Fix get_available_pages() for SemiSpace (#721)

Closes #615.

Author: k-sareen

src/plan/generational/copying/global.rs

@@ -143,7 +143,7 @@ impl<VM: VMBinding> Plan for GenCopy<VM> {
 
     /// Return the number of pages available for allocation. Assuming all future allocations goes to nursery.
     fn get_available_pages(&self) -> usize {
-        // super.get_pages_avail() / 2 to reserve pages for copying
+        // super.get_available_pages() / 2 to reserve pages for copying
         (self
             .get_total_pages()
             .saturating_sub(self.get_reserved_pages()))

src/plan/generational/immix/global.rs

@@ -183,9 +183,9 @@ impl<VM: VMBinding> Plan for GenImmix<VM> {
         self.gen.get_used_pages() + self.immix.reserved_pages()
     }
 
-    /// Return the number of pages avilable for allocation. Assuming all future allocations goes to nursery.
+    /// Return the number of pages available for allocation. Assuming all future allocations goes to nursery.
     fn get_available_pages(&self) -> usize {
-        // super.get_pages_avail() / 2 to reserve pages for copying
+        // super.get_available_pages() / 2 to reserve pages for copying
         (self
             .get_total_pages()
             .saturating_sub(self.get_reserved_pages()))

src/plan/semispace/global.rs

@@ -118,6 +118,13 @@ impl<VM: VMBinding> Plan for SemiSpace<VM> {
         self.tospace().reserved_pages() + self.common.get_used_pages()
     }
 
+    fn get_available_pages(&self) -> usize {
+        (self
+            .get_total_pages()
+            .saturating_sub(self.get_reserved_pages()))
+            >> 1
+    }
+
     fn base(&self) -> &BasePlan<VM> {
         &self.common.base
     }