#14622: Add `joinState# :: State# s -> State# s -> State# s` primop -------------------------------------+------------------------------------- Reporter: reinerp | Owner: (none) Type: feature | Status: new request | Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The state threading in GHC.Prim requires the user to impose a total order on memory reads and writes within a state thread. In some cases, this total order constrains the compiler more than necessary. For example, if the programmer knows that `addr1` and `addr2` don't alias, then they might like to express that the two writes in the following can be reordered relative to each other: {{{#!hs doTwoWrites :: Addr# -> Addr# -> State# s -> State# s doTwoWrites addr1 addr2 s = writeWord8OffAddr# addr2 0# 123## ( writeWord8OffAddr# addr1 0# 234## s) }}} If we had the following primop, then we could express that flexibility in reordering: {{{#!hs -- | Produces a state that has a data dependency on the two input -- states. Generates no machine code. joinState# :: State# s -> State# s -> State# s }}} That is, we'd write the above function as: {{{#!hs doTwoWrites :: Addr# -> Addr# -> State# s -> State# s doTwoWrites addr1 addr2 s = joinState# (writeWord8OffAddr# addr2 0# 123## s) (writeWord8OffAddr# addr1 0# 234## s) }}} This would give us fine-grained fork/join of `State#` threads, with fork provided by copying of `State# s` objects and join provided by `joinState#`. This lets the programmer express arbitrary DAGs of dependencies between memory operations, rather than just linear chains of dependencies. Low priority; I don't have a specific use case with a missed optimization. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14622 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler