> The compile can transform your code if the result of the computations it makes is the same (plus any ordering guarantees you've encoded with the correct primitives, etc.). "Interact with the outside world in exactly the same way" is way too strong a guarantee.

The word "result" is doing a lot of work there. `printf ("%d\n", 2+2);` isn't interesting because 4 appears in a memory cell; it's interesting because 4 appears on stdout. Which one is the "result"?

If you're going to make assumptions about what's "inside" the program and what's "outside", you have to make them explicit. And they have to be reasonable assumptions. An assumption that memory is "inside" has to be justified in the presence of shared memory, virtual memory, debuggers, or whatever. You have to actually explain what you mean in a lot more detail than I think the average spec has a chance of doing.

If I create an unlinked temp file, and the compiler can observe that I'm holding the only FD open on that file, should that file be seen as "inside the computation", or as "a collection of results and inputs"?

Without reading the specs, I can be 95 percent sure that they don't nail down all the issues... and 100 percent sure that if they do nail down all the issues, or even all the possibly important issues, the corner cases are unknown to almost all actual programmers. Which means either that it's not appropriate for the compiler to rely on just any rule regardless of what the spec says, or that it's not reasonable to write code in the language.