[clang] [Driver] Teach Barmetal toolchain about GCC installation (PR #121829)

[Peter Smith via cfe-commits]

================
@@ -110,20 +110,81 @@ static std::string computeBaseSysRoot(const Driver &D, bool IncludeTriple) {
   return std::string(SysRootDir);
 }
 
+static bool hasGCCToolChainAlongSideClang(const Driver &D) {
+  SmallString<128> GCCDir;
+  llvm::sys::path::append(GCCDir, D.Dir, "..", D.getTargetTriple(),
+                          "lib/crt0.o");
+  return llvm::sys::fs::exists(GCCDir);
+}
+
+std::string BareMetal::computeSysRoot() const {
+  if (!SysRoot.empty())
+    return SysRoot;
+
+  const Driver &D = getDriver();
+  if (!D.SysRoot.empty())
+    return D.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.
+  SmallString<128> SysRootDir;
+  if (GCCInstallation.isValid()) {
+    StringRef LibDir = GCCInstallation.getParentLibPath();
+    StringRef TripleStr = GCCInstallation.getTriple().str();
+    llvm::sys::path::append(SysRootDir, LibDir, "..", TripleStr);
+  } else if (hasGCCToolChainAlongSideClang(D)) {
+    // 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, D.Dir, "..", D.getTargetTriple());
+  }
+
+  if (llvm::sys::fs::exists(SysRootDir))
+    return std::string(SysRootDir);
+  return computeBaseSysRoot(D, /*IncludeTriple*/ true);
+}
+
+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();
+  if (GCCInstallation.isValid()) {
+    Multilibs = GCCInstallation.getMultilibs();
+    SelectedMultilibs.assign({GCCInstallation.getMultilib()});
----------------
smithp35 wrote:

Are we delegating GCC multilibs entirely to the GNU toolchain driver? As I understand it these are hard-coded at the moment, so I expect the GNU arm-none-eabi toolchains won't work unless their multilibs are added.

Have I got that right? Or will this only work with toolchains, possibly RISCV, that already have their multilib structure encoded in GCCInstallation.

https://github.com/llvm/llvm-project/pull/121829