rustc_metadata: safely read/write the index positions.

Author: eddyb

src/librustc_metadata/index.rs

@@ -2,7 +2,6 @@ use crate::schema::*;
 
 use rustc::hir::def_id::{DefId, DefIndex, DefIndexAddressSpace};
 use rustc_serialize::opaque::Encoder;
-use std::slice;
 use std::u32;
 use log::debug;
 
@@ -14,14 +13,14 @@ use log::debug;
 /// appropriate spot by calling `record_position`. We should never
 /// visit the same index twice.
 pub struct Index {
-    positions: [Vec<u32>; 2]
+    positions: [Vec<u8>; 2]
 }
 
 impl Index {
     pub fn new((max_index_lo, max_index_hi): (usize, usize)) -> Index {
         Index {
-            positions: [vec![u32::MAX; max_index_lo],
-                        vec![u32::MAX; max_index_hi]],
+            positions: [vec![0xff; max_index_lo * 4],
+                        vec![0xff; max_index_hi * 4]],
         }
     }
 
@@ -36,26 +35,27 @@ impl Index {
         let space_index = item.address_space().index();
         let array_index = item.as_array_index();
 
-        assert!(self.positions[space_index][array_index] == u32::MAX,
+        let destination = &mut self.positions[space_index][array_index * 4..];
+        assert!(read_le_u32(destination) == u32::MAX,
                 "recorded position for item {:?} twice, first at {:?} and now at {:?}",
                 item,
-                self.positions[space_index][array_index],
+                read_le_u32(destination),
                 position);
 
-        self.positions[space_index][array_index] = position.to_le();
+        write_le_u32(destination, position);
     }
 
     pub fn write_index(&self, buf: &mut Encoder) -> LazySeq<Index> {
         let pos = buf.position();
 
         // First we write the length of the lower range ...
-        buf.emit_raw_bytes(words_to_bytes(&[(self.positions[0].len() as u32).to_le()]));
+        buf.emit_raw_bytes(&(self.positions[0].len() as u32 / 4).to_le_bytes());
         // ... then the values in the lower range ...
-        buf.emit_raw_bytes(words_to_bytes(&self.positions[0][..]));
+        buf.emit_raw_bytes(&self.positions[0]);
         // ... then the values in the higher range.
-        buf.emit_raw_bytes(words_to_bytes(&self.positions[1][..]));
+        buf.emit_raw_bytes(&self.positions[1]);
         LazySeq::with_position_and_length(pos as usize,
-            self.positions[0].len() + self.positions[1].len() + 1)
+            (self.positions[0].len() + self.positions[1].len()) / 4 + 1)
     }
 }
 
@@ -64,24 +64,20 @@ impl<'tcx> LazySeq<Index> {
     /// DefIndex (if any).
     #[inline(never)]
     pub fn lookup(&self, bytes: &[u8], def_index: DefIndex) -> Option<Lazy<Entry<'tcx>>> {
-        let words = &bytes_to_words(&bytes[self.position..])[..self.len];
-
-        debug!("Index::lookup: index={:?} words.len={:?}",
+        debug!("Index::lookup: index={:?} len={:?}",
                def_index,
-               words.len());
+               self.len);
 
-        let positions = match def_index.address_space() {
-            DefIndexAddressSpace::Low => &words[1..],
+        let i = def_index.as_array_index() + match def_index.address_space() {
+            DefIndexAddressSpace::Low => 0,
             DefIndexAddressSpace::High => {
                 // This is a DefIndex in the higher range, so find out where
                 // that starts:
-                let lo_count = u32::from_le(words[0].get()) as usize;
-                &words[lo_count + 1 .. ]
+                read_le_u32(&bytes[self.position..]) as usize
             }
         };
 
-        let array_index = def_index.as_array_index();
-        let position = u32::from_le(positions[array_index].get());
+        let position = read_le_u32(&bytes[self.position + (1 + i) * 4..]);
         if position == u32::MAX {
             debug!("Index::lookup: position=u32::MAX");
             None
@@ -92,26 +88,12 @@ impl<'tcx> LazySeq<Index> {
     }
 }
 
-#[repr(packed)]
-#[derive(Copy)]
-struct Unaligned<T>(T);
-
-// The derived Clone impl is unsafe for this packed struct since it needs to pass a reference to
-// the field to `T::clone`, but this reference may not be properly aligned.
-impl<T: Copy> Clone for Unaligned<T> {
-    fn clone(&self) -> Self {
-        *self
-    }
-}
-
-impl<T> Unaligned<T> {
-    fn get(self) -> T { self.0 }
-}
-
-fn bytes_to_words(b: &[u8]) -> &[Unaligned<u32>] {
-    unsafe { slice::from_raw_parts(b.as_ptr() as *const Unaligned<u32>, b.len() / 4) }
+fn read_le_u32(b: &[u8]) -> u32 {
+    let mut bytes = [0; 4];
+    bytes.copy_from_slice(&b[..4]);
+    u32::from_le_bytes(bytes)
 }
 
-fn words_to_bytes(w: &[u32]) -> &[u8] {
-    unsafe { slice::from_raw_parts(w.as_ptr() as *const u8, w.len() * 4) }
+fn write_le_u32(b: &mut [u8], x: u32) {
+    b[..4].copy_from_slice(&x.to_le_bytes());
 }