[GHC] #2028: STM slightly conservative on write-only transactions

[GHC]

#2028: STM slightly conservative on write-only transactions
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    Reporter:  JulesBean  |       Owner:         
        Type:  proposal   |      Status:  new    
    Priority:  normal     |   Milestone:         
   Component:  Compiler   |     Version:  6.8.1  
    Severity:  normal     |    Keywords:         
  Difficulty:  Unknown    |    Testcase:         
Architecture:  Unknown    |          Os:  Unknown
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 The STM appears to be slightly too conservative on write-only
 transactions.

 It will store a copy of the current (that is, old) value on a writeTVar,
 even if it was never read from. This can cause a spurious retry.

 E.g. {{{atomically $ writeTVar tv 0}}} really has no reason to ever retry.
 Neither, in fact, does {{{atomically $ mapM (\tv -> writeTVar tv 0)
 [tv1,tv2,tv3,tv4]}}}.

 My suggestion is to instead indicate "no prior value" for that TVar, and
 then make no consistency checks on TVars marked as no prior value...

 Jules

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2028>
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