The highly condensed version is that the compiler optimized away an if block that was responsible for throwing an error as impossible to reach in correctly functioning code, when the whole purpose of that if block was to check that condition and error so that the program did not continue in an invalid state.

Specifically:

  word* target = segmentStart + farPointer.offset;
  if (target < segmentStart || target >= segmentEnd) {
    throwBoundsError();
  }
  doSomething(*target);

However, as it turns out, pointer arithmetic that overflows is undefined behavior under the C standard. As a result, the compiler is allowed to assume that the addition on the first line never overflows. Since farPointer.offset is an unsigned number, the compiler is able to conclude that target < segmentStart always evaluates false. Thus, the compiler removes this part of the check. Unfortunately, in the case of overflow, this is exactly the part of the check that we need.

The post that's from (the sumbmitted article to the HN discussion I linked) is fairly accessible in my view. I highly recommend reading it.

Another discussion that probably has good info is this one.[2]

1: https://news.ycombinator.com/item?id=14163111

2: https://news.ycombinator.com/item?id=14785867