5 ms·
Fair enough! > And if it's not succeeded for 54 years, "try harder", or "just never make a mistake", is at least not the solution. And I 100% agree. UB is way
by muvlon 5mo ago
Fair enough!
> And if it's not succeeded for 54 years, "try harder", or "just never make a mistake", is at least not the solution.
And I 100% agree. UB is way overused by these standards for how dangerous it is, and as a consequence using C (and C++) for anything nontrivial amounts to navigating a minefield.
- saagarjha 5mo agoWhat should the behavior above be defined to do?
- lll-o-lll 5mo agoHCF
- JadeNB 5mo agoWhat is that?
- FabHK 5mo agoA fictitious assembly instruction (and pretty good TV series). https://en.wikipedia.org/wiki/Halt_and_Catch_Fire_(computing) https://en.wikipedia.org/wiki/Halt_and_Catch_Fire_(computing...
- SAI_Peregrinus 5mo agoHalt and Catch Fire
- saagarjha 5mo agoI have good news about what UB allows
- jeffffff 5mo agoCompilation error
- echoangle 5mo agoCouldn’t you just define that function arguments are evaluated left to right? Or just throw an error.
- saagarjha 5mo agoI meant reading the uninitialized variable
- muvlon 5mo agoThere is no uninitialized variable, I explicitly initialized it to 5. And yes indeed, C could do what Rust does and define the order of evaluation for function arguments. If the argument expressions are indeed side-effect-less, the compiler can always make use of the "as-if" rule and legally reorder the computation anyway, for example to alleviate register pressure.
- jfoks 5mo agoWhy? Just for this edge case? It could be faster and/or allow smaller code size to allow this to be undefined. Undefined is also different from "depends on the compiler", because which behavior is chosen can even depend on the circumstances, whatever code appears before and/or after it. That said, UB in code, such as this example of ordering of reads of volatile parameters being undefined, does not automatically mean that code that uses it is bad. It may very well be that the function being called doesn't misbehave either way.
- echoangle 5mo agoThat’s the point of the whole article. It’s not worth the speed gain to have a language that nobody can safely use because you can’t really prevent UB when you write it. > It may very well be that the function being called doesn't misbehave either way. The function being good or bad has nothing to do with the UB. The UB occurs before the function is called.
- Filligree 5mo agoPrint x twice. Not all “side effects” care about order. Better yet, define an order for parameter evaluation.
- poppadom1982 5mo agoYou're missing the point. Volatile forces two loads of a value that may have changed in the middle. So the value of "x" may depend on the time/order of load.
- hmry 5mo agoWhy is that missing the point? Loading it twice, possibly with different values, is the intended behavior. It's only undefined because the C spec doesn't specify the order of the loads (unlike most other languages which have a perfectly well-defined order for side effects in a single expression).
- rowanG077 5mo agoWhat you are describing is implementation defined behavior. Using that is perfectly safe and reasonable. Undefined means this programs is malformed.
- hmry 5mo agoNo I'm just repeating what the original comment said, which is that it's explicitly UB: "5.1.2.4.1 says any volatile access - including just reading it - is a side effect. 6.5.1.2 says that unsequenced side effects on the same scalar object (in this case, x) are UB. 6.5.3.3.8 tells us that the evaluations of function arguments are indeterminately sequenced w.r.t. each other." If function arguments were sequenced with respect to each other, it wouldn't be a problem. But actually, maybe the original comment is wrong. Presumably "indeterminately sequenced" and "unsequenced" mean different things, although I don't have a copy of the standard at hand to check.
- AnimalMuppet 5mo ago
- tsukikage 5mo ago“Implementation defined behaviour”: compiler author chooses, and documents the choice. A lot of UB should be implementation defined behaviour instead; this would much better match programmers’ intuitions as they reason about their code - you can even see it in the comments of this post: it’s always things like “this hardware supports / doesn’t support unaligned accesses”, it’s never nasal demons.
- tardedmeme 5mo agoI told someone at a conference that UB actually means "implementation-defined, no documentation required". He started to refute me and then stopped.
- marcosdumay 5mo agoExcept that UB doesn't mean that. UB means "the developer must never write this".
- munch117 5mo agoBoth are wrong. It means "this standard does not constrain the behaviour of code that does this". It's entirely legal for implementations to have predictable behaviour, documented or not, for code that is undefined by the standard. In their quest for maxxing benchmark performance they generally choose not to, but there's really nothing in any standard that stops you from making an implementation that prioritises safety.
- tardedmeme 5mo agoEvery implementation so far has predictable behavior in all cases. Sometimes the rules for predicting it are very obscure. But it's all fully defined within the compiler's binary code. And none of them link to nasal portals.
- AlotOfReading 5mo ago
- stellamariesays 5mo ago[flagged]
- webstrand 5mo agoI think as compilers got smarter, UB changed somewhat in meaning. Originally the compilers didn't perform such complex analysis, and while invoking UB could break your program, it would still do something reasonable.
- marcosdumay 5mo agoYes, but compilers got smart enough for it to be a problem around 30 years ago, and we are still arguing about what to do.
- pjmlp 5mo agoYou see a reasoning here, basically when all those C compiler benchmarks started, vendors moved from what Frank Allen described, to anything goes to win SPEC something benchmarks. "Oh, it was quite a while ago. I kind of stopped when C came out. That was a big blow. We were making so much good progress on optimizations and transformations. We were getting rid of just one nice problem after another. When C came out, at one of the SIGPLAN compiler conferences, there was a debate between Steve Johnson from Bell Labs, who was supporting C, and one of our people, Bill Harrison, who was working on a project that I had at that time supporting automatic optimization...The nubbin of the debate was Steve's defense of not having to build optimizers anymore because the programmer would take care of it. That it was really a programmer's issue.... Seibel: Do you think C is a reasonable language if they had restricted its use to operating-system kernels? Allen: Oh, yeah. That would have been fine. And, in fact, you need to have something like that, something where experts can really fine-tune without big bottlenecks because those are key problems to solve. By 1960, we had a long list of amazing languages: Lisp, APL, Fortran, COBOL, Algol 60. These are higher-level than C. We have seriously regressed, since C developed. C has destroyed our ability to advance the state of the art in automatic optimization, automatic parallelization, automatic mapping of a high-level language to the machine. This is one of the reasons compilers are ... basically not taught much anymore in the colleges and universities." -- Fran Allen interview, Excerpted from: Peter Seibel. Coders at Work: Reflections on the Craft of Programming