4 files changed, 65 insertions, 0 deletions
diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 05942f4..17c1c91 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -898,6 +898,20 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
   if (D && D->hasAttr<NoProfileFunctionAttr>())
     Fn->addFnAttr(llvm::Attribute::NoProfile);
 
+  if (D) {
+    // Function attributes take precedence over command line flags.
+    if (auto *A = D->getAttr<FunctionReturnThunksAttr>()) {
+      switch (A->getThunkType()) {
+      case FunctionReturnThunksAttr::Kind::Keep:
+        break;
+      case FunctionReturnThunksAttr::Kind::Extern:
+        Fn->addFnAttr(llvm::Attribute::FnRetThunkExtern);
+        break;
+      }
+    } else if (CGM.getCodeGenOpts().FunctionReturnThunks)
+      Fn->addFnAttr(llvm::Attribute::FnRetThunkExtern);
+  }
+
   if (FD && (getLangOpts().OpenCL ||
              (getLangOpts().HIP && getLangOpts().CUDAIsDevice))) {
     // Add metadata for a kernel function.
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index ec1a31f..b5bfea1 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -6344,6 +6344,10 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (IsUsingLTO)
     Args.AddLastArg(CmdArgs, options::OPT_mibt_seal);
 
+  if (Arg *A = Args.getLastArg(options::OPT_mfunction_return_EQ))
+    CmdArgs.push_back(
+        Args.MakeArgString(Twine("-mfunction-return=") + A->getValue()));
+
   // Forward -f options with positive and negative forms; we translate these by
   // hand.  Do not propagate PGO options to the GPU-side compilations as the
   // profile info is for the host-side compilation only.
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index abef4cf..50aaab6 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1485,6 +1485,9 @@ void CompilerInvocation::GenerateCodeGenArgs(
   if (Opts.IBTSeal)
     GenerateArg(Args, OPT_mibt_seal, SA);
 
+  if (Opts.FunctionReturnThunks)
+    GenerateArg(Args, OPT_mfunction_return_EQ, "thunk-extern", SA);
+
   for (const auto &F : Opts.LinkBitcodeFiles) {
     bool Builtint = F.LinkFlags == llvm::Linker::Flags::LinkOnlyNeeded &&
                     F.PropagateAttrs && F.Internalize;
@@ -1825,6 +1828,27 @@ bool CompilerInvocation::ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args,
       Diags.Report(diag::err_drv_invalid_value) << A->getAsString(Args) << Name;
   }
 
+  if (const Arg *A = Args.getLastArg(OPT_mfunction_return_EQ)) {
+    auto Val = llvm::StringSwitch<llvm::FunctionReturnThunksKind>(A->getValue())
+                   .Case("keep", llvm::FunctionReturnThunksKind::Keep)
+                   .Case("thunk-extern", llvm::FunctionReturnThunksKind::Extern)
+                   .Default(llvm::FunctionReturnThunksKind::Invalid);
+    // SystemZ might want to add support for "expolines."
+    if (!T.isX86())
+      Diags.Report(diag::err_drv_argument_not_allowed_with)
+          << A->getSpelling() << T.getTriple();
+    else if (Val == llvm::FunctionReturnThunksKind::Invalid)
+      Diags.Report(diag::err_drv_invalid_value)
+          << A->getAsString(Args) << A->getValue();
+    else if (Val == llvm::FunctionReturnThunksKind::Extern &&
+             Args.getLastArgValue(OPT_mcmodel_EQ).equals("large"))
+      Diags.Report(diag::err_drv_argument_not_allowed_with)
+          << A->getAsString(Args)
+          << Args.getLastArg(OPT_mcmodel_EQ)->getAsString(Args);
+    else
+      Opts.FunctionReturnThunks = static_cast<unsigned>(Val);
+  }
+
   if (Opts.PrepareForLTO && Args.hasArg(OPT_mibt_seal))
     Opts.IBTSeal = 1;
 
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 5138971..de8d795 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -8000,6 +8000,26 @@ static void handleZeroCallUsedRegsAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(ZeroCallUsedRegsAttr::Create(S.Context, Kind, AL));
 }
 
+static void handleFunctionReturnThunksAttr(Sema &S, Decl *D,
+                                           const ParsedAttr &AL) {
+  StringRef KindStr;
+  SourceLocation LiteralLoc;
+  if (!S.checkStringLiteralArgumentAttr(AL, 0, KindStr, &LiteralLoc))
+    return;
+
+  FunctionReturnThunksAttr::Kind Kind;
+  if (!FunctionReturnThunksAttr::ConvertStrToKind(KindStr, Kind)) {
+    S.Diag(LiteralLoc, diag::warn_attribute_type_not_supported)
+        << AL << KindStr;
+    return;
+  }
+  // FIXME: it would be good to better handle attribute merging rather than
+  // silently replacing the existing attribute, so long as it does not break
+  // the expected codegen tests.
+  D->dropAttr<FunctionReturnThunksAttr>();
+  D->addAttr(FunctionReturnThunksAttr::Create(S.Context, Kind, AL));
+}
+
 static void handleSYCLKernelAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   // The 'sycl_kernel' attribute applies only to function templates.
   const auto *FD = cast<FunctionDecl>(D);
@@ -8866,6 +8886,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_ZeroCallUsedRegs:
     handleZeroCallUsedRegsAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_FunctionReturnThunks:
+    handleFunctionReturnThunksAttr(S, D, AL);
+    break;
 
   // Microsoft attributes:
   case ParsedAttr::AT_LayoutVersion: