[compiler-rt] b8f04a6 - [builtins] Try to ensure single copy of emulated TLS state

[Shoaib Meenai via llvm-commits]

Author: Shoaib Meenai
Date: 2021-07-30T14:52:38-07:00
New Revision: b8f04a670f27a84412099dd025fa762ee58f4c1a

URL: https://github.com/llvm/llvm-project/commit/b8f04a670f27a84412099dd025fa762ee58f4c1a
DIFF: https://github.com/llvm/llvm-project/commit/b8f04a670f27a84412099dd025fa762ee58f4c1a.diff

LOG: [builtins] Try to ensure single copy of emulated TLS state

Multiple copies of emulated TLS state means inconsistent results when
accessing the same thread-local variable from different shared objects
(https://github.com/android/ndk/issues/1551). Making `__emutls_get_address`
be a weak default visibility symbol should make the dynamic linker
ensure only a single copy gets used at runtime. This is best-effort, but
the more robust approach of putting emulated TLS into its own shared
object would (a) be a much bigger change, and (b) shared objects are
pretty heavyweight, and adding a new one to a space-constrained
environment isn't an easy sell. Given the expected rarity of direct
accesses to emulated TLS variables across different shared objects, the
best-effort approach should suffice.

Reviewed By: danalbert, rprichard

Differential Revision: https://reviews.llvm.org/D107127

Added: 
    

Modified: 
    compiler-rt/lib/builtins/emutls.c

Removed: 
    


################################################################################
diff  --git a/compiler-rt/lib/builtins/emutls.c b/compiler-rt/lib/builtins/emutls.c
index 98cabd917d6c..88af348c9e22 100644
--- a/compiler-rt/lib/builtins/emutls.c
+++ b/compiler-rt/lib/builtins/emutls.c
@@ -374,6 +374,21 @@ 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 
diff erent shared libraries,
+// accesses to the same TLS variable from 
diff erent shared libraries will yield
+// 
diff erent 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--);