[PATCH] D31539: Hoisting invariant.group in LICM

[Piotr Padlewski via Phabricator via llvm-commits]

Prazek added a comment.



> I thought your mailing list post was about //speculating// loads.  Here we're not speculating anything -- we're only sinking.  In any case, you do not need the `!invariant.group` metadata on the sunk load for the example above to "work".

The dereferenceable is usefull for speculative loads, but in tha last mail I showed the problem with other metadata:
If I will remove invariant.group md while hoisting from loop, then I probably won't be able to devirtualize it further. Consider:

  void loop(A* a, int p) {
    for (int i = 0; i < p; i++)
      a->foo():
  }
  
  void call() {
    A a;
    clobber(&a); // external function
    loop(&a, 15);
  }
  external void clobber(A *a);

if I will hoist vtable load of out the loop before inlining, then it won't be able to change it to A::foo(), because invariant.group will be removed. 
If I will not hoist it, then it will be devirtualized after inlining, but for callsites of loop() that wasn't inlined it will be worse.

That means we need a way of specifing that one propery holds globally, which would mean that is not dependent on branch. for vtables and virtual function the invariant.group, invariant.load and !dereferenceable is a global property, and I can't loose this information.


https://reviews.llvm.org/D31539