Use C++ exceptions unconditionally for Objective-C[++] on MinGW #267
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Current status:
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I presume that we can use the implementation from here on MinGW for the first point? Line 276 in 3c42c64 |
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OK, I've added a dummy implementation of A lot of tests are currently failing like this:
For example: Any thoughts on what could cause that? |
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Can you trace into the C++ personality function? If clang is correctly generating the LSDA, then this should work. Actually, try sticking a breakpoint on: And: These should be being called by the C++ personality function to check for matches. The error message you're seeing is the one that happens if the C++ runtime throws an exception and nothing catches it (unwind reaches the end of the stack). |
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From #260 (comment)
This doesn't reproduce after rebuilding, but leaving the comment here for future reference. |
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I added additional logging and this is the current output (contradicting my previous comment): |
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Can you step through the do-catch methods and see where the match is failing? |
qmfrederik
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OK - some progress. The actual exception object wasn't being copied in With that, most tests seem to pass; only |
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w.r.t. ExceptionTest failing, it looks like this may be related to rethrowing an exception Here's the output (with a bunch of additional logging statements): |
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For |
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Okay, it looks as if the first throw and catch is working. That's a good start: throwing an ObjC object and catching To remove the not-MinGW condition and then implement Can you share the LLVM IR for compiling this with the modified clang? |
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Here's the clang change: llvm/llvm-project@d68db09 That gives this output: Looks like the application now halts earlier. I'll get you the intermediate representation in a follow-up. |
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Here's the intermediate representation (assuming I did this right): |
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Hmm, it looks as if clang isn't emitting the begin and end catch calls. In a finally block, I think it should be calling the _Unwind_rethrow thing (I think it was before the last change?). If I'm reading the IR correctly, I'm a bit confused because it looks as if the finally code should be emitting the begin / end carch blocks: Can you see what's going on there? |
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OK - so we were entering the This causes the test to proceed a bit further: Looks like the final block as a wrong pointer to the exception (I assume it should have been |
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The segfault comes from the logging code which calls resulting in an invalid exception object being thrown? |
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OK - so with llvm/llvm-project@53132da I get all tests, except for |
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Nice! Can you add a clang test and raise a PR? I’ll review it. You can base the test on this: https://github.com/llvm/llvm-project/blob/main/clang/test/CodeGenObjC/exceptions-nonfragile.m Pass some different target triples and runtime versions and make sure that it does the right thing in each case. You can run a single test with llvm-lit. |
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Will do, thanks. Here's all current changes squashed into a single commit: llvm/llvm-project@fa97083 |
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@davidchisnall Here you go: llvm/llvm-project#77255 |
This was referenced Jan 7, 2024
davidchisnall
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Jan 8, 2024
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Any ideas why the uncaught exception tests are failing? Does the VEH hook not do the right thing? |
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The hook is not implemented yet, all it does (for now) is to return I haven't had the time yet to read up on the semantics of Vectored Exception Handlers (the documentation seems sparse) and the expected behavior of Where I left it at the moment:
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I think they're called before SEH, which is why I was unsure how it worked. I'm not sure if you get an unhandled-exception exception that you can catch with VEH?
Well, that gets complicated. Probably only for ObjC ones, because C++ already has it defined that it calls It would be nice to call
I think the type info is embedded in the thrown object. It's a The corner case here is that you can use |
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@davidchisnall Looks like the easiest way to do is to:
I've pushed a commit which implements this. Feels a bit clunky (but it's similar to what the Apple runtime does); but I couldn't find an easier way to get this information. I believe |
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That’s probably easier. The unwind exception holds a pointer to the C++ exception, which holds the type info and has the object appended, but different C++ runtimes have subtly different versions of this structure. Given that this is a very slow path (basically only ever used to pop up a diagnostic window when a thread crashes) it’ probably fine for it to do the rethrow thing. |
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@davidchisnall I've made some last changes to this PR:
With that, all tests on MinGW now pass (100% tests passed, 0 tests failed out of 98): I think this PR is now in a pretty good shape, let me know if there's anything else you need from me. |
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Looks mostly fine. I don’t understand the VEH bits fully (I thought those handlers happened first, why isn’t it called for every thrown exception?) so a few commends explaining the model would be nice for the next person. Also a couple of changes needed to make it work properly in multithreaded contexts.
eh_win32_mingw.m
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| extern void __cxa_rethrow(); | ||
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| BOOL handler_installed = NO; | ||
| BOOL in_handler = NO; |
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These probably need to be thread local.
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in_handler: yes, handler_installed tracks the call to AddVectoredExceptionHandler which should called only once per process?
| objc_uncaught_exception_handler previousHandler = __atomic_exchange_n(&_objc_unexpected_exception, handler, __ATOMIC_SEQ_CST); | ||
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| // Add a vectored exception handler to support the hook. We only need to do this once. | ||
| if (!handler_installed) { |
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This probably also needs to be atomic.
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Hmm - this is to protect against a race condition where objc_setUncaughtExceptionHandler might be called from multiple threads at the same time? I guess we could add a mutex around this block but is that a realistic scenario?
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Please can you squash before you merge? |
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@davidchisnall I left some additional notes, and I updated |
davidchisnall
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Jan 10, 2024
I squashed but don't have merge permissions in this repo ;-) |
Fixed. |
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Cool, thanks! |
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The GNUstep Objective C runtime (libobjc2) is adding support for the GNU ABI on Windows (more specifically, MinGW). The libobjc2 runtime uses C++ exceptions in that configuration; this PR updates clang to act accordingly. The corresponding change to libobjc2 is here: gnustep/libobjc2#267
justinfargnoli
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The GNUstep Objective C runtime (libobjc2) is adding support for the GNU ABI on Windows (more specifically, MinGW). The libobjc2 runtime uses C++ exceptions in that configuration; this PR updates clang to act accordingly. The corresponding change to libobjc2 is here: gnustep/libobjc2#267
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Alternative implementation of #260. Requires changes to clang - see qmfrederik/llvm-project@88ce1ba