[LLVMdev] [WinEH] Cloning blocks that reference non-cloned PHI nodes

[Reid Kleckner]

On Mon, Apr 13, 2015 at 11:25 AM, Kaylor, Andrew <andrew.kaylor at intel.com>
wrote:

>  Comments below…
>
>
>
> *From:* Reid Kleckner [mailto:rnk at google.com]
> *Sent:* Friday, April 10, 2015 5:01 PM
> *To:* Kaylor, Andrew
> *Cc:* David Majnemer <david.majnemer at gmail.com> (david.majnemer at gmail.com);
> LLVM Developers Mailing List
> *Subject:* Re: [WinEH] Cloning blocks that reference non-cloned PHI nodes
>
>
>
> Yeah, wrapping up the whole selector comparison into one intrinsic would
> help. Thanks for the idea. :)
>
>
> The current strategy also seems pretty fragile around complex cleanups. If
> the cleanup isn't some simple destructor call with easily traceable
> unconditional branch control flow, we get lost and can't find our way to
> the next selector comparison. Having an explicit 'resume' or some other
> intrinsic at the end of the cleanup would be pretty helpful here.
>
>
>
> ---
>
>
>
> We also talked a lot about making WinEHPrepare try to less work all at
> once, and split it into phases. This is separable from the landingswitch
> proposal, which is more about making it easier to identify handler start
> points. Here's a sketch of the phases we came up with:
>
>
>
> Step 1: Identify selector dispatch conditional branches. Identify all
> cleanup actions, and split basic blocks to make them start and end on a
> basic block boundary. Build a list of all basic blocks that start a handler.
>
>
>
> [Andy Kaylor] This sounds an awful lot like what we’re doing now except
> for the block splitting part.  I guess that you are talking about doing
> this for all landing pads before we go any further, right?  I agree that
> the current code to identify cleanup blocks is too fragile.  We can
> probably fix it though.  Intrinsics to mark the start and/or end of cleanup
> code would help immensely.
>

Yep, this is basically the selector pattern matching that we do today.
Maybe the block splitting isn't necessary, but I feel like it would be nice
to say that every handler starts at a BB and no two handlers start at the
same BB. Today it is possible for a cleanup and catch handler to start at
the same BB in this example:

some_eh_bb:
  call void @dtor()
  ... typeid...
  %matches = icmp eq ...
  br i1 %matches, label %more_dispatch, label %catch_int

 Step 2: Do a forwards-backwards dataflow analysis to mark basic blocks as
> reachable and unreachable from each handler start block. The forwards part
> is an easy DFS, the backwards part is basically about pruning unreachable
> blocks like 'ret' from a handler. We can probably skip the backwards part
> initially.
>
>
>
> [Andy Kaylor] I have some concerns about this part.   When blocks have
> been merged so that handlers are sharing blocks with non-exception code
> things are going to get fairly messy.  The existing cloning code takes care
> of this (in theory at least) by scrubbing things after resolving PHIs based
> on which blocks didn’t get cloned.  I suppose this is a potential fault
> line if the code we didn’t want loops back into the shared block.  I’m
> guessing that this is the sort of problem you’re thinking of that has you
> wanting to change things.  Do you have a test case that exposes the
> problem?  More importantly, do you have a solution in mind?
>

Specifically, I'm dealing with __finally blocks today, which have this
control flow structure of branch to shared cleanup, branch on phi out to EH
or normal flow. This made WinEHPrepare pretty unhappy. I need some way to
fix this. I've been toying with doing the __finally outlining in Clang, but
if we're serious about making WinEHPrepare handle this, then maybe we don't
need that.


> With catch handlers we have clear markers for the beginning and end of the
> catch (possibly including multiple end calls).  Having similar markers for
> cleanup would help a lot.  I still think there’s potential for getting lost.
>

We can add markers, but I'm worried that it might be an optimization
barrier and we could still get lost. What differentiates the end of one
cleanup from the end of another? Should the end cleanup call consume an SSA
value produced by the begin cleanup call maybe? I guess nesting catches
isn't a problem because the only way to enter another catch is to do an
invoke, which goes via a landingpad, which we don't outline. The same
should be true for cleanup end markers.

 Step 3: Clone all blocks that are reachable from more than one handler
> until all blocks belong to exactly one handler, and sink code from
> landingpads back into handlers if necessary. We don't have many code
> commoning optimizations, so this step is more of a defense against
> theoretical future optimization passes that do more than tail merging.
>
>
>
> [Andy Kaylor] I’m not sure I see how this is better than the current
> cloning approach.
>

These last 3 steps are basically an optimization. Cloning and then deleting
all IR reachable from landingpads, which we currently do at -O0, is fairly
wasteful. When no inlining occurs at -O0 and -O1, the IR should be easily
analyzable, and we just need to splice the handler BBs into new functions
and rewrite uses of allocas.


>  Step 4: Demote all SSA values live from one handler into or out of
> another. The only SSA values referenceable across handlers are static
> allocas.
>
>
>
> [Andy Kaylor] Again, if I underststand correctly we’re doing this now,
> just with a fairly distributed mechanism.
>
>
>
>
>
> Step 5: Extract the blocks belonging to each handler into their own
> functions. Rewrite static alloca references in each new handler to use
> llvm.framerecover.
>
>
>
> Step 6: Profit?
>
>
>
> Does this seem like a reasonable plan? IMO simplifying the overarching
> design is a higher priority right now than fixing individual test cases.
>
>
>
> [Andy Kaylor] I think there’s significant value in getting some core suite
> of tests working so that we can get out of our current pattern of rewriting
> all of our tests every time we make a significant change and into a
> situation where we’ll have some stable tests that will verify that any
> redesign isn’t breaking something.  I also think that in chasing down
> problems with the implementation we have now we’ll learn more about the
> problems that need to be solved.
>
>
>
> I don’t have a strong objection to your working on a redesign while I try
> to work through some bugs in the current implementation, but I’d like to
> ask you to hold off on committing significant restructuring for a little
> while unless I run into something that can’t be solved within the current
> design.  I feel like I can have the small test suite that I’m working on
> passing within a week or so.
>

Sure. By breaking things down into steps, I'm not trying to fundamentally
change anything like I was with landingswitch. This is just trying to
separate IR analysis from IR transformation so that we can reason about
things a bit more easily.
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