[llvm-bugs] [Bug 40996] New: [clang-cl] Implement /d2FH4 to emit compact Win64 xdata

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=40996

            Bug ID: 40996
           Summary: [clang-cl] Implement /d2FH4 to emit compact Win64
                    xdata
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: rnk at google.com
                CC: craig.topper at gmail.com, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, spatel+llvm at rotateright.com

Visual C++ folks appear to have optimized x64 EH data (.xdata), as described
here:
https://devblogs.microsoft.com/cppblog/making-cpp-exception-handling-smaller-x64/

The new flag is available in VS 2019 preview 3. The new __CxxFrameHandler4
personality function uses more compact tables for smaller binary size. We
should follow suit at some point.

For users that don't use exceptions, I am curious about how they reduced the
size of the unwind opcodes, which are used for normal stack unwinding. That is
something that *everybody* uses, and reducing its size therefore useful to
everyone.

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