add threadStatus# primop, for querying the status of a ThreadId#

compiler/prelude/primops.txt.pp

@@ -1531,6 +1531,11 @@ primop  NoDuplicateOp "noDuplicate#" GenPrimOp
    with
    out_of_line = True
 
+primop  ThreadStatusOp "threadStatus#" GenPrimOp
+   ThreadId# -> State# RealWorld -> (# State# RealWorld, Int# #)
+   with
+   out_of_line = True
+
 ------------------------------------------------------------------------
 section "Weak pointers"
 ------------------------------------------------------------------------

includes/Constants.h

@@ -193,6 +193,7 @@
 
 /*
  * Constants for the why_blocked field of a TSO
+ * NB. keep these in sync with GHC/Conc.lhs: threadStatus
  */
 #define NotBlocked          0
 #define BlockedOnMVar       1

includes/StgMiscClosures.h

@@ -585,6 +585,7 @@ RTS_FUN(unblockAsyncExceptionszh_fast);
 RTS_FUN(myThreadIdzh_fast);
 RTS_FUN(labelThreadzh_fast);
 RTS_FUN(isCurrentThreadBoundzh_fast);
+RTS_FUN(threadStatuszh_fast);
 
 RTS_FUN(mkWeakzh_fast);
 RTS_FUN(finalizzeWeakzh_fast);

rts/Linker.c

@@ -768,6 +768,7 @@ typedef struct _RtsSymbolVal {
       SymX(stg_upd_frame_info)			\
       SymX(suspendThread)			\
       SymX(takeMVarzh_fast)			\
+      SymX(threadStatuszh_fast)			\
       SymX(timesIntegerzh_fast)			\
       SymX(tryPutMVarzh_fast)			\
       SymX(tryTakeMVarzh_fast)			\

rts/PrimOps.cmm

@@ -1034,6 +1034,39 @@ isCurrentThreadBoundzh_fast
   RET_N(r);
 }
 
+threadStatuszh_fast
+{
+    /* args: R1 :: ThreadId# */
+    W_ tso;
+    W_ why_blocked;
+    W_ what_next;
+    W_ ret;
+
+    tso = R1;
+    loop:
+      if (TO_W_(StgTSO_what_next(tso)) == ThreadRelocated) {
+          tso = StgTSO__link(tso);
+          goto loop;
+      }
+
+    what_next   = TO_W_(StgTSO_what_next(tso));
+    why_blocked = TO_W_(StgTSO_why_blocked(tso));
+    // Note: these two reads are not atomic, so they might end up
+    // being inconsistent.  It doesn't matter, since we
+    // only return one or the other.  If we wanted to return the
+    // contents of block_info too, then we'd have to do some synchronisation.
+
+    if (what_next == ThreadComplete) {
+        ret = 16;  // NB. magic, matches up with GHC.Conc.threadStatus
+    } else {
+        if (what_next == ThreadKilled) {
+            ret = 17;
+        } else {
+            ret = why_blocked;
+        }
+    }
+    RET_N(ret);
+}
 
 /* -----------------------------------------------------------------------------
  * TVar primitives