[llvm-branch-commits] [llvm] 328a6ec - Force visibility of llvm::Any to external

[Tom Stellard via llvm-branch-commits]

Author: serge-sans-paille
Date: 2021-05-24T21:39:54-07:00
New Revision: 328a6ec955327c6d56b6bc3478c723dd3cd468ef

URL: https://github.com/llvm/llvm-project/commit/328a6ec955327c6d56b6bc3478c723dd3cd468ef
DIFF: https://github.com/llvm/llvm-project/commit/328a6ec955327c6d56b6bc3478c723dd3cd468ef.diff

LOG: Force visibility of llvm::Any to external

llvm::Any::TypeId::Id relies on the uniqueness of the address of a static
variable defined in a template function. hidden visibility implies vague linkage
for that variable, which does not guarantee the uniqueness of the address across
a binary and a shared library. This totally breaks the implementation of
llvm::Any.

Ideally, setting visibility to llvm::Any::TypeId::Id should be enough,
unfortunately this doesn't work as expected and we lack time (before 12.0.1
release) to understand why setting the visibility to llvm::Any does work.

See https://gcc.gnu.org/wiki/Visibility and
https://gcc.gnu.org/onlinedocs/gcc/Vague-Linkage.html
for more information on that topic.

Differential Revision: https://reviews.llvm.org/D101972

(cherry picked from commit 3d3abc22b3ef189813a3b9061c2a90ba86a32f44)

Added: 
    

Modified: 
    llvm/include/llvm/ADT/Any.h

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/ADT/Any.h b/llvm/include/llvm/ADT/Any.h
index 0aded628cda4a..1e3abca706792 100644
--- a/llvm/include/llvm/ADT/Any.h
+++ b/llvm/include/llvm/ADT/Any.h
@@ -23,7 +23,12 @@
 
 namespace llvm {
 
-class Any {
+class LLVM_EXTERNAL_VISIBILITY Any {
+
+  // The `Typeid<T>::Id` static data member below is a globally unique
+  // identifier for the type `T`. It is explicitly marked with default
+  // visibility so that when `-fvisibility=hidden` is used, the loader still
+  // merges duplicate definitions across DSO boundaries.
   template <typename T> struct TypeId { static const char Id; };
 
   struct StorageBase {