The cost to supress a warning (and how not pay for it)

In the previous post, i pointed that passing a managed type (dynamic array) as a var parameter is more efficient than returning the value as a function result. However this technique have a known side effect: the compiler outputs a message  (Warning: Local variable "XXX" does not seem to be initialized) each time a call to the procedure is compiled.

The direct way to suppress the warning is change the parameter from var to out. Pretty simple but out does more than inhibit the compiler message. It implicitly initialize managed types parameters to nil or add a call FPC_INITIALIZE if the parameter is a record that has at least a field of a managed type. It does not add implicit code to simple types like Integer or class instances (TObject etc).

Although the performance impact is mostly negligible, is extra code anyway. In my case i initialize the parameter explicitly so out would add redundant code. There's an alternative to suppress the message: add the directive {%H-} in front of the variable that is being passed to the procedure. In the example of the previous post would be:

BuildRecArray({%H-}Result);

It can be annoying if the function is called often or the routine is part of a public API, otherwise is fine. At least for me.

Update: out does not generate initialization code for records that contains only fields which type is not automatically managed by the compiler, e.g., Integer.

Does it matter how dynamic arrays are passed/returned to/from a routine?

I was implementing a routine that should return a dynamic array and wondered if the produced code of a function and a procedure with a var parameter are different. So, i setup a simple test:

type
  TMyRec = record
    O: TObject;
    S: String;
  end;

  TMyRecArray = array of TMyRec;

function BuildRecArray: TMyRecArray;
begin
  SetLength(Result, 1);
  Result[0].O := nil;
  Result[0].S := 'x';
end;

procedure BuildRecArray(var Result: TMyRecArray);
begin
  SetLength(Result, 1);
  Result[0].O := nil;
  Result[0].S := 'x';
end;

var
  Result: TMyRecArray;

begin
  BuildRecArray(Result); //or Result := BuildRecArray
end.

Looking at the generated assembly revealed that the function version (returns the array in the result) leads to bigger code when compared with the procedure version (pass the array as a var parameter). More: the code difference is due to an implicit exception frame which is known to impact performance.

And what about the caller code? Again the function version generates more code (creates a temporary variable and calls FPC_DYNARRAY_DECR_REF).

In short: yes, it matters.