rts: Implement concurrent collection in the nonmoving collector

This extends the non-moving collector to allow concurrent collection.

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 extension involves the introduction of a capability-local
remembered set, known as the /update remembered set/, which tracks
objects which may no longer be visible to the collector due to mutation.
To maintain this remembered set we introduce a write barrier on
mutations which is enabled while a concurrent mark is underway.

The update remembered set representation is similar to that of the
nonmoving mark queue, being a chunked array of `MarkEntry`s. Each
`Capability` maintains a single accumulator chunk, which it flushed
when it (a) is filled, or (b) when the nonmoving collector enters its
post-mark synchronization phase.

While the write barrier touches a significant amount of code it is
conceptually straightforward: the mutator must ensure that the referee
of any pointer it overwrites is added to the update remembered set.
However, there are a few details:

 * In the case of objects with a dirty flag (e.g. `MVar`s) we can
   exploit the fact that only the *first* mutation requires a write
   barrier.

 * Weak references, as usual, complicate things. In particular, we must
   ensure that the referee of a weak object is marked if dereferenced by
   the mutator. For this we (unfortunately) must introduce a read
   barrier, as described in Note [Concurrent read barrier on deRefWeak#]
   (in `NonMovingMark.c`).

 * Stable names are also a bit tricky as described in Note [Sweeping
   stable names in the concurrent collector] (`NonMovingSweep.c`).

We take quite some pains to ensure that the high thread count often seen
in parallel Haskell applications doesn't affect pause times. To this end
we allow thread stacks to be marked either by the thread itself (when it
is executed or stack-underflows) or the concurrent mark thread (if the
thread owning the stack is never scheduled). There is a non-trivial
handshake to ensure that this happens without racing which is described
in Note [StgStack dirtiness flags and concurrent marking].

Co-Authored-by: Ömer Sinan Ağacan <omer@well-typed.com>

3 files changed, 99 insertions, 5 deletions

diff --git a/compiler/GHC/StgToCmm/Bind.hs b/compiler/GHC/StgToCmm/Bind.hs
index 44316cacb0..9e192a0ac8 100644
--- a/compiler/GHC/StgToCmm/Bind.hs
+++ b/compiler/GHC/StgToCmm/Bind.hs
@@ -631,6 +631,7 @@ emitBlackHoleCode node = do
              -- work with profiling.
 
   when eager_blackholing $ do
+    whenUpdRemSetEnabled dflags $ emitUpdRemSetPushThunk node
     emitStore (cmmOffsetW dflags node (fixedHdrSizeW dflags)) currentTSOExpr
     -- See Note [Heap memory barriers] in SMP.h.
     emitPrimCall [] MO_WriteBarrier []
diff --git a/compiler/GHC/StgToCmm/Prim.hs b/compiler/GHC/StgToCmm/Prim.hs
index cdbc8d9fd9..155cdcbf80 100644
--- a/compiler/GHC/StgToCmm/Prim.hs
+++ b/compiler/GHC/StgToCmm/Prim.hs
@@ -42,6 +42,7 @@ import BlockId
 import MkGraph
 import StgSyn
 import Cmm
+import Module   ( rtsUnitId )
 import Type     ( Type, tyConAppTyCon )
 import TyCon
 import CLabel
@@ -339,14 +340,20 @@ dispatchPrimop dflags = \case
     emitAssign (CmmLocal res) (cmmLoadIndexW dflags mutv (fixedHdrSizeW dflags) (gcWord dflags))
 
   WriteMutVarOp -> \[mutv, var] -> OpDest_AllDone $ \res@[] -> do
+    old_val <- CmmLocal <$> newTemp (cmmExprType dflags var)
+    emitAssign old_val (cmmLoadIndexW dflags mutv (fixedHdrSizeW dflags) (gcWord dflags))
+
     -- Without this write barrier, other CPUs may see this pointer before
     -- the writes for the closure it points to have occurred.
+    -- Note that this also must come after we read the old value to ensure
+    -- that the read of old_val comes before another core's write to the
+    -- MutVar's value.
     emitPrimCall res MO_WriteBarrier []
     emitStore (cmmOffsetW dflags mutv (fixedHdrSizeW dflags)) var
     emitCCall
             [{-no results-}]
             (CmmLit (CmmLabel mkDirty_MUT_VAR_Label))
-            [(baseExpr, AddrHint), (mutv,AddrHint)]
+            [(baseExpr, AddrHint), (mutv, AddrHint), (CmmReg old_val, AddrHint)]
 
 --  #define sizzeofByteArrayzh(r,a) \
 --     r = ((StgArrBytes *)(a))->bytes
@@ -1983,17 +1990,21 @@ doWritePtrArrayOp :: CmmExpr
 doWritePtrArrayOp addr idx val
   = do dflags <- getDynFlags
        let ty = cmmExprType dflags val
+           hdr_size = arrPtrsHdrSize dflags
+       -- Update remembered set for non-moving collector
+       whenUpdRemSetEnabled dflags
+           $ emitUpdRemSetPush (cmmLoadIndexOffExpr dflags hdr_size ty addr ty idx)
        -- This write barrier is to ensure that the heap writes to the object
        -- referred to by val have happened before we write val into the array.
        -- See #12469 for details.
        emitPrimCall [] MO_WriteBarrier []
-       mkBasicIndexedWrite (arrPtrsHdrSize dflags) Nothing addr ty idx val
+       mkBasicIndexedWrite hdr_size Nothing addr ty idx val
        emit (setInfo addr (CmmLit (CmmLabel mkMAP_DIRTY_infoLabel)))
-  -- the write barrier.  We must write a byte into the mark table:
-  -- bits8[a + header_size + StgMutArrPtrs_size(a) + x >> N]
+       -- the write barrier.  We must write a byte into the mark table:
+       -- bits8[a + header_size + StgMutArrPtrs_size(a) + x >> N]
        emit $ mkStore (
          cmmOffsetExpr dflags
-          (cmmOffsetExprW dflags (cmmOffsetB dflags addr (arrPtrsHdrSize dflags))
+          (cmmOffsetExprW dflags (cmmOffsetB dflags addr hdr_size)
                          (loadArrPtrsSize dflags addr))
           (CmmMachOp (mo_wordUShr dflags) [idx,
                                            mkIntExpr dflags (mUT_ARR_PTRS_CARD_BITS dflags)])
@@ -2584,6 +2595,9 @@ emitCopyArray copy src0 src_off dst0 dst_off0 n =
         dst     <- assignTempE dst0
         dst_off <- assignTempE dst_off0
 
+        -- Nonmoving collector write barrier
+        emitCopyUpdRemSetPush dflags (arrPtrsHdrSizeW dflags) dst dst_off n
+
         -- Set the dirty bit in the header.
         emit (setInfo dst (CmmLit (CmmLabel mkMAP_DIRTY_infoLabel)))
 
@@ -2646,6 +2660,9 @@ emitCopySmallArray copy src0 src_off dst0 dst_off n =
         src     <- assignTempE src0
         dst     <- assignTempE dst0
 
+        -- Nonmoving collector write barrier
+        emitCopyUpdRemSetPush dflags (smallArrPtrsHdrSizeW dflags) dst dst_off n
+
         -- Set the dirty bit in the header.
         emit (setInfo dst (CmmLit (CmmLabel mkSMAP_DIRTY_infoLabel)))
 
@@ -2774,6 +2791,12 @@ doWriteSmallPtrArrayOp :: CmmExpr
 doWriteSmallPtrArrayOp addr idx val = do
     dflags <- getDynFlags
     let ty = cmmExprType dflags val
+
+    -- Update remembered set for non-moving collector
+    tmp <- newTemp ty
+    mkBasicIndexedRead (smallArrPtrsHdrSize dflags) Nothing ty tmp addr ty idx
+    whenUpdRemSetEnabled dflags $ emitUpdRemSetPush (CmmReg (CmmLocal tmp))
+
     emitPrimCall [] MO_WriteBarrier [] -- #12469
     mkBasicIndexedWrite (smallArrPtrsHdrSize dflags) Nothing addr ty idx val
     emit (setInfo addr (CmmLit (CmmLabel mkSMAP_DIRTY_infoLabel)))
@@ -2953,3 +2976,31 @@ emitCtzCall res x width = do
         [ res ]
         (MO_Ctz width)
         [ x ]
+
+---------------------------------------------------------------------------
+-- Pushing to the update remembered set
+---------------------------------------------------------------------------
+
+-- | Push a range of pointer-array elements that are about to be copied over to
+-- the update remembered set.
+emitCopyUpdRemSetPush :: DynFlags
+                      -> WordOff    -- ^ array header size
+                      -> CmmExpr    -- ^ destination array
+                      -> CmmExpr    -- ^ offset in destination array (in words)
+                      -> Int        -- ^ number of elements to copy
+                      -> FCode ()
+emitCopyUpdRemSetPush _dflags _hdr_size _dst _dst_off 0 = return ()
+emitCopyUpdRemSetPush dflags hdr_size dst dst_off n =
+    whenUpdRemSetEnabled dflags $ do
+        updfr_off <- getUpdFrameOff
+        graph <- mkCall lbl (NativeNodeCall,NativeReturn) [] args updfr_off []
+        emit graph
+  where
+    lbl = mkLblExpr $ mkPrimCallLabel
+          $ PrimCall (fsLit "stg_copyArray_barrier") rtsUnitId
+    args =
+      [ mkIntExpr dflags hdr_size
+      , dst
+      , dst_off
+      , mkIntExpr dflags n
+      ]
diff --git a/compiler/GHC/StgToCmm/Utils.hs b/compiler/GHC/StgToCmm/Utils.hs
index 30e37bb930..0b3a8d8b08 100644
--- a/compiler/GHC/StgToCmm/Utils.hs
+++ b/compiler/GHC/StgToCmm/Utils.hs
@@ -39,6 +39,11 @@ module GHC.StgToCmm.Utils (
         mkWordCLit,
         newStringCLit, newByteStringCLit,
         blankWord,
+
+        -- * Update remembered set operations
+        whenUpdRemSetEnabled,
+        emitUpdRemSetPush,
+        emitUpdRemSetPushThunk,
   ) where
 
 #include "HsVersions.h"
@@ -576,3 +581,40 @@ assignTemp' e
        let reg = CmmLocal lreg
        emitAssign reg e
        return (CmmReg reg)
+
+
+---------------------------------------------------------------------------
+-- Pushing to the update remembered set
+---------------------------------------------------------------------------
+
+whenUpdRemSetEnabled :: DynFlags -> FCode a -> FCode ()
+whenUpdRemSetEnabled dflags code = do
+    do_it <- getCode code
+    the_if <- mkCmmIfThenElse' is_enabled do_it mkNop (Just False)
+    emit the_if
+  where
+    enabled = CmmLoad (CmmLit $ CmmLabel mkNonmovingWriteBarrierEnabledLabel) (bWord dflags)
+    zero = zeroExpr dflags
+    is_enabled = cmmNeWord dflags enabled zero
+
+-- | Emit code to add an entry to a now-overwritten pointer to the update
+-- remembered set.
+emitUpdRemSetPush :: CmmExpr   -- ^ value of pointer which was overwritten
+                  -> FCode ()
+emitUpdRemSetPush ptr = do
+    emitRtsCall
+      rtsUnitId
+      (fsLit "updateRemembSetPushClosure_")
+      [(CmmReg (CmmGlobal BaseReg), AddrHint),
+       (ptr, AddrHint)]
+      False
+
+emitUpdRemSetPushThunk :: CmmExpr -- ^ the thunk
+                       -> FCode ()
+emitUpdRemSetPushThunk ptr = do
+    emitRtsCall
+      rtsUnitId
+      (fsLit "updateRemembSetPushThunk_")
+      [(CmmReg (CmmGlobal BaseReg), AddrHint),
+       (ptr, AddrHint)]
+      False