[lmm]

I'm reminded of http://mjg59.livejournal.com/108257.html :

"POSIX says this is fine, so any application that expected this behaviour is already broken by definition. But this is rules lawyering. POSIX says that many things that are not useful are fine, but doesn't exist for the pleasure of sadistic OS implementors. POSIX exists to allow application writers to write useful applications. If you interpret POSIX in such a way that gains you some benefit but shafts a large number of application writers then people are going to be reluctant to use your code. You're no longer a general purpose filesystem - you're a filesystem that's only suitable for people who write code with the expectation that their OS developers are actively trying to fuck them over."

GCC doesn't, or at least shouldn't, exist to implement the ANSI standard. It exists to help people to write useful programs.

[pgaddict]

And it does that by implementing a compiler for a language with existing standard, which is ANSI. If you don't like the ANSI C standard (and I admit it's not perfect), don't use a compiler for ANSI C.

Also, this is not just GCC problem, all the existing C compilers have the issue to some extent. After all, STACK (the MIT tool to detect undefined behavior) is based on clang. And ICC exploits the same UB tricks AFAIK.

[lmm]

> And it does that by implementing a compiler for a language with existing standard, which is ANSI. If you don't like the ANSI C standard (and I admit it's not perfect), don't use a compiler for ANSI C.

I'm not arguing that GCC should violate the ANSI standard; rather it should provide additional guarantees above the what ANSI requires (which was always the intent of the standard; the standard defines the absolute minimum that cross-platform programs can depend on, the reason so much is undefined is to allow compilers to have their own strategies for what should happen in those cases, not to require that compilers blow up in those cases). Honestly I think the ANSI side of things is a red herring; when given the option of some change that will slightly improve performance on some benchmarks, but make a lot of user code silently fail, a responsible developer should know to reject that change whether or not that change violates some standard.

> Also, this is not just GCC problem, all the existing C compilers have the issue to some extent.

The post is claiming that GCC is the worst of them. Certainly my impression is that clang is substantially less aggressive at exploiting UB; I don't know ICC well enough to comment.

[pgaddict]

> I'm not arguing that GCC should violate the ANSI standard; rather it should provide additional guarantees above the what ANSI requires ...

The problem with that approach is that it introduces dependency on the compiler. The original code was ANSI C and thus should compile fine on all compilers compatible with ANSI C, the new code is not as each compiler will decide to handle undefined behavior differently. Either you'll make the exact compiler a hard dependency (i.e. it always has to be compiled with gcc and fails to build with everything else), or it will produce "correct" binaries on some compilers and "incorrect" binaries on others. That's hardly an improvement.

The only way out of this is either to abandon C and use a language with stronger guarantees, or make the ANSI C more strict by adding the guarantees to the standard. Which is not going to happen, I guess.

> The post is claiming that GCC is the worst of them. Certainly my impression is that clang is substantially less aggressive at exploiting UB; I don't know ICC well enough to comment.

GCC is also the most widely, so people tend to spot issues more often.

All this "problem" is a direct consequence of using C without really understanding what guarantees it does and does not provide, and instead driving by a simplified model of the environment. And then getting angry that the simplified model is not really correct.

[lmm]

> The original code was ANSI C and thus should compile fine on all compilers compatible with ANSI C, the new code is not as each compiler will decide to handle undefined behavior differently.

Except 40% of the original code already wasn't ANSI C.

> Either you'll make the exact compiler a hard dependency (i.e. it always has to be compiled with gcc and fails to build with everything else), or it will produce "correct" binaries on some compilers and "incorrect" binaries on others. That's hardly an improvement.

Having code that was broken under GCC not be broken under GCC absolutely is an improvement, particularly since in fact this kind of code often works on every other extant compiler.

> make the ANSI C more strict by adding the guarantees to the standard. Which is not going to happen, I guess.

Standards tend to codify existing practice. There's no reason the standard couldn't be made stricter - but the way we get to there from here is if the major compilers implement stricter restrictions and can show that they can be implemented consistently and users find them useful. GCC has been willing to do that kind of innovation for other parts of the standard.

[pgaddict]

> Except 40% of the original code already wasn't ANSI C.

Then why complain that ANSI C compiler gets confused by it?

> Having code that was broken under GCC not be broken under GCC absolutely is an improvement, particularly since in fact this kind of code often works on every other extant compiler.

No, the code does not work on every other compiler. And if it is, there's no guarantee it will stay like that.

> Standards tend to codify existing practice. There's no reason the standard couldn't be made stricter - but the way we get to there from here is if the major compilers implement stricter restrictions and can show that they can be implemented consistently and users find them useful. GCC has been willing to do that kind of innovation for other parts of the standard.

AFAIK some of the limitations are there because of non-traditional platforms - some of them may be a thing of the past so removing them would be OK, but some are not (and thus won't be removed from the standard). And one of the points of ANSI C (and POSIX) is to define global guarantees, not per-platform ones.

[lmm]

> Then why complain that ANSI C compiler gets confused by it?

Because I didn't ask for an ANSI C-and-not-a-penny-more compiler. Nobody wants that. Back in the day the GNU project made a point of going against standards when the standardized behaviour was user-unfriendly (POSIX_ME_HARDER etc.)

> No, the code does not work on every other compiler.

In many of these cases it does work on all other major compilers, or all other relevant platforms for that particular codebase.

> And if it is, there's no guarantee it will stay like that.

So what? That doesn't make it better to break it now.

> one of the points of ANSI C (and POSIX) is to define global guarantees, not per-platform ones.

Which is why it's GCC's (or any other compiler's) responsibility to define the per-platform guarantees.