[Driver] Support using toolchain libc and libc++ for baremetal (#96736)

We want to support using a complete Clang/LLVM toolchain that includes
LLVM libc and libc++ for baremetal targets. To do so, we need the driver
to add the necessary include paths.

clang/include/clang/Driver/ToolChain.h

@@ -526,6 +526,9 @@ class ToolChain {
   // Returns target specific standard library path if it exists.
   std::optional<std::string> getStdlibPath() const;
 
+  // Returns target specific standard library include path if it exists.
+  std::optional<std::string> getStdlibIncludePath() const;
+
   // Returns <ResourceDir>/lib/<OSName>/<arch> or <ResourceDir>/lib/<triple>.
   // This is used by runtimes (such as OpenMP) to find arch-specific libraries.
   virtual path_list getArchSpecificLibPaths() const;

clang/lib/Driver/ToolChain.cpp

@@ -811,6 +811,12 @@ std::optional<std::string> ToolChain::getStdlibPath() const {
   return getTargetSubDirPath(P);
 }
 
+std::optional<std::string> ToolChain::getStdlibIncludePath() const {
+  SmallString<128> P(D.Dir);
+  llvm::sys::path::append(P, "..", "include");
+  return getTargetSubDirPath(P);
+}
+
 ToolChain::path_list ToolChain::getArchSpecificLibPaths() const {
   path_list Paths;
 

clang/lib/Driver/ToolChains/BareMetal.cpp

@@ -270,15 +270,19 @@ void BareMetal::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
     addSystemInclude(DriverArgs, CC1Args, Dir.str());
   }
 
-  if (!DriverArgs.hasArg(options::OPT_nostdlibinc)) {
-    const SmallString<128> SysRoot(computeSysRoot());
-    if (!SysRoot.empty()) {
-      for (const Multilib &M : getOrderedMultilibs()) {
-        SmallString<128> Dir(SysRoot);
-        llvm::sys::path::append(Dir, M.includeSuffix());
-        llvm::sys::path::append(Dir, "include");
-        addSystemInclude(DriverArgs, CC1Args, Dir.str());
-      }
+  if (DriverArgs.hasArg(options::OPT_nostdlibinc))
+    return;
+
+  if (std::optional<std::string> Path = getStdlibIncludePath())
+    addSystemInclude(DriverArgs, CC1Args, *Path);
+
+  const SmallString<128> SysRoot(computeSysRoot());
+  if (!SysRoot.empty()) {
+    for (const Multilib &M : getOrderedMultilibs()) {
+      SmallString<128> Dir(SysRoot);
+      llvm::sys::path::append(Dir, M.includeSuffix());
+      llvm::sys::path::append(Dir, "include");
+      addSystemInclude(DriverArgs, CC1Args, Dir.str());
     }
   }
 }
@@ -296,6 +300,40 @@ void BareMetal::AddClangCXXStdlibIncludeArgs(const ArgList &DriverArgs,
     return;
 
   const Driver &D = getDriver();
+  std::string Target = getTripleString();
+
+  auto AddCXXIncludePath = [&](StringRef Path) {
+    std::string Version = detectLibcxxVersion(Path);
+    if (Version.empty())
+      return;
+
+    {
+      // First the per-target include dir: include/<target>/c++/v1.
+      SmallString<128> TargetDir(Path);
+      llvm::sys::path::append(TargetDir, Target, "c++", Version);
+      addSystemInclude(DriverArgs, CC1Args, TargetDir);
+    }
+
+    {
+      // Then the generic dir: include/c++/v1.
+      SmallString<128> Dir(Path);
+      llvm::sys::path::append(Dir, "c++", Version);
+      addSystemInclude(DriverArgs, CC1Args, Dir);
+    }
+  };
+
+  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;
+  }
+
   std::string SysRoot(computeSysRoot());
   if (SysRoot.empty())
     return;

clang/test/Driver/baremetal.cpp

@@ -13,7 +13,7 @@
 // CHECK-V6M-C-SAME: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
 // CHECK-V6M-C-SAME: "-isysroot" "[[SYSROOT:[^"]*]]"
 // CHECK-V6M-C-SAME: "-internal-isystem" "[[SYSROOT]]{{[/\\]+}}include{{[/\\]+}}c++{{[/\\]+}}v1"
-// CHECk-V6M-C-SAME: "-internal-isystem" "[[SYSROOT]]{{[/\\]+}}include"
+// CHECK-V6M-C-SAME: "-internal-isystem" "[[SYSROOT]]{{[/\\]+}}include"
 // CHECK-V6M-C-SAME: "-x" "c++" "{{.*}}baremetal.cpp"
 // CHECK-V6M-C-NEXT: ld{{(.exe)?}}" "{{.*}}.o" "-Bstatic" "-EL"
 // CHECK-V6M-C-SAME: "-T" "semihosted.lds" "-Lsome{{[/\\]+}}directory{{[/\\]+}}user{{[/\\]+}}asked{{[/\\]+}}for"
@@ -26,6 +26,20 @@
 // RUN:     --sysroot=%S/Inputs/baremetal_arm | FileCheck --check-prefix=CHECK-V6M-LIBINC %s
 // CHECK-V6M-LIBINC-NOT: "-internal-isystem"
 
+// RUN: %clang %s -### --target=armv6m-none-eabi -o %t.out 2>&1 \
+// RUN:     -ccc-install-dir %S/Inputs/basic_baremetal_tree/bin \
+// RUN:   | FileCheck --check-prefix=CHECK-V6M-TREE %s
+// CHECK-V6M-TREE:      InstalledDir: [[INSTALLED_DIR:.+]]
+// CHECK-V6M-TREE:      "-cc1" "-triple" "thumbv6m-unknown-none-eabi"
+// CHECK-V6M-TREE-SAME: "-resource-dir" "[[RESOURCE_DIR:[^"]+]]"
+// CHECK-V6M-TREE-SAME: "-internal-isystem" "[[INSTALLED_DIR]]{{[/\\]+}}..{{[/\\]+}}include{{[/\\]+}}armv6m-unknown-none-eabi{{[/\\]+}}c++{{[/\\]+}}v1"
+// CHECK-V6M-TREE-SAME: {{^}} "-internal-isystem" "[[INSTALLED_DIR]]{{[/\\]+}}..{{[/\\]+}}include{{[/\\]+}}c++{{[/\\]+}}v1"
+// CHECK-V6M-TREE-SAME: "-internal-isystem" "[[INSTALLED_DIR]]{{[/\\]+}}..{{[/\\]+}}include{{[/\\]+}}armv6m-unknown-none-eabi"
+// CHECK-V6M-TREE-SAME: "-x" "c++" "{{.*}}baremetal.cpp"
+// CHECK-V6M-TREE-NEXT: ld{{(.exe)?}}" "{{.*}}.o" "-Bstatic" "-EL"
+// CHECK-V6M-TREE-SAME: "-L[[INSTALLED_DIR]]{{[/\\]+}}..{{[/\\]+}}lib{{[/\\]+}}armv6m-unknown-none-eabi"
+// CHECK-V6M-TREE-SAME: "-lc" "-lm" "{{[^"]*}}libclang_rt.builtins.a" "--target2=rel" "-o" "{{.*}}.tmp.out"
+
 // RUN: %clang %s -### --target=armv7m-vendor-none-eabi -rtlib=compiler-rt 2>&1 \
 // RUN:     --sysroot=%S/Inputs/baremetal_arm \
 // RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \