unwrap_panicked test fails (unwind support)

[cynecx]

The generator::unwrap_panicked test fails. It seems like both catch_unwinds (inside unwind_wrapper and inside the test) are not catching the panic. Seems like the unwinder somehow fails to properly unwind the stack.

😔

Ref PR: edef1c#70

[cynecx]

It seems like some bug sneaked into my branch as I can reproduce the test failure in debug mode. However the original unwind branch doesn't seem to have this bug. But the original unwind branch experiences release mode test failures. It looks like a compiler "regression" because I was able to bisect the issue to the follwing PR: rust-lang/rust#67502.

[cynecx]

Okay, the issue seems to be that no landingpads exist when the context switch, which is done through the inline assembly, is (swap_link/swap) fully inlined. Without inlining, a landingpad would exist because without swap being inlined, swap would've been called with invoke <target> to label <dst> unwind <catch> , hence having a landingpad. Inlining basically removes this because swap contains inline assembly which doesn't have a landingpad (llvm doesn't support that rn), so llvm doesn't generate the required exception (lsda) tables.

There are two solutions:

• Annotate swap and swap_link with #[inline(never)] (Not sure if its guaranteed by the compiler though).
• Introduce inline asm unwinding support on llvm's side and patch rustc.

In its current state unwinding support is basically unsound and broken.

[cynecx]

Re: #[inline(never)], according to https://doc.rust-lang.org/reference/attributes/codegen.html#the-inline-attribute, it seems that the attribute is merely a suggestion. That leaves the first solution unsound.

[cynecx]

LLVM patch which should allow unwinding from inline asm: https://github.com/cynecx/llvm-project/tree/asm-landingpads (The patch only contains support for SelectionDAG. Adding support for GlobalIsel should be straightforward too).

[cynecx]

Let's see where this goes: https://reviews.llvm.org/D95745. Support for unwinding from inline-asm has been upstreamed! 🎉

Next step: Wait for rust to upgrade to LLVM 13.

[cynecx]

@Amanieu JFYI: I've been reviving your old "unwinding support" for libfringe branch here: https://github.com/cynecx/libfringe/tree/asm as a test case for rust-lang/rust#88439.
The test suite passes with a "small rustc hack" (adding back rbx clobber for the x86_64 backend), alternatively, I thought about just "push/pop"-ing rbx. However, I haven't figured out the correct cfi directives yet, so the unwinder actually restores rbx.