[GHC] #693: dynamic locking

[GHC]

#693: dynamic locking
----------------------------+----------------------------------------------
        Reporter:           |            Owner:  simonmar
  simonmar                  |           Status:  closed
            Type:  task     |        Milestone:  7.8.1
        Priority:  low      |          Version:  6.4.1
       Component:  Runtime  |         Keywords:
  System                    |     Architecture:  Unknown/Multiple
      Resolution:  fixed    |       Difficulty:  Moderate (less than a day)
Operating System:           |       Blocked By:
  Unknown/Multiple          |  Related Tickets:
 Type of failure:           |
  None/Unknown              |
       Test Case:  N/A      |
        Blocking:           |
----------------------------+----------------------------------------------

Comment (by parcs):

 I think `atomicModifyMutVar#` and `noDuplicate#` could be optimized in a
 similar fashion.

 When `n_capabilities == 1`, `atomicModifyMutVar#` could update the
 `MutVar` in a faster (1.8x), non-atomic fashion instead of using a
 compare-and-swap. Similarly, `noDuplicate#` could elide the call to
 `threadPaused` since there is no possibility of work being duplicated. Are
 these safe assumptions?

 How does this patch look? `./validate` seems content with it, at least.

 {{{
 diff --git a/rts/PrimOps.cmm b/rts/PrimOps.cmm
 index ced15ee..863c352 100644
 --- a/rts/PrimOps.cmm
 +++ b/rts/PrimOps.cmm
 @@ -340,6 +340,15 @@ stg_atomicModifyMutVarzh ( gcptr mv, gcptr f )
     LDV_RECORD_CREATE(r);
     StgThunk_payload(r,0) = z;

 +#ifdef THREADED_RTS
 +   if (CInt[n_capabilities] == 1 :: CInt) {
 +     x = StgMutVar_var(mv);
 +     StgThunk_payload(z,1) = x;
 +     StgMutVar_var(mv) = y;
 +     goto out;
 +   }
 +#endif
 +
   retry:
     x = StgMutVar_var(mv);
     StgThunk_payload(z,1) = x;
 @@ -350,6 +359,7 @@ stg_atomicModifyMutVarzh ( gcptr mv, gcptr f )
     StgMutVar_var(mv) = y;
  #endif

 +out:
     if (GET_INFO(mv) == stg_MUT_VAR_CLEAN_info) {
       ccall dirty_MUT_VAR(BaseReg "ptr", mv "ptr");
     }
 @@ -2008,6 +2018,10 @@ INFO_TABLE_RET(stg_noDuplicate, RET_SMALL, W_
 info_ptr)

  stg_noDuplicatezh /* no arg list: explicit stack layout */
  {
 +    if (CInt[n_capabilities] == 1 :: CInt) {
 +        jump %ENTRY_CODE(Sp(0)) [];
 +    }
 +
      STK_CHK(WDS(1), stg_noDuplicatezh);

      // leave noDuplicate frame in case the current
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/693#comment:16>
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