[m-rev.] for post-commit review: Explicitly start parallel marker threads.

[Peter Wang]

As of Boehm GC v8.2.2 (commit b8ee3ca626d6bd67c8c2c94d517f2f7da7cd5a8c)
parallel marker threads are not started during GC_INIT(). Instead, the
marker threads are only started when the client program creates the
first (user) thread, or if GC_start_mark_threads() is called explicitly.

runtime/mercury_wrapper.c:
    Call GC_allow_register_threads() as part of GC initialisation.
    This will enable parallel marking in par.gc C grades from the start
    of the Mercury program, even if the program has not created any
    threads (yet).
---
 runtime/mercury_wrapper.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/runtime/mercury_wrapper.c b/runtime/mercury_wrapper.c
index d5015bfa0..44cd8d4c9 100644
--- a/runtime/mercury_wrapper.c
+++ b/runtime/mercury_wrapper.c
@@ -843,6 +843,13 @@ MR_init_conservative_GC(void)
 
     GC_set_oom_fn(MR_oom_func);
 
+    #ifdef MR_THREAD_SAFE
+    // Explicitly put the GC into multithreaded mode. This also launches any
+    // parallel marker threads. Without this, parallel marker threads will not
+    // be started until the first user thread is created.
+    GC_allow_register_threads();
+    #endif // MR_THREAD_SAFE
+
   #endif // MR_BOEHM_GC
 }
 #endif // MR_CONSERVATIVE_GC
-- 
2.39.0