I think your detailed investigation demonstrates why the code I posted in repsonse to creamygoodness's post behaves the way it does.

After much poking and scratching and re-reading of suggestions that have been kindly and thoughtfully presented here, I've eventually come up with using this approach in the PDL code: 
#include <stdio.h>

#if defined _MSC_VER && _MSC_VER < 1400
#pragma optimize("", off)
#endif 

int main(void) {

#if defined _MSC_VER && _MSC_VER < 1400
   double nv = 2.0 / 3;
   float foo = 2.0 / 3;
   float dummy = (float)nv;

   if(foo == dummy) printf("True ");
   else printf("False ");

#else
   double nv = 2.0 / 3;
   float foo = 2.0 / 3;

   if(foo == (float)nv) printf("True ");
   else printf("False ");

#endif

   return 0;
}

[download]
It seems to be doing the right thing with all of my compilers.
I don't think that C script needs to have the optimization turned off - the creation of the dummy variable is alone sufficient to get the behaviour I'm after. But for some reason, in the PDL code, creation of the dummy variable is *not*, by itself, sufficient - optimization also needs to be disabled. (I turn it off for the setvaltobad functions, then turn it back on again.)

Thanks to *all* who replied.

Cheers,
Rob

In reply to Re^2: [Win32, C, and way OT] C floats, doubles, and their equivalence by syphilis
in thread [Win32, C, and way OT] C floats, doubles, and their equivalence by syphilis

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