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3444 :: I'm getting a type mismatch despite using a type cast

• monster (orginal) [2017-12-29T16:25:08+01:00] view original

  I’m getting a "type mismatch" when compiling my code, and I cannot see what I’m doing wrong. Since the code is somewhat large, here is a summarized version:

  # MODULE atomiks
  
  type
    VolatilePtr*[T] = distinct ptr T
  
  template declVolatile*(name: untyped, T: typedesc, init: T) {.dirty.} =
    ## Defines an hidden volatile value, with an initial value,  and a pointer to it
    var `volatile XXX name XXX value` {.global,volatile.} = init
    let `name` {.global.} = cast[VolatilePtr[T]](addr `volatile XXX name XXX value`)
  
  # ...
  
  proc atomicIncRelaxed*[T: AtomType](p: VolatilePtr[T], x: T = 1): T =
    ## Increments a volatile (32/64 bit) value, and returns the new value.
    ## Performed in RELAXED/No-Fence memory-model.
    ## Will only compile on Windows 8+!
    let pp = cast[pointer](p)
    when sizeof(T) == 8:
      cast[T](interlockedExchangeAddNoFence64(pp, cast[int64](x)))
    elif sizeof(T) == 4:
      cast[T](interlockedExchangeAddNoFence32(pp, cast[int32](x)))
    else:
      static: assert(false, "invalid parameter size: " & $sizeof(T))
  
  # MODULE kueues
  
  type
    MsgSeqID* = int32
  
  declVolatile(myProcessGlobalSeqID, MsgSeqID, MsgSeqID(0))
  # Current (global) per Process message sequence ID.
  
  proc seqidProvider(): MsgSeqID =
    ## Generates a message sequence ID for the Thread.
    result = atomicIncRelaxed(myProcessGlobalSeqID) # LINE NO 213
  Run

  The line with atomicIncRelaxed() doesn’t compile with this message:

  kueues.nim(213, 14) Error: type mismatch: got (int) but expected 'MsgSeqID = int32'
  Run

  I think that the type of “myProcessGlobalSeqID” should be “VolatilePtr[MsgSeqID]”, therefore the call to “atomicIncRelaxed(myProcessGlobalSeqID)” should receive a “VolatilePtr[MsgSeqID]” as input parameter, and return a “MsgSeqID” as output, since I cast the result of "interlockedExchangeAddNoFenceXX()" to "T". Since “result” is of type “MsgSeqID”, everything should be fine, IMO.

  EDIT:

  If I add a cast, just so it compiles, like this:

  result = MsgSeqID(atomicIncRelaxed(myProcessGlobalSeqID))
  Run

  I get this warning:

  warning C4133: 'function': incompatible types - from 'NI32 *' to 'NI *'
  Run

  And the generated code at that location looks like:

  NI32 volatile volatileXXXmyProcessGlobalSeqIDXXXvalue_E9bTOMWoAg8GLV5f71h9b3tg;
  NI32* myProcessGlobalSeqID_08rTXbtOEJJH36WoNR65hw;
  #...
  N_NIMCALL(NI32, seqidProvider_C9apW3EupUtW1ohORfpWXXw)(void) {
          NI32 result;
          NI T1_;
          nimfr_("seqidProvider", "kueues.nim");
          result = (NI32)0;
          nimln_(213, "kueues.nim");
          T1_ = (NI)0;
          T1_ = atomicIncRelaxed_7kAYyH5nySTDnWgC39bO1AQ(myProcessGlobalSeqID_08rTXbtOEJJH36WoNR65hw, ((NI) 1));
          result = ((NI32)chckRange(T1_, ((NI32) (-2147483647 -1)), ((NI32) 2147483647)));
          popFrame();
          return result;
  }
  Run

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• Udiknedormin (orginal) [2017-12-30T13:45:54+01:00] view original

  # Replace:
  proc atomicIncRelaxed*[T: AtomType](p: VolatilePtr[T], x: T = 1): T
  # with:
  proc atomicIncRelaxed*[T: AtomType](p: VolatilePtr[T], x: T = 1.int32): T
  Run

  Note it will still work for int64 thanks to the conversion.

  The reason of this problem is that int32 is int at the same time. Because you used 1, which is int, and the VolatilePtr[int32] could be VolatilePtr[int] as well (thanks to int=int32), I take it the compiler was happy to assume T=int. When you use 1.int32 explicitly, either in the call or default value, it does the opposite: if it takes VolatilePtr[int], it assumes it is VolatilePtr[int32] (because it matched the default 1.int32) and happily assigns an int32 result to an int.

  So the general conclusion is: type equality is one-way in some contexts.

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• monster (orginal) [2017-12-30T14:07:22+01:00] view original

  @Udiknedormin I can't say 100% for sure, because now I get some error that it cannot find the required "Interlocked" function (maybe wrong version of Visual Studio installed?), but at least the original error message is gone, so I think your solution worked. :)

  EDIT:

  I have now found the definition of InterlockedExchangeAddNoFence() in winnt.h (and In fact, what I needed was InterlockedAddNoFence(), which is also there). And it looks like this:

  #define InterlockedExchangeAddNoFence _InterlockedExchangeAdd

  Argh! The "no fence" is a joke!

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