From 07149e03f45194a8fdb12c616c1f221d5186e1e5 Mon Sep 17 00:00:00 2001
From: Michael Woerister <michaelwoerister@posteo>
Date: Wed, 9 Aug 2017 18:18:48 +0200
Subject: [PATCH] trans-scheduler: Let main thread take over for other worker.

---
 src/librustc_trans/back/write.rs | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/src/librustc_trans/back/write.rs b/src/librustc_trans/back/write.rs
index 0d5fe6c0ae95..94cae37761fe 100644
--- a/src/librustc_trans/back/write.rs
+++ b/src/librustc_trans/back/write.rs
@@ -1356,6 +1356,16 @@ fn start_executing_work(sess: &Session,
                             maybe_start_llvm_timer(&item, &mut llvm_start_time);
                             main_thread_worker_state = MainThreadWorkerState::LLVMing;
                             spawn_work(cgcx, item);
+                        } else {
+                            // There is no unstarted work, so let the main thread
+                            // take over for a running worker. Otherwise the
+                            // implicit token would just go to waste.
+                            // We reduce the `running` counter by one. The
+                            // `tokens.truncate()` below will take care of
+                            // giving the Token back.
+                            debug_assert!(running > 0);
+                            running -= 1;
+                            main_thread_worker_state = MainThreadWorkerState::LLVMing;
                         }
                     }
                     MainThreadWorkerState::Translating => {