Move trace_object() to SFT

[qinsoon]

As raised in #108 (comment), invoking initialize_header() from a Space trait object would result in dynamic dispatching in the fast path. We can move initialize_header() to SFT, this would solve the problem.

[qinsoon]

We want to do a similar refactoring for trace_object() as well.

[qinsoon]

initialize_header() is in SFT. However, we cannot easily move trace_object() to SFT without further refactoring. SFT is a trait object, which means its methods need to be simple. However, trace_object() is a generic method (with T: TransitiveClosure), which is not simple. We need refactoring.

[steveblackburn]

I'd like to up the priority on this. Perhaps this can be done as part of the next bugfix/cleanup release.

[qinsoon]

initialize_header() is in SFT. However, we cannot easily move trace_object() to SFT without further refactoring. SFT is a trait object, which means its methods need to be simple. However, trace_object() is a generic method (with T: TransitiveClosure), which is not simple. We need refactoring.

trace_object() uses TransitiveClosure and CopyContext as type parameters. However, once we remove the plan-specific process edges (instead use a general SFT-based process edges), the type TransitiveClosure can be removed. The type parameter for CopyContext doesn't really matter. We can simply use dyn CopyContext - as long as we need to use CopyContext (we are copying which is slow), the overhead of dynamic dispatch on dyn CopyContext shouldn't affect much.

[wks]

trace_object() has two type parameters, but the types themselves are never explicitly used in the function body. So they can be replaced with impl TransitiveClosure and impl CopyContext. They are semantically equivalent, but less verbose. impl SomeTrait means the concrete type of the parameter can still be inferred at compile time, but we just don't mention the concrete type name. Therefore it still has the performance advantage (including opportunities to inline) of generic parameters.

diff --git a/src/policy/copyspace.rs b/src/policy/copyspace.rs
index 77e75321..d6ad9154 100644
--- a/src/policy/copyspace.rs
+++ b/src/policy/copyspace.rs
@@ -182,12 +182,12 @@ impl<VM: VMBinding> CopySpace<VM> {
     }
 
     #[inline]
-    pub fn trace_object<T: TransitiveClosure, C: CopyContext>(
+    pub fn trace_object(
         &self,
-        trace: &mut T,
+        trace: &mut impl TransitiveClosure,
         object: ObjectReference,
         semantics: AllocationSemantics,
-        copy_context: &mut C,
+        copy_context: &mut impl CopyContext,
     ) -> ObjectReference {
         trace!("copyspace.trace_object(, {:?}, {:?})", object, semantics,);
         debug_assert!(

[qinsoon]

trace_object() has two type parameters, but the types themselves are never explicitly used in the function body. So they can be replaced with impl TransitiveClosure and impl CopyContext. They are semantically equivalent, but less verbose. impl SomeTrait means the concrete type of the parameter can still be inferred at compile time, but we just don't mention the concrete type name. Therefore it still has the performance advantage (including opportunities to inline) of generic parameters.

diff --git a/src/policy/copyspace.rs b/src/policy/copyspace.rs
index 77e75321..d6ad9154 100644
--- a/src/policy/copyspace.rs
+++ b/src/policy/copyspace.rs
@@ -182,12 +182,12 @@ impl<VM: VMBinding> CopySpace<VM> {
     }
 
     #[inline]
-    pub fn trace_object<T: TransitiveClosure, C: CopyContext>(
+    pub fn trace_object(
         &self,
-        trace: &mut T,
+        trace: &mut impl TransitiveClosure,
         object: ObjectReference,
         semantics: AllocationSemantics,
-        copy_context: &mut C,
+        copy_context: &mut impl CopyContext,
     ) -> ObjectReference {
         trace!("copyspace.trace_object(, {:?}, {:?})", object, semantics,);
         debug_assert!(

I think impl Trait is considered the same as generic type parameters in terms of object safety. That means if we would like to use a trait object, its method cannot use <T> as well as a: impl Trait as parameters.