Rewrite precompute_borrows_out_of_scope for fewer hash table lookups.

It now does one hash table lookup per basic block, instead of one per
statement. This is worthwhile because this function is hot for NLL
builds of `ucd`.

Author: nnethercote

src/librustc_mir/dataflow/impls/borrows.rs

@@ -21,7 +21,7 @@ use rustc::ty::{RegionKind, RegionVid};
 use rustc::ty::RegionKind::ReScope;
 
 use rustc_data_structures::bitslice::{BitwiseOperator, Word};
-use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::indexed_set::IdxSet;
 use rustc_data_structures::indexed_vec::IndexVec;
 use rustc_data_structures::sync::Lrc;
@@ -53,6 +53,13 @@ pub struct Borrows<'a, 'gcx: 'tcx, 'tcx: 'a> {
     _nonlexical_regioncx: Rc<RegionInferenceContext<'tcx>>,
 }
 
+struct StackEntry {
+    bb: mir::BasicBlock,
+    lo: usize,
+    hi: usize,
+    first_part_only: bool
+}
+
 fn precompute_borrows_out_of_scope<'tcx>(
     mir: &Mir<'tcx>,
     regioncx: &Rc<RegionInferenceContext<'tcx>>,
@@ -61,48 +68,79 @@ fn precompute_borrows_out_of_scope<'tcx>(
     borrow_region: RegionVid,
     location: Location,
 ) {
-    // Keep track of places we've locations to check and locations that we have checked.
-    let mut stack = vec![ location ];
-    let mut visited = FxHashSet();
-    visited.insert(location);
-
-    debug!(
-        "borrow {:?} has region {:?} with value {:?}",
-        borrow_index,
-        borrow_region,
-        regioncx.region_value_str(borrow_region),
-    );
-    debug!("borrow {:?} starts at {:?}", borrow_index, location);
-    while let Some(location) = stack.pop() {
-        // If region does not contain a point at the location, then add to list and skip
-        // successor locations.
-        if !regioncx.region_contains(borrow_region, location) {
-            debug!("borrow {:?} gets killed at {:?}", borrow_index, location);
-            borrows_out_of_scope_at_location
-                .entry(location)
-                .or_default()
-                .push(borrow_index);
-            continue;
+    // We visit one BB at a time. The complication is that we may start in the
+    // middle of the first BB visited (the one containing `location`), in which
+    // case we may have to later on process the first part of that BB if there
+    // is a path back to its start.
+
+    // For visited BBs, we record the index of the first statement processed.
+    // (In fully processed BBs this index is 0.) Note also that we add BBs to
+    // `visited` once they are added to `stack`, before they are actually
+    // processed, because this avoids the need to look them up again on
+    // completion.
+    let mut visited = FxHashMap();
+    visited.insert(location.block, location.statement_index);
+
+    let mut stack = vec![];
+    stack.push(StackEntry {
+        bb: location.block,
+        lo: location.statement_index,
+        hi: mir[location.block].statements.len(),
+        first_part_only: false,
+    });
+
+    while let Some(StackEntry { bb, lo, hi, first_part_only }) = stack.pop() {
+        let mut finished_early = first_part_only;
+        for i in lo ..= hi {
+            let location = Location { block: bb, statement_index: i };
+            // If region does not contain a point at the location, then add to list and skip
+            // successor locations.
+            if !regioncx.region_contains(borrow_region, location) {
+                debug!("borrow {:?} gets killed at {:?}", borrow_index, location);
+                borrows_out_of_scope_at_location
+                    .entry(location)
+                    .or_default()
+                    .push(borrow_index);
+                finished_early = true;
+                break;
+            }
         }
 
-        let bb_data = &mir[location.block];
-        // If this is the last statement in the block, then add the
-        // terminator successors next.
-        if location.statement_index == bb_data.statements.len() {
-            // Add successors to locations to visit, if not visited before.
-            if let Some(ref terminator) = bb_data.terminator {
-                for block in terminator.successors() {
-                    let loc = block.start_location();
-                    if visited.insert(loc) {
-                        stack.push(loc);
-                    }
-                }
-            }
-        } else {
-            // Visit next statement in block.
-            let loc = location.successor_within_block();
-            if visited.insert(loc) {
-                stack.push(loc);
+        if !finished_early {
+            // Add successor BBs to the work list, if necessary.
+            let bb_data = &mir[bb];
+            assert!(hi == bb_data.statements.len());
+            for &succ_bb in bb_data.terminator.as_ref().unwrap().successors() {
+                visited.entry(succ_bb)
+                    .and_modify(|lo| {
+                        // `succ_bb` has been seen before. If it wasn't
+                        // fully processed, add its first part to `stack`
+                        // for processing.
+                        if *lo > 0 {
+                            stack.push(StackEntry {
+                                bb: succ_bb,
+                                lo: 0,
+                                hi: *lo - 1,
+                                first_part_only: true,
+                            });
+                        }
+                        // And update this entry with 0, to represent the
+                        // whole BB being processed.
+                        *lo = 0;
+                    })
+                    .or_insert_with(|| {
+                        // succ_bb hasn't been seen before. Add it to
+                        // `stack` for processing.
+                        stack.push(StackEntry {
+                            bb: succ_bb,
+                            lo: 0,
+                            hi: mir[succ_bb].statements.len(),
+                            first_part_only: false,
+                        });
+                        // Insert 0 for this BB, to represent the whole BB
+                        // being processed.
+                        0
+                    });
             }
         }
     }