add a cache for MIR predecessors

src/librustc/lib.rs

@@ -102,6 +102,7 @@ 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
+}

src/librustc/mir/repr.rs

@@ -20,12 +20,15 @@ use rustc_back::slice;
 use hir::InlineAsm;
 use std::ascii;
 use std::borrow::{Cow};
+use std::cell::Ref;
 use std::fmt::{self, Debug, Formatter, Write};
 use std::{iter, u32};
 use std::ops::{Index, IndexMut};
 use syntax::ast::{self, Name};
 use syntax::codemap::Span;
 
+use super::cache::Cache;
+
 macro_rules! newtype_index {
     ($name:ident, $debug_name:expr) => (
         #[derive(Copy, Clone, PartialEq, Eq, Hash, PartialOrd, Ord,
@@ -88,6 +91,9 @@ pub struct Mir<'tcx> {
 
     /// A span representing this MIR, for error reporting
     pub span: Span,
+
+    /// A cache for various calculations
+    cache: Cache
 }
 
 /// where execution begins
@@ -113,7 +119,8 @@ impl<'tcx> Mir<'tcx> {
             arg_decls: arg_decls,
             temp_decls: temp_decls,
             upvar_decls: upvar_decls,
-            span: span
+            span: span,
+            cache: Cache::new()
         }
     }
 
@@ -124,8 +131,19 @@ impl<'tcx> Mir<'tcx> {
 
     #[inline]
     pub fn basic_blocks_mut(&mut self) -> &mut IndexVec<BasicBlock, BasicBlockData<'tcx>> {
+        self.cache.invalidate();
         &mut self.basic_blocks
     }
+
+    #[inline]
+    pub fn predecessors(&self) -> Ref<IndexVec<BasicBlock, Vec<BasicBlock>>> {
+        self.cache.predecessors(self)
+    }
+
+    #[inline]
+    pub fn predecessors_for(&self, bb: BasicBlock) -> Ref<Vec<BasicBlock>> {
+        Ref::map(self.predecessors(), |p| &p[bb])
+    }
 }
 
 impl<'tcx> Index<BasicBlock> for Mir<'tcx> {