Add pass to ensure predecessors cache is generated after optimization

src/librustc/mir/mod.rs

@@ -207,27 +207,14 @@ impl<'tcx> Body<'tcx> {
     }
 
     pub fn basic_block_terminator_opt_mut(&mut self, bb: BasicBlock) -> &mut Option<Terminator<'tcx>> {
+        // FIXME we should look into improving the cache invalidation
         self.predecessors_cache = None;
         &mut self.basic_blocks[bb].terminator
     }
 
     pub fn basic_block_terminator_mut(&mut self, bb: BasicBlock) -> &mut Terminator<'tcx> {
+        // FIXME we should look into improving the cache invalidation
         self.predecessors_cache = None;
-/*
-        let data = &mut self.basic_blocks[bb];
-        if let Some(cache) = self.predecessors_cache.as_mut() {
-            for successor in data.terminator().successors() {
-                let successor_vec = &mut cache[*successor];
-                for i in (0..successor_vec.len()).rev() {
-                    if successor_vec[i] == bb {
-                        successor_vec.swap_remove(i);
-                        break;
-                    }
-                }
-            }
-        }
-*/
-
         self.basic_blocks[bb].terminator_mut()
     }
 
@@ -245,6 +232,7 @@ impl<'tcx> Body<'tcx> {
     }
 
     #[inline]
+    /// This will recompute the predecessors cache if it is not available
     pub fn predecessors(&mut self) -> &IndexVec<BasicBlock, Vec<BasicBlock>> {
         if self.predecessors_cache.is_none() {
             self.predecessors_cache = Some(self.calculate_predecessors())

src/librustc_mir/transform/ensure_predecessors_cache.rs

@@ -0,0 +1,15 @@
+use rustc::mir::Body;
+use rustc::ty::TyCtxt;
+use crate::transform::{MirPass, MirSource};
+
+pub struct EnsurePredecessorsCache;
+
+impl<'tcx> MirPass<'tcx> for EnsurePredecessorsCache {
+    fn run_pass(&self, _: TyCtxt<'tcx>, _: MirSource<'tcx>, body: &mut Body<'tcx>) {
+        // predecessors is lazily calculated. We want to ensure that the cache is properly filled
+        // before the next stages of compilation, since thise following stages will only be allowed
+        // to read the cache and not generate it. If the cache is already up to date, this line is
+        // a nop.
+        let _predecessors = body.predecessors();
+    }
+}

src/librustc_mir/transform/mod.rs

@@ -20,6 +20,7 @@ pub mod check_unsafety;
 pub mod simplify_branches;
 pub mod simplify_try;
 pub mod simplify;
+pub mod ensure_predecessors_cache;
 pub mod erase_regions;
 pub mod no_landing_pads;
 pub mod rustc_peek;
@@ -313,6 +314,7 @@ fn run_optimization_passes<'tcx>(
         &simplify::SimplifyLocals,
 
         &add_call_guards::CriticalCallEdges,
+        &ensure_predecessors_cache::EnsurePredecessorsCache,
         &dump_mir::Marker("PreCodegen"),
     ]);
 }