Nasty inline codegen bug in BDS 2006

I'm using assign-test-and-do-something code pattern quite frequently. You know, code like this:

Result := CalculateSomething(some, param); // assign
if not Result then // test
  Exit; // do something

As all real programmers, I'm lazy and tend to rewrite such code as:

if Asgn(Result, CalculateSomething(some, param)) then
  Exit;

Most of the dirty work is done by the Asgn function, which acts as a splitter - it copies one value to two destinations - output parameter and function result.

function Asgn(var output: integer; const value: integer): integer;
begin
  output := value;
  Result := value;
end;

(Actually, I have four overloaded Asgn functions, but that's beside the point.)

Yesterday, I did two things almost at the same time (which was, in hindsight, very lucky - it helped me to find the problem in "mere" 30 minutes). Firstly, I changed all Asgn functions to be inlined on modern Delphis and secondly, I used same variable as part of the value calculation and as the output parameter. In other words, I did this: 

function Asgn(var output: integer; const value: integer): integer; inline;
begin
  output := value;
  Result := value;
end;

copy := Asgn(v1, v1+v2);

(It was slightly more complicated than that but the code above nicely summarizes the problem.)

This code should calculate a sum of two variables and assign it to both v1 and copy. It should, I said, but it didn't. Correct value was assigned to v1, but something completely different was assigned to the copy variable. When I removed the inline, Delphi immediately started to generate correct code.

So, for now, I removed inline from all my Asgn functions. I also know that I'll be extremely causious when using inline in the future.

CodeGear has been notified of the problem (QC #41166) and I hope they'll be able to fix the codegen before Delphi 2007 is out.

If you don't believe me, try it for yourself. Demo is below. It should output four numbers 17, but it doesn't.

program Project8;

{$APPTYPE CONSOLE}

uses
  SysUtils;

function Asgn1(var output: integer; const value: integer): integer;
begin
  output := value;
  Result := value;
end;

function Asgn2(var output: integer; const value: integer): integer; inline;
begin
  output := value;
  Result := value;
end;

var
  v1, v2: integer;
  
begin
  v1 := 10;
  v2 := 7;
  Writeln(Asgn1(v1, v1+v2));
  Writeln(v1);
  v1 := 10;
  v2 := 7;
  Writeln(Asgn2(v1, v1+v2));
  Writeln(v1);
  Readln;
end.