[clang] [Fuchsia] Build with -fvisibility=default (PR #67067)

[Alex Brachet via cfe-commits]

https://github.com/abrachet updated https://github.com/llvm/llvm-project/pull/67067

>From 62f1432f09ba4844fb2e826c73ce5748c34c6daf Mon Sep 17 00:00:00 2001
From: Alex Brachet <abrachet at google.com>
Date: Wed, 20 Sep 2023 17:55:44 -0400
Subject: [PATCH] [Fuchsia] Build with -fvisibility=default

There was an issue with relative vtables when two TU's which define
the same vtable object are built with different default visibilities.
Some TU's are built with -fvisibility=hidden in the code base, grep
for CMAKE_CXX_VISIBILITY_PRESET to find them. Our whole toolchain,
is statically linked, and built with -fPIE anyway, so the cost of
overriding local CMAKE_CXX_VISIBILITY_PRESET properties is not high.
I've counted that adding this flag increases our llvm binary by 13
relocations. Frankly I'm not sure where those are even coming from.

It would be preferable to use hidden visibility, but that breaks
liblld. This can be solved by setting LLDB_EXPORT_ALL_SYMBOLS.
After that some ORC tests fail which do symbolic lookup in the
tests. It seems that setting CMAKE_CXX_VISIBILITY_PRESET=hidden
will not be worth the maintenance burden. Setting it to default
works to unblock using relative vtables, so we can just go with
that.
---
 clang/cmake/caches/Fuchsia-stage2.cmake | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/clang/cmake/caches/Fuchsia-stage2.cmake b/clang/cmake/caches/Fuchsia-stage2.cmake
index bc4d9f1462b1814..a58d3823eeeaf9c 100644
--- a/clang/cmake/caches/Fuchsia-stage2.cmake
+++ b/clang/cmake/caches/Fuchsia-stage2.cmake
@@ -51,6 +51,12 @@ set(CLANG_PLUGIN_SUPPORT OFF CACHE BOOL "")
 set(ENABLE_LINKER_BUILD_ID ON CACHE BOOL "")
 set(ENABLE_X86_RELAX_RELOCATIONS ON CACHE BOOL "")
 
+# TODO(#67176): relative-vtables doesn't play well with different default
+# visibilities. Making everything hidden visibility causes other complications
+# let's choose default visibility for our entire toolchain.
+set(CMAKE_C_VISIBILITY_PRESET default CACHE STRING "")
+set(CMAKE_CXX_VISIBILITY_PRESET default CACHE STRING "")
+
 set(CMAKE_BUILD_TYPE Release CACHE STRING "")
 if (APPLE)
   set(CMAKE_OSX_DEPLOYMENT_TARGET "10.13" CACHE STRING "")