Are brainfuck[1] or malbolge[2] sane? They do exactly what they says they will do, so following your logic they are sane, aren't they?
> The problem is with ever believing what you are describing with C has some 1-to-1 relationship with what the compiler is producing for a given real world hardware implementation.
I'm not sure, that it is possible to be a good C programmer and to not have any clue of what the compiler will produce. You need to know, for example, that if you pass something big by value it will lead to copying this value into the stack. You'd better have some idea how many registers compiler uses to pass arguments to functions, and how it uses them to pass structs. Or, as another example, you need to know how C packs structures, so you will not end up with a structure that uses several times more memory than it needs.
Besides of this you need to think of cache locality and of other concepts that are not mentioned in the specification of C abstract machine. And these things have much greater impact on the performance then the optimizations that become possible only when compiler allows itself to go crazy with UB.
[1] https://en.wikipedia.org/wiki/Brainfuck [2] https://en.wikipedia.org/wiki/Malbolge
