Comment by xorblurb | Hacker News Reader

Comment by xorblurb | Hacker News Reader

Parent Full thread

It is not the particular situation people are upset about (well, at least me). It is the reasoning, especially in the context of "you shall not write undefined behavior according to the standard even if it should makes sens given the computer you are actually programming" that is usually sent on us regular programmer by compiler authors... :/

Yet they do exactly the same thing, and pretty much casually. And I actually don't really blame them for doing it. Everybody makes mistakes. But I blame them for the double standard, the risks, and the attitude. The problem in the first place: engineering is also about mitigating the fact that mistakes will happen, often mitigating even the mistake of others, and the modern approach in regard with undefined behavior does not help and is even ultra-hurtful for that. I don't see how stability will emerge with that kind of thought process.

You are tarring all compiler authors with a very broad brush here.

Myself and other compiler authors I work with are much more specific in their requests. We do actually define some behavior that the standard doesn't require, and we do document this and tell our users its fine to rely on it.

Unfortunately, we also have a lot of really clear experience with this stuff that lead us to two conclusions:

1) When we identify UB in code and go look at that code, for very large bodies of code what we find are overwhelmingly bugs. Bad, complete bugs. Defining the behavior wouldn't fix the bug, it was just one of the many symptoms.

2) We have real hardware differences for a lot of simple things that make them hard or impossible to give defined behavior for. As an example, shifting left by more bits than the size of the value gives different results on several different platforms.

None of this means that we don't need to help programmers cope with this. We do indeed also make mistakes, and in fact it was these compiler authors that built a tool to help programmers find UB in their code: http://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html

> We have real hardware differences for a lot of simple things that make them hard or impossible to give defined behavior for.

Hard or impossible to give _performant_ defined behavior for.

> As an example, shifting left by more bits than the size of the value gives different results on several different platforms.

You could, for instance, always mod the shift by the word size, but possibly at a significant cost.

Reply on news.ycombinator.com