Does “Undefined Behavior” really permit *anything* to happen? [duplicate]

Answer 1

Yes, it permits anything to happen. The note is just giving examples. The definition is pretty clear:

Undefined behavior: behavior for which this International Standard imposes no requirements.

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Frequent point of confusion:

You should understand that "no requirement" also means means the implementation is NOT required to leave the behavior undefined or do something bizarre/nondeterministic!

The implementation is perfectly allowed by the C++ standard to document some sane behavior and behave accordingly.¹ So, if your compiler claims to wrap around on signed overflow, logic (sanity?) would dictate that you're welcome to rely on that behavior on that compiler. Just don't expect another compiler to behave the same way if it doesn't claim to.

_{¹Heck, it's even allowed to document one thing and do another. That'd be stupid, and it'd probably make you toss it into the trash—why would you trust a compiler whose documentation lies to you?—but it's not against the C++ standard.}

Answer 2

One of the reasons for leaving behavior undefined is to allow the compiler to make whatever assumptions it wants when optimizing.

If there exists some condition that must hold if an optimization is to be applied, and that condition is dependent on undefined behavior in the code, then the compiler may assume that it's met, since a conforming program can't depend on undefined behavior in any way. Importantly, the compiler does not need to be consistent in these assumptions. (which is not the case for implementation-defined behavior)

So suppose your code contains an admittedly contrived example like the one below:

int bar = 0;
int foo = (undefined behavior of some kind);
if (foo) {
   f();
   bar = 1;
}
if (!foo) {
   g();
   bar = 1;
}
assert(1 == bar);

The compiler is free to assume that !foo is true in the first block and foo is true in the second, and thus optimize the entire chunk of code away. Now, logically either foo or !foo must be true, and so looking at the code, you would reasonably be able to assume that bar must equal 1 once you've run the code. But because the compiler optimized in that manner, bar never gets set to 1. And now that assertion becomes false and the program terminates, which is behavior that would not have happened if foo hadn't relied on undefined behavior.

Now, is it possible for the compiler to actually insert completely new code if it sees undefined behavior? If doing so will allow it to optimize more, absolutely. Is it likely to happen often? Probably not, but you can never guarantee it, so operating on the assumption that nasal demons are possible is the only safe approach.

Answer 3

One of the historical purposes of Undefined Behavior was to allow for the possibility that certain actions may have different potentially-useful effects on different platforms. For example, in the early days of C, given

int i=INT_MAX;
i++;
printf("%d",i);

some compilers could guarantee that the code would print some particular value (for a two's-complement machine it would typically be INT_MIN), while others would guarantee that the program would terminate without reaching the printf. Depending upon the application requirements, either behavior could be useful. Leaving the behavior undefined meant that an application where abnormal program termination was an acceptable consequence of overflow but producing seemingly-valid-but-wrong output would not be, could forgo overflow checking if run on a platform which would reliably trap it, and an application where abnormal termination in case of overflow would not be acceptable, but producing arithmetically-incorrect output would be, could forgo overflow checking if run on a platform where overflows weren't trapped.

Recently, however, some compiler authors seem to have gotten into a contest to see who can most efficiently eliminate any code whose existence would not be mandated by the standard. Given, for example...

#include <stdio.h>

int main(void)
{
  int ch = getchar();
  if (ch < 74)
    printf("Hey there!");
  else
    printf("%d",ch*ch*ch*ch*ch);
}

a hyper-modern compiler may conclude that if ch is 74 or greater, the computation of ch*ch*ch*ch*ch would yield Undefined Behavior, and as a consequence the program should print "Hey there!" unconditionally regardless of what character was typed.

Answer 4

I thought I'd answer just one of your points, since the other answers answer the general question quite well, but have left this unaddressed.

"Ignoring the situation -- Yes, the standard goes on to say that this will have "unpredictable results", but that's not the same as the compiler inserting code (which I assume would be a prerequisite for, you know, nasal demons)."

A situation in which nasal demons could very reasonably be expected to occur with a sensible compiler, without the compiler inserting ANY code, would be the following:

if(!spawn_of_satan)
    printf("Random debug value: %i\n", *x); // oops, null pointer deference
    nasal_angels();
else
    nasal_demons();

A compiler, if it can prove that that *x is a null pointer dereference, is perfectly entitled, as part of some optimisation, to say "OK, so I see that they've dereferenced a null pointer in this branch of the if. Therefore, as part of that branch I'm allowed to do anything. So I can therefore optimise to this:"

if(!spawn_of_satan)
    nasal_demons();
else
    nasal_demons();

"And from there, I can optimise to this:"

nasal_demons();

You can see how this sort of thing can in the right circumstances prove very useful for an optimising compiler, and yet cause disaster. I did see some examples a while back of cases where actually it IS important for optimisation to be able to optimise this sort of case. I might try to dig them out later when I have more time.

EDIT: One example that just came from the depths of my memory of such a case where it's useful for optimisation is where you very frequently check a pointer for being NULL (perhaps in inlined helper functions), even after having already dereferenced it and without having changed it. The optimising compiler can see that you've dereferenced it and so optimise out all the "is NULL" checks, since if you've dereferenced it and it IS null, anything is allowed to happen, including just not running the "is NULL" checks. I believe that similar arguments apply to other undefined behaviour.

Answer 5

Undefined behaviors allow compilers to generate faster code in some cases. Consider two different processor architectures that ADD differently: Processor A inherently discards the carry bit upon overflow, while processor B generates an error. (Of course, Processor C inherently generates Nasal Demons - its just the easiest way to discharge that extra bit of energy in a snot-powered nanobot...)

If the standard required that an error be generated, then all code compiled for processor A would basically be forced to include additional instructions, to perform some sort of check for overflow, and if so, generate an error. This would result in slower code, even if the developer know that they were only going to end up adding small numbers.

Undefined behavior sacrifices portability for speed. By allowing 'anything' to happen, the compiler can avoid writing safety-checks for situations that will never occur. (Or, you know... they might.)

Additionally, when a programmer knows exactly what an undefined behavior will actually cause in their given environment, they are free to exploit that knowledge to gain additional performance.

If you want to ensure that your code behaves exactly the same on all platforms, you need to ensure that no 'undefined behavior' ever occurs - however, this may not be your goal.

Edit: (In respons to OPs edit) Implementation Defined behavior would require the consistent generation of nasal demons. Undefined behavior allows the sporadic generation of nasal demons.

That's where the advantage that undefined behavior has over implementation specific behavior appears. Consider that extra code may be needed to avoid inconsistent behavior on a particular system. In these cases, undefined behavior allows greater speed.

Answer 6

The definition of undefined behaviour, in every C and C++ standard, is essentially that the standard imposes no requirements on what happens.

Yes, that means any outcome is permitted. But there are no particular outcomes that are required to happen, nor any outcomes that are required to NOT happen. It does not matter if you have a compiler and library that consistently yields a particular behaviour in response to a particular instance of undefined behaviour - such a behaviour is not required, and may change even in a future bugfix release of your compiler - and the compiler will still be perfectly correct according to each version of the C and C++ standards.

If your host system has hardware support in the form of connection to probes that are inserted in your nostrils, it is within the realms of possibility that an occurrence of undefined behaviour will cause undesired nasal effects.