[WalterGillman]

I was thinking more along the lines of detectable instances with compiler introducing "optimizations", but as a C "programmer" I do not mind bounds checks and any other runtime improvements that stay true to the language.

If it's implementation-defined that you can turn them off when you're building for the PDP11, I'm sold.

[muldvarp]

Compilers already warn when they detect _unconditional_ undefined behavior. They just don't warn on _conditional_ undefined behavior because doing so would introduce far too many warnings.

Exploiting undefined behavior for optimization only requires local analysis, detecting whether that undefined behavior arises (either unconditionally or at all) requires global analysis. To put it differentially: The compiler often simply doesn't know whether the undefined behavior arises, it only knows that the optimization it introduces is valid anyway.