Add EntityMap::with_capacity.

Create a secondary entity map with space reserved for a known range of entity
references.

Add dfg.num_ebbs() and dfg.num_insts() methods to provide capacities.

cranelift/src/libcretonne/entity_map.rs

@@ -102,6 +102,21 @@ impl<K, V> EntityMap<K, V>
     where K: EntityRef,
           V: Clone + Default
 {
+    /// Create a new secondary `EntityMap` that is prepared to hold `n` elements.
+    ///
+    /// Use this when the length of the primary map is known:
+    /// ```
+    /// let secondary_map = EntityMap::with_capacity(primary_map.len());
+    /// ```
+    pub fn with_capacity(n: usize) -> Self {
+        let mut map = EntityMap {
+            elems: Vec::with_capacity(n),
+            unused: PhantomData,
+        };
+        map.elems.resize(n, V::default());
+        map
+    }
+
     /// Ensure that `k` is a valid key but adding default entries if necesssary.
     ///
     /// Return a mutable reference to the corresponding entry.

cranelift/src/libcretonne/ir/dfg.rs

@@ -42,6 +42,22 @@ impl DataFlowGraph {
             extended_values: Vec::new(),
         }
     }
+
+    /// Get the total number of instructions created in this function, whether they are currently
+    /// inserted in the layout or not.
+    ///
+    /// This is intended for use with `EntityMap::with_capacity`.
+    pub fn num_insts(&self) -> usize {
+        self.insts.len()
+    }
+
+    /// Get the total number of extended basic blocks created in this function, whether they are
+    /// currently inserted in the layout or not.
+    ///
+    /// This is intended for use with `EntityMap::with_capacity`.
+    pub fn num_ebbs(&self) -> usize {
+        self.ebbs.len()
+    }
 }
 
 /// Handling values.