Comment by quelsolaar
5 hours ago
A lot of the Central UB can not be defined, because they rely on detection. In order to have a well defined behaviour (by the standard or the compiler) the implementation needs to first detect that the behaviour is triggered, this is often very tricky or expensive. Its easy to define that a program should halt, if it writes outside an array, but detecting if it does can be both slow and hard to implement. There are implementations that do, but they are rarely used outside of debugging.
A better way to think about UB is as a contract between developer and implementation, so that the implementations can more easily reason about the code. How would you optimize:
(x * 2) / 2
An optimizer can optimize this out for a signed integer, because it doesn't have to consider overflow, but with a unsigned integer it can not. UB is a big reason why C is the most power efficient high level language.
> How would you optimize: (x * 2) / 2
I'd do the math myself and just write x.
I don't even use * for multiplication anymore, I use __builtin_mul_overflow and then check the result. Anyone who doesn't is gonna hit the overflow case one day, and they'll be lucky if their program isn't exploited because of it. I've been making an effort to use all the overflow checking builtins by default in most if not all cases. I've also been making Claude audit every single bare arithmetic operation in my projects. He's caught quite a few security issues already, and overflow checking dealt with them all.
This particular contract between developer and implementation is totally worthless and doing more harm than good. It encompasses regular everyday normal things like multiplication and addition. All things that our brains literally rely on in order to reason about the code. Can't even add numbers without the compiler screwing it up.
Programmers need to deal with overflow at all times. Can't calculate an offset without dealing with overflow. Can't calculate a size without dealing with overflow. It's simply everywhere in systems programming, which is what C was designed to do. The consequence of ignoring this is usually that your program gets mercilessly exploited.
All this for some efficiency gains. The cost/benefit analysis is way off here. Things should be correct, first and foremost. Then the compiler should give us the necessary sharp tools to make it fast, if needed. It shouldn't be making it fast at the cost of turning the entire language into a memetic vulnerability machine.
The things you want from C isn't C. Id advice you to use another language.
No. I like C. I've learned about a dozen languages by now. I always end up coming back to C. I've just accepted it.
There is no reason whatsoever that C can't be improved. Compiler attributes and builtins are already doing quite a lot of heavy lifting. Recent addition: counted_by, an attribute that allows compilers to properly track the size of memory referenced by pointers. All C programmers should be making liberal use of this stuff.