[PATCH] D31261: [IR] De-virtualize ~Value to save a vptr

[Reid Kleckner via llvm-commits]

On Wed, Mar 22, 2017 at 4:41 PM, Craig Topper <craig.topper at gmail.com>
wrote:

> How will this interact with this trick http://llvm.org/docs/Pro
> grammersManual.html#tagging-considerations where we make an assumption
> that the vtable ptr lives at the start of the User object. Is the
> assumption now on the Type* that will now be at the start of Value?
>

Yes, I think you're right. This code here reinterprets the vptr (or Type*
now) as a UserRef, checks the bottom bit, and if it is unset, reinterprets
it as a User. Otherwise, it masks the bit and loads from it to get back to
the User:

User *Use::getUser() const {
  const Use *End = getImpliedUser();
  const UserRef *ref = reinterpret_cast<const UserRef *>(End);
  return ref->getInt() ? ref->getPointer()
                       : reinterpret_cast<User *>(const_cast<Use *>(End));
}

So, we were relying on vtables being two byte aligned. Now we're relying on
Types being at least two byte aligned.

We should... clean this up at some point to operate within the language. It
shouldn't be hard.
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