[ruby/mmtk] Remove unused constant

Remove the unused constant HAS_MOVED_GFIELDSTBL and related methods.

In the mmtk/mmtk-ruby repo, we are now able to find the global field
(IV) table of a moved object during copying GC without using the
HAS_MOVED_GFIELDSTBL bit.  We synchronize some of the code, although we
haven't implemented moving GC in ruby/mmtk, yet.

See: 13080acdf5

400ba4e747

gc/mmtk/Cargo.toml

@@ -36,4 +36,7 @@ default = []
 # When moving an object, clear its original copy.
 clear_old_copy = []
 
+# Enable extra assertions in release build.  For debugging.
+extra_assert = []
+
 [workspace]

gc/mmtk/src/abi.rs

@@ -1,5 +1,5 @@
 use crate::api::RubyMutator;
-use crate::Ruby;
+use crate::{extra_assert, Ruby};
 use libc::c_int;
 use mmtk::scheduler::GCWorker;
 use mmtk::util::{Address, ObjectReference, VMMutatorThread, VMWorkerThread};
@@ -10,7 +10,6 @@ pub const MIN_OBJ_ALIGN: usize = 8; // Even on 32-bit machine.  A Ruby object is
 
 pub const GC_THREAD_KIND_WORKER: libc::c_int = 1;
 
-const HAS_MOVED_GFIELDSTBL: usize = 1 << 63;
 const HIDDEN_SIZE_MASK: usize = 0x0000FFFFFFFFFFFF;
 
 // Should keep in sync with C code.
@@ -20,6 +19,32 @@ const RUBY_FL_EXIVAR: usize = 1 << 10;
 #[allow(non_camel_case_types)]
 pub struct st_table;
 
+#[repr(C)]
+pub struct HiddenHeader {
+    pub prefix: usize,
+}
+
+impl HiddenHeader {
+    #[inline(always)]
+    pub fn is_sane(&self) -> bool {
+        self.prefix & !HIDDEN_SIZE_MASK == 0
+    }
+
+    #[inline(always)]
+    fn assert_sane(&self) {
+        extra_assert!(
+            self.is_sane(),
+            "Hidden header is corrupted: {:x}",
+            self.prefix
+        );
+    }
+
+    pub fn payload_size(&self) -> usize {
+        self.assert_sane();
+        self.prefix & HIDDEN_SIZE_MASK
+    }
+}
+
 /// Provide convenient methods for accessing Ruby objects.
 /// TODO: Wrap C functions in `RubyUpcalls` as Rust-friendly methods.
 pub struct RubyObjectAccess {
@@ -47,32 +72,17 @@ impl RubyObjectAccess {
         self.suffix_addr() + Self::suffix_size()
     }
 
-    fn hidden_field(&self) -> Address {
-        self.obj_start()
+    fn hidden_header(&self) -> &'static HiddenHeader {
+        unsafe { self.obj_start().as_ref() }
     }
 
-    fn load_hidden_field(&self) -> usize {
-        unsafe { self.hidden_field().load::<usize>() }
-    }
-
-    fn update_hidden_field<F>(&self, f: F)
-    where
-        F: FnOnce(usize) -> usize,
-    {
-        let old_value = self.load_hidden_field();
-        let new_value = f(old_value);
-        unsafe {
-            self.hidden_field().store(new_value);
-        }
+    #[allow(unused)] // Maybe we need to mutate the hidden header in the future.
+    fn hidden_header_mut(&self) -> &'static mut HiddenHeader {
+        unsafe { self.obj_start().as_mut_ref() }
     }
 
     pub fn payload_size(&self) -> usize {
-        self.load_hidden_field() & HIDDEN_SIZE_MASK
-    }
-
-    pub fn set_payload_size(&self, size: usize) {
-        debug_assert!((size & HIDDEN_SIZE_MASK) == size);
-        self.update_hidden_field(|old| old & !HIDDEN_SIZE_MASK | size & HIDDEN_SIZE_MASK);
+        self.hidden_header().payload_size()
     }
 
     fn flags_field(&self) -> Address {
@@ -87,18 +97,6 @@ impl RubyObjectAccess {
         (self.load_flags() & RUBY_FL_EXIVAR) != 0
     }
 
-    pub fn has_moved_gfields_tbl(&self) -> bool {
-        (self.load_hidden_field() & HAS_MOVED_GFIELDSTBL) != 0
-    }
-
-    pub fn set_has_moved_gfields_tbl(&self) {
-        self.update_hidden_field(|old| old | HAS_MOVED_GFIELDSTBL)
-    }
-
-    pub fn clear_has_moved_gfields_tbl(&self) {
-        self.update_hidden_field(|old| old & !HAS_MOVED_GFIELDSTBL)
-    }
-
     pub fn prefix_size() -> usize {
         // Currently, a hidden size field of word size is placed before each object.
         OBJREF_OFFSET

gc/mmtk/src/lib.rs

@@ -131,3 +131,14 @@ pub(crate) fn set_panic_hook() {
         }
     }));
 }
+
+/// This kind of assertion is enabled if either building in debug mode or the
+/// "extra_assert" feature is enabled.
+#[macro_export]
+macro_rules! extra_assert {
+    ($($arg:tt)*) => {
+        if std::cfg!(any(debug_assertions, feature = "extra_assert")) {
+            std::assert!($($arg)*);
+        }
+    };
+}