Undefined Behavior
It's really not that complicated
There’s a lot of confusion among programmers about what C’s “undefined behavior”, or “UB” as it is commonly abbreviated, is for, and why C compilers are allowed to assume UB never happens. This post won’t talk about why the spec contains UB, but will attempt to shed some light on what from my perspective is a rather confusing aspect of how compilers work with it.
A particularly tricky aspect of UB for the typical C programmer is that it sometimes causes the compiler to do counterintuitive things, even to seemingly unrelated parts of the program. It’s a confusing moment at first when disabling optimizations changes the visible effects of your program. If UB represents some kind of abstract “out-of-spec” error condition, why is the compiler allowed to change the behavior of statements leading up to it? There is a very reasonable explanation you could give here, how the spec covers the meaning of whole programs and not just individual statements, but I think it’s much easier understood via time travel.
The unreachable() macro is one way for a program to explicitly invoke
undefined behavior. Because the behavior is undefined, the implementation of
unreachable() could be a program that sends a robot back in time to prevent
the code from running, then destroys the entire world so that none of us are
alive in a timeline where unreachable() finishes execution. A final
printf("Undefined behavior can *never* occur.\n"); is the last thing the
universe sees.
This solution may of course have some retroactive effects that at first seem
unrelated. The time traveling robot may choose not to simply terminate the
program right before it enters unreachable(), and may instead prevent the
program from being started in the first place. It might even delete the code
that was compiled to produce the program, leaving a cryptic commit message
before deactivating itself in a car crusher. It might go even further back in
time to prevent the programmer’s parents from meeting. An oversight in the
robot’s programming might even cause it to be overzealous in its interpretation
of its mission, as it points the time machine to “1971, Bell Labs.” Technically,
the specification prohibits none of these things.
Pending the development of time travel, though, foresight will have to do. Timelines containing an invocation of UB can be safely ignored, and while compilers are not mandated to prevent its occurrence, or even informed that they should assume it never occurs, it’s a useful model for what a conforming compiler is allowed to do in the remaining timelines. The futures where the optimizer breaks its neck doing assembly parkour are not futures we expect to find ourselves in. Or at least, their behavior is undefined.