Masking floating point exceptions with Set8087CW, SetMXCSR and TWebBrowser

Question

As I am receiving "Floating point division by zero" exception when using TWebBrowser and TEmbeddedWB from time to time, I discovered that I need to mask division by zero exceptions Set8087CW or SetMXCSR.

Q1: What would be the best approach to do this:

1. to mask such exceptions early in the application startup and never touch them again (the app is multithreaded)?
2. to use OnBeforeNavigate and OnDocumentComplete events to mask / unmask exceptions? (is there a chance that exception could occur after the document is loaded?)

Q2: What would be the best "command" to mask only "division by zero" and nothing else - if application is 32-bit is there a need to mask 64 bit exception too?

The application I am using it it has TWebBrowser control available all the time for displaying email contents.

Also, if anyone can clarify - is this a particular bug with TWebBrowser control from Microsoft or just difference between Delphi/C++ Builder and Microsoft tools? What would happen if I would host TWebBrowser inside Visual C++ application if division by zero error would appear - it wouldn't be translated into exception but what would happen then - how would Visual C++ handle "division by zero" exception then?

It is kind of strange that Microsoft didn't notice this problem for such a long time - also it is strange that Embarcardero never noticed it too. Because masking floating point exception effectively also masks your own program exception for that particular purpose.

UPDATE

My final solution after some examination is:

SetExceptionMask(GetExceptionMask() << exZeroDivide);

The default state from GetExceptionMask() returns: TFPUExceptionMask() << exDenormalized << exUnderflow << exPrecision. So obviously, some exceptions are already masked - this just adds exZeroDivide to the masked exceptions.

As a result every division by zero now results with +INF in floating point instead of exception. I can live with that - for the production version of the code it will me masked to avoid errors and for the debug version it will be unmasked to detect floating point division by zero.

Answer 1

Assuming that you have no need for floating point exceptions to be unmasked in your application code, far and away the simplest thing to do is to mask exceptions at some point in your initialization code.

The best way to do this is as so:

SetExceptionMask(exAllArithmeticExceptions);

This will set the 8087 control word on 32 bit targets, and the MXCSR on 64 bit targets. You will find SetExceptionMask in the Math unit.

If you wish to have floating point exceptions unmasked in your code then it gets tricky. One strategy would be to run your floating point code in a dedicated thread that unmasks the exceptions. This certainly can work, but not if you rely on the RTL functions Set8087CW and SetMXCSR. Note that everything in the RTL that controls FP units routes through these functions. For example SetExceptionMask does.

The problem is that Set8087CW and SetMXCSR are not threadsafe. It seems hard to believe that Embarcadero could be so inept as to produce fundamental routines that operate on thread context and yet fail to be threadsafe. But that is what they have done.

It's surprisingly hard to undo the mess that they have left, and to do so involves quite a bit of code patching. The lack of thread safety is down to the (mis)use of the global variables Default8087CW and DefaultMXCSR. If two threads call Set8087CW or SetMXCSR at the same time then these global variables can have the effect of leaking the value from one thread to the other.

You could replace Set8087CW and SetMXCSR with versions that did not change global state, but it's sadly not that simple. The global state is used in various other places. This may seem immodest, but if you want to learn more about this matter, read my document attached to this QC report: http://qc.embarcadero.com/wc/qcmain.aspx?d=107411