[LLVMdev] RFC: Native Windows C++ exception handling

[Reid Kleckner]

These are exactly the sorts of code transformations we want to allow by
delaying the outlining until later. By keeping such code inlined in the
parent function until after optimization, we enable a lot of core
optimizations like SROA. For example, we should be able to completely
eliminate wrappers like unique_ptr that would otherwise stay around due to
the pointer escaped to the destructor call that gets executed on exception.

On Tue, Feb 10, 2015 at 6:21 PM, Joseph Tremoulet <jotrem at microsoft.com>
wrote:

>  Hi,
>
>
>
> Sorry if I'm late to the party.  I'm curious whether you need to be
> concerned about code being moved into the landing pad area with this
> approach (i.e. whether "real code" might wind up in the eh.actions call's
> block which [if I'm following correctly] won't actually get executed at
> runtime).
>
>
>
> For example, given something like
>
>
>
> int foo (int a, int b) {
>
>   int x;
>
>   try {
>
>     x = a + b;
>
>     maybe_throw();
>
>     return 0;
>
>   } catch (int) {
>
>     return x;
>
>   } catch (float) {
>
>     return x + 1;
>
>   }
>
> }
>
>
>
> Do you need to worry that something like partial deadcode elimination
> could want to move the definition of x down into the landing pad code (but
> not all the way down into the catch handlers)?
>
>
>
> Or conversely, given something like
>
>
>
> int foo(int a, int b) {
>
>   try {
>
>     maybe_throw();
>
>     return 0;
>
>   } catch (int) {
>
>     return a + b;
>
>   } catch (float) {
>
>     return (a + b) + 1;
>
>   }
>
> }
>
>
>
> Do you need to worry that something like very busy expressions could want
> to hoist the "a + b" computation up into the landing pad code (but not all
> the way up into the try block)?
>
>
>
> Should those sorts of code motion be legal?  If not, what's preventing
> them, and if so, how will the moved code be executed at runtime?
>
>
>
> Thanks
>
> -Joseph
>
>
>
>
>
> *From:* llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] *On
> Behalf Of *Kaylor, Andrew
> *Sent:* Wednesday, January 28, 2015 7:49 PM
> *To:* Reid Kleckner
> *Cc:* Bataev, Alexey; Reid Kleckner (reid at kleckner.net); LLVM Developers
> Mailing List
> *Subject:* Re: [LLVMdev] RFC: Native Windows C++ exception handling
>
>
>
> Hi Reid,
>
>
>
> I’ve worked through a few scenarios, and I think this is converging.  I’m
> attaching a new example, which extends the one we’ve been working through
> to handle a few more cases.
>
>
>
> I wasn’t sure what you intended the first i32 argument in an
> llvm.eh.actions entry to be.  I’ve been using it as a place to store the eh
> state that I’m associating with the action, but that’s kind of circular
> because I’m using the information in these tables to calculate that value.
> I’ll be able to do this calculation in the MSVCEHPrepare pass before these
> instructions are added, so I can put it there (it’s nice for readability),
> but the later pass that generates the tables (assuming we leave that in a
> later pass) will need information not represented in the action table (at
> least as I’ve been using it).  I’m not 100% sure that this is necessary,
> but it seemed like the way things ought to be.  I’ll experiment more before
> anything gets set in stone.
>
>
>
> In addition to the IP-to-state table that we’ve talked about, we also need
> an unwind table (which has each state, the state it transitions to when it
> expires and the unwind handler, if any) and a catch handler table (which
> has the range of states that can throw to the handlers, the state of the
> handlers and a map of types handled to their handlers).  I’ve got examples
> of these in the attached example.
>
>
>
> So I now have a firm plan for how to compute these tables from the
> outlined IR.
>
>
>
> I think the algorithm you proposed for computing eh states doesn’t quite
> work.  In particular, if multiple catch handlers get added by the same
> landing pad we’ll want to start by assuming that they have the same state.
> If they end up getting popped at different times then we’ll need to update
> the eh state of the one that gets popped first.  Unfortunately my example
> doesn’t cover this case, but I worked through it and my new algorithm
> (based on your but slightly tweaked) works for that case.  Also, unwind
> handlers get discrete states (they happen when a transition crosses the
> state, but in the .xdata tables they are represented with a single state).
> Catch handlers, on the other hand, do get a range.
>
>
>
> Anyway, here’s what I’m doing (by hand so far, I don’t have it coded yet).
>
>
>
> 1. Start with an empty stack of actions and an empty master list of eh
> state.
>
> 2. Visit each landing pad in succession, processing the actions at that
> landing pad back to front.
>
> 3. Pop actions from the current stack that aren’t in the current landing
> pad’s action table
>
>    a. If a catch is popped that had been assumed to have the same state as
> a catch that isn’t being popped
>
>            its state number and all state numbers above it need to be
> incremented
>
>    b. When a catch is popped, the next available eh_state is assigned to
> its handler
>
> 4. As an action is pushed to the current stack, it is assigned an eh_state
> in the master list
>
>    a. If the action was an unwind or if it was a cacth after an unwind,
> the next available eh_state is incremented
>
>    b. If the action was a catch following a catch that was also just
> added, it gets the same eh_state as the previous catch
>
> 5. When all landing pads have been handled, the remaining actions are
> popped and processed as above.
>
>
>
> The “next” state for each eh_state can also be computed during the above
> process by observing the state of the action on the top of the current
> action stack when the action associated with the state is popped.  In the
> case of catch handlers, I think the next state will always be the same as
> the next state of the corresponding catch action.
>
>
>
> So, I think it makes sense to compute the unwind and catch tables during
> the MSVCEHPrepare pass, but I wasn’t sure how best to preserve the
> information once it was computed.  Is it reasonable to stick this in
> metadata?
>
>
>
> We can keep fine tuning this if you like, but I think it’s looking solid
> enough that I’m going to start revising my outlining patch to produce the
> results in the attached example.
>
>
>
> -Andy
>
>
>
>
>
> *From:* Reid Kleckner [mailto:rnk at google.com <rnk at google.com>]
> *Sent:* Tuesday, January 27, 2015 1:55 PM
> *To:* Kaylor, Andrew
> *Cc:* Bataev, Alexey; Reid Kleckner (reid at kleckner.net); LLVM Developers
> Mailing List; Anton Korobeynikov; Kreitzer, David L
> *Subject:* Re: [LLVMdev] RFC: Native Windows C++ exception handling
>
>
>
> On Tue, Jan 27, 2015 at 12:58 PM, Kaylor, Andrew <andrew.kaylor at intel.com>
> wrote:
>
> Thanks, Reid.   These are good points.
>
>
>
> So I guess that does take us back to something more like my original
> proposal.
>
>
>
> I like your suggestion of having some kind of “eh.actions” intrinsic to
> represent the outlining rather than the extension to landingpad that I had
> proposed.  I was just working on something like that in conjunction with my
> second alternative idea.
>
>
>
> Great!
>
>
>
>  What I’d really like is to have the “eh.actions” intrinsic take a shape
> that makes it really easy to construct the .xdata table directly from these
> calls.  As I think I mentioned in my original post, I think I have any idea
> for how to reconstruct functionally correct eh states (at least for
> synchronous EH purposes) from the invoke and landingpad instructions.  I
> would like to continue, as in my original proposal, limiting the unwind
> representations to those that are unique to a given landing pad.  I think
> with enough documentation I can make that seem sensible.
>
>
>
> My thinking is that the "eh.actions" list can be transformed into a
> compact xdata table later, after we've done machine basic block layout.
>
>
>
> I think the algorithm will be something like
>
>
>
> 1. Input: already laid out MachineFunction
>
> 2. Call EHStreamer::computeCallSiteTable to populate a LandingPadInfo
> vector sorted by ascending PC values
>
> 4. Iterate the LandingPadInfos, comparing the action list of each landing
> pad with the previous landing pad, assuming an empty action list at
> function start and end.
>
> 5. Model the action list as a stack, and compute the common suffix of the
> landing pad action lists
>
> 6. Each new action pushed represents a new EH state number
>
> 7. Pushing a cleanup action adds a state table entry that transitions from
> the current EH state to the previous with a cleanup handler
>
> 8. Pushing a catch action adds a new unwind table entry with an open range
> from the current EH state to an unknown EH state. The state after catching
> is... ???
>
> 9. Popping a catch action closes an open unwind table range
>
>
>
> So, I think the action list is at least not totally crazy. =)
>
>
>
>  I’ll start working on a revised proposal.  Let me know if you have any
> more solid ideas.
>
>
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