*From:* Reid Kleckner [mailto:firstname.lastname@example.org]
*Sent:* Friday, April 10, 2015 5:01 PM
*To:* Kaylor, Andrew
*Cc:* David Majnemer <email@example.com> (firstname.lastname@example.org);
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:
call void @dtor()
%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
[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
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
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
[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
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.