[LLVMdev] New EH representation for MSVC compatibility

Reid Kleckner
Mon, 18 May 2015 18:53:42 -0000

On Fri, May 15, 2015 at 5:27 PM, Kaylor, Andrew <andrew.kaylor at
intel.com>
wrote:

>  I like the way this sorts out with regard to funclet code generation.
> It feels very natural for Windows EH, though obviously not as natural for
> non-Windows targets and I think it is likely to block some optimizations
> that are currently possible with those targets.
>

Right, it will block some of today's optimizations by default. I'm OK
with
this because we can add those optimizations back by checking if the
personality is Itanium-family (sjlj, arm, or dwarf), and optimizing EH
codepaths is not usually performance critical.


> > If the unwind label is missing, then control leaves the function after
> the EH action is completed. If a function is inlined, EH blocks with
> missing unwind labels are wired up to the unwind label used by the inlined
> call site.
>
>
>
> Is this saying that a “missing” unwind label corresponds to telling the
> runtime to continue the search at the next frame?
>

Yep. For the C++ data structure it would simply be a missing or null
operand.


> Your example looks wrong in this regard, unless I’m misunderstanding it.
> It looks like any exceptions that aren’t caught in that function will lead
> to a terminate call.
>

Well, those are the intended semantics of noexcept, unless I'm mistaken.
And the inliner *should* wire up the unwind edge of the terminateblock to
the unwind edge of the inlined invoke instruction, because it's natural to
lower terminateblock to a catch-all plus termination call block. I wanted
to express that as data, though, so that in the common case that the
noexcept function is not inlined, we can simply flip the "noexcept"
bit in
the EH info. There's a similar optimization we can do for Itanium that we
miss today.

> Invokes that are reached after a catchblock without following any unwind
> edges must transitively unwind to the first catchend block that the
> catchblock unwinds to.
>
>
>
> I’m not sure I understand this correctly.  In particular, I’m confused
> about the roles of resume and catchend.
>

catchendblock is really there to support figuring out which calls were
inside the catch scope. resume has two roles: moving to the next EH action
after a cleanup, and transitioning from the catch block back to normal
control flow. Some of my coworkers said it should be split into two
instructions for each purpose, and I could go either way.


> > %val = cleanupblock <valty> unwind label %nextaction
>
>
>
> Why isn’t this a terminator?  It seems like it performs the same sort of
> role as catchblock, except presumably it is always entered.  I suppose
> that’s probably the answer to my question, but it strikes me as an
> ambiguity in the scheme.  The catchblock instruction is more or less a
> conditional branch whereas the cleanupblock is more like a label with a
> hint as to an unconditional branch that will happen later.  And I guess
> that’s another thing that bothers me -- a resume instruction at the end of
> a catch implementation means something subtly different than a resume
> instruction at the end of a cleanup implementation.
>

Yeah, reusing the resume instruction for both these things might not be
good. I liked not having to add more terminator instructions, though. I
think most optimizations will not care about the differences between the
two kinds of resume. For CFG formation purposes, it either has one
successor or none, and that's enough for most users.

I felt that cleanupblock should not be a terminator because it keeps the IR
more concise. The smaller an IR construct is, the more people seem to
understand it, so I tried to go with that.
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