Auto merge of #119259 - cjgillot:single-crate-id, r=Mark-Simulacrum

Only store StableCrateId once in DefPathTable.

https://github.com/rust-lang/rust/pull/119238 made me think of this.

cc `@Mark-Simulacrum`

4 files changed, 40 insertions, 32 deletions

diff --git a/compiler/rustc_data_structures/src/hashes.rs b/compiler/rustc_data_structures/src/hashes.rs
index ad068cdbc98..291ee5bbe26 100644
--- a/compiler/rustc_data_structures/src/hashes.rs
+++ b/compiler/rustc_data_structures/src/hashes.rs
@@ -25,7 +25,7 @@ impl Hash64 {
     pub const ZERO: Hash64 = Hash64 { inner: 0 };
 
     #[inline]
-    pub(crate) fn new(n: u64) -> Self {
+    pub fn new(n: u64) -> Self {
         Self { inner: n }
     }
 
diff --git a/compiler/rustc_hir/src/def_path_hash_map.rs b/compiler/rustc_hir/src/def_path_hash_map.rs
index 8bfb47af26f..9a6dee1e511 100644
--- a/compiler/rustc_hir/src/def_path_hash_map.rs
+++ b/compiler/rustc_hir/src/def_path_hash_map.rs
@@ -1,21 +1,22 @@
-use rustc_data_structures::fingerprint::Fingerprint;
-use rustc_span::def_id::{DefIndex, DefPathHash};
+use rustc_data_structures::stable_hasher::Hash64;
+use rustc_span::def_id::DefIndex;
 
 #[derive(Clone, Default)]
 pub struct Config;
 
 impl odht::Config for Config {
-    type Key = DefPathHash;
+    // This hash-map is single-crate, so we only need to key by the local hash.
+    type Key = Hash64;
     type Value = DefIndex;
 
-    type EncodedKey = [u8; 16];
+    type EncodedKey = [u8; 8];
     type EncodedValue = [u8; 4];
 
     type H = odht::UnHashFn;
 
     #[inline]
-    fn encode_key(k: &DefPathHash) -> [u8; 16] {
-        k.0.to_le_bytes()
+    fn encode_key(k: &Hash64) -> [u8; 8] {
+        k.as_u64().to_le_bytes()
     }
 
     #[inline]
@@ -24,8 +25,8 @@ impl odht::Config for Config {
     }
 
     #[inline]
-    fn decode_key(k: &[u8; 16]) -> DefPathHash {
-        DefPathHash(Fingerprint::from_le_bytes(*k))
+    fn decode_key(k: &[u8; 8]) -> Hash64 {
+        Hash64::new(u64::from_le_bytes(*k))
     }
 
     #[inline]
diff --git a/compiler/rustc_hir/src/definitions.rs b/compiler/rustc_hir/src/definitions.rs
index 2ab9a6ef32c..9fb1fc19bf4 100644
--- a/compiler/rustc_hir/src/definitions.rs
+++ b/compiler/rustc_hir/src/definitions.rs
@@ -20,27 +20,42 @@ use std::hash::Hash;
 /// Internally the `DefPathTable` holds a tree of `DefKey`s, where each `DefKey`
 /// stores the `DefIndex` of its parent.
 /// There is one `DefPathTable` for each crate.
-#[derive(Clone, Default, Debug)]
+#[derive(Debug)]
 pub struct DefPathTable {
+    stable_crate_id: StableCrateId,
     index_to_key: IndexVec<DefIndex, DefKey>,
-    def_path_hashes: IndexVec<DefIndex, DefPathHash>,
+    // We do only store the local hash, as all the definitions are from the current crate.
+    def_path_hashes: IndexVec<DefIndex, Hash64>,
     def_path_hash_to_index: DefPathHashMap,
 }
 
 impl DefPathTable {
+    fn new(stable_crate_id: StableCrateId) -> DefPathTable {
+        DefPathTable {
+            stable_crate_id,
+            index_to_key: Default::default(),
+            def_path_hashes: Default::default(),
+            def_path_hash_to_index: Default::default(),
+        }
+    }
+
     fn allocate(&mut self, key: DefKey, def_path_hash: DefPathHash) -> DefIndex {
+        // Assert that all DefPathHashes correctly contain the local crate's StableCrateId.
+        debug_assert_eq!(self.stable_crate_id, def_path_hash.stable_crate_id());
+        let local_hash = def_path_hash.local_hash();
+
         let index = {
             let index = DefIndex::from(self.index_to_key.len());
             debug!("DefPathTable::insert() - {:?} <-> {:?}", key, index);
             self.index_to_key.push(key);
             index
         };
-        self.def_path_hashes.push(def_path_hash);
+        self.def_path_hashes.push(local_hash);
         debug_assert!(self.def_path_hashes.len() == self.index_to_key.len());
 
         // Check for hash collisions of DefPathHashes. These should be
         // exceedingly rare.
-        if let Some(existing) = self.def_path_hash_to_index.insert(&def_path_hash, &index) {
+        if let Some(existing) = self.def_path_hash_to_index.insert(&local_hash, &index) {
             let def_path1 = DefPath::make(LOCAL_CRATE, existing, |idx| self.def_key(idx));
             let def_path2 = DefPath::make(LOCAL_CRATE, index, |idx| self.def_key(idx));
 
@@ -58,13 +73,6 @@ impl DefPathTable {
             );
         }
 
-        // Assert that all DefPathHashes correctly contain the local crate's
-        // StableCrateId
-        #[cfg(debug_assertions)]
-        if let Some(root) = self.def_path_hashes.get(CRATE_DEF_INDEX) {
-            assert!(def_path_hash.stable_crate_id() == root.stable_crate_id());
-        }
-
         index
     }
 
@@ -73,19 +81,19 @@ impl DefPathTable {
         self.index_to_key[index]
     }
 
+    #[instrument(level = "trace", skip(self), ret)]
     #[inline(always)]
     pub fn def_path_hash(&self, index: DefIndex) -> DefPathHash {
         let hash = self.def_path_hashes[index];
-        debug!("def_path_hash({:?}) = {:?}", index, hash);
-        hash
+        DefPathHash::new(self.stable_crate_id, hash)
     }
 
     pub fn enumerated_keys_and_path_hashes(
         &self,
-    ) -> impl Iterator<Item = (DefIndex, &DefKey, &DefPathHash)> + ExactSizeIterator + '_ {
+    ) -> impl Iterator<Item = (DefIndex, &DefKey, DefPathHash)> + ExactSizeIterator + '_ {
         self.index_to_key
             .iter_enumerated()
-            .map(move |(index, key)| (index, key, &self.def_path_hashes[index]))
+            .map(move |(index, key)| (index, key, self.def_path_hash(index)))
     }
 }
 
@@ -96,9 +104,6 @@ impl DefPathTable {
 pub struct Definitions {
     table: DefPathTable,
     next_disambiguator: UnordMap<(LocalDefId, DefPathData), u32>,
-
-    /// The [StableCrateId] of the local crate.
-    stable_crate_id: StableCrateId,
 }
 
 /// A unique identifier that we can use to lookup a definition
@@ -329,11 +334,11 @@ impl Definitions {
         let def_path_hash = key.compute_stable_hash(parent_hash);
 
         // Create the root definition.
-        let mut table = DefPathTable::default();
+        let mut table = DefPathTable::new(stable_crate_id);
         let root = LocalDefId { local_def_index: table.allocate(key, def_path_hash) };
         assert_eq!(root.local_def_index, CRATE_DEF_INDEX);
 
-        Definitions { table, next_disambiguator: Default::default(), stable_crate_id }
+        Definitions { table, next_disambiguator: Default::default() }
     }
 
     /// Adds a definition with a parent definition.
@@ -375,10 +380,10 @@ impl Definitions {
         hash: DefPathHash,
         err: &mut dyn FnMut() -> !,
     ) -> LocalDefId {
-        debug_assert!(hash.stable_crate_id() == self.stable_crate_id);
+        debug_assert!(hash.stable_crate_id() == self.table.stable_crate_id);
         self.table
             .def_path_hash_to_index
-            .get(&hash)
+            .get(&hash.local_hash())
             .map(|local_def_index| LocalDefId { local_def_index })
             .unwrap_or_else(|| err())
     }
diff --git a/compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs b/compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs
index 4f280bb9d80..9950bc1c31f 100644
--- a/compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs
+++ b/compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs
@@ -19,7 +19,9 @@ impl DefPathHashMapRef<'_> {
     #[inline]
     pub fn def_path_hash_to_def_index(&self, def_path_hash: &DefPathHash) -> DefIndex {
         match *self {
-            DefPathHashMapRef::OwnedFromMetadata(ref map) => map.get(def_path_hash).unwrap(),
+            DefPathHashMapRef::OwnedFromMetadata(ref map) => {
+                map.get(&def_path_hash.local_hash()).unwrap()
+            }
             DefPathHashMapRef::BorrowedFromTcx(_) => {
                 panic!("DefPathHashMap::BorrowedFromTcx variant only exists for serialization")
             }