[LLVMdev] New EH representation for MSVC compatibility

[Philip Reames]

On 05/18/2015 11:53 AM, Reid Kleckner wrote:
> On Fri, May 15, 2015 at 5:27 PM, Kaylor, Andrew 
> <andrew.kaylor at intel.com <mailto: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.
Leaving aside the rest of the thread, I feel the need to refute this 
point in isolation.  I've found that optimizing (usually simplifying and 
eliminating) exception paths ends up being *extremely* important for my 
workloads.  Failing to optimize exception paths sufficiently tends to 
indirectly hurt things like inlining for example.  Any design which 
starts with the assumption that optimizing exception paths isn't 
important is going to be extremely problematic for me.
>
>     > 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.
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150518/10827bc6/attachment.html>