Can a runtime environment detect an infinite loop?

Answered By : Kyle Strand

It might be theoretically possible for a runtime environment to check for such loops using the following procedure:

After ever instruction executed, the runtime environment would make a complete image of the state of a running process (i.e. all memory associated with it, including registers, P.C., stack, heap, and globals), save that image somewhere, and then check to see whether it matches any of its previously saved images for that process. If there is a match, then the process is stuck in an infinite loop. Otherwise, the next instruction is executed and the process is repeated.

In fact, rather than performing this check after every single instruction, the runtime environment could simply pause the process periodically and make a save-state. If the process is stuck in an infinite loop involving n states, then after at most n checks, a duplicate state will be observed.

Note, of course, that this is not a solution to the halting problem; the distinction is discussed here.

But such a feature would be an enormous waste of resources; continually pausing a process to save all memory associated with it would slow it down tremendously and consume an enormous amount of memory very quickly. (Although old images could be deleted after a while, it would be risky to limit the total number of images that could be saved because a large infinite loop--i.e., one with many states--might not get caught if there are too few states kept in memory.) Moreover, this feature wouldn't actually provide that much benefit, since its ability to catch errors would be extremely limited and because it's relatively simple to find infinite loops with other debugging methods (such as simply stepping through the code and recognizing the logic error).

Therefore, I doubt that such a runtime environment exists or that it will ever exist, unless someone programs it just for kicks. (Which I am somewhat tempted to do now.)

Problem Detail: 

Would it be possible for a runtime environment to detect infinite loops and subsequently stop the associated process, or would implementing such logic be equivalent to solving the halting problem?

For the purpose of this question, I define an "infinite loop" to mean a series of instructions and associated starting stack/heap data that, when executed, return the process to exactly the same state (including the data) as it was in before initiating the infinite loop. (In other words, a program generating an indefinitely long decimal expansion of pi isn't "stuck" in an "infinite loop," because at every iteration, it has more digits of pi somewhere in its associated memory.)

(Ported from http://stackoverflow.com/q/16250472/1858225)

Asked By : Kyle Strand

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Question Source : http://cs.stackexchange.com/questions/11645