notes.txt

To avoid depending on libgcc.a when using Clang's runtime library compiler-rt we should use
-Wl,--exclude-libs,libgcc.a when building stage 2 and onwards.

Otherwise building protobuf in yugabyte-db-thirdparty fails to find _Unwind_Resume.
_Unwind_Resume is ultimately defined in /lib64/libgcc_s.so.1.

nm -gD /lib64/libgcc_s.so.1  | grep Unwind_Resume
000000000000fd80 T _Unwind_Resume
000000000000fe50 T _Unwind_Resume_or_Rethrow

libgcc.a brings in the _Unwind_Resume dependency.

+ nm -gC /opt/rh/devtoolset-8/root/usr/lib/gcc/x86_64-redhat-linux/8/libgcc.a
+ grep Unwind
nm: _trampoline.o: no symbols
nm: __main.o: no symbols
nm: _mulhc3.o: no symbols
nm: _divhc3.o: no symbols
                 U _Unwind_Resume

This symbol is ultimately defined in libgcc_s.so.

+ cat /opt/rh/devtoolset-8/root/usr/lib/gcc/x86_64-redhat-linux/8/libgcc_s.so
/* GNU ld script
   Use the shared library, but some functions are only in
   the static library, so try that secondarily.  */
OUTPUT_FORMAT(elf64-x86-64)
GROUP ( /lib64/libgcc_s.so.1 libgcc.a )

---

It also turns out that _Unwind_Resume is defined in LLVM-built libunwind:

find . -name "*.so" -or -name "*.a" | while read f; do if nm -gD "$f" 2>/dev/null | grep "_Unwind_Resume"; then echo "$f"; fi; done
000000000000c620 T _Unwind_Resume
000000000000ccd0 T _Unwind_Resume_or_Rethrow
./lib/libunwind.so
                 U _Unwind_Resume
./lib/libc++abi.so

(This is in a build with -Wl,--exclude-libs,libgcc.a specified).

---

Well, there is not much difference.

cd /opt/yb-build/llvm/llvm-v10.0.1-cw7/stage-3/installed;
find . -name "*.so" -or -name "*.a" | while read f; do if nm -gD "$f" 2>/dev/null | grep "_Unwind_Resume"; then echo "$f"; fi; done

000000000000c620 T _Unwind_Resume
000000000000ccd0 T _Unwind_Resume_or_Rethrow
./lib/libunwind.so
                 U _Unwind_Resume
./lib/libc++abi.so

---

Some libraries also depend on libgcc_s.so while they should not.

find . -name "*.so" -or -name "*.a" | while read f; do
  if ldd "$f" 2>/dev/null | grep -q "libgcc_s"; then echo "$f"; fi
done

./lib/clang/10.0.1/lib/linux/libclang_rt.ubsan_standalone-x86_64.so
./lib/clang/10.0.1/lib/linux/libclang_rt.dyndd-x86_64.so
./lib/clang/10.0.1/lib/linux/libclang_rt.scudo_minimal-x86_64.so
./lib/clang/10.0.1/lib/linux/libclang_rt.scudo-x86_64.so
./lib/clang/10.0.1/lib/linux/libclang_rt.ubsan_minimal-x86_64.so
./lib/clang/10.0.1/lib/linux/libclang_rt.asan-x86_64.so
./lib/clang/10.0.1/lib/linux/libclang_rt.hwasan-x86_64.so


---

CentOS 8 dependencies:

cmake
ninja-build