Masking floating point exceptions with Set8087CW, SetMXCSR and TWebBrowser

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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

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