[LLVMdev] New EH representation for MSVC compatibility

[Kaylor, Andrew]

I hadn’t noticed the “noexcept” specifier in your example.  That clears up part of my concerns, but I still have some problems.


With regard to the multiple meanings of ‘resume’ I am more concerning about developers who are reading the IR understanding it than about passes operating on it.  Apart from making it harder to debug problems related to control flow at resume instructions I think this makes it more likely that code which mishandles it will be introduced down the road.  If I’m reading things correctly, a resume instruction in your proposal could mean:

a) We’re done handling this exception, continue normal execution at this label.
b) We’re done handling this exception, continue execution in an enclosing catch handler at this label.
c) We’re done executing this termination handler, check the catch handler at this label to see if it can handle the current exception.
d) We’re done executing this termination handler, now execute the termination handler at this label.
e) We’re done executing this termination handler, continue handling the exception in the runtime.

I suppose (a) and (b) are more or less the same and it doesn’t entirely matter whether the destination is in normal code or exception code.  In practical terms (c) and (d) may be the same also, but logically, in terms of how the runtime works, they are different.  I’m pretty sure there’s a gap in my understanding of your proposal because I don’t understand how e() is represented at all.

As an exercise, I tried to work through the IR that would be produced in the non-optimized case for the following code:

void test() {
  try {
    Obj o1;
    try {
      f();
    } catch (int) {}
    Obj o2;
    try {
      g();
    } catch (int) {}
    h();
  } catch (int) {}
}

Here’s what I came up with:

define void @foo() personality i32 (...)* @__CxxFrameHandler3 {
  %e.addr = alloca i32
  invoke void @f(i32 1)
    to label %cont1 unwind label %cleanup.Obj
cont1:
  invoke void @g(i32 2)
    to label %cont2 unwind label %cleanup.Obj.1
cont2:
  invoke void @h(i32 2)
    to label %cont3 unwind label %cleanup.Obj.2
cont3:
  call void @~Obj()
  call void @~Obj()
  br label %return
return:
  ret void

cleanup.Obj:
  cleanupblock unwind label %maycatch.int
  call void @~Obj()
  resume label %maycatch.int

maycatch.int:
  catchblock void [i8* @typeid.int, i32 7, i32* %e.addr]
    to label %catch.int unwind label %catchend
catch.int:
  resume label %cont1
catchend:
  resume

cleanup.Obj.1:
  cleanupblock unwind label %maycatch.int.1
  call void @~Obj()
  call void @~Obj()
  resume label %maycatch.int.1

maycatch.int.1:
  catchblock void [i8* @typeid.int, i32 7, i32* %e.addr]
    to label %catch.int.1 unwind label %catchend.1
catch.int.1:
  resume label %cont2
catchend.1:
  resume


cleanup.Obj.2:
  cleanupblock unwind label %maycatch.int.2
  call void @~Obj()
  call void @~Obj()
  resume label %maycatch.int.2

maycatch.int.2:
  catchblock void [i8* @typeid.int, i32 7, i32* %e.addr]
    to label %catch.int.2 unwind label %catchend.2
catch.int.2:
  resume label %return
catchend.2:
  resume
}

I don’t know if I got that right, but it seems to me that there are a couple of problems with this.  Most obviously, there is a good bit of duplicated code here (which the optimization passes will probably want to combine).

More significantly though is that it doesn’t correctly describe what happens if a non-int exception is thrown in any of the called functions.  For instance, if a non-int exception is thrown from g() that is caught somewhere further down the stack, the runtime should call a terminate handler that destructs o1 and then call a terminate handler that destructs o2.  However, my IR doesn’t describe a terminate handler that destructs just o2 and I don’t know how I could get it to do so within the scheme that you have proposed.

Do you have a way to handle this case that I haven’t perceived?


In a mostly unrelated matter, have you thought about what needs to be done to prevent catchblock blocks from being combined?  For example, suppose you have code that looks like this:

void test() {
  try {
    f();
  } catch (int) {
    x();
    y();
    z();
  }
  try {
    g();
  } catch (…) {
  }
  try {
    h();
  } catch (int) {
    x();
    y();
    z();
  }
}

I think it’s very likely that if we don’t do anything to prevent it the IR generated for this will be indistinguishable from the IR generated for this:

void test() {
  try {
    f();
    try {
      g();
    } catch (…) {
    }
    h();
  } catch (int) {
    x();
    y();
    z();
  }
}

In this case that might be OK, but theoretically the calls to f() and h() should get different states and there are almost certainly cases where failing to recognize that will cause problems.  What’s more, the same basic pattern arises for this case:

void test() {
  try {
    f();
  } catch (int) {
    x();
    y();
    z();
  }
  try {
    g();
  } catch (float) {
  }
  try {
    h();
  } catch (int) {
    x();
    y();
    z();
  }
}

But in this case, if we get the state numbering wrong an int-exception from g() could end up being incorrectly caught by the xyz handler.

BTW, finding cases like this is the primary reason that I’ve been trying to push my current in-flight patch onto the sinking ship that is our current implementation.  I mentioned to you before that the test suite I’m using passes with my proposed patch, but that’s only true with optimizations disabled.  With optimizations turned on I’m seeing all kinds of fun things like similar handlers being combined and common instructions being hoisted above a shared(!) eh_begincatch call in if-else paired handlers.  I don’t know if it will be worth trying to fix these problems, but seeing them in action has been very instructive.

-Andy


From: Reid Kleckner [mailto:rnk at google.com]
Sent: Monday, May 18, 2015 11:54 AM
To: Kaylor, Andrew
Cc: LLVM Developers Mailing List; Bill Wendling; Nick Lewycky
Subject: Re: New EH representation for MSVC compatibility

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.

> 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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