I think the equivalent for the current practice would basically be:<p>1. setting &quot;-foverflow=promote&quot; on the compiler&#x27;s parsing-and-IR-generating stage, thus generating code that acts like all integers occupy infinitely-sized abstract machine registers, and expects this to get shimmed as checked promotion to pointers-to-structs from a bignum library on most architectures;<p>and then, seperately,<p>2. setting an optimization flag like &quot;-Onopromote&quot; for the target-recompilation stage, that throws away all the bignum promotion shims that <i>would</i> have been inserted, and pretends that it&#x27;s adhering to bignum-promotion semantics anyway (thus creating code that will be optimized the way C code currently is.)<p>That&#x27;s quite a bit more complicated, but also a lot more explicit about what&#x27;s going on: you have to specify (or pick a profile that specifies) the actual semantics you want, and then you have to explicitly opt into <i>assuming</i> those semantics rather than <i>enforcing</i> them. There&#x27;d probably be a similar pair of {semantic, assumption} flags for each currently-undefined behavior.