Drop AST on a separate thread and prefetch hir_crate

Author: Zoxc

compiler/rustc_ast_lowering/src/lib.rs

@@ -50,7 +50,7 @@ use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
-use rustc_data_structures::sync::Lrc;
+use rustc_data_structures::sync::{spawn, Lrc};
 use rustc_errors::{DiagArgFromDisplay, DiagCtxt, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, LifetimeRes, Namespace, PartialRes, PerNS, Res};
@@ -445,9 +445,14 @@ pub fn lower_to_hir(tcx: TyCtxt<'_>, (): ()) -> hir::Crate<'_> {
         .lower_node(def_id);
     }
 
-    // Drop AST to free memory
     drop(ast_index);
-    sess.time("drop_ast", || drop(krate));
+
+    // Drop AST to free memory. It can be expensive so try to drop it on a separate thread.
+    let prof = sess.prof.clone();
+    spawn(move || {
+        let _timer = prof.verbose_generic_activity("drop_ast");
+        drop(krate);
+    });
 
     // Don't hash unless necessary, because it's expensive.
     let opt_hir_hash =

compiler/rustc_data_structures/src/sync.rs

@@ -53,7 +53,7 @@ pub use worker_local::{Registry, WorkerLocal};
 mod parallel;
 #[cfg(parallel_compiler)]
 pub use parallel::scope;
-pub use parallel::{join, par_for_each_in, par_map, parallel_guard, try_par_for_each_in};
+pub use parallel::{join, par_for_each_in, par_map, parallel_guard, spawn, try_par_for_each_in};
 
 pub use vec::{AppendOnlyIndexVec, AppendOnlyVec};
 

compiler/rustc_data_structures/src/sync/parallel.rs

@@ -61,6 +61,10 @@ mod disabled {
         }}
     }
 
+    pub fn spawn(func: impl FnOnce() + 'static) {
+        func()
+    }
+
     pub fn join<A, B, RA, RB>(oper_a: A, oper_b: B) -> (RA, RB)
     where
         A: FnOnce() -> RA,
@@ -138,6 +142,17 @@ mod enabled {
         };
     }
 
+    pub fn spawn(func: impl FnOnce() + DynSend + 'static) {
+        if mode::is_dyn_thread_safe() {
+            let func = FromDyn::from(func);
+            rayon::spawn(|| {
+                (func.into_inner())();
+            });
+        } else {
+            func()
+        }
+    }
+
     // This function only works when `mode::is_dyn_thread_safe()`.
     pub fn scope<'scope, OP, R>(op: OP) -> R
     where

compiler/rustc_interface/src/passes.rs

@@ -688,6 +688,9 @@ pub fn create_global_ctxt<'tcx>(
 fn analysis(tcx: TyCtxt<'_>, (): ()) -> Result<()> {
     rustc_passes::hir_id_validator::check_crate(tcx);
 
+    // Prefetch this to prevent multiple threads from blocking on it later.
+    tcx.ensure_with_value().hir_crate(());
+
     let sess = tcx.sess;
 
     sess.time("misc_checking_1", || {