[llvm-dev] FYI: gc relocations on exception path w/RS4GC currently broken

[Reid Kleckner via llvm-dev]

So, here's a crazy idea. What if we change the definition of dominance for
invokes that produce tokens so that the token return value is live out the
exceptional edge?

If that's too crazy, what if we used operand bundles to make a new token
that "forward declares" the statepoint token:
  %exceptional_token = call token @llvm.gc.exceptional.token()
  %normal_token = invoke @llvm.gc.experimental.statepoint(....) [
"eh_token" token %exceptional_token ]
    to label %normal_dest unwind label %lpad_dest
...
lpad_dest:
  %ehvals = { i8*, i32 } landingpad ... like usual
  %p1 = @llvm.gc.relocate(token %exceptional_token)

A given exceptional token can be used with exactly one or zero GC
statepoint calls, so a late pass can map from one to the other and insert
reloads in the usual way.

That said, I imagine you want to use this exceptional token with more than
one invoke, so that you don't end up needing a landingpad per potentially
throwing call site. I think this design can be extended to handle that,
though.

On Fri, Jan 22, 2016 at 12:35 PM, Philip Reames via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> For anyone following along on ToT using the gc.statepoint mechanism, you
> should know that ToT is currently not able to express arbitrary exceptional
> control flow and relocations along exceptional edges. This is a direct
> result of moving the gc.statepoint representation to using a token type
> landingpad.  Essentially, we have a design inconsistency where we expect to
> be able to "resume" a phi of arbitrary landing pads, but we expect
> relocations to be tied specifically to a particular invoke.
>
> Chen, Joseph, and I have spent some time talking about how to resolve
> this.  All of the schemes we've come up with representing relocations using
> gc.relocates on the exceptional path require either a change to how we
> define an invoke instruction (something we'd really like to avoid) or a new
> intrinsic with special treatment in the optimizer so that it basically
> "becomes part of" the landing pad without actually being the landing pad.
> None of us were particular thrilled by the changes involved.
>
> Given exceptional paths are nearly by definition cold, we're currently
> exploring another option.  We're considering having RS4GC insert explicit
> spill slots at the IR level (via allocas) for values live along exceptional
> paths, and leaving all of the normal path values represented as
> gc.relocates.  This avoids the need for another IR extension, makes it
> slightly easier to meet an ABI requirement Joseph has, and provides a
> better platform for lowering experimentation.  Joseph is working on
> implementing this and will probably have something up for review next week
> or the week after. Once that's in, we're going to run some performance
> experiments to see if it's a viable lowering strategy even without Joseph's
> particular ABI requirement, and if so, make that the standard way of
> representing relocations on exceptional edges.
>
> Assuming this approach works, we're going to defer solving the problem of
> how to cleanly represent explicit relocations along the exceptional path
> until a later point in time.  In particular, the value of the explicit
> relocations comes mainly from being able to lower them efficiently to
> register uses.  Since the work to integrate relocations with the register
> allocator hasn't happened and doesn't look like it's going to happen in the
> near term (*), this seems like a reasonable compromise.
>
> Philip
>
> (*) To give some context on this, it turns out one of our initial starting
> assumptions was wrong in practice.  We expected the quality of lowering for
> the gc arguments at statepoint/safepoint to be very important for overall
> code quality.  While this may some day become true, we've found that
> whenever we encounter a hot safepoint, the problem is usually that we
> didn't inline appropriately.  As a result, we've ended up fixing (out of
> tree) inlining or devirtualization bugs rather than working on the lowering
> itself. For us, a truly hot megamorphic call site has turned out to be a
> very rare beast.  Worth noting is that this is only true because we're a
> high tier JIT with good profiling information.  It's likely that other
> users who don't have the same design point may find the lowering far more
> problematic; in fact, we have some evidence this may already be true.
>
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