Auto merge of #56983 - ljedrz:parallel_query_tweaks, r=Zoxc

Parallel query tweaks

- faster stack reversal in `remove_cycle`
- insert visited queries more eagerly
- simplify stack trimming in `cycle_check`
- minor refactoring in 2 spots

2 files changed, 10 insertions, 22 deletions

diff --git a/src/librustc/ty/query/job.rs b/src/librustc/ty/query/job.rs
index 6e513d68f60..ba722661097 100644
--- a/src/librustc/ty/query/job.rs
+++ b/src/librustc/ty/query/job.rs
@@ -303,12 +303,12 @@ fn cycle_check<'tcx>(query: Lrc<QueryJob<'tcx>>,
                      stack: &mut Vec<(Span, Lrc<QueryJob<'tcx>>)>,
                      visited: &mut FxHashSet<*const QueryJob<'tcx>>
 ) -> Option<Option<Waiter<'tcx>>> {
-    if visited.contains(&query.as_ptr()) {
+    if !visited.insert(query.as_ptr()) {
         return if let Some(p) = stack.iter().position(|q| q.1.as_ptr() == query.as_ptr()) {
             // We detected a query cycle, fix up the initial span and return Some
 
             // Remove previous stack entries
-            stack.splice(0..p, iter::empty());
+            stack.drain(0..p);
             // Replace the span for the first query with the cycle cause
             stack[0].0 = span;
             Some(None)
@@ -317,8 +317,7 @@ fn cycle_check<'tcx>(query: Lrc<QueryJob<'tcx>>,
         }
     }
 
-    // Mark this query is visited and add it to the stack
-    visited.insert(query.as_ptr());
+    // Query marked as visited is added it to the stack
     stack.push((span, query.clone()));
 
     // Visit all the waiters
@@ -343,7 +342,7 @@ fn connected_to_root<'tcx>(
     visited: &mut FxHashSet<*const QueryJob<'tcx>>
 ) -> bool {
     // We already visited this or we're deliberately ignoring it
-    if visited.contains(&query.as_ptr()) {
+    if !visited.insert(query.as_ptr()) {
         return false;
     }
 
@@ -352,8 +351,6 @@ fn connected_to_root<'tcx>(
         return true;
     }
 
-    visited.insert(query.as_ptr());
-
     visit_waiters(query, |_, successor| {
         if connected_to_root(successor, visited) {
             Some(None)
@@ -403,11 +400,9 @@ fn remove_cycle<'tcx>(
                                       DUMMY_SP,
                                       &mut stack,
                                       &mut visited) {
-        // Reverse the stack so earlier entries require later entries
-        stack.reverse();
-
-        // The stack is a vector of pairs of spans and queries
-        let (mut spans, queries): (Vec<_>, Vec<_>) = stack.into_iter().unzip();
+        // The stack is a vector of pairs of spans and queries; reverse it so that
+        // the earlier entries require later entries
+        let (mut spans, queries): (Vec<_>, Vec<_>) = stack.into_iter().rev().unzip();
 
         // Shift the spans so that queries are matched with the span for their waitee
         spans.rotate_right(1);
@@ -424,7 +419,7 @@ fn remove_cycle<'tcx>(
 
         // Find the queries in the cycle which are
         // connected to queries outside the cycle
-        let entry_points: Vec<_> = stack.iter().filter_map(|(span, query)| {
+        let entry_points = stack.iter().filter_map(|(span, query)| {
             if query.parent.is_none() {
                 // This query is connected to the root (it has no query parent)
                 Some((*span, query.clone(), None))
@@ -449,10 +444,7 @@ fn remove_cycle<'tcx>(
                     Some((*span, query.clone(), Some(waiter)))
                 }
             }
-        }).collect();
-
-        let entry_points: Vec<(Span, Lrc<QueryJob<'tcx>>, Option<(Span, Lrc<QueryJob<'tcx>>)>)>
-         = entry_points;
+        }).collect::<Vec<(Span, Lrc<QueryJob<'tcx>>, Option<(Span, Lrc<QueryJob<'tcx>>)>)>>();
 
         // Deterministically pick an entry point
         let (_, entry_point, usage) = pick_query(tcx, &entry_points, |e| (e.0, e.1.clone()));
diff --git a/src/librustc/ty/query/on_disk_cache.rs b/src/librustc/ty/query/on_disk_cache.rs
index 04c880826fe..94456e3e773 100644
--- a/src/librustc/ty/query/on_disk_cache.rs
+++ b/src/librustc/ty/query/on_disk_cache.rs
@@ -398,11 +398,7 @@ impl<'sess> OnDiskCache<'sess> {
                              -> Option<T>
         where T: Decodable
     {
-        let pos = if let Some(&pos) = index.get(&dep_node_index) {
-            pos
-        } else {
-            return None
-        };
+        let pos = index.get(&dep_node_index).cloned()?;
 
         // Initialize the cnum_map using the value from the thread which finishes the closure first
         self.cnum_map.init_nonlocking_same(|| {