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1248 :: Allocation of return value

• andrea (orginal) [2015-05-21T19:04:11+02:00] view original

  Say I have a proc returning a complex data structure. A common idiom in C would be to allocate space for the data in the calling function, and pass a pointer to that as an out parameter, so that the callee can build the result directly there, and no copying is necessary when returning.

  I think I read this is done implicitly in Nim, so that for instance the following

  proc callee(x: int): BigDataStructure =
    for i in 0 .. x:
      result[i] = something(i)
  
  proc caller(): auto =
    let b = callee(5)
  Run

  would have callee fill the memory allocated by the declaration of b, which is on the stack frame of caller (or on the heap).

  Is this understanding correct? Does this feature depend on using the result implicit variable or would it be the same if I created another variable to hold the result and return that?

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• Jehan (orginal) [2015-05-21T20:39:38+02:00] view original

  First, var x = initFoo(), where the type is a large value, is compiled as follows:

  1. x is initialized to zero.
  2. A pointer to x is passed to the function implementing initFoo.
  3. This pointer is used as the place where result is stored. Hence, no further copying is needed if you are assigning directly to result.
  4. Both clang and gcc backends will frequently be able to even inline the whole thing.

  By using an intermediate variable that you then return, you introduce an intermediate copy, because return tmp means basically result = tmp; return. Backends may still be able to optimize it away, but it can't be guaranteed.

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• filcuc (orginal) [2015-05-22T00:51:26+02:00] view original

  Nice info. i'll keep it mind from now one :)

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• andrea (orginal) [2015-05-22T09:46:37+02:00] view original

  Thanks Jehan, that clears things a lot! One more question: what happens when the proc is a single expression? Is this

  proc callee(): Foo = expression
  Run

  any different from this?

  proc callee(): Foo =
    result = expression
  Run

  I tend to prefer the first style when usable, since it looks much more declarative, but I would not want to inadvertently introduce a copy

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• Araq (orginal) [2015-05-22T10:16:38+02:00] view original

  There is no difference. The compiler rewrites = expression to result = expression.

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• andrea (orginal) [2015-05-22T10:32:22+02:00] view original

  Great, thanks!

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