Eliminating a statement by assuming UB will never happen is a compile-time condition.

I think the problem is more that it’s not as if there’s a single place in the compiler saying “aha! UB! let’s surprise the developer!”. It’s the effect of propagation through multiple optimization steps.

Not quite, it’s the fact that if you have to assume the UB condition and make behavior defined for that condition, then you can’t apply certain optimizations, and/or you have to generate extra code to detect and handle the UB condition.

In any case, it would mean that existing programs that do not exhibit UB (but that contain expressions that could be UB when executed in the context of a different program) would suddenly compile to less efficient code. It’s not surprising that compiler vendors have little interest in agreeing to such changes to the C standard, which effectively would mean a performance regression for their compilers.

You can’t change the situation without either forfeiting some performance or changing existing programs.

This is how UB came about in the first place, in the first ANSI C version. Everything the compiler vendors couldn’t agree on to specify even just an implementation-defined behavior for, became UB.