From 60fa5351a3f2891cafd6f67dda6e7bb121d6f7c0 Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <nnethercote@mozilla.com>
Date: Thu, 26 Sep 2019 07:06:00 +1000
Subject: [PATCH] Avoid `chain()` in `find_constraint_paths_between_regions()`.

This iterator can be hot, and chained iterators are slow. The second
half of the chain is almost always empty, so this commit specializes the
code to avoid the chained iteration.

This change reduces instruction counts for the `wg-grammar` benchmark by
up to 1.5%.
---
 .../nll/region_infer/error_reporting/mod.rs   | 54 +++++++++++--------
 1 file changed, 33 insertions(+), 21 deletions(-)

diff --git a/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs
index 26a89b4e7a8d1..849ba781001c8 100644
--- a/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs
+++ b/src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs
@@ -253,28 +253,40 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             let outgoing_edges_from_graph = self.constraint_graph
                 .outgoing_edges(r, &self.constraints, fr_static);
 
-
-            // But member constraints can also give rise to `'r: 'x`
-            // edges that were not part of the graph initially, so
-            // watch out for those.
-            let outgoing_edges_from_picks = self.applied_member_constraints(r)
-                .iter()
-                .map(|&AppliedMemberConstraint { min_choice, member_constraint_index, .. }| {
-                    let p_c = &self.member_constraints[member_constraint_index];
-                    OutlivesConstraint {
-                        sup: r,
-                        sub: min_choice,
-                        locations: Locations::All(p_c.definition_span),
-                        category: ConstraintCategory::OpaqueType,
+            macro_rules! handle_constraint {
+                ($constraint:ident) => {
+                    debug_assert_eq!($constraint.sup, r);
+                    let sub_region = $constraint.sub;
+                    if let Trace::NotVisited = context[sub_region] {
+                        context[sub_region] = Trace::FromOutlivesConstraint($constraint);
+                        deque.push_back(sub_region);
                     }
-                });
-
-            for constraint in outgoing_edges_from_graph.chain(outgoing_edges_from_picks) {
-                debug_assert_eq!(constraint.sup, r);
-                let sub_region = constraint.sub;
-                if let Trace::NotVisited = context[sub_region] {
-                    context[sub_region] = Trace::FromOutlivesConstraint(constraint);
-                    deque.push_back(sub_region);
+                }
+            }
+
+            for constraint in outgoing_edges_from_graph {
+                handle_constraint!(constraint);
+            }
+
+            // But member constraints can also give rise to `'r: 'x` edges that
+            // were not part of the graph initially, so watch out for those.
+            // (But they are extremely rare.)
+            let applied_member_constraints = self.applied_member_constraints(r);
+            if !applied_member_constraints.is_empty() {
+                let outgoing_edges_from_picks = self.applied_member_constraints(r)
+                    .iter()
+                    .map(|&AppliedMemberConstraint { min_choice, member_constraint_index, .. }| {
+                        let p_c = &self.member_constraints[member_constraint_index];
+                        OutlivesConstraint {
+                            sup: r,
+                            sub: min_choice,
+                            locations: Locations::All(p_c.definition_span),
+                            category: ConstraintCategory::OpaqueType,
+                        }
+                    });
+
+                for constraint in outgoing_edges_from_picks {
+                    handle_constraint!(constraint);
                 }
             }
         }