Auto merge of #78448 - rylev:cache-foreign_modules, r=wesleywiser

foreign_modules query hash table lookups

When compiling a large monolithic crate we're seeing huge times in the `foreign_modules` query due to repeated iteration over foreign modules (in order to find a module by its id). This implements hash table lookups so that which massively reduces time spent in that query in this particular case. We'll need to see if the overhead of creating the hash table has a negative impact on performance in more normal compilation scenarios.

I'm working with `@wesleywiser` on this.

3 files changed, 13 insertions, 8 deletions

diff --git a/compiler/rustc_metadata/src/rmeta/decoder.rs b/compiler/rustc_metadata/src/rmeta/decoder.rs
index b01a55b48da..c031e0e2e19 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder.rs
@@ -1414,12 +1414,14 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         }
     }
 
-    fn get_foreign_modules(&self, tcx: TyCtxt<'tcx>) -> &'tcx [ForeignModule] {
+    fn get_foreign_modules(&self, tcx: TyCtxt<'tcx>) -> Lrc<FxHashMap<DefId, ForeignModule>> {
         if self.root.is_proc_macro_crate() {
             // Proc macro crates do not have any *target* foreign modules.
-            &[]
+            Lrc::new(FxHashMap::default())
         } else {
-            tcx.arena.alloc_from_iter(self.root.foreign_modules.decode((self, tcx.sess)))
+            let modules: FxHashMap<DefId, ForeignModule> =
+                self.root.foreign_modules.decode((self, tcx.sess)).map(|m| (m.def_id, m)).collect();
+            Lrc::new(modules)
         }
     }
 
diff --git a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
index b5fb850e92e..ddd85ab7aaa 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
@@ -6,12 +6,14 @@ use crate::rmeta::{self, encoder};
 
 use rustc_ast as ast;
 use rustc_ast::expand::allocator::AllocatorKind;
+use rustc_data_structures::stable_map::FxHashMap;
 use rustc_data_structures::svh::Svh;
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_hir::definitions::{DefKey, DefPath, DefPathHash};
 use rustc_middle::hir::exports::Export;
+use rustc_middle::middle::cstore::ForeignModule;
 use rustc_middle::middle::cstore::{CrateSource, CrateStore, EncodedMetadata};
 use rustc_middle::middle::exported_symbols::ExportedSymbol;
 use rustc_middle::middle::stability::DeprecationEntry;
@@ -266,9 +268,8 @@ pub fn provide(providers: &mut Providers) {
                         Some(id) => id,
                         None => return false,
                     };
-                    tcx.foreign_modules(id.krate)
-                        .iter()
-                        .find(|m| m.def_id == fm_id)
+                    let map = tcx.foreign_modules(id.krate);
+                    map.get(&fm_id)
                         .expect("failed to find foreign module")
                         .foreign_items
                         .contains(&id)
@@ -281,7 +282,9 @@ pub fn provide(providers: &mut Providers) {
         },
         foreign_modules: |tcx, cnum| {
             assert_eq!(cnum, LOCAL_CRATE);
-            &tcx.arena.alloc(foreign_modules::collect(tcx))[..]
+            let modules: FxHashMap<DefId, ForeignModule> =
+                foreign_modules::collect(tcx).into_iter().map(|m| (m.def_id, m)).collect();
+            Lrc::new(modules)
         },
         link_args: |tcx, cnum| {
             assert_eq!(cnum, LOCAL_CRATE);
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 2a81737e168..a7cf1079b8f 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -1502,7 +1502,7 @@ impl EncodeContext<'a, 'tcx> {
     fn encode_foreign_modules(&mut self) -> Lazy<[ForeignModule]> {
         empty_proc_macro!(self);
         let foreign_modules = self.tcx.foreign_modules(LOCAL_CRATE);
-        self.lazy(foreign_modules.iter().cloned())
+        self.lazy(foreign_modules.iter().map(|(_, m)| m).cloned())
     }
 
     fn encode_hygiene(&mut self) -> (SyntaxContextTable, ExpnDataTable) {