Add a JumpTableData::with_capacity and use it.

As with Vec::with_capacity, this helps reduce intermediate heap allocation.

lib/cretonne/src/ir/jumptable.rs

@@ -31,6 +31,14 @@ impl JumpTableData {
         }
     }
 
+    /// Create a new empty jump table with the specified capacity.
+    pub fn with_capacity(capacity: usize) -> JumpTableData {
+        JumpTableData {
+            table: Vec::with_capacity(capacity),
+            holes: 0,
+        }
+    }
+
     /// Set a table entry.
     ///
     /// The table will grow as needed to fit `idx`.

lib/wasm/src/code_translator.rs

@@ -364,7 +364,7 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
             if jump_args_count == 0 {
                 // No jump arguments
                 let val = state.pop1();
-                let mut data = JumpTableData::new();
+                let mut data = JumpTableData::with_capacity(depths.len());
                 for depth in depths {
                     let i = state.control_stack.len() - 1 - (depth as usize);
                     let frame = &mut state.control_stack[i];
@@ -385,7 +385,7 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                 // We then proceed to split the edges going out of the br_table
                 let val = state.pop1();
                 let cut_index = state.stack.len() - jump_args_count;
-                let mut data = JumpTableData::new();
+                let mut data = JumpTableData::with_capacity(depths.len());
                 let dest_ebbs: HashMap<usize, Ebb> = depths.iter().fold(HashMap::new(), |mut acc,
                  &depth| {
                     if acc.get(&(depth as usize)).is_none() {