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A performance hit compared to what?  There are half a 
dozen variations stacked up now.   _Py_CheckInterval is 
back to 10 in the latest patch, and perhaps that has 
something to do with it.

_Py_CheckInterval needn't be delcared volatile, BTW; i.e., 
take the "volatile" out of

+ PyAPI_DATA(volatile int) _Py_CheckInterval;

I can't time this today, but you should be just as keen to 
get x-platform verification when claiming a performance hit 
as when claiming a performance boost.  Chances are that it 
will slobber all over the place across compilers; ceval is 
extremely touchy.  I'm sitting on a major slowdown under 
MSCV6 after the SET_LINENO thing, and I'm not panicked 
about that <wink>.