Cache the DepNodeIndex of upstream crates in order to avoid multiple locks and table lookups on each access of crate metadata.

src/librustc/dep_graph/graph.rs

@@ -30,7 +30,7 @@ rustc_index::newtype_index! {
 }
 
 impl DepNodeIndex {
-    const INVALID: DepNodeIndex = DepNodeIndex::MAX;
+    pub const INVALID: DepNodeIndex = DepNodeIndex::MAX;
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]

src/librustc_metadata/creader.rs

@@ -3,10 +3,11 @@
 use crate::cstore::{self, CStore, CrateSource, MetadataBlob};
 use crate::locator::{self, CratePaths};
 use crate::schema::{CrateRoot, CrateDep};
-use rustc_data_structures::sync::{Lrc, RwLock, Lock};
+use rustc_data_structures::sync::{Lrc, RwLock, Lock, AtomicCell};
 
 use rustc::hir::def_id::CrateNum;
 use rustc_data_structures::svh::Svh;
+use rustc::dep_graph::DepNodeIndex;
 use rustc::middle::cstore::DepKind;
 use rustc::mir::interpret::AllocDecodingState;
 use rustc::session::{Session, CrateDisambiguator};
@@ -271,7 +272,8 @@ impl<'a> CrateLoader<'a> {
             },
             private_dep,
             span,
-            raw_proc_macros
+            raw_proc_macros,
+            dep_node_index: AtomicCell::new(DepNodeIndex::INVALID),
         };
 
         let cmeta = Lrc::new(cmeta);

src/librustc_metadata/cstore.rs

@@ -2,6 +2,7 @@
 // crates and libraries
 
 use crate::schema;
+use rustc::dep_graph::DepNodeIndex;
 use rustc::hir::def_id::{CrateNum, DefIndex};
 use rustc::hir::map::definitions::DefPathTable;
 use rustc::middle::cstore::{DepKind, ExternCrate, MetadataLoader};
@@ -9,7 +10,7 @@ use rustc::mir::interpret::AllocDecodingState;
 use rustc_index::vec::IndexVec;
 use rustc::util::nodemap::{FxHashMap, NodeMap};
 
-use rustc_data_structures::sync::{Lrc, RwLock, Lock};
+use rustc_data_structures::sync::{Lrc, RwLock, Lock, AtomicCell};
 use syntax::ast;
 use syntax::ext::base::SyntaxExtension;
 use syntax_pos;
@@ -83,6 +84,13 @@ pub struct CrateMetadata {
     pub span: Span,
 
     pub raw_proc_macros: Option<&'static [ProcMacro]>,
+
+    /// The `DepNodeIndex` of the `DepNode` representing this upstream crate.
+    /// It is initialized on the first access in `get_crate_dep_node_index()`.
+    /// Do not access the value directly, as it might not have been initialized
+    /// yet.
+    /// The field must always be initialized to `DepNodeIndex::INVALID`.
+    pub(super) dep_node_index: AtomicCell<DepNodeIndex>,
 }
 
 pub struct CStore {

src/librustc_metadata/cstore_impl.rs

@@ -51,19 +51,15 @@ macro_rules! provide {
                 let ($def_id, $other) = def_id_arg.into_args();
                 assert!(!$def_id.is_local());
 
-                let def_path_hash = $tcx.def_path_hash(DefId {
-                    krate: $def_id.krate,
-                    index: CRATE_DEF_INDEX
-                });
-                let dep_node = def_path_hash
-                    .to_dep_node(rustc::dep_graph::DepKind::CrateMetadata);
-                // The DepNodeIndex of the DepNode::CrateMetadata should be
-                // cached somewhere, so that we can use read_index().
-                $tcx.dep_graph.read(dep_node);
-
                 let $cdata = $tcx.crate_data_as_rc_any($def_id.krate);
                 let $cdata = $cdata.downcast_ref::<cstore::CrateMetadata>()
                     .expect("CrateStore created data is not a CrateMetadata");
+
+                if $tcx.dep_graph.is_fully_enabled() {
+                    let crate_dep_node_index = $cdata.get_crate_dep_node_index($tcx);
+                    $tcx.dep_graph.read_index(crate_dep_node_index);
+                }
+
                 $compute
             })*
 

src/librustc_metadata/decoder.rs

@@ -13,6 +13,7 @@ use rustc::hir::def::{self, Res, DefKind, CtorOf, CtorKind};
 use rustc::hir::def_id::{CrateNum, DefId, DefIndex, LocalDefId, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::FxHashMap;
+use rustc::dep_graph::{DepNodeIndex, DepKind};
 use rustc::middle::lang_items;
 use rustc::mir::{self, interpret};
 use rustc::mir::interpret::AllocDecodingSession;
@@ -1365,6 +1366,30 @@ impl<'a, 'tcx> CrateMetadata {
         // This shouldn't borrow twice, but there is no way to downgrade RefMut to Ref.
         self.source_map_import_info.borrow()
     }
+
+    /// Get the `DepNodeIndex` corresponding this crate. The result of this
+    /// method is cached in the `dep_node_index` field.
+    pub(super) fn get_crate_dep_node_index(&self, tcx: TyCtxt<'tcx>) -> DepNodeIndex {
+        let mut dep_node_index = self.dep_node_index.load();
+
+        if dep_node_index == DepNodeIndex::INVALID {
+            // We have not cached the DepNodeIndex for this upstream crate yet,
+            // so use the dep-graph to find it out and cache it.
+            // Note that multiple threads can enter this block concurrently.
+            // That is fine because the DepNodeIndex remains constant
+            // throughout the whole compilation session, and multiple stores
+            // would always write the same value.
+
+            let def_path_hash = self.def_path_hash(CRATE_DEF_INDEX);
+            let dep_node = def_path_hash.to_dep_node(DepKind::CrateMetadata);
+
+            dep_node_index = tcx.dep_graph.dep_node_index_of(&dep_node);
+            assert!(dep_node_index != DepNodeIndex::INVALID);
+            self.dep_node_index.store(dep_node_index);
+        }
+
+        dep_node_index
+    }
 }
 
 // Cannot be implemented on 'ProcMacro', as libproc_macro