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

[Kaylor, Andrew]

Just as an off-the-cuff idea that I haven’t thought through at all, what would you think of the possibility of replacing this:

  %12 = call i32 @llvm.eh.typeid.for(i8* bitcast (%eh.CatchHandlerType* @llvm.eh.handlertype.H.0 to i8*))
  %matches7 = icmp eq i32 %sel6, %12

with this:

  %matches7 = call i32 @llvm.eh.cmp.selector(i32 %sel6, i8* bitcast (%eh.CatchHandlerType* @llvm.eh.handlertype.H.0 to i8*))

If we did that, the outlining code could just assume %sel6 was the current selector (because we don’t clone into nested landing pads) and we wouldn’t need to track it at all.

-Andy

From: Reid Kleckner [mailto:rnk at google.com]
Sent: Friday, April 10, 2015 4:06 PM
To: Kaylor, Andrew
Cc: David Majnemer <david.majnemer at gmail.com> (david.majnemer at gmail.com); LLVM Developers Mailing List
Subject: Re: [WinEH] Cloning blocks that reference non-cloned PHI nodes

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<mailto: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


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