X-Git-Url: https://git.lizzy.rs/?a=blobdiff_plain;f=compiler%2Frustc_mir%2Fsrc%2Fdataflow%2Fframework%2Fengine.rs;h=3f9f558223bb00dabc2d89860953553237da139e;hb=0242f963c631a130a3c405d7e54f4695ef10a139;hp=1b7264f86a2d159348da7d67d0e24c944ef59f56;hpb=83ee3198220b9738f6e4f04bfbdcf3481d5c7b59;p=rust.git

diff --git a/compiler/rustc_mir/src/dataflow/framework/engine.rs b/compiler/rustc_mir/src/dataflow/framework/engine.rs
index 1b7264f86a2..3f9f558223b 100644
--- a/compiler/rustc_mir/src/dataflow/framework/engine.rs
+++ b/compiler/rustc_mir/src/dataflow/framework/engine.rs
@@ -208,12 +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),