[nickelpro]

If you want slow code that behaves exactly the way you expect, turn off optimizations. Congrats, you have the loose assembly wrapper language you always wanted C to be.

For the rest of us, we're going to keep getting every last drop out of performance that we can wring out of the compiler. I do not want my compiler to produce an "obvious" or "reasonable" interpretation of my code, I want it to produce the fastest possible "as if" behavior for what I described within the bounds of the standard.

If I went outside the bounds of the standard, that's my problem, not the compiler's.

[krackers]

Just like -ffast-math, that should be an opt-in flag. I'd bet most people want (and even expect) the compiler to do the sane thing, especially if it only costs them 2% performance. The quest for mythical "performance" over correctness is precisely why we've landed into this situation in the first place.

[nickelpro]

The compiler does sane things. I also fundamentally disagree with the OP that the situations that invoke UB are surprising.

This largely derives from C developers believing they understand the language by thinking of it as a loose wrapper around assembly, instead of an abstract machine with specific rules and requirements.

Within the bounds of the abstract machine described by the standard, things like signed integer overflow, pointer aliasing, etc, don't make any sense. They are intuitively undefined. If C developers actually read the standard instead of pretending C describes the 8086 they learned in undergrad, they wouldn't be worried about the compiler doing "sane" things with their unsound code because they wouldn't gravitate towards that code in the first place. No one thinks that dereferencing an integer makes sense, no one accidentally writes that code, because it intuitively doesn't work even in misguided internal models.

This doesn't solve problems like buffer overflows of course, which are much more about the logical structure of the program than its language rules. For that style of logical error there's no hope for C in the general case, although static analyzers help.

[krackers]

Isn't this tautologically saying the compiler does sane things if you define "sane" as what the compiler allows? As Linus said standards are written on toilet paper, in the real world you have a need to do signed overflow and type punning. In the real-world pretty much every system of note uses 2s complement, and if there was a concern about maintaining compatibility with archaic systems it could be made "implementation defined" behavior instead.

To get something approximating "sane behavior" you have to set a dozen flags to disable various types of questionable optimizations.

[nickelpro]

No, C is totally sane for the machine it describes. If you want to describe a different machine, one in which objects can alias, where signed overflow has a specific meaning, etc, you can choose to describe that machine instead using compiler flags. In that case it is on you to understand what new, unstandardized machine you are describing. This is fine and good, nothing wrong with it, and often very useful.

The problem is with ever believing what you are describing with C has some 1-to-1 relationship with what the compiler is producing for a given real world hardware implementation. This delusion is unique to C. Fortran, Python, Java, Go, etc, programmers don't ever think about what the underlying hardware is doing with their code. They're writing code for an abstract machine defined by the language in implementation or standard.

[ordu]

> No, C is totally sane for the machine it describes.

Are brainfuck[1] or malbolge[2] sane? They do exactly what they says they will do, so following your logic they are sane, aren't they?

> The problem is with ever believing what you are describing with C has some 1-to-1 relationship with what the compiler is producing for a given real world hardware implementation.

I'm not sure, that it is possible to be a good C programmer and to not have any clue of what the compiler will produce. You need to know, for example, that if you pass something big by value it will lead to copying this value into the stack. You'd better have some idea how many registers compiler uses to pass arguments to functions, and how it uses them to pass structs. Or, as another example, you need to know how C packs structures, so you will not end up with a structure that uses several times more memory than it needs.

Besides of this you need to think of cache locality and of other concepts that are not mentioned in the specification of C abstract machine. And these things have much greater impact on the performance then the optimizations that become possible only when compiler allows itself to go crazy with UB.

[1] https://en.wikipedia.org/wiki/Brainfuck [2] https://en.wikipedia.org/wiki/Malbolge

[nickelpro]

> Are brainfuck[1] or malbolge[2] sane?

In that they are sound and consistent, yes, any esoteric language is sane. Merely esoteric.

> I'm not sure, that it is possible to be a good C programmer and to not have any clue of what the compiler will produce...

The compiler is under no requirement to pack structures in a specific way, or under any requirement to produce accesses in a cache pattern that mirrors your intent in the code. The compiler is required to produce a very specific set of observable behaviors, specifically:

> Volatile accesses to objects are evaluated strictly according to the rules of the abstract machine.

> At program termination, all data written into files shall be identical to the result that execution of the program according to the abstract semantics would have produced.

> The input and output dynamics of interactive devices shall take place as specified in 7.23.3. The intent of these requirements is that unbuffered or line-buffered output appear as soon as possible, to ensure that prompting messages appear prior to a program waiting for input.

And that's it. When thinking about the correctness of one's code, things like layout and cache access patterns are irrelevant because the abstract machine does not provide for such things. Thinking in that frame can only lead to errors.

Now, as a fully separate and apart lens to view one's code, of course performance matters. Of course struct layout (as guaranteed by ABI standards like SysV and Windows) matters, of course you should make the optimizer's job easier by doing sequential rather than scattered accesses. That's true of any programming language.

It's true for COBOL and Go and Ada, and yet when using those languages one does not try to reason about accessing objects via incompatible lvalues like C programmers often try to do. This machine-optimization frame of thinking is not a frame you can use to think about the behavior of the program with, behavior needs to be conceived in the frame of the abstract machine.

[RevEng]

The problem is in how compiler writers have interpreted this as license to do some bizarre things. Does it contain UB? Skip that code entirely without saying a word. Regardless of whether or not the code is malformed, this kind of response is hostile to the user.

[Filligree]

For the rest of us, actually we will be using different languages that don't pretend UB is what we want. I'd be using C a lot more if I could possibly trust myself to write correct C.