feature = "object_pinning")]
+                    if let MetadataSpec::OnSide(side) = *VM::VMObjectModel::LOCAL_PINNING_BIT_SPEC {
+                        side.bzero_metadata(line.start(), Line::BYTES);
+                    }
+
                     prev_line_is_marked = false;
                 }
             }