[commit: ghc] master: Optimize casMutVar# for single-threaded RTS (ff7a3c4)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/ff7a3c4f9034af0aca1119c1c1e8f7187460bbad/ghc

>---------------------------------------------------------------

commit ff7a3c4f9034af0aca1119c1c1e8f7187460bbad
Author: David Feuer <david.feuer at gmail.com>
Date:   Mon May 8 17:44:37 2017 -0400

    Optimize casMutVar# for single-threaded RTS
    
    The single-threaded RTS shouldn't actually need to use CAS to
    implement `casMutVar#`; there are no other threads to coordinate
    with.
    
    Reviewers: austin, bgamari, erikd, simonmar
    
    Reviewed By: simonmar
    
    Subscribers: rwbarton, thomie
    
    Differential Revision: https://phabricator.haskell.org/D3544


>---------------------------------------------------------------

ff7a3c4f9034af0aca1119c1c1e8f7187460bbad
 rts/PrimOps.cmm | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/rts/PrimOps.cmm b/rts/PrimOps.cmm
index 7dd88b8..dddba39 100644
--- a/rts/PrimOps.cmm
+++ b/rts/PrimOps.cmm
@@ -528,6 +528,7 @@ stg_newMutVarzh ( gcptr init )
 stg_casMutVarzh ( gcptr mv, gcptr old, gcptr new )
  /* MutVar# s a -> a -> a -> State# s -> (# State#, Int#, Any a #) */
 {
+#if defined(THREADED_RTS)
     gcptr h;
 
     (h) = prim %cmpxchgW(mv + SIZEOF_StgHeader + OFFSET_StgMutVar_var, old, new);
@@ -539,6 +540,20 @@ stg_casMutVarzh ( gcptr mv, gcptr old, gcptr new )
         }
         return (0,new);
     }
+#else
+    gcptr prev_val;
+
+    prev_val = StgMutVar_var(mv);
+    if (prev_val != old) {
+        return (1,prev_val);
+    } else {
+        StgMutVar_var(mv) = new;
+        if (GET_INFO(mv) == stg_MUT_VAR_CLEAN_info) {
+            ccall dirty_MUT_VAR(BaseReg "ptr", mv "ptr");
+        }
+        return (0,new);
+    }
+#endif
 }
 
 stg_atomicModifyMutVarzh ( gcptr mv, gcptr f )