Why an application starts with FPU Control Word different than Default8087CW?

Question

Could you please help me to understand what is going on with FPU Control Word in my Delphi application, on Win32 platform.

When we create a new VCL application, the control word is set up to 1372h. This is the first thing I don't understand, why it is 1372h instead of 1332h which is the Default8087CW defined in System unit.

The difference between these two:

1001101110010  //1372h
1001100110010  //1332h

is the 6th bit which according to documentation is reserved or not used.

The second question regards CreateOleObject.

function CreateOleObject(const ClassName: string): IDispatch;
var
  ClassID: TCLSID;
begin
  try
    ClassID := ProgIDToClassID(ClassName);
{$IFDEF CPUX86}
    try
      Set8087CW( Default8087CW or $08);
{$ENDIF CPUX86}
      OleCheck(CoCreateInstance(ClassID, nil, CLSCTX_INPROC_SERVER or
        CLSCTX_LOCAL_SERVER, IDispatch, Result));
{$IFDEF CPUX86}
    finally
      Reset8087CW;
    end;
{$ENDIF CPUX86}
  except
    on E: EOleSysError do
      raise EOleSysError.Create(Format('%s, ProgID: "%s"',[E.Message, ClassName]),E.ErrorCode,0) { Do not localize }
  end;    
end;

The above function is changing control word to 137Ah, so it is turning on the 3rd bit (Overflow Mask). I don't understand why it is calling Reset8087CW after, instead of restoring the state of the word which was before entering into the function?

Answer 1

The 6th bit is reserved and ignored. Those two control words are in fact equal in the sense that the FPU behaves the same. The system just happens to set the reserved bit. Even if you attempt to set the value to $1332, the system will set it to $1372. No matter what value you ask the 6th bit to have, it will always be set. So, when comparing these values you have to ignore that bit. Nothing to worry about here.

As for CreateOleObject the authors decided that if you are going to use that function then you are also going to mask overflow when using the COM object, and indeed beyond. Who knows why they did so, and for 32 bit code only? Probably they found a bunch of COM objects that routinely overflowed, and so added this sticking plaster. It wasn't enough to mask overflow on creation, it also need to be done when using the object so The RTL designers chose to unmask overflow henceforth.

Or perhaps it was a bug. They decided not to fix it for 32 bit code because people relied on the behaviour, but they did fix for 64 bit code.

In any case this function does nothing very special. You don't need to use it. You can write your own that does what you want it to do.

Floating point control is a problem when working with interop. Delphi code expects unmasked exceptions. Code built with other tools typically masks them. Ideally you would mask exceptions when you call out of your Delphi code and unmask them on return. Expect other libraries to arbitrarily change the control word. Also be aware that Set8087CW is not thread safe which is a massive problem that Embarcadero have refused to address for many years.

There's no easy way forward. If you aren't using floating point in your program then you could simply mask exceptions and probably be fine. Otherwise you need to make sure that the control word is set appropriately at all points in all threads. In general that is close to impossible using the standard Delphi RTL. I personally handle this by replacing the key parts of the RTL with threadsafe versions. I have documented how to do so in this QC report: QC#107411.

Answer 2

Disclaimer: I debugged the questions in Delphi XE.

First, the second question.

If you look at the code of Set8087CW you will see that it stores the new FPU CW value in Default8087CW variable, and Reset8087CW restores FPU CW from Default8087CW; so the Reset8087CW call after Set8087CW does nothing at all, which is demonstrated by

Memo1.Lines.Clear;
Memo1.Lines.Add(IntToHex(Get8087CW, 4));   // 1372
Set8087CW( Default8087CW or $08);
Memo1.Lines.Add(IntToHex(Get8087CW, 4));   // 137A
Reset8087CW;
Memo1.Lines.Add(IntToHex(Get8087CW, 4));   // 137A

Evidently a bug.

Now the first question - it was interesting debugging exercise.

The Default8087CW value of Delphi VCL application is changed from hex 1332 to 1372 by Windows.CreateWindowEx function, called from Classes.AllocateHWnd, called from TApplication.Create, called from initialization section of Controls.pas unit.

Have a look at CreateWindowEx code - it explains what happens. I don't really want to discuss it further - the FPU support in Delphi is too messy and buggy.