Add comments to explain memory usage optimization

[rust.git] / compiler / rustc_mir / src / dataflow / framework / engine.rs

diff --git a/compiler/rustc_mir/src/dataflow/framework/engine.rs b/compiler/rustc_mir/src/dataflow/framework/engine.rs
index c1e500d01b1d34cb070d550ae820ad31be561c89..3f9f558223bb00dabc2d89860953553237da139e 100644 (file)
--- a/compiler/rustc_mir/src/dataflow/framework/engine.rs
+++ b/compiler/rustc_mir/src/dataflow/framework/engine.rs
@@ -62,15 +62,6 @@ pub fn visit_reachable_with(
         let blocks = mir::traversal::reachable(body);
         visit_results(body, blocks.map(|(bb, _)| bb), self, vis)
     }
-
-    pub fn visit_in_rpo_with(
-        &self,
-        body: &'mir mir::Body<'tcx>,
-        vis: &mut impl ResultsVisitor<'mir, 'tcx, FlowState = A::Domain>,
-    ) {
-        let blocks = mir::traversal::reverse_postorder(body);
-        visit_results(body, blocks.map(|(bb, _)| bb), self, vis)
-    }
 }
 
 /// A solver for dataflow problems.
@@ -217,12 +208,19 @@ pub fn iterate_to_fixpoint(self) -> Results<'tcx, A>
             }
         }
 
+        // `state` is not actually used between iterations;
+        // this is just an optimization to avoid reallocating
+        // every iteration.
         let mut state = analysis.bottom_value(body);
         while let Some(bb) = dirty_queue.pop() {
             let bb_data = &body[bb];
 
-            // Apply the block transfer function, using the cached one if it exists.
+            // Set the state to the entry state of the block.
+            // This is equivalent to `state = entry_sets[bb].clone()`,
+            // but it saves an allocation, thus improving compile times.
             state.clone_from(&entry_sets[bb]);
+
+            // Apply the block transfer function, using the cached one if it exists.
             match &apply_trans_for_block {
                 Some(apply) => apply(bb, &mut state),
                 None => A::Direction::apply_effects_in_block(&analysis, &mut state, bb, bb_data),