[llvm-dev] Dereferenceable load semantics & LICM

[Sanjoy Das via llvm-dev]

Hi Piotr,

On March 31, 2017 at 9:07:42 AM, Piotr Padlewski
(piotr.padlewski at gmail.com) wrote:
> Hi all,
> I have a question about dereferenceable metadata on load instruction. I
> have a patch (https://reviews.llvm.org/D31539) for LICM that hoists loads
> with !invariant.group.
> The motivation example is devirtualization:
> ...
> [snip]
>
> On the other hand, after performing my LICM of !invariant.group load, GVN
> hoists the second load and I am not sure why it is legal then.

I suspect what's going on is that we first canonicalize the loop to:

if (precondition) {
  do {
    vptr = load vtable;
    fptr = *vptr;
    ...
  } while (backedge_condition);
}

after which it is safe to transform the program to (modulo aliasing):

if (precondition) {
  vptr = load vtable;
  fptr = *vptr;
  do {
    ...
  } while (backedge_condition);
}

since the we moved a load from a ("strongly") postdominating location.
We know that once we were in the preheader we know we're definitely
going to execute the vptr and fptr loads, so they better be
dereferenceable.  In other words, we're "exploiting undefined
behavior" here.

-- Sanjoy