[llvm-dev] Possible soundness issue with available_externally (split from "RFC: Add guard intrinsics")

[Sanjoy Das via llvm-dev]

Hal Finkel wrote:

 > But it is not all optimizations that are the problem. Rather, it
 > seems like a select few (e.g. things involving collapsing allowed
 > non-determinism in atomics), and losing those optimizations seems
 > better than generally losing function-attribute deduction.

If we go by the langref, then optimizations that fold undef are also
problematic (though most C/C++ programs resulting in such IR would
have UB in practice).

I think there are non-(non-deterministic) problematic cases too.  The
following won't happen today since `readnone` does not imply
`safe_to_speculate`, but if we add a `safe_to_speculate` property some
day:

   int foo(bool C) available_externally {
     if (C)
       ((int *) null)++; // UB
     ret 42;
   }

   void bar() {
     if (<some cond>)
      foo(true);
   }

Now, normally you can just delete the `if (C)` branch in foo, and it
would become just a `ret 42`, and would look like it is speculatable
above the `<some cond>` check.  But if you then link with an -O0
version, you'll have introduced UB if `<some cond>` is always false at
runtime.

Today this won't happen since we don't speculate `readnone nounwind`
functions, but could become a problem in the future.

-- Sanjoy