From 0242f963c631a130a3c405d7e54f4695ef10a139 Mon Sep 17 00:00:00 2001 From: Camelid Date: Sun, 8 Nov 2020 13:52:15 -0800 Subject: [PATCH] Add comments to explain memory usage optimization --- compiler/rustc_mir/src/dataflow/framework/engine.rs | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) 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 @@ where } } + // `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),