[dzaima]

Take for example the compiler optimizing:

    void foo(bool cond) {
      int a = 0;
      if (cond) a = 10;
      if (cond) printf("%d\n", 10 / a);
    }

into:

    void foo(bool cond) {
      int a = 0;
      if (cond) {
        a = 10;
        if (cond) printf("%d\n", 10 / a);
      } else {
        if (cond) printf("%d\n", 10 / a);
      }
    }

and then screaming that (in its generated 'else' block) there's a very clear '10 / 0'. Now, of course, you'd hope that the compiler would also recognize that that's in a never-taken-here 'if' (and in most cases it will), but in various situations it might not be able to (perhaps most notably 'cond' instead being a function call that will give the same result in both places but the compiler doesn't know that).

Now, maybe there are some passes across which false-positives would not be introduced, but that's a rather small subset, and you'd have to reorder the passes such that you run all of them before any "destructive" ones, potentially resulting in having to duplicate passes to restore previous optimization behavior, at which point it's not really "reusing".