Fix and enable object unloading in GHCi

Fixes #16525 by tracking dependencies between object file symbols and
marking symbol liveness during garbage collection

See Note [Object unloading] in CheckUnload.c for details.

1 files changed, 27 insertions, 5 deletions

diff --git a/rts/sm/GC.c b/rts/sm/GC.c
index a5aa7e1f4e..0dc92a29a0 100644
--- a/rts/sm/GC.c
+++ b/rts/sm/GC.c
@@ -97,6 +97,13 @@
  * See also: Note [STATIC_LINK fields] in Storage.h.
  */
 
+/* Hot GC globals
+ * ~~~~~~~~~~~~~~
+ * The globals below are quite hot during GC but read-only, initialized during
+ * the beginning of collection. It is important that they reside in the same
+ * cache-line to minimize unnecessary cache misses.
+ */
+
 /* N is the oldest generation being collected, where the generations
  * are numbered starting at 0.  A major GC (indicated by the major_gc
  * flag) is when we're collecting all generations.  We only attempt to
@@ -105,6 +112,7 @@
 uint32_t N;
 bool major_gc;
 bool deadlock_detect_gc;
+bool unload_mark_needed;
 
 /* Data used for allocation area sizing.
  */
@@ -314,6 +322,12 @@ GarbageCollect (uint32_t collect_gen,
           static_flag == STATIC_FLAG_A ? STATIC_FLAG_B : STATIC_FLAG_A;
   }
 
+  if (major_gc) {
+      unload_mark_needed = prepareUnloadCheck();
+  } else {
+      unload_mark_needed = false;
+  }
+
 #if defined(THREADED_RTS)
   work_stealing = RtsFlags.ParFlags.parGcLoadBalancingEnabled &&
                   N >= RtsFlags.ParFlags.parGcLoadBalancingGen;
@@ -831,9 +845,12 @@ GarbageCollect (uint32_t collect_gen,
 
   resetNurseries();
 
- // mark the garbage collected CAFs as dead
 #if defined(DEBUG)
-  if (major_gc && !RtsFlags.GcFlags.useNonmoving) { gcCAFs(); }
+  // Mark the garbage collected CAFs as dead. Done in `nonmovingGcCafs()` when
+  // non-moving GC is enabled.
+  if (major_gc && !RtsFlags.GcFlags.useNonmoving) {
+      gcCAFs();
+  }
 #endif
 
   // Update the stable name hash table
@@ -844,9 +861,14 @@ GarbageCollect (uint32_t collect_gen,
   // hs_free_stable_ptr(), both of which access the StablePtr table.
   stablePtrUnlock();
 
-  // Must be after stablePtrUnlock(), because it might free stable ptrs.
-  if (major_gc) {
-      checkUnload (gct->scavenged_static_objects);
+  // Unload dynamically-loaded object code after a major GC.
+  // See Note [Object unloading] in CheckUnload.c for details.
+  //
+  // TODO: Similar to `nonmovingGcCafs` non-moving GC should have its own
+  // collector for these objects, but that's currently not implemented, so we
+  // simply don't unload object code when non-moving GC is enabled.
+  if (major_gc && !RtsFlags.GcFlags.useNonmoving) {
+      checkUnload();
   }
 
 #if defined(PROFILING)