[LLVMdev] RFC: How to represent SEH (__try / __except) in LLVM IR

[Kaylor, Andrew]

Thanks for the additional information.

Right now I’m experimenting with a mix of code compiled with MSVC and code compiled with clang, trying to get a C++ exception thrown and caught by the MSVC-compiled code across a function in the clang-compiled code.  My goal here is to isolate a small part of what needs to be done in a way that lends itself to tinkering.  I think this might lead me to the outlining of EH blocks that you describe below.

If the clang code doesn’t have and exception handler (and it can’t since clang won’t compile that right now) and doesn’t need to do any clean-up, this works fine.  If the clang code does need to do cleanup, clang currently emits the same landingpad stuff that it would emit for mingw and since I’m trying to link with the MSVC environment I end up with unresolved externals.  So I’m playing around with the clang-generated IR to see if I can turn it into something that will handle the cleanup and let the exception pass.  I’ve got it calling my custom SEH-style personality function and it’s trivial to get that to let the exception pass without doing the cleanup.  Now I just need to figure out how to get it to execute the cleanup code.

I haven’t spent a lot of time on this yet, so if this overlaps with what you’ve been doing I can step back and approach it from a different direction.  Otherwise, I’ll proceed and see if I can make use of your suggestions below with regard to outlining, probably starting with manual changes to the IR that simulate the process.

-Andy


From: Reid Kleckner [mailto:rnk at google.com]
Sent: Thursday, November 13, 2014 11:51 AM
To: Kaylor, Andrew
Cc: LLVM Developers Mailing List; John McCall
Subject: Re: [LLVMdev] RFC: How to represent SEH (__try / __except) in LLVM IR

Cool! Apologies for the following stream of consciousness brain dump...

On Wed, Nov 12, 2014 at 5:07 PM, Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>> wrote:
Hi Reid,

I’ve been following your proposal, and I’d be interested in helping out if I can.  My main interest right now is in enabling C++ exception handling in clang for native (i.e. not mingw/cygwin) Windows targets (both 32-bit and 64-bit), but if I understand things correctly that will be closely related to your SEH work under the hood.

Great! I agree, any changes to LLVM IR made to support SEH will also be needed to support C++ exceptions on Windows, in particular the outlining.

In the current LLVM model, all the exception handling code lives in the landing pad. The Windows unwinder doesn't actually return control to the landingpad until very late. Instead, it creates new stack frames to invoke the cleanup, catch handler (C++ EH only), or filter function (SEH only). This is why we need to have outlining somewhere. The question is, where should we do it? Personally, I want to do this on LLVM IR during CodeGenPrepare.

The major challenge that outlining anywhere presents is that now the outlined code has to "know" something about the frame layout of the function it was outlined from in order to access local variables. I think we can add `i8* @llvm.eh.get_capture_block(i8* %function, i8* %parent_rbp)` and `void @llvm.eh.set_capture_block(i8* %captures)` intrinsics to make this work. Any SSA values or allocas captured by the outlined landing pad code will be demoted to memory and stored in the capture block, and the layout will be encoded in a struct used by the outlined handlers and the parent function. However, once you do this, you cannot inline the IR without some heroics. It probably isn't that important to be able to inline functions with try/catch, but a good acid test for any new LLVM IR construct is "will it inline?", and this construct fails. I think we can live with this construct as long as we only introduce it after CodeGenPrepare.

The remaining wrinkle in the capture block scheme is stack realignment prologues. In this case, we have three pointers to the stack: the SP, the base pointer (esi/rbx), and the frame pointer (ebp/rbp). Is the capture block stored at a known constant offset from ebp/rbp or esi/rbx? Or do we load and store a dynamic offset saved somewhere near ebp/rbp? This needs study.

I’m still trying to get up to speed on what is and is not implemented, but I think I’m starting to get a clear picture.  My understanding is that LLVM has the necessary support to emit exception handling records that Windows will be able to work with (for Win64 EH) but some work may be required to get the IR properly wired up, and that there’s basically nothing in place to support Win32 EH and nothing in clang to generate the IR for either case.  Is that more or less accurate?

We can emit valid pdata and xdata sections on Win64, and this supports basic stack unwinding. On top of that, we currently follow mingw64 and use Itanium-style LSDA tables and the __gxx_personality_seh0 personality function to run EH handlers. This means the standard exception handling IR emitted by clang and other frontends "just works" on Windows, and I want to keep it that way. I think most of the changes should be on the LLVM side to lower the standard EH IR down to something that is more compatible with MSVC EH.

I’ve been looking at the work Kai Nacke did in ldc to implement exception handling there, but it isn’t clear to me yet how relevant that is to clang.

Can you tell me more about what your plans are?  Specifically, do you intend to support both 32 and 64 bit targets?  And were you also planning to work toward C++ exception handling support in clang once you had the general SEH support in place?

I want to do Win64 first because it is easier and better documented, and then look at 32-bit next. 32-bit SEH does things like "take the address of a BB label from the middle of the parent function and 'call' it with a special ebp value passed in", but that is basically equivalent to the Win64 way of doing things with a very special calling convention.

I know some people are also interested in ARM (WoA), which should be similar to Win64, as it also uses pdata/xdata style unwind info.

Finally, and most importantly, what can I do to help?

I think there are some separable tasks here.

The EH capture block intrinsics can probably be built in isolation from the outlining. We can probably make `get_capture_block` work with the result of `@llvm.frameaddress(i32 0)`. The inliner also has to be taught to avoid inlining functions that set up a capture block.

Doing outlining will be similar what `llvm::CloneAndPruneFunctionInto` does, except it will start at the landing pad instead of the entry block. Instead of mapping from parameters to arguments, the outliner would map the selector to a constant and propagate that value forwards, pruning conditional branches as it goes. The `resume` instruction would end outlining and become a `ret`. Any cloned `ret` instructions are the result of cloning something that is statically reachable but dynamically unreachable. We can transform them to `unreachable` and run standard cleanup passes to propagate that backwards.

32-bit x86 EH will require installing an alloca onto the fs:00 chain of EH handlers. I suppose this could be emitted during CodeGenPrepare as regular LLVM IR instructions, since we have a way of writing `load/store fs:00` with address space 257. This alloca should probably be the same as the capture block, since it has to be at some known offset from ebp.
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