Auto merge of #34149 - arielb1:remove-remove-dead-blocks, r=nikomatsakis

MIR cleanups and predecessor cache

This PR cleans up a few things in MIR and adds a predecessor cache to allow graph algorithms to be run easily.

r? @nikomatsakis

Author: bors

src/librustc/lib.rs

@@ -28,6 +28,7 @@
 #![feature(box_syntax)]
 #![feature(collections)]
 #![feature(const_fn)]
+#![feature(core_intrinsics)]
 #![feature(enumset)]
 #![feature(iter_arith)]
 #![feature(libc)]
@@ -102,6 +103,7 @@ pub mod middle {
 }
 
 pub mod mir {
+    mod cache;
     pub mod repr;
     pub mod tcx;
     pub mod visit;

src/librustc/mir/cache.rs

@@ -0,0 +1,69 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::cell::{Ref, RefCell};
+use rustc_data_structures::indexed_vec::IndexVec;
+
+use mir::repr::{Mir, BasicBlock};
+
+use rustc_serialize as serialize;
+
+#[derive(Clone)]
+pub struct Cache {
+    predecessors: RefCell<Option<IndexVec<BasicBlock, Vec<BasicBlock>>>>
+}
+
+
+impl serialize::Encodable for Cache {
+    fn encode<S: serialize::Encoder>(&self, s: &mut S) -> Result<(), S::Error> {
+        serialize::Encodable::encode(&(), s)
+    }
+}
+
+impl serialize::Decodable for Cache {
+    fn decode<D: serialize::Decoder>(d: &mut D) -> Result<Self, D::Error> {
+        serialize::Decodable::decode(d).map(|_v: ()| Self::new())
+    }
+}
+
+
+impl Cache {
+    pub fn new() -> Self {
+        Cache {
+            predecessors: RefCell::new(None)
+        }
+    }
+
+    pub fn invalidate(&self) {
+        // FIXME: consider being more fine-grained
+        *self.predecessors.borrow_mut() = None;
+    }
+
+    pub fn predecessors(&self, mir: &Mir) -> Ref<IndexVec<BasicBlock, Vec<BasicBlock>>> {
+        if self.predecessors.borrow().is_none() {
+            *self.predecessors.borrow_mut() = Some(calculate_predecessors(mir));
+        }
+
+        Ref::map(self.predecessors.borrow(), |p| p.as_ref().unwrap())
+    }
+}
+
+fn calculate_predecessors(mir: &Mir) -> IndexVec<BasicBlock, Vec<BasicBlock>> {
+    let mut result = IndexVec::from_elem(vec![], mir.basic_blocks());
+    for (bb, data) in mir.basic_blocks().iter_enumerated() {
+        if let Some(ref term) = data.terminator {
+            for &tgt in term.successors().iter() {
+                result[tgt].push(bb);
+            }
+        }
+    }
+
+    result
+}