Hello all,

I have discovered this example of a bug with ASM code
generation: basically, Delphi 2.0 doesn't always know
when (e.g.) the EDX register is being silently trashed.

This matters in constructors and destructors, since DL
is used to pass a silent parameter which is needed by
both the entry and exit code.

------------------------------------------------------

program TestAsm;

{
  ** WARNING ** Buggy Code!!! Do Not Run!!!

{$DEFINE Bug}

uses
  SysUtils;

type
  TBadAsm = class
            private
              FField: Comp;
            public
              constructor Create(const Number: Integer);
              destructor Destroy; override;
            end;

  TCompRec = packed record
               Lo, Hi: Integer
             end;

constructor TBadAsm.Create(const Number: Integer);
begin
  asm
    MOV EAX, 50
  {
    The MUL ECX instruction places the product in EDX:EAX. However, the
values
    in both EAX and EDX are needed by Delphi. Delphi is smart enough to
know
    to preserve EAX, but it forgets to save EDX as well **unless explicitly
    reminded that EDX is destroyed** (using INC EDX) ...
  }
  {$IFNDEF Bug}
    INC EDX
  {$ENDIF}
    MUL ECX
    MOV ECX, Self
    MOV TCompRec[ECX+OFFSET TBadAsm.FField].Lo, EAX
    MOV TCompRec[ECX+OFFSET TBadAsm.FField].Hi, EDX
  end
end;

destructor TBadAsm.Destroy;
begin
  asm
    MOV ECX, EAX
{
  Destructors have identical problems to constructors: their entry and exit
  code depend on the values in EAX and DL, and Delphi forgets to preserve
  these values under certain conditions ...

  Trying to use the `assembler' directive with destructors also fails-
  Internal Error C3254 ...

  {$IFNDEF Bug}
    INC EDX
  {$ENDIF}
    MUL TCompRec[ECX+OFFSET TBadAsm.FField].Lo
  end
end;

var
  BadAsm: TBadAsm;
begin
  BadAsm := TBadAsm.Create(100);
  BadAsm.Free
end.