[LLVMdev] Windows EH support

[Kaylor, Andrew]

Hi Reid,

I'm trying to get a "big picture" view of everything that needs to be done for clang and llvm to support SEH and C++ exception handling for the "msvc" environment.  I've put together a list, including work in progress, but I'm guessing that I've missed some details.  Can you look this over and tell me if it matches your idea of what needs to be done?

Structured Exception Handling
---------------------------------------
* Add support for __try and __except in clang.

It looks like this is mostly implemented by your changes in the D5607 review, but there hasn't been any activity on that review for quite a while.  You're going to rewrite a lot of that to change the way filters are implemented, right?


* Add support for __finally in clang

I haven't seen anything for this yet.  Based on various discussions, it seems like you intend to mix C++ scope cleanup with SEH handlers in ways that MSVC specifically punts.  Is that right?


* Add support for unwinding in frames with no exception handlers

I'm not clear as to what needs to be done for this.  I know clang is intentionally generating errors when you compile code with an MSVC-environment target.  I'm guessing that just removing this block will be sufficient for clang to generate the IR we want.  Then it will be up to the back end to produce the correct unwinding support, which I think it may already be doing.


* Add support for frame allocation intrinsics.

You've started this in review D6493, which seems to be progressing.  I don't know how much remains to be done there, but I trust that you have it under control.


* Add support for outlining filter functions.

This is partially implemented in my D6556 review.  I think that will be ready to check in once the expected for of the input IR settles.  I still need to add support for removing outlined code.  It will also need to make use of the frame allocation intrinsics to reference frame variables in the outlined code.  You've mentioned that we should also do analysis to minimize the amount of duplicated code.  I agree but I was hoping that could wait until everything else is in place and working.


* Add support for outlining __try bodies.

I don't know what your plan is for identifying the beginning and end of these regions, but I expect that we'll be able to leverage the outlining code from the task above to do most of the work for this task.


* Add support to emit .xdata table for SEH for 64-bit targets.

This is implemented in your D6300 review, but that looks like it has stalled a bit.  Is there anything holding this up other than lack of reviews?


* Add support to generate stack-based EH tables for 32-bit targets.

I haven't seen any work on this.  Am I correct in thinking that other than the placement of the EH tables the 32-bit implementation will be able to re-use all of the other SEH code (IR generation, outlining, etc.)?


* Add more tests.


C++ Exception Handling
-------------------------------
* Add support for MSVC-specific try-except IR emission to clang.

This seems fairly straight-forward.  I saw some code in your D5607 review to add a way to access the correct personality function.  Other than that, it looks like this will just be a matter of removing the "ErrorUnspported" blocks and adding MSVC-specific support to the parts of the code that are currently generating libc++abi calls for throwing and catching exceptions.  The general shape of the IR generated seems to be correct.


* Add support for outlining C++ catch handlers

This will be very similar to the filter outlining in the D6556 review.  I think a bit of extra effort will be required to correctly identify and group cleanup handlers and get them mixed with the exception handlers correctly, but overall I feel like I have a handle on this.


* Add support to emit .xdata table for C++EH for 64-bit targets.

The analogous SEH code should provide a stable location for where these changes need to be made.  From there it seems to be a matter of plumbing to get the handler addresses and new code to emit the table in the correct format for the MSVC C++ personality function.


* Add support to generate stack-based C++EH tables for 32-bit targets


* Add more tests


------------------------------------------------

I think I've mentioned before that C++ exception handling is my main interest here.  I've been planning to follow through on the remaining work for the function outlining for both SEH and C++EH.  Beyond that, I guess we need to talk about what work you've already got in flight.  Would you want me to work on the other C++EH tasks while you finish up SEH, or is there something there that overlaps with work you've already started?

-Andy

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