Merge non-moving garbage collector

This introduces a concurrent mark & sweep garbage collector to manage the old
generation. The concurrent nature of this collector typically results in
significantly reduced maximum and mean pause times in applications with large
working sets.

Due to the large and intricate nature of the change I have opted to
preserve the fully-buildable history, including merge commits, which is
described in the "Branch overview" section below.

Collector design
================

The full design of the collector implemented here is described in detail
in a technical note

> B. Gamari. "A Concurrent Garbage Collector For the Glasgow Haskell
> Compiler" (2018)

This document can be requested from @bgamari.
The basic heap structure used in this design is heavily inspired by

> K. Ueno & A. Ohori. "A fully concurrent garbage collector for
> functional programs on multicore processors." /ACM SIGPLAN Notices/
> Vol. 51. No. 9 (presented at ICFP 2016)

This design is intended to allow both marking and sweeping
concurrent to execution of a multi-core mutator. Unlike the Ueno design,
which requires no global synchronization pauses, the collector
introduced here requires a stop-the-world pause at the beginning and end
of the mark phase.

To avoid heap fragmentation, the allocator consists of a number of
fixed-size /sub-allocators/. Each of these sub-allocators allocators into
its own set of /segments/, themselves allocated from the block
allocator. Each segment is broken into a set of fixed-size allocation
blocks (which back allocations) in addition to a bitmap (used to track
the liveness of blocks) and some additional metadata (used also used
to track liveness).

This heap structure enables collection via mark-and-sweep, which can be
performed concurrently via a snapshot-at-the-beginning scheme (although
concurrent collection is not implemented in this patch).

Implementation structure
========================

The majority of the collector is implemented in a handful of files:

 * `rts/Nonmoving.c` is the heart of the beast. It implements the entry-point
   to the nonmoving collector (`nonmoving_collect`), as well as the allocator
   (`nonmoving_allocate`) and a number of utilities for manipulating the heap.

 * `rts/NonmovingMark.c` implements the mark queue functionality, update
   remembered set, and mark loop.

 * `rts/NonmovingSweep.c` implements the sweep loop.

 * `rts/NonmovingScav.c` implements the logic necessary to scavenge the
   nonmoving heap.

Branch overview
===============

```
 * wip/gc/opt-pause:
 |   A variety of small optimisations to further reduce pause times.
 |
 * wip/gc/compact-nfdata:
 |   Introduce support for compact regions into the non-moving
 |\  collector
 | \
 |  \
 | | * wip/gc/segment-header-to-bdescr:
 | | |   Another optimization that we are considering, pushing
 | | |   some segment metadata into the segment descriptor for
 | | |   the sake of locality during mark
 | | |
 | * | wip/gc/shortcutting:
 | | |   Support for indirection shortcutting and the selector optimization
 | | |   in the non-moving heap.
 | | |
 * | | wip/gc/docs:
 | |/    Work on implementation documentation.
 | /
 |/
 * wip/gc/everything:
 |   A roll-up of everything below.
 |\
 | \
 | |\
 | | \
 | | * wip/gc/optimize:
 | | |   A variety of optimizations, primarily to the mark loop.
 | | |   Some of these are microoptimizations but a few are quite
 | | |   significant. In particular, the prefetch patches have
 | | |   produced a nontrivial improvement in mark performance.
 | | |
 | | * wip/gc/aging:
 | | |   Enable support for aging in major collections.
 | | |
 | * | wip/gc/test:
 | | |   Fix up the testsuite to more or less pass.
 | | |
 * | | wip/gc/instrumentation:
 | | |   A variety of runtime instrumentation including statistics
 | | /   support, the nonmoving census, and eventlog support.
 | |/
 | /
 |/
 * wip/gc/nonmoving-concurrent:
 |   The concurrent write barriers.
 |
 * wip/gc/nonmoving-nonconcurrent:
 |   The nonmoving collector without the write barriers necessary
 |   for concurrent collection.
 |
 * wip/gc/preparation:
 |   A merge of the various preparatory patches that aren't directly
 |   implementing the GC.
 |
 |
 * GHC HEAD
 .
 .
 .
```

1 files changed, 90 insertions, 34 deletions

diff --git a/rts/Schedule.c b/rts/Schedule.c
index eced4d4fb6..9323915dfe 100644
--- a/rts/Schedule.c
+++ b/rts/Schedule.c
@@ -44,6 +44,8 @@
 #include "StablePtr.h"
 #include "StableName.h"
 #include "TopHandler.h"
+#include "sm/NonMoving.h"
+#include "sm/NonMovingMark.h"
 
 #if defined(HAVE_SYS_TYPES_H)
 #include <sys/types.h>
@@ -110,6 +112,19 @@ Mutex sched_mutex;
 #define FORKPROCESS_PRIMOP_SUPPORTED
 #endif
 
+/*
+ * sync_finished_cond allows threads which do not own any capability (e.g. the
+ * concurrent mark thread) to participate in the sync protocol. In particular,
+ * if such a thread requests a sync while sync is already in progress it will
+ * block on sync_finished_cond, which will be signalled when the sync is
+ * finished (by releaseAllCapabilities).
+ */
+#if defined(THREADED_RTS)
+static Condition sync_finished_cond;
+static Mutex sync_finished_mutex;
+#endif
+
+
 /* -----------------------------------------------------------------------------
  * static function prototypes
  * -------------------------------------------------------------------------- */
@@ -130,7 +145,6 @@ static void scheduleYield (Capability **pcap, Task *task);
 static bool requestSync (Capability **pcap, Task *task,
                          PendingSync *sync_type, SyncType *prev_sync_type);
 static void acquireAllCapabilities(Capability *cap, Task *task);
-static void releaseAllCapabilities(uint32_t n, Capability *cap, Task *task);
 static void startWorkerTasks (uint32_t from USED_IF_THREADS,
                               uint32_t to USED_IF_THREADS);
 #endif
@@ -150,7 +164,8 @@ static void scheduleHandleThreadBlocked( StgTSO *t );
 static bool scheduleHandleThreadFinished( Capability *cap, Task *task,
                                           StgTSO *t );
 static bool scheduleNeedHeapProfile(bool ready_to_gc);
-static void scheduleDoGC(Capability **pcap, Task *task, bool force_major);
+static void scheduleDoGC( Capability **pcap, Task *task,
+                          bool force_major, bool deadlock_detect );
 
 static void deleteThread (StgTSO *tso);
 static void deleteAllThreads (void);
@@ -250,7 +265,7 @@ schedule (Capability *initialCapability, Task *task)
     case SCHED_INTERRUPTING:
         debugTrace(DEBUG_sched, "SCHED_INTERRUPTING");
         /* scheduleDoGC() deletes all the threads */
-        scheduleDoGC(&cap,task,true);
+        scheduleDoGC(&cap,task,true,false);
 
         // after scheduleDoGC(), we must be shutting down.  Either some
         // other Capability did the final GC, or we did it above,
@@ -547,7 +562,7 @@ run_thread:
     }
 
     if (ready_to_gc || scheduleNeedHeapProfile(ready_to_gc)) {
-      scheduleDoGC(&cap,task,false);
+      scheduleDoGC(&cap,task,false,false);
     }
   } /* end of while() */
 }
@@ -921,7 +936,7 @@ scheduleDetectDeadlock (Capability **pcap, Task *task)
         // they are unreachable and will therefore be sent an
         // exception.  Any threads thus released will be immediately
         // runnable.
-        scheduleDoGC (pcap, task, true/*force major GC*/);
+        scheduleDoGC (pcap, task, true/*force major GC*/, true/*deadlock detection*/);
         cap = *pcap;
         // when force_major == true. scheduleDoGC sets
         // recent_activity to ACTIVITY_DONE_GC and turns off the timer
@@ -991,7 +1006,7 @@ scheduleProcessInbox (Capability **pcap USED_IF_THREADS)
     while (!emptyInbox(cap)) {
         // Executing messages might use heap, so we should check for GC.
         if (doYouWantToGC(cap)) {
-            scheduleDoGC(pcap, cap->running_task, false);
+            scheduleDoGC(pcap, cap->running_task, false, false);
             cap = *pcap;
         }
 
@@ -1368,17 +1383,24 @@ scheduleNeedHeapProfile( bool ready_to_gc )
  * change to the system, such as altering the number of capabilities, or
  * forking.
  *
+ * pCap may be NULL in the event that the caller doesn't yet own a capability.
+ *
  * To resume after stopAllCapabilities(), use releaseAllCapabilities().
  * -------------------------------------------------------------------------- */
 
 #if defined(THREADED_RTS)
-static void stopAllCapabilities (Capability **pCap, Task *task)
+void stopAllCapabilities (Capability **pCap, Task *task)
+{
+    stopAllCapabilitiesWith(pCap, task, SYNC_OTHER);
+}
+
+void stopAllCapabilitiesWith (Capability **pCap, Task *task, SyncType sync_type)
 {
     bool was_syncing;
     SyncType prev_sync_type;
 
     PendingSync sync = {
-        .type = SYNC_OTHER,
+        .type = sync_type,
         .idle = NULL,
         .task = task
     };
@@ -1387,9 +1409,10 @@ static void stopAllCapabilities (Capability **pCap, Task *task)
         was_syncing = requestSync(pCap, task, &sync, &prev_sync_type);
     } while (was_syncing);
 
-    acquireAllCapabilities(*pCap,task);
+    acquireAllCapabilities(pCap ? *pCap : NULL, task);
 
     pending_sync = 0;
+    signalCondition(&sync_finished_cond);
 }
 #endif
 
@@ -1400,6 +1423,16 @@ static void stopAllCapabilities (Capability **pCap, Task *task)
  * directly, instead use stopAllCapabilities().  This is used by the GC, which
  * has some special synchronisation requirements.
  *
+ * Note that this can be called in two ways:
+ *
+ * - where *pcap points to a capability owned by the caller: in this case
+ *   *prev_sync_type will reflect the in-progress sync type on return, if one
+ *   *was found
+ *
+ *  - where pcap == NULL: in this case the caller doesn't hold a capability.
+ *    we only return whether or not a pending sync was found and prev_sync_type
+ *    is unchanged.
+ *
  * Returns:
  *    false if we successfully got a sync
  *    true  if there was another sync request in progress,
@@ -1424,13 +1457,25 @@ static bool requestSync (
         // After the sync is completed, we cannot read that struct any
         // more because it has been freed.
         *prev_sync_type = sync->type;
-        do {
-            debugTrace(DEBUG_sched, "someone else is trying to sync (%d)...",
-                       sync->type);
-            ASSERT(*pcap);
-            yieldCapability(pcap,task,true);
-            sync = pending_sync;
-        } while (sync != NULL);
+        if (pcap == NULL) {
+            // The caller does not hold a capability (e.g. may be a concurrent
+            // mark thread). Consequently we must wait until the pending sync is
+            // finished before proceeding to ensure we don't loop.
+            // TODO: Don't busy-wait
+            ACQUIRE_LOCK(&sync_finished_mutex);
+            while (pending_sync) {
+                waitCondition(&sync_finished_cond, &sync_finished_mutex);
+            }
+            RELEASE_LOCK(&sync_finished_mutex);
+        } else {
+            do {
+                debugTrace(DEBUG_sched, "someone else is trying to sync (%d)...",
+                          sync->type);
+                ASSERT(*pcap);
+                yieldCapability(pcap,task,true);
+                sync = pending_sync;
+            } while (sync != NULL);
+        }
 
         // NOTE: task->cap might have changed now
         return true;
@@ -1445,9 +1490,9 @@ static bool requestSync (
 /* -----------------------------------------------------------------------------
  * acquireAllCapabilities()
  *
- * Grab all the capabilities except the one we already hold.  Used
- * when synchronising before a single-threaded GC (SYNC_SEQ_GC), and
- * before a fork (SYNC_OTHER).
+ * Grab all the capabilities except the one we already hold (cap may be NULL is
+ * the caller does not currently hold a capability). Used when synchronising
+ * before a single-threaded GC (SYNC_SEQ_GC), and before a fork (SYNC_OTHER).
  *
  * Only call this after requestSync(), otherwise a deadlock might
  * ensue if another thread is trying to synchronise.
@@ -1477,29 +1522,30 @@ static void acquireAllCapabilities(Capability *cap, Task *task)
             }
         }
     }
-    task->cap = cap;
+    task->cap = cap == NULL ? tmpcap : cap;
 }
 #endif
 
 /* -----------------------------------------------------------------------------
- * releaseAllcapabilities()
+ * releaseAllCapabilities()
  *
- * Assuming this thread holds all the capabilities, release them all except for
- * the one passed in as cap.
+ * Assuming this thread holds all the capabilities, release them all (except for
+ * the one passed in as keep_cap, if non-NULL).
  * -------------------------------------------------------------------------- */
 
 #if defined(THREADED_RTS)
-static void releaseAllCapabilities(uint32_t n, Capability *cap, Task *task)
+void releaseAllCapabilities(uint32_t n, Capability *keep_cap, Task *task)
 {
     uint32_t i;
 
     for (i = 0; i < n; i++) {
-        if (cap->no != i) {
-            task->cap = capabilities[i];
-            releaseCapability(capabilities[i]);
+        Capability *tmpcap = capabilities[i];
+        if (keep_cap != tmpcap) {
+            task->cap = tmpcap;
+            releaseCapability(tmpcap);
         }
     }
-    task->cap = cap;
+    task->cap = keep_cap;
 }
 #endif
 
@@ -1507,9 +1553,11 @@ static void releaseAllCapabilities(uint32_t n, Capability *cap, Task *task)
  * Perform a garbage collection if necessary
  * -------------------------------------------------------------------------- */
 
+// N.B. See Note [Deadlock detection under nonmoving collector] for rationale
+// behind deadlock_detect argument.
 static void
 scheduleDoGC (Capability **pcap, Task *task USED_IF_THREADS,
-              bool force_major)
+              bool force_major, bool deadlock_detect)
 {
     Capability *cap = *pcap;
     bool heap_census;
@@ -1801,9 +1849,10 @@ delete_threads_and_gc:
     // reset pending_sync *before* GC, so that when the GC threads
     // emerge they don't immediately re-enter the GC.
     pending_sync = 0;
-    GarbageCollect(collect_gen, heap_census, gc_type, cap, idle_cap);
+    signalCondition(&sync_finished_cond);
+    GarbageCollect(collect_gen, heap_census, deadlock_detect, gc_type, cap, idle_cap);
 #else
-    GarbageCollect(collect_gen, heap_census, 0, cap, NULL);
+    GarbageCollect(collect_gen, heap_census, deadlock_detect, 0, cap, NULL);
 #endif
 
     // If we're shutting down, don't leave any idle GC work to do.
@@ -2453,7 +2502,11 @@ resumeThread (void *task_)
     tso = incall->suspended_tso;
     incall->suspended_tso = NULL;
     incall->suspended_cap = NULL;
-    tso->_link = END_TSO_QUEUE; // no write barrier reqd
+    // we will modify tso->_link
+    IF_NONMOVING_WRITE_BARRIER_ENABLED {
+        updateRemembSetPushClosure(cap, (StgClosure *)tso->_link);
+    }
+    tso->_link = END_TSO_QUEUE;
 
     traceEventRunThread(cap, tso);
 
@@ -2627,6 +2680,8 @@ initScheduler(void)
   /* Initialise the mutex and condition variables used by
    * the scheduler. */
   initMutex(&sched_mutex);
+  initMutex(&sync_finished_mutex);
+  initCondition(&sync_finished_cond);
 #endif
 
   ACQUIRE_LOCK(&sched_mutex);
@@ -2662,9 +2717,10 @@ exitScheduler (bool wait_foreign USED_IF_THREADS)
     // If we haven't killed all the threads yet, do it now.
     if (sched_state < SCHED_SHUTTING_DOWN) {
         sched_state = SCHED_INTERRUPTING;
+        nonmovingStop();
         Capability *cap = task->cap;
         waitForCapability(&cap,task);
-        scheduleDoGC(&cap,task,true);
+        scheduleDoGC(&cap,task,true,false);
         ASSERT(task->incall->tso == NULL);
         releaseCapability(cap);
     }
@@ -2732,7 +2788,7 @@ performGC_(bool force_major)
     // TODO: do we need to traceTask*() here?
 
     waitForCapability(&cap,task);
-    scheduleDoGC(&cap,task,force_major);
+    scheduleDoGC(&cap,task,force_major,false);
     releaseCapability(cap);
     boundTaskExiting(task);
 }