Mmap the incremental data instead of reading it.

compiler/rustc_incremental/src/persist/file_format.rs

@@ -14,6 +14,7 @@
 use std::io::{self, Read};
 use std::path::Path;
 
+use rustc_data_structures::memmap::Mmap;
 use rustc_serialize::opaque::{FileEncodeResult, FileEncoder};
 use rustc_serialize::Encoder;
 
@@ -54,14 +55,15 @@ pub fn read_file(
     report_incremental_info: bool,
     path: &Path,
     nightly_build: bool,
-) -> io::Result<Option<(Vec<u8>, usize)>> {
-    let data = match fs::read(path) {
-        Ok(data) => data,
+) -> io::Result<Option<(Mmap, usize)>> {
+    let file = match fs::File::open(path) {
+        Ok(file) => file,
         Err(err) if err.kind() == io::ErrorKind::NotFound => return Ok(None),
         Err(err) => return Err(err),
     };
+    let mmap = unsafe { Mmap::map(file) }?;
 
-    let mut file = io::Cursor::new(data);
+    let mut file = io::Cursor::new(&*mmap);
 
     // Check FILE_MAGIC
     {
@@ -103,7 +105,7 @@ pub fn read_file(
     }
 
     let post_header_start_pos = file.position() as usize;
-    Ok(Some((file.into_inner(), post_header_start_pos)))
+    Ok(Some((mmap, post_header_start_pos)))
 }
 
 fn report_format_mismatch(report_incremental_info: bool, file: &Path, message: &str) {

compiler/rustc_incremental/src/persist/load.rs

@@ -1,6 +1,7 @@
 //! Code to save/load the dep-graph from files.
 
 use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::memmap::Mmap;
 use rustc_middle::dep_graph::{SerializedDepGraph, WorkProduct, WorkProductId};
 use rustc_middle::ty::OnDiskCache;
 use rustc_serialize::opaque::Decoder;
@@ -48,7 +49,7 @@ fn load_data(
     report_incremental_info: bool,
     path: &Path,
     nightly_build: bool,
-) -> LoadResult<(Vec<u8>, usize)> {
+) -> LoadResult<(Mmap, usize)> {
     match file_format::read_file(report_incremental_info, path, nightly_build) {
         Ok(Some(data_and_pos)) => LoadResult::Ok { data: data_and_pos },
         Ok(None) => {

compiler/rustc_middle/src/ty/context.rs

@@ -27,6 +27,7 @@
 use rustc_ast as ast;
 use rustc_attr as attr;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use rustc_data_structures::memmap::Mmap;
 use rustc_data_structures::profiling::SelfProfilerRef;
 use rustc_data_structures::sharded::{IntoPointer, ShardedHashMap};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
@@ -71,7 +72,7 @@
 
 pub trait OnDiskCache<'tcx>: rustc_data_structures::sync::Sync {
     /// Creates a new `OnDiskCache` instance from the serialized data in `data`.
-    fn new(sess: &'tcx Session, data: Vec<u8>, start_pos: usize) -> Self
+    fn new(sess: &'tcx Session, data: Mmap, start_pos: usize) -> Self
     where
         Self: Sized;
 

compiler/rustc_query_impl/src/on_disk_cache.rs

@@ -1,5 +1,6 @@
 use crate::QueryCtxt;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
+use rustc_data_structures::memmap::Mmap;
 use rustc_data_structures::sync::{HashMapExt, Lock, Lrc, OnceCell};
 use rustc_data_structures::unhash::UnhashMap;
 use rustc_hir::def_id::{CrateNum, DefId, DefIndex, LocalDefId, StableCrateId, LOCAL_CRATE};
@@ -42,7 +43,7 @@
 /// any side effects that have been emitted during a query.
 pub struct OnDiskCache<'sess> {
     // The complete cache data in serialized form.
-    serialized_data: Vec<u8>,
+    serialized_data: Option<Mmap>,
 
     // Collects all `QuerySideEffects` created during the current compilation
     // session.
@@ -182,7 +183,8 @@ fn new(tcx: TyCtxt<'_>, file: &SourceFile) -> EncodedSourceFileId {
 }
 
 impl<'sess> rustc_middle::ty::OnDiskCache<'sess> for OnDiskCache<'sess> {
-    fn new(sess: &'sess Session, data: Vec<u8>, start_pos: usize) -> Self {
+    /// Creates a new `OnDiskCache` instance from the serialized data in `data`.
+    fn new(sess: &'sess Session, data: Mmap, start_pos: usize) -> Self {
         debug_assert!(sess.opts.incremental.is_some());
 
         // Wrap in a scope so we can borrow `data`.
@@ -204,7 +206,7 @@ fn new(sess: &'sess Session, data: Vec<u8>, start_pos: usize) -> Self {
         };
 
         Self {
-            serialized_data: data,
+            serialized_data: Some(data),
             file_index_to_stable_id: footer.file_index_to_stable_id,
             file_index_to_file: Default::default(),
             cnum_map: OnceCell::new(),
@@ -225,7 +227,7 @@ fn new(sess: &'sess Session, data: Vec<u8>, start_pos: usize) -> Self {
 
     fn new_empty(source_map: &'sess SourceMap) -> Self {
         Self {
-            serialized_data: Vec::new(),
+            serialized_data: None,
             file_index_to_stable_id: Default::default(),
             file_index_to_file: Default::default(),
             cnum_map: OnceCell::new(),
@@ -577,7 +579,10 @@ fn with_decoder<'a, 'tcx, T, F: FnOnce(&mut CacheDecoder<'sess, 'tcx>) -> T>(
 
         let mut decoder = CacheDecoder {
             tcx,
-            opaque: opaque::Decoder::new(&self.serialized_data[..], pos.to_usize()),
+            opaque: opaque::Decoder::new(
+                self.serialized_data.as_deref().unwrap_or(&[]),
+                pos.to_usize(),
+            ),
             source_map: self.source_map,
             cnum_map,
             file_index_to_file: &self.file_index_to_file,