[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

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);

llvm/test/ThinLTO/X86/linkonce_odr_unnamed_addr.ll

@@ -0,0 +1,10 @@
+; This test ensures that when linkonce_odr + unnamed_addr symbols promoted to
+; weak symbols, it preserves the auto hide property.
+
+; RUN: opt -module-summary %s -o %t.bc
+; RUN: opt -module-summary %s -o %t2.bc
+; RUN: llvm-lto -thinlto-action=thinlink -o %t3.bc %t.bc %t2.bc
+; RUN: llvm-lto -thinlto-action=promote %t.bc -thinlto-index=%t3.bc -o - | llvm-dis -o - | FileCheck %s
+
+; CHECK: @linkonceodrunnamed = weak_odr hidden unnamed_addr constant i32 0
+@linkonceodrunnamed = linkonce_odr unnamed_addr constant i32 0

llvm/test/ThinLTO/X86/linkonce_resolution_comdat.ll

@@ -10,7 +10,7 @@
 ; Copy from first module is prevailing and converted to weak_odr, copy
 ; from second module is preempted and converted to available_externally and
 ; removed from comdat.
-; IMPORT1: define weak_odr i32 @f(i8*) unnamed_addr comdat($c1) {
+; IMPORT1: define weak_odr hidden i32 @f(i8*) unnamed_addr comdat($c1) {
 ; IMPORT2: define available_externally i32 @f(i8*) unnamed_addr {
 
 ; RUN: llvm-nm -o - < %t1.bc.thinlto.o | FileCheck %s --check-prefix=NM1