[LLVMdev] New EH representation for MSVC compatibility

[Reid Kleckner]

On Mon, May 18, 2015 at 3:28 PM, Kaylor, Andrew <andrew.kaylor at intel.com>
wrote:

>  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.
>

Yeah, Joseph agreed this was overloading resume too much. The new idea is
that a-b would be handled by 'endcatch' (I previously called this
'recover', but endcatch will be more readable) and c-e are represented with
an 'endcleanup' instruction. The endcleanup will have an unwind label
operand indicating which EH action runs next, which handles both c and d,
depending on the nature of the block. If the unwind label is missing, then
execution continues in the calling function's actions.


> 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).
>

First, each destructor gets it's own cleanupblock, and ultimately ends up
in it's own outlined funclet. This cuts down on ~Obj duplication.

Second, you seem to always have the invokes unwinding to cleanups which
then unwind to catch handlers, even if the try is more closely nested than
the cleanup. If f() throws int, we don't actually run cleanup.Obj, we run
the catch and rejoin normal control flow, which will call ~Obj for o1.

Here's how I'd translate that to IR, assuming Obj is empty and has no ctor:

define void @foo() personality i32 (...)* @__CxxFrameHandler3 {
entry:
  %o1 = alloca i8
  %o2 = alloca i8
  invoke void @f()
    to label %cont1 unwind label %maycatch.int2
cont1:
  invoke void @g()
    to label %cont2 unwind label %maycatch.int3
cont2:
  invoke void @h()
    to label %cont3 unwind label %cleanup.Obj1
cont3:
  call void @~Obj(i8* %o2)
  call void @~Obj(i8* %o1)
  br label %return
return:
  ret void

maycatch.int2:
  catchblock [i8* @typeid.int]
    to label %catch.int2 unwind label %catchendblock2
catch.int2:
  endcatch label %cont2
catchendblock2:
  catchendblock unwind label %cleanup.Obj1

maycatch.int3:
  catchblock [i8* @typeid.int]
    to label %catch.int3 unwind label %catchendblock3
catch.int3:
  endcatch label %cont3
catchendblock3:
  catchendblock unwind label %cleanup.Obj2

cleanup.Obj2:
  cleanupblock unwind label %cleanup.Obj1
  call void @~Obj(i8* %o2)
  endcleanup from label %cleanup.Obj2 unwind label %cleanup.Obj1

cleanup.Obj1:
  cleanupblock unwind label %maycatch.int1
  call void @~Obj(i8* %o1)
  endcleanup from label %cleanup.Obj1 unwind label %maycatch.int1

maycatch.int1:
  catchblock [i8* @typeid.int]
    to label %catch.int1 unwind label %catchendblock1
catch.int1:
  endcatch label %return
catchendblock1:
  catchendblock ; no unwind label, uncaught exceptions leave the function
}

This has a total of 4 calls to ~Obj, so each cleanup is emitted exactly
twice: once for normal flow and once for exceptional flow.


> 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.
>

The idea is that the program notionally "executes" the EH blocks and each
block describes the intended successor. In the Itanium landingpad model,
that's pretty close to what actually happens, with an optimization to skip
execution of frames that have no cleanups and have no matching catches.

If h throws a float exception, we should run, in order, maycatch.int3,
catchendblock3, cleanup.Obj2, cleanup.Obj1, maycatch.int1, catchendblock4,
and then unwind to the parent. Make sense?


> 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 the original source, the catch-all block unwinds to the parent frame. In
the transformed example, it doesn't. Mid-level optimization passes should
keep the catch-all in the first example unwinding out to the parent,
otherwise things will break down.

Practically, the current middle-end will be unable to merge the catchblocks
because of catchendblock and this rule: "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."

The catchblocks will look like:

maycatch.int1:
  catchblock [i8* @typeid.int]
    to label %catch.int1 ; unwind to parent
catch.int1:
  invoke void @x() to label %cont1 unwind label %catchendblock1
cont1:
  invoke void @y() to label %cont2 unwind label %catchendblock1
cont2:
  invoke void @z() to label %cont3 unwind label %catchendblock1
cont4:
  endcatch label %...
catchendblock1:
  catchendblock ; unwind to parent

Any code merging optimization would have to ensure that those xyz calls
unwind to the catchendblock referenced by the catchblock. Practically, this
will require merging the entire catchblock, which *is* a legal
transformation, and we can compensate for it in the ip2state tables.
Imagine assigning these states:

call f() // state 0
call g() // state 2
catchall // state 3
call h() // state 0
catchint // state 1

So, f and h can have the same states even though they are split across the
g call in source order.

I need to write more about the state numbering algorithm we came up with,
it's the primary constraint on the design.


> 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.
>
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