tree b59a8a2dd81fdd8ea136e63dc506dfa45c3da22b
parent 15b67dacf3685d986bf9def2b885315a2604b4c8
author Sergej Jaskiewicz <jaskiewiczs@icloud.com> 1572954444 +0300
committer Copybara-Service <copybara-worker@google.com> 1598638501 -0700

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

GitOrigin-RevId: b3fdf33ba6aa7ef80621696f74aaf2f6f8e1d1de