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3431 :: noSideEffect pragma is not doing that much

• Stefan_Salewski (orginal) [2017-12-24T21:02:24+01:00] view original

  proc p(i: int): int {.noSideEffect.} =
    debugEcho("p() called")
    return i * 2
  
  proc p2 =
    echo p(3)
    echo 2 * p(3)
  
  p2()
  
  Run

  
  ./t
  p() called
  6
  p() called
  12
  Run

  I was always hoping that above code would call proc p() only once, as p has no side effects and is called two times with same parameter. So caching result should occur.

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• cdome (orginal) [2017-12-24T21:21:38+01:00] view original

  Interesting to check if it is sufficient to add __attribute__ ((pure)) to p declaration in c codegen to make it happen. If yes, then it is easy to implement.

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• GULPF (orginal) [2017-12-24T23:06:36+01:00] view original

  {.noSideEffect.} allows the arguments to be mutated, so it isn't the same as _attribute((pure)).

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• Udiknedormin (orginal) [2017-12-26T21:58:09+01:00] view original

  {.noSideEffect, codeGenDecl: "__attribute__((pure)) $# $#$#".} still doesn't help for my nim 0.17.2 and gcc 5.4.0.

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