[Clang][Driver]Pass LLVM options to lld in case of LTO

[bin.narwal]

Summary

Hi,
When debugging ThinLTO with clang -mllvm -debug-only=function-import -flto=thin -fuse-ld=lld ..., it gives me below warning message:
warning: argument unused during compilation: '-mllvm -debug-only=function-import' [-Wunused-command-line-argument]

Actually lld already supports passing arguments to llvm using -mllvm, it would be easier for debug if driver passes it along to lld.

This simple patch does this. Note it also silently eats up such options if other linkers (ld.bfd, gold) are used.

Any comments? Thanks

[MaskRay]

-mllvm,foobar is for compilation (.c/.cc -> .o)

-Wl,-mllvm,foobar for LTO options. For linking, -mllvm,foobar is not used and thus warned.

clang/lib/Driver/ToolChains/CommonArgs.cpp

363

This is wrong. The intention is not to pass -plugin when the linker is clearly lld. lld does not need -plugin for LTO.

[bin.narwal]

In D79629#2026974, @MaskRay wrote:

-mllvm,foobar is for compilation (.c/.cc -> .o)

-Wl,-mllvm,foobar for LTO options. For linking, -mllvm,foobar is not used and thus warned.

Right, that's reasonable design. The wrong UseLLD condition is a typo, anyway, I will discard this patch.

Thanks

clang/lib/Driver/ToolChains/CommonArgs.cpp

 bool tools::isUseSeparateSections(const llvm::Triple &Triple) {
   return Triple.getOS() == llvm::Triple::CloudABI;
 }
 
 void tools::addLTOOptions(const ToolChain &ToolChain, const ArgList &Args,
                           ArgStringList &CmdArgs, const InputInfo &Output,
                           const InputInfo &Input, bool IsThinLTO) {
   const char *Linker = Args.MakeArgString(ToolChain.GetLinkerPath());
-  if (llvm::sys::path::filename(Linker) != "ld.lld" &&
-      llvm::sys::path::stem(Linker) != "ld.lld") {
+  bool UseLLD = (llvm::sys::path::filename(Linker) == "ld.lld" ||
+                 llvm::sys::path::stem(Linker) == "ld.lld");
+  if (UseLLD) {
     // Tell the linker to load the plugin. This has to come before
     // AddLinkerInputs as gold requires -plugin to come before any -plugin-opt
     // that -Wl might forward.
     CmdArgs.push_back("-plugin");
 
 #if defined(_WIN32)
     const char *Suffix = ".dll";
 #elif defined(__APPLE__)
     const char *Suffix = ".dylib";
 #else
     const char *Suffix = ".so";
 #endif
 
     SmallString<1024> Plugin;
     llvm::sys::path::native(Twine(ToolChain.getDriver().Dir) +
                                 "/../lib" CLANG_LIBDIR_SUFFIX "/LLVMgold" +
                                 Suffix,
                             Plugin);
     CmdArgs.push_back(Args.MakeArgString(Plugin));
   }
 
+  // Silence warning for "clang -mllvm ...".
+  for (const Arg *A : Args.filtered(options::OPT_mllvm)) {
+    A->claim();
+
+    // Pass llvm options to lld.
+    if (!UseLLD)
+      continue;
+
+    // FIXME: This is now deprecated and should be removed.
+    if (StringRef(A->getValue(0)) == "-disable-llvm-optzns") {
+      CmdArgs.push_back("-disable-llvm-optzns");
+    } else {
+      A->render(Args, CmdArgs);
+    }
+  }
+
   // Try to pass driver level flags relevant to LTO code generation down to
   // the plugin.
 
   // Handle flags for selecting CPU variants.
   std::string CPU = getCPUName(Args, ToolChain.getTriple());
   if (!CPU.empty())
     CmdArgs.push_back(Args.MakeArgString(Twine("-plugin-opt=mcpu=") + CPU));