4 files changed, 96 insertions, 7 deletions
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index 6c139df0a85..0c958de64fa 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -244,7 +244,6 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     fx.debug_introduce_locals(&mut bx);
 
     // Codegen the body of each block using reverse postorder
-    // FIXME(eddyb) reuse RPO iterator between `analysis` and this.
     for (bb, _) in traversal::reverse_postorder(&mir) {
         fx.codegen_block(bb);
     }
diff --git a/compiler/rustc_const_eval/src/transform/promote_consts.rs b/compiler/rustc_const_eval/src/transform/promote_consts.rs
index faea2111d92..1052d588fad 100644
--- a/compiler/rustc_const_eval/src/transform/promote_consts.rs
+++ b/compiler/rustc_const_eval/src/transform/promote_consts.rs
@@ -13,7 +13,7 @@
 //! move analysis runs after promotion on broken MIR.
 
 use rustc_hir as hir;
-use rustc_middle::mir::traversal::ReversePostorder;
+use rustc_middle::mir::traversal::ReversePostorderIter;
 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::cast::CastTy;
@@ -170,7 +170,7 @@ impl<'tcx> Visitor<'tcx> for Collector<'_, 'tcx> {
 
 pub fn collect_temps_and_candidates<'tcx>(
     ccx: &ConstCx<'_, 'tcx>,
-    rpo: &mut ReversePostorder<'_, 'tcx>,
+    rpo: &mut ReversePostorderIter<'_, 'tcx>,
 ) -> (IndexVec<Local, TempState>, Vec<Candidate>) {
     let mut collector = Collector {
         temps: IndexVec::from_elem(TempState::Undefined, &ccx.body.local_decls),
diff --git a/compiler/rustc_middle/src/mir/mod.rs b/compiler/rustc_middle/src/mir/mod.rs
index 883fc72cd56..45999f87658 100644
--- a/compiler/rustc_middle/src/mir/mod.rs
+++ b/compiler/rustc_middle/src/mir/mod.rs
@@ -62,7 +62,9 @@ pub mod spanview;
 mod switch_sources;
 pub mod tcx;
 pub mod terminator;
+use crate::mir::traversal::PostorderCache;
 pub use terminator::*;
+
 pub mod traversal;
 mod type_foldable;
 pub mod visit;
@@ -323,6 +325,7 @@ pub struct Body<'tcx> {
     predecessor_cache: PredecessorCache,
     switch_source_cache: SwitchSourceCache,
     is_cyclic: GraphIsCyclicCache,
+    postorder_cache: PostorderCache,
 
     pub tainted_by_errors: Option<ErrorGuaranteed>,
 }
@@ -372,6 +375,7 @@ impl<'tcx> Body<'tcx> {
             predecessor_cache: PredecessorCache::new(),
             switch_source_cache: SwitchSourceCache::new(),
             is_cyclic: GraphIsCyclicCache::new(),
+            postorder_cache: PostorderCache::new(),
             tainted_by_errors,
         };
         body.is_polymorphic = body.has_param_types_or_consts();
@@ -401,6 +405,7 @@ impl<'tcx> Body<'tcx> {
             predecessor_cache: PredecessorCache::new(),
             switch_source_cache: SwitchSourceCache::new(),
             is_cyclic: GraphIsCyclicCache::new(),
+            postorder_cache: PostorderCache::new(),
             tainted_by_errors: None,
         };
         body.is_polymorphic = body.has_param_types_or_consts();
@@ -422,6 +427,7 @@ impl<'tcx> Body<'tcx> {
         self.predecessor_cache.invalidate();
         self.switch_source_cache.invalidate();
         self.is_cyclic.invalidate();
+        self.postorder_cache.invalidate();
         &mut self.basic_blocks
     }
 
@@ -432,6 +438,7 @@ impl<'tcx> Body<'tcx> {
         self.predecessor_cache.invalidate();
         self.switch_source_cache.invalidate();
         self.is_cyclic.invalidate();
+        self.postorder_cache.invalidate();
         (&mut self.basic_blocks, &mut self.local_decls)
     }
 
@@ -446,6 +453,7 @@ impl<'tcx> Body<'tcx> {
         self.predecessor_cache.invalidate();
         self.switch_source_cache.invalidate();
         self.is_cyclic.invalidate();
+        self.postorder_cache.invalidate();
         (&mut self.basic_blocks, &mut self.local_decls, &mut self.var_debug_info)
     }
 
diff --git a/compiler/rustc_middle/src/mir/traversal.rs b/compiler/rustc_middle/src/mir/traversal.rs
index d08bede1d73..8d831cc73b8 100644
--- a/compiler/rustc_middle/src/mir/traversal.rs
+++ b/compiler/rustc_middle/src/mir/traversal.rs
@@ -1,4 +1,7 @@
+use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
+use rustc_data_structures::sync::OnceCell;
 use rustc_index::bit_set::BitSet;
+use rustc_serialize as serialize;
 
 use super::*;
 
@@ -268,10 +271,6 @@ impl<'a, 'tcx> ReversePostorder<'a, 'tcx> {
     }
 }
 
-pub fn reverse_postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> ReversePostorder<'a, 'tcx> {
-    ReversePostorder::new(body, START_BLOCK)
-}
-
 impl<'a, 'tcx> Iterator for ReversePostorder<'a, 'tcx> {
     type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
 
@@ -307,3 +306,86 @@ pub fn reachable_as_bitset<'tcx>(body: &Body<'tcx>) -> BitSet<BasicBlock> {
     (&mut iter).for_each(drop);
     iter.visited
 }
+
+#[derive(Clone)]
+pub struct ReversePostorderIter<'a, 'tcx> {
+    body: &'a Body<'tcx>,
+    blocks: &'a Vec<BasicBlock>,
+    idx: usize,
+}
+
+impl<'a, 'tcx> Iterator for ReversePostorderIter<'a, 'tcx> {
+    type Item = (BasicBlock, &'a BasicBlockData<'tcx>);
+
+    fn next(&mut self) -> Option<(BasicBlock, &'a BasicBlockData<'tcx>)> {
+        if self.idx == 0 {
+            return None;
+        }
+        self.idx -= 1;
+
+        self.blocks.get(self.idx).map(|&bb| (bb, &self.body[bb]))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.idx, Some(self.idx))
+    }
+}
+
+impl<'a, 'tcx> ExactSizeIterator for ReversePostorderIter<'a, 'tcx> {}
+
+pub fn reverse_postorder<'a, 'tcx>(body: &'a Body<'tcx>) -> ReversePostorderIter<'a, 'tcx> {
+    let blocks = body.postorder_cache.compute(body);
+
+    let len = blocks.len();
+
+    ReversePostorderIter { body, blocks, idx: len }
+}
+
+#[derive(Clone, Debug)]
+pub(super) struct PostorderCache {
+    cache: OnceCell<Vec<BasicBlock>>,
+}
+
+impl PostorderCache {
+    #[inline]
+    pub(super) fn new() -> Self {
+        PostorderCache { cache: OnceCell::new() }
+    }
+
+    /// Invalidates the postorder cache.
+    #[inline]
+    pub(super) fn invalidate(&mut self) {
+        self.cache = OnceCell::new();
+    }
+
+    /// Returns the &Vec<BasicBlocks> represents the postorder graph for this MIR.
+    #[inline]
+    pub(super) fn compute(&self, body: &Body<'_>) -> &Vec<BasicBlock> {
+        self.cache.get_or_init(|| Postorder::new(body, START_BLOCK).map(|(bb, _)| bb).collect())
+    }
+}
+
+impl<S: serialize::Encoder> serialize::Encodable<S> for PostorderCache {
+    #[inline]
+    fn encode(&self, s: &mut S) -> Result<(), S::Error> {
+        s.emit_unit()
+    }
+}
+
+impl<D: serialize::Decoder> serialize::Decodable<D> for PostorderCache {
+    #[inline]
+    fn decode(_: &mut D) -> Self {
+        Self::new()
+    }
+}
+
+impl<CTX> HashStable<CTX> for PostorderCache {
+    #[inline]
+    fn hash_stable(&self, _: &mut CTX, _: &mut StableHasher) {
+        // do nothing
+    }
+}
+
+TrivialTypeFoldableAndLiftImpls! {
+    PostorderCache,
+}