Add some comments to the frontend code.

cranelift/frontend/src/frontend.rs

@@ -41,8 +41,17 @@ pub struct FunctionBuilder<'a> {
 
 #[derive(Clone, Default)]
 struct EbbData {
-    filled: bool,
+    /// An Ebb is "pristine" iff no instructions have been added since the last
+    /// call to `switch_to_block()`.
     pristine: bool,
+
+    /// An Ebb is "filled" iff a terminator instruction has been inserted since
+    /// the last call to `switch_to_block()`.
+    ///
+    /// A filled block cannot be pristine.
+    filled: bool,
+
+    /// Count of parameters not supplied implicitly by the SSABuilder.
     user_param_count: usize,
 }
 

cranelift/wasm/src/func_translator.rs

@@ -89,7 +89,8 @@ impl FuncTranslator {
         let entry_block = builder.create_ebb();
         builder.append_ebb_params_for_function_params(entry_block);
         builder.switch_to_block(entry_block); // This also creates values for the arguments.
-        builder.seal_block(entry_block);
+        builder.seal_block(entry_block); // Declare all predecessors known.
+
         // Make sure the entry block is inserted in the layout before we make any callbacks to
         // `environ`. The callback functions may need to insert things in the entry block.
         builder.ensure_inserted_ebb();