[ThinLTO] Add an auto-hide feature

When a symbol is not exported outside of the
DSO, it is can be hidden. Usually we try to internalize
as much as possible, but it is not always possible, for
instance a symbol can be referenced outside of the LTO
unit, or there can be cross-module reference in ThinLTO.

This is a recommit of r293912 after fixing build failures,
and a recommit of r293918 after fixing LLD tests.

Differential Revision: https://reviews.llvm.org/D28978

llvm-svn: 293970

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index 5fa99863135..2966be2d46d 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -644,6 +644,13 @@ void llvm::thinLTOInternalizeModule(Module &TheModule,
     return !GlobalValue::isLocalLinkage(Linkage);
   };
 
+  // Try to auto-hide the symbols.
+  for (auto &GO : TheModule.global_objects()) {
+    const auto &GS = DefinedGlobals.find(GO.getGUID());
+    if (GS != DefinedGlobals.end() && GS->second->flags().AutoHide)
+      GO.setVisibility(GlobalValue::HiddenVisibility);
+  }
+
   // FIXME: See if we can just internalize directly here via linkage changes
   // based on the index, rather than invoking internalizeModule.
   llvm::internalizeModule(TheModule, MustPreserveGV);