[ThinLTO] Teach ThinLTO about auto hide symbols

Summary:
For symbols that has linkonce_odr linkage and unnamed_addr, it can be
auto hide by linker to avoid weak external symbols. Teach ThinLTO to
perform auto hide so it can safely promote linkonce_odr to weak symbols
without breaking this nice property.

Reviewers: tejohnson, mehdi_amini

Reviewed By: tejohnson

Subscribers: inglorion, eraman, rnk, pcc, llvm-commits

Differential Revision: https://reviews.llvm.org/D43130

llvm-svn: 324757

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index a5903e27a9d..b92f798c4ab 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -683,6 +683,13 @@ void llvm::thinLTOResolveWeakForLinkerModule(
         // changed to enable this for aliases.
         llvm_unreachable("Expected GV to be converted");
     } else {
+      // If the original symbols has global unnamed addr and linkonce_odr linkage,
+      // it should be an auto hide symbol. Add hidden visibility to the symbol to
+      // preserve the property.
+      if (GV.hasLinkOnceODRLinkage() && GV.hasGlobalUnnamedAddr() &&
+          NewLinkage == GlobalValue::WeakODRLinkage)
+        GV.setVisibility(GlobalValue::HiddenVisibility);
+
       DEBUG(dbgs() << "ODR fixing up linkage for `" << GV.getName() << "` from "
                    << GV.getLinkage() << " to " << NewLinkage << "\n");
       GV.setLinkage(NewLinkage);