[clang] [llvm] [win][x64] Unwind v2 3/n: Add support for emitting unwind v2 information (equivalent to MSVC /d2epilogunwind) (PR #129142)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

So if an unwind table has an epilog opcode, and you chain another unwind table to it, the chained unwind table inherits the epilog opcode?  You could use that.

The primary unwind table ends at the first epilog, you mark it with an "atend" epilog opcode. The next region starts immediately after that, and continues until the next epilog.  The subsequent region uses a chained unwind table.  The chained unwind table inherits the atend marking, so it has a correct epilog marking.  Repeat for each epilog, and you can cover a function with an arbitrary number of epilogs without repeating the opcodes.

That's not great for codesize, and it excludes any other use of chaining... but it's better than just giving up.  Bailing out is a hazard: minor code changes can trigger the error, and the connection between the user's code and the error is hard to understand.

https://github.com/llvm/llvm-project/pull/129142