[RISCV] Merging RISCVToolChain and BareMetal toolchains

Currently, LLVM has two RISC-V toolchain classes in Clang for baremetal
development, creating unnecessary maintenance overhead. This patch extends the
BareMetal toolchain to support an existing GCC installation, resolving this issue.

The latest patchset preserves the behavior of both toolchain objects with minor
differences. If no --sysroot option is passed on the command line or if the GCC
installation is invalid, the sysroot will first be formed as per the
RISCVToolChain baremetal object. If this path does not exist, the sysroot will
be formed as per the BareMetal toolchain object.

Additionally, the presence of --gcc-toolchain or --gcc-install-dir will imply
that GNU linker is the default linker unless otherwise a differnt linker is
passed through `-fuse-ld` flag.

RFC - https://discourse.llvm.org/t/merging-riscvtoolchain-and-baremetal-toolchains/75524

Author: Garvit Gupta

clang/lib/Driver/Driver.cpp

@@ -6521,10 +6521,6 @@ const ToolChain &Driver::getToolChain(const ArgList &Args,
         break;
       case llvm::Triple::riscv32:
       case llvm::Triple::riscv64:
-        if (toolchains::RISCVToolChain::hasGCCToolchain(*this, Args))
-          TC =
-              std::make_unique<toolchains::RISCVToolChain>(*this, Target, Args);
-        else
           TC = std::make_unique<toolchains::BareMetal>(*this, Target, Args);
         break;
       case llvm::Triple::ve:

clang/lib/Driver/ToolChains/BareMetal.cpp

@@ -97,7 +97,8 @@ static bool findRISCVMultilibs(const Driver &D,
   return false;
 }
 
-static std::string computeBaseSysRoot(const Driver &D, bool IncludeTriple) {
+static std::string computeInstalledToolchainSysRoot(const Driver &D,
+                                                    bool IncludeTriple) {
   if (!D.SysRoot.empty())
     return D.SysRoot;
 
@@ -110,20 +111,94 @@ static std::string computeBaseSysRoot(const Driver &D, bool IncludeTriple) {
   return std::string(SysRootDir);
 }
 
+// GCC sysroot here means form sysroot from either --gcc-install-dir, or from
+// --gcc-toolchain or if the toolchain is installed alongside clang in
+// bin/../<TargetTriple> directory if it is not explicitly specified on the command
+//  line through `--sysroot` option. libc here will be newlib.
+std::string BareMetal::computeGCCSysRoot() const {
+  if (!getDriver().SysRoot.empty())
+    return getDriver().SysRoot;
+
+  SmallString<128> SysRootDir;
+  if (GCCInstallation.isValid()) {
+    StringRef LibDir = GCCInstallation.getParentLibPath();
+    StringRef TripleStr = GCCInstallation.getTriple().str();
+    llvm::sys::path::append(SysRootDir, LibDir, "..", TripleStr);
+  } else {
+    // Use the triple as provided to the driver. Unlike the parsed triple
+    // this has not been normalized to always contain every field.
+    llvm::sys::path::append(SysRootDir, getDriver().Dir, "..",
+                            getDriver().getTargetTriple());
+  }
+
+  if (!llvm::sys::fs::exists(SysRootDir))
+    return std::string();
+
+  return std::string(SysRootDir);
+}
+
+std::string BareMetal::computeSysRoot() const {
+  if (!SysRoot.empty())
+    return SysRoot;
+
+  std::string SysRoot = getDriver().SysRoot;
+  if (!SysRoot.empty() && llvm::sys::fs::exists(SysRoot))
+    return SysRoot;
+
+  // Verify the GCC installation from -gcc-install-dir, --gcc-toolchain, or
+  // alongside clang. If valid, form the sysroot. Otherwise, check
+  // lib/clang-runtimes above the driver.
+  SysRoot = computeGCCSysRoot();
+  if (!SysRoot.empty())
+    return SysRoot;
+
+  SysRoot =
+      computeInstalledToolchainSysRoot(getDriver(), /*IncludeTriple*/ true);
+
+  return SysRoot;
+}
+
+static void addMultilibsFilePaths(const Driver &D, const MultilibSet &Multilibs,
+                                  const Multilib &Multilib,
+                                  StringRef InstallPath,
+                                  ToolChain::path_list &Paths) {
+  if (const auto &PathsCallback = Multilibs.filePathsCallback())
+    for (const auto &Path : PathsCallback(Multilib))
+      addPathIfExists(D, InstallPath + Path, Paths);
+}
+
 BareMetal::BareMetal(const Driver &D, const llvm::Triple &Triple,
                      const ArgList &Args)
-    : ToolChain(D, Triple, Args),
-      SysRoot(computeBaseSysRoot(D, /*IncludeTriple=*/true)) {
-  getProgramPaths().push_back(getDriver().Dir);
-
-  findMultilibs(D, Triple, Args);
-  SmallString<128> SysRoot(computeSysRoot());
-  if (!SysRoot.empty()) {
-    for (const Multilib &M : getOrderedMultilibs()) {
-      SmallString<128> Dir(SysRoot);
-      llvm::sys::path::append(Dir, M.osSuffix(), "lib");
-      getFilePaths().push_back(std::string(Dir));
-      getLibraryPaths().push_back(std::string(Dir));
+    : Generic_ELF(D, Triple, Args){
+  GCCInstallation.init(Triple, Args);
+  SysRoot = computeSysRoot();
+  UseLD = Args.getLastArgValue(options::OPT_fuse_ld_EQ).equals_insensitive("ld");
+  if (GCCInstallation.isValid()) {
+    Multilibs = GCCInstallation.getMultilibs();
+    SelectedMultilibs.assign({GCCInstallation.getMultilib()});
+    path_list &Paths = getFilePaths();
+    // Add toolchain/multilib specific file paths.
+    addMultilibsFilePaths(D, Multilibs, SelectedMultilibs.back(),
+                          GCCInstallation.getInstallPath(), Paths);
+    getFilePaths().push_back(GCCInstallation.getInstallPath().str());
+    ToolChain::path_list &PPaths = getProgramPaths();
+    // Multilib cross-compiler GCC installations put ld in a triple-prefixed
+    // directory off of the parent of the GCC installation.
+    PPaths.push_back(Twine(GCCInstallation.getParentLibPath() + "/../" +
+                           GCCInstallation.getTriple().str() + "/bin")
+                         .str());
+    PPaths.push_back((GCCInstallation.getParentLibPath() + "/../bin").str());
+    getFilePaths().push_back(computeSysRoot() + "/lib");
+  } else {
+    getProgramPaths().push_back(getDriver().Dir);
+    findMultilibs(D, Triple, Args);
+    if (!SysRoot.empty()) {
+      for (const Multilib &M : getOrderedMultilibs()) {
+        SmallString<128> Dir(SysRoot);
+        llvm::sys::path::append(Dir, M.osSuffix(), "lib");
+        getFilePaths().push_back(std::string(Dir));
+        getLibraryPaths().push_back(std::string(Dir));
+      }
     }
   }
 }
@@ -236,7 +311,7 @@ getMultilibConfigPath(const Driver &D, const llvm::Triple &Triple,
       return {};
     }
   } else {
-    MultilibPath = computeBaseSysRoot(D, /*IncludeTriple=*/false);
+    MultilibPath = computeInstalledToolchainSysRoot(D, /*IncludeTriple=*/false);
     llvm::sys::path::append(MultilibPath, MultilibFilename);
   }
   return MultilibPath;
@@ -254,7 +329,7 @@ void BareMetal::findMultilibs(const Driver &D, const llvm::Triple &Triple,
   if (D.getVFS().exists(*MultilibPath)) {
     // If multilib.yaml is found, update sysroot so it doesn't use a target
     // specific suffix
-    SysRoot = computeBaseSysRoot(D, /*IncludeTriple=*/false);
+    SysRoot = computeInstalledToolchainSysRoot(D, /*IncludeTriple=*/false);
     findMultilibsFromYAML(*this, D, *MultilibPath, Args, Result);
     SelectedMultilibs = Result.SelectedMultilibs;
     Multilibs = Result.Multilibs;
@@ -279,8 +354,6 @@ Tool *BareMetal::buildStaticLibTool() const {
   return new tools::baremetal::StaticLibTool(*this);
 }
 
-std::string BareMetal::computeSysRoot() const { return SysRoot; }
-
 BareMetal::OrderedMultilibs BareMetal::getOrderedMultilibs() const {
   // Get multilibs in reverse order because they're ordered most-specific last.
   if (!SelectedMultilibs.empty())
@@ -291,6 +364,36 @@ BareMetal::OrderedMultilibs BareMetal::getOrderedMultilibs() const {
   return llvm::reverse(Default);
 }
 
+ToolChain::CXXStdlibType BareMetal::GetDefaultCXXStdlibType() const {
+  if (getTriple().isRISCV()) {
+    return GCCInstallation.isValid() ? ToolChain::CST_Libstdcxx
+                                     : ToolChain::CST_Libcxx;
+  }
+  return ToolChain::CST_Libcxx;
+}
+
+ToolChain::RuntimeLibType BareMetal::GetDefaultRuntimeLibType() const {
+  if (getTriple().isRISCV()) {
+    return GCCInstallation.isValid() ? ToolChain::RLT_Libgcc
+                                     : ToolChain::RLT_CompilerRT;
+  }
+  return ToolChain::RLT_CompilerRT;
+}
+
+ToolChain::UnwindLibType
+BareMetal::GetUnwindLibType(const llvm::opt::ArgList &Args) const {
+  if (getTriple().isRISCV())
+    return ToolChain::UNW_None;
+
+  return ToolChain::GetUnwindLibType(Args);
+}
+
+const char *BareMetal::getDefaultLinker() const {
+  if(isUsingLD())
+    return "ld";
+  return "ld.lld";
+}
+
 void BareMetal::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
                                           ArgStringList &CC1Args) const {
   if (DriverArgs.hasArg(options::OPT_nostdinc))
@@ -325,6 +428,19 @@ void BareMetal::addClangTargetOptions(const ArgList &DriverArgs,
   CC1Args.push_back("-nostdsysteminc");
 }
 
+void BareMetal::addLibStdCxxIncludePaths(
+    const llvm::opt::ArgList &DriverArgs,
+    llvm::opt::ArgStringList &CC1Args) const {
+  if (GCCInstallation.isValid()) {
+    const GCCVersion &Version = GCCInstallation.getVersion();
+    StringRef TripleStr = GCCInstallation.getTriple().str();
+    const Multilib &Multilib = GCCInstallation.getMultilib();
+    addLibStdCXXIncludePaths(computeSysRoot() + "/include/c++/" + Version.Text,
+                            TripleStr, Multilib.includeSuffix(), DriverArgs,
+                            CC1Args);
+  }
+}
+
 void BareMetal::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
                                              ArgStringList &CC1Args) const {
   if (DriverArgs.hasArg(options::OPT_nostdinc, options::OPT_nostdlibinc,
@@ -355,15 +471,15 @@ void BareMetal::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
   };
 
   switch (GetCXXStdlibType(DriverArgs)) {
-    case ToolChain::CST_Libcxx: {
-      SmallString<128> P(D.Dir);
-      llvm::sys::path::append(P, "..", "include");
-      AddCXXIncludePath(P);
-      break;
-    }
-    case ToolChain::CST_Libstdcxx:
-      // We only support libc++ toolchain installation.
-      break;
+  case ToolChain::CST_Libcxx: {
+    SmallString<128> P(D.Dir);
+    llvm::sys::path::append(P, "..", "include");
+    AddCXXIncludePath(P);
+    break;
+  }
+  case ToolChain::CST_Libstdcxx:
+    addLibStdCxxIncludePaths(DriverArgs, CC1Args);
+    break;
   }
 
   std::string SysRoot(computeSysRoot());
@@ -428,6 +544,10 @@ void BareMetal::AddCXXStdlibLibArgs(const ArgList &Args,
     CmdArgs.push_back("-lsupc++");
     break;
   }
+
+  if (getTriple().isRISCV() && GCCInstallation.isValid())
+    return;
+
   CmdArgs.push_back("-lunwind");
 }
 
@@ -503,12 +623,22 @@ void baremetal::Linker::ConstructJob(Compilation &C, const JobAction &JA,
   const llvm::Triple::ArchType Arch = TC.getArch();
   const llvm::Triple &Triple = getToolChain().getEffectiveTriple();
 
-  AddLinkerInputs(TC, Inputs, Args, CmdArgs, JA);
+  if (!D.SysRoot.empty())
+    CmdArgs.push_back(Args.MakeArgString("--sysroot=" + D.SysRoot));
 
+  Args.addAllArgs(CmdArgs, {options::OPT_u});
   CmdArgs.push_back("-Bstatic");
 
-  if (TC.getTriple().isRISCV() && Args.hasArg(options::OPT_mno_relax))
-    CmdArgs.push_back("--no-relax");
+  if (TC.getTriple().isRISCV()) {
+    if (Args.hasArg(options::OPT_mno_relax))
+      CmdArgs.push_back("--no-relax");
+    if (TC.isUsingLD()) {
+      CmdArgs.push_back("-m");
+      CmdArgs.push_back(TC.getArch() == llvm::Triple::riscv64 ? "elf64lriscv"
+                                                              : "elf32lriscv");
+    }
+    CmdArgs.push_back("-X");
+  }
 
   if (Triple.isARM() || Triple.isThumb()) {
     bool IsBigEndian = arm::isARMBigEndian(Triple, Args);
@@ -519,9 +649,24 @@ void baremetal::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     CmdArgs.push_back(Arch == llvm::Triple::aarch64_be ? "-EB" : "-EL");
   }
 
-  if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles,
-                   options::OPT_r)) {
-    CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crt0.o")));
+  bool WantCRTs =
+      !Args.hasArg(options::OPT_nostdlib, options::OPT_nostartfiles);
+
+  const char *crtbegin, *crtend;
+  if (WantCRTs) {
+    if (!Args.hasArg(options::OPT_r))
+      CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath("crt0.o")));
+    auto RuntimeLib = TC.GetRuntimeLibType(Args);
+    if (RuntimeLib == ToolChain::RLT_Libgcc) {
+      crtbegin = "crtbegin.o";
+      crtend = "crtend.o";
+    } else {
+      assert(RuntimeLib == ToolChain::RLT_CompilerRT);
+      crtbegin =
+          TC.getCompilerRTArgString(Args, "crtbegin", ToolChain::FT_Object);
+      crtend = TC.getCompilerRTArgString(Args, "crtend", ToolChain::FT_Object);
+    }
+    CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath(crtbegin)));
   }
 
   Args.addAllArgs(CmdArgs, {options::OPT_L, options::OPT_T_Group,
@@ -536,12 +681,20 @@ void baremetal::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     TC.AddCXXStdlibLibArgs(Args, CmdArgs);
 
   if (!Args.hasArg(options::OPT_nostdlib, options::OPT_nodefaultlibs)) {
-    CmdArgs.push_back("-lc");
     CmdArgs.push_back("-lm");
-
+    if (TC.isUsingLD())
+      CmdArgs.push_back("--start-group");
+    CmdArgs.push_back("-lc");
+    if (TC.isUsingLD()) {
+      CmdArgs.push_back("-lgloss");
+      CmdArgs.push_back("--end-group");
+    }
     TC.AddLinkRuntimeLib(Args, CmdArgs);
   }
 
+  if (WantCRTs)
+    CmdArgs.push_back(Args.MakeArgString(TC.GetFilePath(crtend)));
+
   if (D.isUsingLTO()) {
     assert(!Inputs.empty() && "Must have at least one input.");
     // Find the first filename InputInfo object.
@@ -555,8 +708,8 @@ void baremetal::Linker::ConstructJob(Compilation &C, const JobAction &JA,
     addLTOOptions(TC, Args, CmdArgs, Output, *Input,
                   D.getLTOMode() == LTOK_Thin);
   }
-  if (TC.getTriple().isRISCV())
-    CmdArgs.push_back("-X");
+
+  AddLinkerInputs(TC, Inputs, Args, CmdArgs, JA);
 
   // The R_ARM_TARGET2 relocation must be treated as R_ARM_REL32 on arm*-*-elf
   // and arm*-*-eabi (the default is R_ARM_GOT_PREL, used on arm*-*-linux and