add immix_zero_on_release (#736)

Add `immix_zero_on_release` feature to assist debugging.

Author: JunmingZhao42

Cargo.toml

@@ -95,6 +95,8 @@ object_pinning = []
 immix_no_defrag = []
 # Reduce block size for ImmixSpace.  This mitigates fragmentation when defrag is disabled.
 immix_smaller_block = []
+# Zero the unmarked lines after a GC cycle in immix. This helps debug untraced objects.
+immix_zero_on_release = []
 
 # Run sanity GC
 sanity = []

src/policy/immix/block.rs

@@ -240,6 +240,10 @@ impl Block {
                     if prev_line_is_marked {
                         holes += 1;
                     }
+
+                    #[cfg(feature = "immix_zero_on_release")]
+                    crate::util::memory::zero(line.start(), Line::BYTES);
+
                     prev_line_is_marked = false;
                 }
             }