Initialization of a variable has no relation to whether it has a memory location. You can legitimately take the address of an uninitialized variable (that is often one step in initializing them, such as when you pass the address to memset), and even an initialized variable may not have a memory location (if it lives only in register, or is compiled away entirely).

int f; f = 2;

To me this says, there is an int pointer f. Let f point to 2. Is this not possible only because 2 does not occupy memory? I don't see why this couldn't be valid.

    int *f; *f = 2;

    *f = 2;

    f = &2;

The first of these is invalid C (undefined behavior) because it uses a value (the contents of f) before initialization.

The second is invalid C (compile error) because 2 is not an l-value (that is, it does not have a location).

Edited to add: I will note that I don't think there is any reason

    *f = 2;

In those cases some compiler override could provide the solution, while still allowing the compiler to flag all the other cases as errors.

If you use the volatile key word, then you are guaranteed that all reads and stores will actually happen and not be optimized out. But volatile still allows non-atomicity and reordering so you probably want something which prohibits that too.

The problem with this democratic approach is that most users are not qualified such that their opinion is particularly valuable.

The small minority who doesn't find it "obviously correct" may actually in some cases be the minority with a clue.

It could work if only those users are given a vote who pass a language lawyer exam.

They aren't advocating a democratic approach, they are saying that the general expectations of users is valuable information.

You don't rely exclusively on what the users want, but users expectations for how compilers behave is very valuable behavior, whether it means finding out how to inform users of the real behavior or changing the behavior.

You cannot implement that efficiently without rejecting valid programs. Consider code like this:

  int x;
  if( f()) x = 1;
  if( g()) h(x);

And that 'reasonable' likely isn't a necessary part of that statement.

For example, your example could legally be rewritten to:

    int x;
    f();
    x = 1;
    if (g()) { h(x); }

    f();
    if (g()) { h(1); }

EDIT: Note the further optimization that looms: if f() can be proven pure (no side effects), then it can be removed. This makes little sense for a function with no arguments (in which case it would just be a constant). If, however, f(y, ...) is some expensive but pure function, it can just be removed completely.

It could apply if f computes a value based on global state but doesn't change anything, which is slightly weaker than "pure".

So changing that behavior means that the compiler now rejects 40 years worth of correctly working legacy code (and some buggy code, as well). Newer languages (e.g. Java, C#) that don't have to support existing code can afford to do what you want and reject programs where a simple heuristic isn't enough to tell whether a variable is initialized or not.

    int x;
    h(x);

On top of that, the C compilers I know more or less have that, as they give warnings for basically cases where Java (with its stricter rules) would refuse to compile equivalent code, and have a flag that turns warnings into errors.

Often there is no sane default value.

g() might have side-effects, and those side-effects have to happen regardless of whether or not f() returned true.

    if (f()) {
        int x = 1;

        if (g()) {
            h(x);
        }

    } else {
        g();
    }

    int g_flag = g();

    if (f()) {
        int x = 1;

        if (g_flag) {
            h(x);
        }
    |

    int f_flag = f(); // force f() to run for ${reasons}
    int g_flag = g(); // force g() to run for ${reasons}

    if (f_flag && g_flag) {
        h(1);
    }

You should never rely on uinitialized values not just because of the problems relating to undefined behavior, but also because you're adding in assumptions about the runtime state. The code depended on the return value of f() but did not fully express that dependency in the code.

This is begging for one question in my opinion. Should we keep using old libraries that nobody is maintaining anymore? Isn't that a big security issue?

    int foo() {
        int bar;
        return bar + 5; /* C4700: local variable 'bar' used without having been initialized */
    }

In the same situation, GCC says "may be used uninitialized in this function" if you enable the warning (-Wmaybe-uninitialized), despite this being a trivial and certain case.

Maybe you are using a old version.

If you enable -Wall which you always should then this flag is automatically enabled.

https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#inde...

You're out of luck. You have to solve the halting problem to statically analyze whether or not a variable will be used when it's undefined. The reason why this is not solved well is because it's impossible to solve perfectly! Java made another approach that I hate: For example if you have

    Object a;
    for(int i = 0; i < 1;++i) a = new Object();
    a.toString();

    error: variable a might not have been initialized  

The parallel to your example would be if the for loop was "for(int i = 0; i < j;++i)". If the compiler was able to determine that there is a code path whereby j might be undefined, should it be allowed to remove the body of the loop, even in those cases where the programmer knows by other means that "j >= 1"?

My request is that it either keep the loop body, or complain about the undefined behavior, but not silently make 'optimizations' based on the fact that it has identified the potential for undefined behavior to occur.

Note that I'm just using an 'uninitialized variable' as a hypothetical example. Given a chance, I always compile with -Wall -Wextra, and in practice, GCC, CLang, and ICC (the compilers I use) do a good job of issuing warnings for the use of uninitialized variables. I like this current behavior, but would prefer a philosophical approach that makes warnings like this more rather than less common.

To put it in classical logic terminology, one case is modus ponens reasoning. Undefined behavior implies the compiler can do whatever it wants. The compiler finds undefined behavior. Therefore it does whatever it wants. This is the case where it'd be better for the compiler to error out than do something nutty.

But many of the optimizations are doing modus tollens reasoning. If X were true, then the program would perform undefined behavior. Conforming programs do not perform undefined behavior. Therefore NOT-X must hold in conforming programs, and this fact can be used in optimizations.

No; rather, the correct logic is that compiler must preserve the body of the loop if there exists the possibility that it can be reached by a valid code path without any undefined behavior (j is defined, and so forth). Only if the compiler can prove that no well-defined execution path can reach the body can it remove it.

(A bad idea to do without any warning, though. If undefined behavior is confirmed, it should be diagnosed.)

That's not always possible. It's not that it makes the optimization when it identifies UB. It's that it makes an optimization that is valid to make if UB doesn't occur, but if UB were to occur then that optimization could cause all kinds of unexpected problems. But the compiler can't necessarily identify those cases.

Please read the "what every C programmer should know about undefined behavior" series of articles from LLVM; they describe the reason why they can't, in general, provide warnings or errors for these cases in which optimizations rely on lack of undefined behavior:

http://blog.llvm.org/2011/05/what-every-c-programmer-should-...

http://blog.llvm.org/2011/05/what-every-c-programmer-should-...

http://blog.llvm.org/2011/05/what-every-c-programmer-should-...

The third article describes why the compiler can't, in general, warn about those cases in which it's relying on lack of UB, but you should read the first two as well.

Note that for some of those cases, clang and GCC have recently added undefined behavior sanitizers, invoked via "-fsanitize=undefined", which can help even more than the warnings they can add. However, what they do is add extra instrumentation to the executable, and then either log a warning or crash when you hit undefined behavior. The runtime aspect helps avoid the "getting this right would involve solving the halting problem" aspect of why they can't, in general, provide appropriate warnings, but it does mean that this is generally only appropriate in test builds, and that you will only find the undefined behavior that you can trigger during test, while there may be more hiding that only show up in obscure circumstances.

If you really don't want undefined behavior, it's best to use a language, like Rust, which does not have any undefined behavior (outside of "unsafe" blocks). The problem with any kind of warnings that are tacked on after the design of the language is that you are either going to get lots of false positives, lots of false negatives, or both. With a language that is designed not to allow undefined behavior, you know that if the code compiles, it doesn't invoke UB.

    let x;
    println!("x:{}", x);

Sometimes in C one might initialize a variable by passing a pointer to it to an init function:

  void f( void )
  {
    int i;
    bool success;

    success = init( &i );

    if ( success )
      do_stuff( i );
  }

    fn f() {
        if let Some(i) = init() {
            do_stuff(i);
        }
    }

The Rust pattern is not only safer, but briefer than yours. It describes the code flow such that you can't remove or repeat a part and end up with inadvertently broken code, and it's memory safe. There is no way for `init` to blow up the stack (whereas in your example, a malicious or buggy init can use the address of i to smash the stack.)

    int i = 0; // or some other default value

    Object a = null;