backport https://reviews.llvm.org/D107127 to fix shared libraries using emutls

Our emulated TLS implementation relies on local state (e.g. for the pthread
key), and if we duplicate this state across different shared libraries,
accesses to the same TLS variable from different shared libraries will yield
different results (see https://github.com/android/ndk/issues/1551 for an
example). __emutls_get_address is the only external entry point for emulated
TLS, and by making it default visibility and weak, we can rely on the dynamic
linker to coalesce multiple copies at runtime and ensure a single unique copy
of TLS state. This is a best effort;

Also bump the libc++abi minor because now it picks up the __emutls_get_address
symbol.

ok kettenis@

1 files changed, 15 insertions, 0 deletions

diff --git a/gnu/llvm/compiler-rt/lib/builtins/emutls.c b/gnu/llvm/compiler-rt/lib/builtins/emutls.c
index e0aa19155f7..b546273d6d3 100644
--- a/gnu/llvm/compiler-rt/lib/builtins/emutls.c
+++ b/gnu/llvm/compiler-rt/lib/builtins/emutls.c
@@ -375,6 +375,21 @@ emutls_get_address_array(uintptr_t index) {
   return array;
 }
 
+#ifndef _WIN32
+// Our emulated TLS implementation relies on local state (e.g. for the pthread
+// key), and if we duplicate this state across different shared libraries,
+// accesses to the same TLS variable from different shared libraries will yield
+// different results (see https://github.com/android/ndk/issues/1551 for an
+// example). __emutls_get_address is the only external entry point for emulated
+// TLS, and by making it default visibility and weak, we can rely on the dynamic
+// linker to coalesce multiple copies at runtime and ensure a single unique copy
+// of TLS state. This is a best effort; it won't work if the user is linking
+// with -Bsymbolic or -Bsymbolic-functions, and it also won't work on Windows,
+// where the dynamic linker has no notion of coalescing weak symbols at runtime.
+// A more robust solution would be to create a separate shared library for
+// emulated TLS, to ensure a single copy of its state.
+__attribute__((visibility("default"), weak))
+#endif
 void *__emutls_get_address(__emutls_control *control) {
   uintptr_t index = emutls_get_index(control);
   emutls_address_array *array = emutls_get_address_array(index--);