Move -fno-semantic-interposition dso_local logic from TargetMachine to Clang CodeGenModule

This simplifies TargetMachine::shouldAssumeDSOLocal and and gives frontend the
decision to use dso_local. For LLVM synthesized functions/globals, they may lose
inferred dso_local but such optimizations are probably not very useful.

Note: the hasComdat() condition in canBenefitFromLocalAlias (D77429) may be dead now.
(llvm/CodeGen/X86/semantic-interposition-comdat.ll)
(Investigate whether we need test coverage when Fuchsia C++ ABI is clearer)

1 files changed, 11 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 93916e85a461..6d14298d9f5f 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -954,8 +954,17 @@ static bool shouldAssumeDSOLocal(const CodeGenModule &CGM,
   const auto &CGOpts = CGM.getCodeGenOpts();
   llvm::Reloc::Model RM = CGOpts.RelocationModel;
   const auto &LOpts = CGM.getLangOpts();
-  if (RM != llvm::Reloc::Static && !LOpts.PIE)
-    return false;
+  if (RM != llvm::Reloc::Static && !LOpts.PIE) {
+    // On ELF, if -fno-semantic-interposition is specified, we can set dso_local
+    // if using a local alias is preferable (can avoid GOT indirection).
+    // Currently only x86 supports local alias.
+    if (!TT.isOSBinFormatELF() ||
+        !CGM.getLangOpts().ExplicitNoSemanticInterposition ||
+        !GV->canBenefitFromLocalAlias())
+      return false;
+    // The allowed targets need to match AsmPrinter::getSymbolPreferLocal.
+    return TT.isX86();
+  }
 
   // A definition cannot be preempted from an executable.
   if (!GV->isDeclarationForLinker())