[llvm-bugs] [Bug 41581] New: SEH unwinding breaks in the presence of a jump-table on Windows/AArch64

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41581

            Bug ID: 41581
           Summary: SEH unwinding breaks in the presence of a jump-table
                    on Windows/AArch64
           Product: new-bugs
           Version: 8.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: gsvelto at mozilla.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

We encountered this issue when building Firefox for Windows/AArch64 using clang
8.0.0 [1]. The issue was introduced in [2].

What's happening is that we install an exception handler with
`SetUnhandledExceptionFilter()` [3] to gather crash dumps. Starting with
clang-8.0.0 this stopped working in many cases; when we encountered an
exception Windows would be unable to unwind the stack and reach the exception
handler, bailing out if it encountered a frame pointing to the JavaScript
interpreter [3] on the stack. The interpreter uses macros to generate a large
jump-table and [2] changed in which section the jump-table contents are stored.
This seems to confuse Windows' unwinding machinery which simply bails out when
encountering a frame that jumped through that table.

I've compared the code emitted w/ and w/o [2] applied and the only difference
are the section directives around the jump table. I can attach the relevant
assembly output if needed but it's very, very large.

I'm working on a reduced test-case and I'll attach it ASAP.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1542827
[2]
https://github.com/llvm/llvm-project/commit/502c6557aa1aa6917fe5ce0ee26cfe936d45eb98
[3]
https://searchfox.org/mozilla-central/rev/ec489aa170b6486891cf3625717d6fa12bcd11c1/js/src/vm/Interpreter.cpp#1653-4424

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