[all-commits] [llvm/llvm-project] b3fdf3: Enable `-funwind-tables` flag when building libunwind

[Sergej Jaskiewicz via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: b3fdf33ba6aa7ef80621696f74aaf2f6f8e1d1de
      https://github.com/llvm/llvm-project/commit/b3fdf33ba6aa7ef80621696f74aaf2f6f8e1d1de
  Author: Sergej Jaskiewicz <jaskiewiczs at icloud.com>
  Date:   2019-12-04 (Wed, 04 Dec 2019)

  Changed paths:
    M libunwind/CMakeLists.txt
    M libunwind/cmake/config-ix.cmake
    M libunwind/test/CMakeLists.txt
    M libunwind/test/libunwind/test/config.py
    M libunwind/test/lit.site.cfg.in
    M libunwind/test/signal_frame.pass.cpp

  Log Message:
  -----------
  Enable `-funwind-tables` flag when building libunwind

Summary:
Or, rather, don't accidentally forget to pass it.

This is aimed to solve the problem discussed in [this thread](http://lists.llvm.org/pipermail/llvm-dev/2019-November/136890.html), and to fix [a year-old bug](https://bugs.llvm.org/show_bug.cgi?id=38468).

TL;DR: when building libunwind for ARM Linux, we **need** libunwind to be built with the `-funwind-tables` flag, because, well ARM EHABI needs unwind info produced by this flag. Without the flag all the procedures in libunwind are marked `.cantunwind`, which causes all sorts of bad things. From `_Unwind_Backtrace` not working, to C++ exceptions not being caught (which is the aforementioned bug is about).

Previously, this flag was not added because the CMake check `add_compile_flags_if_supported(-funwind-tables)` produced a false negative. Why? With this flag, the compiler generates calls to the `__aeabi_unwind_cpp_pr0` symbol, which is defined in libunwind itself and obviously is not available at configure time, before libunwind is built. This led to failure at link time during the CMake check. We handle this by disabling the linker for CMake checks in linbunwind.

Also, this patch introduces a lit feature `libunwind-arm-ehabi`, which is used to mark the `signal_frame.pass.cpp` test as unsupported (as was advised by @miyuki in D70397).

Reviewers: peter.smith, phosek, EricWF, compnerd, jroelofs, saugustine, miyuki, jfb

Subscribers: mgorny, kristof.beyls, christof, libcxx-commits, miyuki

Tags: #libc

Differential Revision: https://reviews.llvm.org/D70815