[Clang] Add option to set alternative toolchain path

In some cases, we need to set alternative toolchain path other than the
default with system (headers, libraries, dynamic linker prefix, ld path,
etc.), e.g., to pick up newer components, but keep sysroot at the same
time (to pick up extra packages).

This change introduces a new option --overlay-platform-toolchain to set
up such alternative toolchain path.

Reviewed By: hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D121992

clang/docs/ClangCommandLineReference.rst

@@ -515,6 +515,10 @@ CUDA offloading device architecture (e.g. sm\_35), or HIP offloading target ID i
 
 Specify comma-separated list of offloading target triples (CUDA and HIP only)
 
+.. option:: --overlay-platform-toolchain=<arg>
+
+Specify a toolchain with higher priority than sysroot in search paths.
+
 .. option:: -p, --profile
 
 .. option:: -pagezero\_size<arg>

clang/include/clang/Driver/Driver.h

@@ -149,6 +149,9 @@ public:
   typedef SmallVector<std::string, 4> prefix_list;
   prefix_list PrefixDirs;
 
+  /// Alternative toolchain path used prior to sysroot.
+  std::string OverlayToolChainPath;
+
   /// sysroot, if present
   std::string SysRoot;
 

clang/include/clang/Driver/Options.td

@@ -4180,6 +4180,8 @@ def _output_class_directory_EQ : Joined<["--"], "output-class-directory=">, Alia
 def _output_class_directory : Separate<["--"], "output-class-directory">, Alias<foutput_class_dir_EQ>;
 def _output_EQ : Joined<["--"], "output=">, Alias<o>;
 def _output : Separate<["--"], "output">, Alias<o>;
+def _overlay_platform_toolchain_EQ : Joined<["--"], "overlay-platform-toolchain=">;
+def _overlay_platform_toolchain : Separate<["--"], "overlay-platform-toolchain">, Alias<_overlay_platform_toolchain_EQ>;
 def _param : Separate<["--"], "param">, Group<CompileOnly_Group>;
 def _param_EQ : Joined<["--"], "param=">, Alias<_param>;
 def _precompile : Flag<["--"], "precompile">, Flags<[NoXarchOption]>,

clang/lib/Driver/Driver.cpp

@@ -1208,6 +1208,11 @@ Compilation *Driver::BuildCompilation(ArrayRef<const char *> ArgList) {
       CompilerPath = Split.second;
     }
   }
+  if (const Arg *A =
+          Args.getLastArg(options::OPT__overlay_platform_toolchain_EQ)) {
+    OverlayToolChainPath = A->getValue();
+    DyldPrefix = A->getValue();
+  }
   if (const Arg *A = Args.getLastArg(options::OPT__sysroot_EQ))
     SysRoot = A->getValue();
   if (const Arg *A = Args.getLastArg(options::OPT__dyld_prefix_EQ))

clang/lib/Driver/ToolChains/Gnu.cpp

@@ -1867,6 +1867,10 @@ static llvm::StringRef getGCCToolchainDir(const ArgList &Args,
   if (A)
     return A->getValue();
 
+  if (const Arg *X = Args.getLastArg(
+          clang::driver::options::OPT__overlay_platform_toolchain_EQ))
+    return X->getValue();
+
   // If we have a SysRoot, ignore GCC_INSTALL_PREFIX.
   // GCC_INSTALL_PREFIX specifies the gcc installation for the default
   // sysroot and is likely not valid with a different sysroot.

clang/lib/Driver/ToolChains/Linux.cpp

@@ -260,6 +260,14 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
 
   const std::string OSLibDir = std::string(getOSLibDir(Triple, Args));
   const std::string MultiarchTriple = getMultiarchTriple(D, Triple, SysRoot);
+  const std::string &ExtraPath = D.OverlayToolChainPath;
+
+  if (!D.OverlayToolChainPath.empty()) {
+    addPathIfExists(D, ExtraPath + "/lib/" + MultiarchTriple, Paths);
+    addPathIfExists(D, ExtraPath + "/lib/../" + OSLibDir, Paths);
+    addPathIfExists(D, ExtraPath + "/usr/lib/" + MultiarchTriple, Paths);
+    addPathIfExists(D, ExtraPath + "/usr/lib/../" + OSLibDir, Paths);
+  }
 
   // mips32: Debian multilib, we use /libo32, while in other case, /lib is
   // used. We need add both libo32 and /lib.
@@ -314,6 +322,11 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
       addPathIfExists(D, D.Dir + "/../" + OSLibDir, Paths);
   }
 
+  if (!D.OverlayToolChainPath.empty()) {
+    addPathIfExists(D, ExtraPath + "/lib", Paths);
+    addPathIfExists(D, ExtraPath + "/usr/lib", Paths);
+  }
+
   addPathIfExists(D, SysRoot + "/lib", Paths);
   addPathIfExists(D, SysRoot + "/usr/lib", Paths);
 }
@@ -567,6 +580,10 @@ void Linux::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
   if (DriverArgs.hasArg(options::OPT_nostdlibinc))
     return;
 
+  if (!D.OverlayToolChainPath.empty())
+    addExternCSystemInclude(DriverArgs, CC1Args,
+                            D.OverlayToolChainPath + "/include");
+
   // LOCAL_INCLUDE_DIR
   addSystemInclude(DriverArgs, CC1Args, SysRoot + "/usr/local/include");
   // TOOL_INCLUDE_DIR

clang/test/Driver/overlay-toolchain.cpp

@@ -0,0 +1,18 @@
+// RUN: %clangxx %s -### --target=powerpc64le-linux-gnu \
+// RUN:   --overlay-platform-toolchain=%S/Inputs/powerpc64le-linux-gnu-tree/gcc-11.2.0 \
+// RUN:   2>&1 | FileCheck %s --check-prefix=OVERLAY
+// RUN: %clangxx %s -### --target=powerpc64le-linux-gnu \
+// RUN:   --sysroot=%S/Inputs/powerpc64le-linux-gnu-tree/gcc-8.3.0 \
+// RUN:   --overlay-platform-toolchain=%S/Inputs/powerpc64le-linux-gnu-tree/gcc-11.2.0 \
+// RUN:   2>&1 | FileCheck %s --check-prefixes=OVERLAY,ROOT
+
+// OVERLAY: "-internal-externc-isystem"
+// OVERLAY: "[[TOOLCHAIN:[^"]+]]/powerpc64le-linux-gnu-tree/gcc-11.2.0/include"
+// ROOT: "-internal-externc-isystem"
+// ROOT: "[[TOOLCHAIN]]/powerpc64le-linux-gnu-tree/gcc-8.3.0/include"
+// OVERLAY: "-dynamic-linker"
+// OVERLAY: "[[TOOLCHAIN]]/powerpc64le-linux-gnu-tree/gcc-11.2.0/lib64/ld64.so.2"
+// OVERLAY: "-L[[TOOLCHAIN]]/powerpc64le-linux-gnu-tree/gcc-11.2.0/lib/../lib64"
+// ROOT: "-L[[TOOLCHAIN]]/powerpc64le-linux-gnu-tree/gcc-8.3.0/lib/../lib64"
+// OVERLAY: "-L[[TOOLCHAIN]]/powerpc64le-linux-gnu-tree/gcc-11.2.0/lib"
+// ROOT: "-L[[TOOLCHAIN]]/powerpc64le-linux-gnu-tree/gcc-8.3.0/lib"