[ThinLTO] Drop dso_local if a GlobalVariable satisfies isDeclarationForLinker()

dso_local leads to direct access even if the definition is not within this compilation unit (it is
still in the same linkage unit). On ELF, such a relocation (e.g. R_X86_64_PC32) referencing a
STB_GLOBAL STV_DEFAULT object can cause a linker error in a -shared link.

If the linkage is changed to available_externally, the dso_local flag should be dropped, so that no
direct access will be generated.

The current behavior is benign, because -fpic does not assume dso_local
(clang/lib/CodeGen/CodeGenModule.cpp:shouldAssumeDSOLocal).
If we do that for -fno-semantic-interposition (D73865), there will be an
R_X86_64_PC32 linker error without this patch.

Reviewed By: tejohnson

Differential Revision: https://reviews.llvm.org/D74751

1 files changed, 9 insertions, 2 deletions

diff --git a/llvm/lib/LTO/LTOBackend.cpp b/llvm/lib/LTO/LTOBackend.cpp
index b749909..dbd6f8c 100644
--- a/llvm/lib/LTO/LTOBackend.cpp
+++ b/llvm/lib/LTO/LTOBackend.cpp
@@ -521,7 +521,13 @@ Error lto::thinBackend(const Config &Conf, unsigned Task, AddStreamFn AddStream,
   if (Conf.PreOptModuleHook && !Conf.PreOptModuleHook(Task, Mod))
     return finalizeOptimizationRemarks(std::move(DiagnosticOutputFile));
 
-  renameModuleForThinLTO(Mod, CombinedIndex);
+  // When linking an ELF shared object, dso_local should be dropped. We
+  // conservatively do this for -fpic.
+  bool ClearDSOLocalOnDeclarations =
+      TM->getTargetTriple().isOSBinFormatELF() &&
+      TM->getRelocationModel() != Reloc::Static &&
+      Mod.getPIELevel() == PIELevel::Default;
+  renameModuleForThinLTO(Mod, CombinedIndex, ClearDSOLocalOnDeclarations);
 
   dropDeadSymbols(Mod, DefinedGlobals, CombinedIndex);
 
@@ -547,7 +553,8 @@ Error lto::thinBackend(const Config &Conf, unsigned Task, AddStreamFn AddStream,
                                    /*IsImporting*/ true);
   };
 
-  FunctionImporter Importer(CombinedIndex, ModuleLoader);
+  FunctionImporter Importer(CombinedIndex, ModuleLoader,
+                            ClearDSOLocalOnDeclarations);
   if (Error Err = Importer.importFunctions(Mod, ImportList).takeError())
     return Err;