[PATCH] D66980: [Windows] Disable TrapUnreachable for Win64, add SEH_NoReturn

[Reid Kleckner via Phabricator via llvm-commits]

rnk added a comment.

In D66980#1653723 <https://reviews.llvm.org/D66980#1653723>, @mstorsjo wrote:

> So I presume that the issue is that the new `int3` isn't at the end of the function where the previous double `ud2` were?


The old ud2 should have become int3, and there was no need to add an int3 after cxa_throw, that was undesired. I ended up reverting this change in rL370829 <https://reviews.llvm.org/rL370829>. There were two issues:

1. SEH_NoReturn expansion needs to look across blocks similar to what SEH_Epilogue does
2. _Unwind_Resume is not marked noreturn

Furthermore, noreturn is supposed to be an optimization hint, not a requirement for correctness. We should really be expanding all unreachable instructions to this SEH_NoReturn pseudo, like a disappearing-ink version of TrapUnreachable. I tried to implement that, but it was hard because it interacts with target independent fast isel. And after all, I'm not sure I like this pseudo instruction that expands to something else based on some non-local analysis. I'm starting to think this should be a standalone pass that inserts int3 as needed at the end of every function and funclet that ends in a call instruction. It would be so simple, and wouldn't need 3 implementations in SDISel, FastISel, and global ISel.


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