[PATCH] Document arithmetic overflow semantics

[Richard Kenner]

    How?  Any new optimization that we introduce that takes advantage
    of the undefinedness will both produce different results from the
    current versions of GCC, 

So what?  There's no requirement that erroneous programs produce the
same result in different versions of GCC.

    and will break the Java front-end.

No, because *those* nodes would be marked as *not* being undefined on
overflow.