[LLVMdev] [WinEH] Cloning blocks that reference non-cloned PHI nodes

[Reid Kleckner]

Sounds reasonable. Honestly, we've been having some serious whiteboard
discussions about how to get out of doing all this selector pattern
matching. I think that's basically the most fragile part of winehprepare,
and if we can add some new constructs that wrap that up in something opaque
to mid-level passes, that would be great.

Unfortunately, I haven't really been able to get enough consensus to write
something down. If I can't convince my coworkers that we need a new
construct in person, there's not much point in writing up a detailed RFC. :(

Here's the gist of the new idea:
- Leave invoke alone, too many optimizations care about it
- Add a new 'landingswitch' (name under debate) instruction that fills the
landingpad role and is also a terminator
- landingswitch clauses have some fixed operands (RTTI, handler label) and
some variable number of operands as required by the EH personality (catch
object, adjectives)
- Allow the 'resume' instruction to have 0, 1, or 2 successors, one of
which can be a landingpad.
- Add a 'resume' clause to landingswitches to allow them to continue the
unwind process at another landingpad in the same function

I've tried to sell this idea, but I'm getting push back suggesting that we
fix it a different way. The most compelling suggestion is to have the
existing landingpad instruction produce an i8* selector instead of an i32
selector, and then we indirectbr on it. This would save the selector
pattern matching that we do today, but I don't particularly like it.
Optimization passes can do basic block commoning, tail merging, and hoist
instructions between the landingpad and the indirectbr, and I don't like
dealing with that.

I also don't like it because we'd need to teach the inliner about
indirectbr, which today it refuses to inline.

On Fri, Apr 10, 2015 at 3:45 PM, Kaylor, Andrew <andrew.kaylor at intel.com>
wrote:

>  Hi Reid and David,
>
>
>
> I just wanted to give you a heads up that I’m currently working on a
> problem where the WinEHPrepare outlining code stumbles and asserts while
> cloning code that references extracted landing pad values via PHI nodes
> that are in intermediate blocks that aren’t being cloned.  The test I’m
> looking at fails with an assertion claiming that llvm.eh.begincatch was
> called twice inside a handler.
>
>
>
> I have an idea for how to address this (giving the cloning director a
> chance to replace operands before an instruction is remapped).  I’ll put
> something up for review as soon as I have it working.  In the mean time I
> wanted to make sure you weren’t working on the same problem.
>
>
>
> I’m seeing it with some C++ EH tests I’m working on, but I think it may be
> possible for it to happen with SEH cleanup.
>
>
>
> If you’re curious, here’s a reproducer.
>
>
>
> int main(void) {
>
>   try {
>
>     try {
>
>       throw 'a';
>
>     } catch (char c) {
>
>       printf("%c\n", c);
>
>     }
>
>     throw 1;
>
>   } catch(int x) {
>
>     printf("%d\n", x);
>
>   } catch(...) {
>
>     printf("...\n");
>
>   }
>
>
>
>   try {
>
>     try {
>
>       throw 'b';
>
>     } catch (char c) {
>
>       printf("%c\n", c);
>
>     }
>
>     throw 2;
>
>   } catch(int x) {
>
>     printf("%d\n", x);
>
>   } catch (char c) {
>
>     printf("%c\n", c);
>
>   } catch(...) {
>
>     printf("...\n");
>
>   }
>
>   return 0;
>
> }
>
>
>
> -Andy
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150410/2a33a49f/attachment.html>