Clang attempts to guess file names based on the OS and distro (yes!),
but unfortunately, that doesn't work for us.

This patch makes it easy to insert libc's $libdir so that Clang passes the
correct absolute file name of crt1.o etc. to 'ld'.  It also disables all
the distro-specific stuff and removes the hard-coded FHS directory names
to make sure Clang also works on foreign distros.

diff --git a/clang/lib/Driver/Distro.cpp b/clang/lib/Driver/Distro.cpp
index 5ac38c34d112..2a704b418fdc 100644
--- a/clang/lib/Driver/Distro.cpp
+++ b/clang/lib/Driver/Distro.cpp
@@ -96,6 +96,10 @@ static Distro::DistroType DetectLsbRelease(llvm::vfs::FileSystem &VFS) {
 }
 
 static Distro::DistroType DetectDistro(llvm::vfs::FileSystem &VFS) {
+  // The compiler should always behave the same, even when used via Guix on a
+  // foreign distro.
+  return Distro::UnknownDistro;
+
   Distro::DistroType Version = Distro::UnknownDistro;
 
   // Newer freedesktop.org's compilant systemd-based systems
diff --git a/clang/lib/Driver/ToolChains/Cuda.cpp b/clang/lib/Driver/ToolChains/Cuda.cpp
index 4a9f6d4c4e3e..0ae13ebebd27 100644
--- a/clang/lib/Driver/ToolChains/Cuda.cpp
+++ b/clang/lib/Driver/ToolChains/Cuda.cpp
@@ -117,6 +117,9 @@ CudaInstallationDetector::CudaInstallationDetector(
     const Driver &D, const llvm::Triple &HostTriple,
     const llvm::opt::ArgList &Args)
     : D(D) {
+  // Don't look for CUDA in /usr.
+  return;
+
   struct Candidate {
     std::string Path;
     bool StrictChecking;
diff --git a/clang/lib/Driver/ToolChains/Linux.cpp b/clang/lib/Driver/ToolChains/Linux.cpp
index 83cb41159de7..6b920e33d1ef 100644
--- a/clang/lib/Driver/ToolChains/Linux.cpp
+++ b/clang/lib/Driver/ToolChains/Linux.cpp
@@ -186,6 +186,10 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
 
   Generic_GCC::PushPPaths(PPaths);
 
+  // Comment out the distro-specific tweaks so that they don't bite when
+  // using Guix on a foreign distro.
+#if 0
+
   Distro Distro(D.getVFS(), Triple);
 
   if (Distro.IsAlpineLinux() || Triple.isAndroid()) {
@@ -250,6 +254,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
 
   if (IsAndroid || Distro.IsOpenSUSE())
     ExtraOpts.push_back("--enable-new-dtags");
+#endif
 
   // The selection of paths to try here is designed to match the patterns which
   // the GCC driver itself uses, as this is part of the GCC-compatible driver.
@@ -270,6 +275,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
   }
   Generic_GCC::AddMultilibPaths(D, SysRoot, OSLibDir, MultiarchTriple, Paths);
 
+#if 0
   addPathIfExists(D, SysRoot + "/lib/" + MultiarchTriple, Paths);
   addPathIfExists(D, SysRoot + "/lib/../" + OSLibDir, Paths);
 
@@ -298,9 +304,11 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
     addPathIfExists(D, SysRoot + "/" + OSLibDir + "/" + ABIName, Paths);
     addPathIfExists(D, SysRoot + "/usr/" + OSLibDir + "/" + ABIName, Paths);
   }
+#endif
 
   Generic_GCC::AddMultiarchPaths(D, SysRoot, OSLibDir, Paths);
 
+#if 0
   // The deprecated -DLLVM_ENABLE_PROJECTS=libcxx configuration installs
   // libc++.so in D.Dir+"/../lib/". Detect this path.
   // TODO Remove once LLVM_ENABLE_PROJECTS=libcxx is unsupported.
@@ -310,6 +318,14 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
 
   addPathIfExists(D, SysRoot + "/lib", Paths);
   addPathIfExists(D, SysRoot + "/usr/lib", Paths);
+#endif
+
+  // Add libc's lib/ directory to the search path, so that crt1.o, crti.o,
+  // and friends can be found.
+  addPathIfExists(D, "@GLIBC_LIBDIR@", Paths);
+
+  // Add GCC's lib/ directory so libstdc++.so can be found.
+  addPathIfExists(D, GCCInstallation.getParentLibPath(), Paths);
 }
 
 ToolChain::RuntimeLibType Linux::GetDefaultRuntimeLibType() const {