Store DefPathHash->DefIndex map in on-disk-hash-table format in crate metadata.

This encoding allows for random access without an expensive upfront decoding
state which in turn allows simplifying the DefPathIndex lookup logic without
regressing performance.

Author: michaelwoerister

Diff for: compiler/rustc_metadata/src/creader.rs

@@ -45,7 +45,7 @@ pub struct CStore {
 
     /// This map is used to verify we get no hash conflicts between
     /// `StableCrateId` values.
-    stable_crate_ids: FxHashMap<StableCrateId, CrateNum>,
+    pub(crate) stable_crate_ids: FxHashMap<StableCrateId, CrateNum>,
 
     /// Unused externs of the crate
     unused_externs: Vec<Symbol>,

Diff for: compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -95,10 +95,8 @@ crate struct CrateMetadata {
     raw_proc_macros: Option<&'static [ProcMacro]>,
     /// Source maps for code from the crate.
     source_map_import_info: OnceCell<Vec<ImportedSourceFile>>,
-    /// For every definition in this crate, maps its `DefPathHash` to its
-    /// `DefIndex`. See `raw_def_id_to_def_id` for more details about how
-    /// this is used.
-    def_path_hash_map: OnceCell<UnhashMap<DefPathHash, DefIndex>>,
+    /// For every definition in this crate, maps its `DefPathHash` to its `DefIndex`.
+    def_path_hash_map: DefPathHashMap<'static>,
     /// Likewise for ExpnHash.
     expn_hash_map: OnceCell<UnhashMap<ExpnHash, ExpnIndex>>,
     /// Used for decoding interpret::AllocIds in a cached & thread-safe manner.
@@ -320,6 +318,11 @@ impl<'a, 'tcx> DecodeContext<'a, 'tcx> {
         self.lazy_state = LazyState::Previous(NonZeroUsize::new(position + min_size).unwrap());
         Ok(Lazy::from_position_and_meta(NonZeroUsize::new(position).unwrap(), meta))
     }
+
+    #[inline]
+    pub fn read_raw_bytes(&mut self, len: usize) -> &'a [u8] {
+        self.opaque.read_raw_bytes(len)
+    }
 }
 
 impl<'a, 'tcx> TyDecoder<'tcx> for DecodeContext<'a, 'tcx> {
@@ -1596,58 +1599,6 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
             .or_insert_with(|| self.root.tables.def_keys.get(self, index).unwrap().decode(self))
     }
 
-    /// Finds the corresponding `DefId` for the provided `DefPathHash`, if it exists.
-    /// This is used by incremental compilation to map a serialized `DefPathHash` to
-    /// its `DefId` in the current session.
-    /// Normally, only one 'main' crate will change between incremental compilation sessions:
-    /// all dependencies will be completely unchanged. In this case, we can avoid
-    /// decoding every `DefPathHash` in the crate, since the `DefIndex` from the previous
-    /// session will still be valid. If our 'guess' is wrong (the `DefIndex` no longer exists,
-    /// or has a different `DefPathHash`, then we need to decode all `DefPathHashes` to determine
-    /// the correct mapping).
-    fn def_path_hash_to_def_id(
-        &self,
-        krate: CrateNum,
-        index_guess: u32,
-        hash: DefPathHash,
-    ) -> Option<DefId> {
-        let def_index_guess = DefIndex::from_u32(index_guess);
-        let old_hash = self
-            .root
-            .tables
-            .def_path_hashes
-            .get(self, def_index_guess)
-            .map(|lazy| lazy.decode(self));
-
-        // Fast path: the definition and its index is unchanged from the
-        // previous compilation session. There is no need to decode anything
-        // else
-        if old_hash == Some(hash) {
-            return Some(DefId { krate, index: def_index_guess });
-        }
-
-        let is_proc_macro = self.is_proc_macro_crate();
-
-        // Slow path: We need to find out the new `DefIndex` of the provided
-        // `DefPathHash`, if its still exists. This requires decoding every `DefPathHash`
-        // stored in this crate.
-        let map = self.cdata.def_path_hash_map.get_or_init(|| {
-            let end_id = self.root.tables.def_path_hashes.size() as u32;
-            let mut map = UnhashMap::with_capacity_and_hasher(end_id as usize, Default::default());
-            for i in 0..end_id {
-                let def_index = DefIndex::from_u32(i);
-                // There may be gaps in the encoded table if we're decoding a proc-macro crate
-                if let Some(hash) = self.root.tables.def_path_hashes.get(self, def_index) {
-                    map.insert(hash.decode(self), def_index);
-                } else if !is_proc_macro {
-                    panic!("Missing def_path_hashes entry for {:?}", def_index);
-                }
-            }
-            map
-        });
-        map.get(&hash).map(|index| DefId { krate, index: *index })
-    }
-
     // Returns the path leading to the thing with this `id`.
     fn def_path(&self, id: DefIndex) -> DefPath {
         debug!("def_path(cnum={:?}, id={:?})", self.cnum, id);
@@ -1670,6 +1621,11 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         self.def_path_hash_unlocked(index, &mut def_path_hashes)
     }
 
+    #[inline]
+    fn def_path_hash_to_def_index(&self, hash: DefPathHash) -> Option<DefIndex> {
+        self.def_path_hash_map.def_path_hash_to_def_index(&hash)
+    }
+
     fn expn_hash_to_expn_id(&self, index_guess: u32, hash: ExpnHash) -> ExpnId {
         debug_assert_eq!(ExpnId::from_hash(hash), None);
         let index_guess = ExpnIndex::from_u32(index_guess);
@@ -1936,13 +1892,18 @@ impl CrateMetadata {
         let alloc_decoding_state =
             AllocDecodingState::new(root.interpret_alloc_index.decode(&blob).collect());
         let dependencies = Lock::new(cnum_map.iter().cloned().collect());
+
+        // Pre-decode the DefPathHash->DefIndex table. This is a cheap operation
+        // that does not copy any data. It just does some data verification.
+        let def_path_hash_map = root.def_path_hash_map.decode(&blob);
+
         CrateMetadata {
             blob,
             root,
             trait_impls,
             raw_proc_macros,
             source_map_import_info: OnceCell::new(),
-            def_path_hash_map: Default::default(),
+            def_path_hash_map,
             expn_hash_map: Default::default(),
             alloc_decoding_state,
             cnum,

Diff for: compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -498,6 +498,10 @@ impl CrateStore for CStore {
         self.get_crate_data(cnum).root.stable_crate_id
     }
 
+    fn stable_crate_id_to_crate_num(&self, stable_crate_id: StableCrateId) -> CrateNum {
+        self.stable_crate_ids[&stable_crate_id]
+    }
+
     /// Returns the `DefKey` for a given `DefId`. This indicates the
     /// parent `DefId` as well as some idea of what kind of data the
     /// `DefId` refers to.
@@ -513,14 +517,10 @@ impl CrateStore for CStore {
         self.get_crate_data(def.krate).def_path_hash(def.index)
     }
 
-    // See `CrateMetadataRef::def_path_hash_to_def_id` for more details
-    fn def_path_hash_to_def_id(
-        &self,
-        cnum: CrateNum,
-        index_guess: u32,
-        hash: DefPathHash,
-    ) -> Option<DefId> {
-        self.get_crate_data(cnum).def_path_hash_to_def_id(cnum, index_guess, hash)
+    fn def_path_hash_to_def_id(&self, cnum: CrateNum, hash: DefPathHash) -> Option<DefId> {
+        self.get_crate_data(cnum)
+            .def_path_hash_to_def_index(hash)
+            .map(|index| DefId { krate: cnum, index })
     }
 
     fn expn_hash_to_expn_id(&self, cnum: CrateNum, index_guess: u32, hash: ExpnHash) -> ExpnId {

Diff for: compiler/rustc_metadata/src/rmeta/def_path_hash_map.rs

@@ -0,0 +1,60 @@
+use crate::rmeta::DecodeContext;
+use crate::rmeta::EncodeContext;
+use crate::rmeta::MetadataBlob;
+use rustc_data_structures::owning_ref::OwningRef;
+use rustc_hir::def_path_hash_map::{
+    Config as HashMapConfig, DefPathHashMap as DefPathHashMapInner,
+};
+use rustc_serialize::{opaque, Decodable, Decoder, Encodable, Encoder};
+use rustc_span::def_id::{DefIndex, DefPathHash};
+
+crate enum DefPathHashMap<'tcx> {
+    OwnedFromMetadata(odht::HashTable<HashMapConfig, OwningRef<MetadataBlob, [u8]>>),
+    BorrowedFromTcx(&'tcx DefPathHashMapInner),
+}
+
+impl DefPathHashMap<'tcx> {
+    #[inline]
+    pub fn def_path_hash_to_def_index(&self, def_path_hash: &DefPathHash) -> Option<DefIndex> {
+        match *self {
+            DefPathHashMap::OwnedFromMetadata(ref map) => map.get(def_path_hash),
+            DefPathHashMap::BorrowedFromTcx(_) => {
+                panic!("DefPathHashMap::BorrowedFromTcx variant only exists for serialization")
+            }
+        }
+    }
+}
+
+impl<'a, 'tcx> Encodable<EncodeContext<'a, 'tcx>> for DefPathHashMap<'tcx> {
+    fn encode(&self, e: &mut EncodeContext<'a, 'tcx>) -> opaque::EncodeResult {
+        match *self {
+            DefPathHashMap::BorrowedFromTcx(def_path_hash_map) => {
+                let bytes = def_path_hash_map.raw_bytes();
+                e.emit_usize(bytes.len())?;
+                e.emit_raw_bytes(bytes)
+            }
+            DefPathHashMap::OwnedFromMetadata(_) => {
+                panic!("DefPathHashMap::OwnedFromMetadata variant only exists for deserialization")
+            }
+        }
+    }
+}
+
+impl<'a, 'tcx> Decodable<DecodeContext<'a, 'tcx>> for DefPathHashMap<'static> {
+    fn decode(d: &mut DecodeContext<'a, 'tcx>) -> Result<DefPathHashMap<'static>, String> {
+        // Import TyDecoder so we can access the DecodeContext::position() method
+        use crate::rustc_middle::ty::codec::TyDecoder;
+
+        let len = d.read_usize()?;
+        let pos = d.position();
+        let o = OwningRef::new(d.blob().clone()).map(|x| &x[pos..pos + len]);
+
+        // Although we already have the data we need via the OwningRef, we still need
+        // to advance the DecodeContext's position so it's in a valid state after
+        // the method. We use read_raw_bytes() for that.
+        let _ = d.read_raw_bytes(len);
+
+        let inner = odht::HashTable::from_raw_bytes(o).map_err(|e| format!("{}", e))?;
+        Ok(DefPathHashMap::OwnedFromMetadata(inner))
+    }
+}

Diff for: compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1,3 +1,4 @@
+use crate::rmeta::def_path_hash_map::DefPathHashMap;
 use crate::rmeta::table::{FixedSizeEncoding, TableBuilder};
 use crate::rmeta::*;
 
@@ -472,6 +473,12 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         }
     }
 
+    fn encode_def_path_hash_map(&mut self) -> Lazy<DefPathHashMap<'tcx>> {
+        self.lazy(DefPathHashMap::BorrowedFromTcx(
+            self.tcx.resolutions(()).definitions.def_path_hash_to_def_index_map(),
+        ))
+    }
+
     fn encode_source_map(&mut self) -> Lazy<[rustc_span::SourceFile]> {
         let source_map = self.tcx.sess.source_map();
         let all_source_files = source_map.files();
@@ -675,6 +682,10 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         let (syntax_contexts, expn_data, expn_hashes) = self.encode_hygiene();
         let hygiene_bytes = self.position() - i;
 
+        i = self.position();
+        let def_path_hash_map = self.encode_def_path_hash_map();
+        let def_path_hash_map_bytes = self.position() - i;
+
         // Encode source_map. This needs to be done last,
         // since encoding `Span`s tells us which `SourceFiles` we actually
         // need to encode.
@@ -722,6 +733,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             syntax_contexts,
             expn_data,
             expn_hashes,
+            def_path_hash_map,
         });
 
         let total_bytes = self.position();
@@ -744,6 +756,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             eprintln!("            impl bytes: {}", impl_bytes);
             eprintln!("    exp. symbols bytes: {}", exported_symbols_bytes);
             eprintln!("  def-path table bytes: {}", def_path_table_bytes);
+            eprintln!(" def-path hashes bytes: {}", def_path_hash_map_bytes);
             eprintln!(" proc-macro-data-bytes: {}", proc_macro_data_bytes);
             eprintln!("             mir bytes: {}", mir_bytes);
             eprintln!("            item bytes: {}", item_bytes);

Diff for: compiler/rustc_metadata/src/rmeta/mod.rs

@@ -1,4 +1,5 @@
 use decoder::Metadata;
+use def_path_hash_map::DefPathHashMap;
 use table::{Table, TableBuilder};
 
 use rustc_ast::{self as ast, MacroDef};
@@ -35,6 +36,7 @@ use encoder::EncodeContext;
 use rustc_span::hygiene::SyntaxContextData;
 
 mod decoder;
+mod def_path_hash_map;
 mod encoder;
 mod table;
 
@@ -231,6 +233,8 @@ crate struct CrateRoot<'tcx> {
     expn_data: ExpnDataTable,
     expn_hashes: ExpnHashTable,
 
+    def_path_hash_map: Lazy<DefPathHashMap<'tcx>>,
+
     source_map: Lazy<[rustc_span::SourceFile]>,
 
     compiler_builtins: bool,

Diff for: compiler/rustc_middle/src/middle/cstore.rs

@@ -199,14 +199,10 @@ pub trait CrateStore: std::fmt::Debug {
     // incr.  comp. uses to identify a CrateNum.
     fn crate_name(&self, cnum: CrateNum) -> Symbol;
     fn stable_crate_id(&self, cnum: CrateNum) -> StableCrateId;
+    fn stable_crate_id_to_crate_num(&self, stable_crate_id: StableCrateId) -> CrateNum;
 
     /// Fetch a DefId from a DefPathHash for a foreign crate.
-    fn def_path_hash_to_def_id(
-        &self,
-        cnum: CrateNum,
-        index_guess: u32,
-        hash: DefPathHash,
-    ) -> Option<DefId>;
+    fn def_path_hash_to_def_id(&self, cnum: CrateNum, hash: DefPathHash) -> Option<DefId>;
     fn expn_hash_to_expn_id(&self, cnum: CrateNum, index_guess: u32, hash: ExpnHash) -> ExpnId;
 
     // utility functions

Diff for: compiler/rustc_middle/src/ty/context.rs

@@ -1316,6 +1316,17 @@ impl<'tcx> TyCtxt<'tcx> {
         }
     }
 
+    /// Maps a StableCrateId to the corresponding CrateNum. This method assumes
+    /// that the crate in question has already been loaded by the CrateStore.
+    #[inline]
+    pub fn stable_crate_id_to_crate_num(self, stable_crate_id: StableCrateId) -> CrateNum {
+        if stable_crate_id == self.sess.local_stable_crate_id() {
+            LOCAL_CRATE
+        } else {
+            self.untracked_resolutions.cstore.stable_crate_id_to_crate_num(stable_crate_id)
+        }
+    }
+
     pub fn def_path_debug_str(self, def_id: DefId) -> String {
         // We are explicitly not going through queries here in order to get
         // crate name and stable crate id since this code is called from debug!()