Subject: Re: [bug#38276] Fix kernel-headers path in clang

From: Carl Dong <[email-masked]>
Date: Wed, 20 Nov 2019 21:28:08 +0000

Hi Mathieu,

I want to apologize for the confusion with that last patch I sent… That was actually the exact opposite patch of what I intended :-)
I’ve attached the correct patch below, and some explanation:

> Plus, in general patching so heavily the sources of a package proves to
> be very hard to maintain.

These are already patches that we carry (see gnu/packages/patches/clang-*-libc-search-path.patch), I’m simply modifying them.

> I'm not sure to understand the benefit of this patch versus hardcoding
> libc and kernel-headers path into clang at build time.

This patch _does_ hardcode libc and kernel-headers path into clang at build time, BUT it does so in a way that also retains sane behaviour when --sysroot is supplied to clang. Explanation from the patch:

> ++  // Check for configure-time "extra" C include directories. When constructing a
> ++  // toolchain, @C_EXTRA_INCLUDE_DIRS@ should be replaced with something like a
> ++  // colon-separated string of the include dirs of libc and kernel headers.
> ++  //
> ++  // The reason why we use this mechanism instead of C_INCLUDE_DIRS above is
> ++  // because when a user supplies clang with --sysroot, the normal expectation
> ++  // is that clang will detect and add the proper $SYSROOT/$MULTIARCHINCL,
> ++  // $SYSROOT/include, and $SYSROOT/usr/include to its list of search paths.
> ++  // However, if C_INCLUDE_DIRS is not empty, this function will return early
> ++  // and not attempt to add the aforementioned search paths, which is not
> ++  // desirable.
> ++  //
> ++  // By adding our configure-time "extra" C include directories here, after
> ++  // we've added $SYSROOT/include and $SYSROOT/usr/include, we make sure that IF
> ++  // --sysroot is supplied on the command line, we pick up the expected search
> ++  // paths in the $SYSROOT, and that they come before our configure-time "extra"
> ++  // C include directories.

Again, sorry for the confusion, and I look forward to hearing what you and others think!

Cheers,
Carl Dong

-----

diff --git a/gnu/packages/llvm.scm b/gnu/packages/llvm.scm
index 33fb53d65e..9ee5062429 100644
--- a/gnu/packages/llvm.scm
+++ b/gnu/packages/llvm.scm
@@ -186,12 +186,7 @@ compiler.  In LLVM this library is called \"compiler-rt\".")
 
              ;; Find libgcc_s, crtbegin.o, and crtend.o.
              (string-append "-DGCC_INSTALL_PREFIX="
-                            (assoc-ref %build-inputs "gcc-lib"))
-
-             ;; Use a sane default include directory.
-             (string-append "-DC_INCLUDE_DIRS="
-                            (assoc-ref %build-inputs "libc")
-                            "/include"))
+                            (assoc-ref %build-inputs "gcc-lib")))
 
        ;; Don't use '-g' during the build to save space.
        #:build-type "Release"
@@ -202,6 +197,8 @@ compiler.  In LLVM this library is called \"compiler-rt\".")
                    (lambda* (#:key inputs #:allow-other-keys)
                      (let ((libc (assoc-ref inputs "libc"))
                            (compiler-rt (assoc-ref inputs "clang-runtime"))
+                           (gcc (assoc-ref inputs "gcc"))
+                           (kernel-headers (assoc-ref inputs "kernel-headers"))
                            (version
                             (string->number
                              ,(version-major (package-version clang-runtime)))))
@@ -221,7 +218,10 @@ compiler.  In LLVM this library is called \"compiler-rt\".")
                             ;; Make sure libc's libdir is on the search path, to
                             ;; allow crt1.o & co. to be found.
                             (("@GLIBC_LIBDIR@")
-                             (string-append libc "/lib"))))
+                             (string-append libc "/lib"))
+                            (("@C_EXTRA_INCLUDE_DIRS@")
+                             (string-append libc "/include" ":"
+                                            kernel-headers "/include"))))
                          (else
                           (substitute* "lib/Driver/Tools.cpp"
                             ;; Patch the 'getLinuxDynamicLinker' function so that
diff --git a/gnu/packages/patches/clang-3.5-libc-search-path.patch b/gnu/packages/patches/clang-3.5-libc-search-path.patch
index 50e4480239..54fb9be7d8 100644
--- a/gnu/packages/patches/clang-3.5-libc-search-path.patch
+++ b/gnu/packages/patches/clang-3.5-libc-search-path.patch
@@ -8,7 +8,7 @@ to make sure Clang also works on non-GuixSD systems.
 
 --- cfe-3.6.0.src/lib/Driver/ToolChains.cpp	2015-02-18 22:03:07.000000000 +0100
 +++ cfe-3.6.0.src/lib/Driver/ToolChains.cpp	2015-06-19 16:37:20.459701044 +0200
-@@ -2931,6 +2931,9 @@ Linux::Linux(const Driver &D, const llvm
+@@ -3040,6 +3040,9 @@
  
    Linker = GetLinkerPath();
  
@@ -18,7 +18,7 @@ to make sure Clang also works on non-GuixSD systems.
    Distro Distro = DetectDistro(Arch);
  
    if (IsOpenSUSE(Distro) || IsUbuntu(Distro)) {
-@@ -2973,6 +2976,7 @@ Linux::Linux(const Driver &D, const llvm
+@@ -3082,6 +3085,7 @@
  
    if (IsOpenSUSE(Distro))
      ExtraOpts.push_back("--enable-new-dtags");
@@ -26,41 +26,49 @@ to make sure Clang also works on non-GuixSD systems.
  
    // 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.
-@@ -3043,14 +3047,12 @@ Linux::Linux(const Driver &D, const llvm
-     addPathIfExists(D.Dir + "/../" + OSLibDir, Paths);
-   }
+@@ -3194,6 +3198,10 @@
  
--  addPathIfExists(SysRoot + "/lib/" + MultiarchTriple, Paths);
--  addPathIfExists(SysRoot + "/lib/../" + OSLibDir, Paths);
--  addPathIfExists(SysRoot + "/usr/lib/" + MultiarchTriple, Paths);
--  addPathIfExists(SysRoot + "/usr/lib/../" + OSLibDir, Paths);
--
-   // Try walking via the GCC triple path in case of biarch or multiarch GCC
-   // installations with strange symlinks.
-   if (GCCInstallation.isValid()) {
-+    // The following code would end up adding things like
-+    // "/usr/lib/x86_64-unknown-linux-gnu/../../lib64" to the search path.
-+#if 0
-     addPathIfExists(SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
-                     "/../../" + OSLibDir, Paths);
- 
-@@ -3060,6 +3062,7 @@ Linux::Linux(const Driver &D, const llvm
-       addPathIfExists(GCCInstallation.getInstallPath() +
-                       BiarchSibling.gccSuffix(), Paths);
-     }
-+#endif
- 
-     // See comments above on the multilib variant for details of why this is
-     // included even from outside the sysroot.
-@@ -3083,8 +3086,9 @@ Linux::Linux(const Driver &D, const llvm
-   if (StringRef(D.Dir).startswith(SysRoot))
-     addPathIfExists(D.Dir + "/../lib", Paths);
- 
--  addPathIfExists(SysRoot + "/lib", Paths);
--  addPathIfExists(SysRoot + "/usr/lib", Paths);
+   addPathIfExists(SysRoot + "/lib", Paths);
+   addPathIfExists(SysRoot + "/usr/lib", Paths);
++
 +  // Add libc's lib/ directory to the search path, so that crt1.o, crti.o,
 +  // and friends can be found.
 +  addPathIfExists("@GLIBC_LIBDIR@", Paths);
  }
  
  bool Linux::HasNativeLLVMSupport() const {
+@@ -3384,6 +3392,34 @@
+   addExternCSystemInclude(DriverArgs, CC1Args, SysRoot + "/include");
+ 
+   addExternCSystemInclude(DriverArgs, CC1Args, SysRoot + "/usr/include");
++
++  // Check for configure-time "extra" C include directories. When constructing a
++  // toolchain, @C_EXTRA_INCLUDE_DIRS@ should be replaced with something like a
++  // colon-separated string of the include dirs of libc and kernel headers.
++  //
++  // The reason why we use this mechanism instead of C_INCLUDE_DIRS above is
++  // because when a user supplies clang with --sysroot, the normal expectation
++  // is that clang will detect and add the proper $SYSROOT/$MULTIARCHINCL,
++  // $SYSROOT/include, and $SYSROOT/usr/include to its list of search paths.
++  // However, if C_INCLUDE_DIRS is not empty, this function will return early
++  // and not attempt to add the aforementioned search paths, which is not
++  // desirable.
++  //
++  // By adding our configure-time "extra" C include directories here, after
++  // we've added $SYSROOT/include and $SYSROOT/usr/include, we make sure that IF
++  // --sysroot is supplied on the command line, we pick up the expected search
++  // paths in the $SYSROOT, and that they come before our configure-time "extra"
++  // C include directories.
++  StringRef CExtraIncludeDirs("@C_EXTRA_INCLUDE_DIRS@");
++  if (CExtraIncludeDirs != "") {
++      SmallVector<StringRef, 5> dirs;
++      CExtraIncludeDirs.split(dirs, ":");
++      for (StringRef dir : dirs) {
++          StringRef Prefix =
++              llvm::sys::path::is_absolute(dir) ? StringRef(SysRoot) : "";
++          addExternCSystemInclude(DriverArgs, CC1Args, Prefix + dir);
++      }
++  }
+ }
+ 
+ /// \brief Helper to add the variant paths of a libstdc++ installation.
diff --git a/gnu/packages/patches/clang-3.8-libc-search-path.patch b/gnu/packages/patches/clang-3.8-libc-search-path.patch
index 0f7d0a4add..243d167755 100644
--- a/gnu/packages/patches/clang-3.8-libc-search-path.patch
+++ b/gnu/packages/patches/clang-3.8-libc-search-path.patch
@@ -29,41 +29,49 @@ changes in clang 3.8.
  
    // 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.
-@@ -3771,14 +3775,12 @@
-     addPathIfExists(D, D.Dir + "/../" + OSLibDir, Paths);
-   }
+@@ -3817,6 +3821,10 @@
  
--  addPathIfExists(D, SysRoot + "/lib/" + MultiarchTriple, Paths);
--  addPathIfExists(D, SysRoot + "/lib/../" + OSLibDir, Paths);
--  addPathIfExists(D, SysRoot + "/usr/lib/" + MultiarchTriple, Paths);
--  addPathIfExists(D, SysRoot + "/usr/lib/../" + OSLibDir, Paths);
--
-   // Try walking via the GCC triple path in case of biarch or multiarch GCC
-   // installations with strange symlinks.
-   if (GCCInstallation.isValid()) {
-+    // The following code would end up adding things like
-+    // "/usr/lib/x86_64-unknown-linux-gnu/../../lib64" to the search path.
-+#if 0
-     addPathIfExists(D,
-                     SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
-                         "/../../" + OSLibDir,
-@@ -3791,6 +3793,7 @@
-                              BiarchSibling.gccSuffix(),
-                       Paths);
-     }
-+#endif
- 
-     // See comments above on the multilib variant for details of why this is
-     // included even from outside the sysroot.
-@@ -3815,8 +3818,9 @@
-   if (StringRef(D.Dir).startswith(SysRoot))
-     addPathIfExists(D, D.Dir + "/../lib", Paths);
- 
--  addPathIfExists(D, SysRoot + "/lib", Paths);
--  addPathIfExists(D, SysRoot + "/usr/lib", Paths);
+   addPathIfExists(D, SysRoot + "/lib", Paths);
+   addPathIfExists(D, SysRoot + "/usr/lib", Paths);
++
 +  // 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);
  }
  
  bool Linux::HasNativeLLVMSupport() const { return true; }
+@@ -4026,6 +4034,34 @@
+   addExternCSystemInclude(DriverArgs, CC1Args, SysRoot + "/include");
+ 
+   addExternCSystemInclude(DriverArgs, CC1Args, SysRoot + "/usr/include");
++
++  // Check for configure-time "extra" C include directories. When constructing a
++  // toolchain, @C_EXTRA_INCLUDE_DIRS@ should be replaced with something like a
++  // colon-separated string of the include dirs of libc and kernel headers.
++  //
++  // The reason why we use this mechanism instead of C_INCLUDE_DIRS above is
++  // because when a user supplies clang with --sysroot, the normal expectation
++  // is that clang will detect and add the proper $SYSROOT/$MULTIARCHINCL,
++  // $SYSROOT/include, and $SYSROOT/usr/include to its list of search paths.
++  // However, if C_INCLUDE_DIRS is not empty, this function will return early
++  // and not attempt to add the aforementioned search paths, which is not
++  // desirable.
++  //
++  // By adding our configure-time "extra" C include directories here, after
++  // we've added $SYSROOT/include and $SYSROOT/usr/include, we make sure that IF
++  // --sysroot is supplied on the command line, we pick up the expected search
++  // paths in the $SYSROOT, and that they come before our configure-time "extra"
++  // C include directories.
++  StringRef CExtraIncludeDirs("@C_EXTRA_INCLUDE_DIRS@");
++  if (CExtraIncludeDirs != "") {
++      SmallVector<StringRef, 5> dirs;
++      CExtraIncludeDirs.split(dirs, ":");
++      for (StringRef dir : dirs) {
++          StringRef Prefix =
++              llvm::sys::path::is_absolute(dir) ? StringRef(SysRoot) : "";
++          addExternCSystemInclude(DriverArgs, CC1Args, Prefix + dir);
++      }
++  }
+ }
+ 
+ 
diff --git a/gnu/packages/patches/clang-6.0-libc-search-path.patch b/gnu/packages/patches/clang-6.0-libc-search-path.patch
index a62e8063c2..d80c798467 100644
--- a/gnu/packages/patches/clang-6.0-libc-search-path.patch
+++ b/gnu/packages/patches/clang-6.0-libc-search-path.patch
@@ -8,18 +8,17 @@ to make sure Clang also works on non-GuixSD systems.
 
 --- cfe-6.0.0.src/lib/Driver/ToolChains/Linux.cpp
 +++ cfe-6.0.0.src/lib/Driver/ToolChains/Linux.cpp
-@@ -207,7 +207,9 @@
-   PPaths.push_back(Twine(GCCInstallation.getParentLibPath() + "/../" +
+@@ -208,6 +208,9 @@
                           GCCInstallation.getTriple().str() + "/bin")
                         .str());
--
+ 
 +  // 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());
  
    if (Distro.IsAlpineLinux()) {
-@@ -255,6 +257,7 @@
+@@ -255,6 +258,7 @@
  
    if (IsAndroid || Distro.IsOpenSUSE())
      ExtraOpts.push_back("--enable-new-dtags");
@@ -27,41 +26,49 @@ to make sure Clang also works on non-GuixSD systems.
  
    // 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.
-@@ -329,14 +332,12 @@
-     addPathIfExists(D, D.Dir + "/../" + OSLibDir, Paths);
-   }
- 
--  addPathIfExists(D, SysRoot + "/lib/" + MultiarchTriple, Paths);
--  addPathIfExists(D, SysRoot + "/lib/../" + OSLibDir, Paths);
--  addPathIfExists(D, SysRoot + "/usr/lib/" + MultiarchTriple, Paths);
--  addPathIfExists(D, SysRoot + "/usr/lib/../" + OSLibDir, Paths);
--
-   // Try walking via the GCC triple path in case of biarch or multiarch GCC
-   // installations with strange symlinks.
-   if (GCCInstallation.isValid()) {
-+    // The following code would end up adding things like
-+    // "/usr/lib/x86_64-unknown-linux-gnu/../../lib64" to the search path.
-+#if 0
-     addPathIfExists(D,
-                     SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
-                         "/../../" + OSLibDir,
-@@ -349,6 +350,7 @@
-                              BiarchSibling.gccSuffix(),
-                       Paths);
-     }
-+#endif
+@@ -375,6 +379,10 @@
  
-     // See comments above on the multilib variant for details of why this is
-     // included even from outside the sysroot.
-@@ -373,8 +375,9 @@
-   if (StringRef(D.Dir).startswith(SysRoot))
-     addPathIfExists(D, D.Dir + "/../lib", Paths);
- 
--  addPathIfExists(D, SysRoot + "/lib", Paths);
--  addPathIfExists(D, SysRoot + "/usr/lib", Paths);
+   addPathIfExists(D, SysRoot + "/lib", Paths);
+   addPathIfExists(D, SysRoot + "/usr/lib", Paths);
++
 +  // 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);
  }
  
  bool Linux::HasNativeLLVMSupport() const { return true; }
+@@ -710,6 +718,34 @@
+   addExternCSystemInclude(DriverArgs, CC1Args, SysRoot + "/include");
+ 
+   addExternCSystemInclude(DriverArgs, CC1Args, SysRoot + "/usr/include");
++
++  // Check for configure-time "extra" C include directories. When constructing a
++  // toolchain, @C_EXTRA_INCLUDE_DIRS@ should be replaced with something like a
++  // colon-separated string of the include dirs of libc and kernel headers.
++  //
++  // The reason why we use this mechanism instead of C_INCLUDE_DIRS above is
++  // because when a user supplies clang with --sysroot, the normal expectation
++  // is that clang will detect and add the proper $SYSROOT/$MULTIARCHINCL,
++  // $SYSROOT/include, and $SYSROOT/usr/include to its list of search paths.
++  // However, if C_INCLUDE_DIRS is not empty, this function will return early
++  // and not attempt to add the aforementioned search paths, which is not
++  // desirable.
++  //
++  // By adding our configure-time "extra" C include directories here, after
++  // we've added $SYSROOT/include and $SYSROOT/usr/include, we make sure that IF
++  // --sysroot is supplied on the command line, we pick up the expected search
++  // paths in the $SYSROOT, and that they come before our configure-time "extra"
++  // C include directories.
++  StringRef CExtraIncludeDirs("@C_EXTRA_INCLUDE_DIRS@");
++  if (CExtraIncludeDirs != "") {
++      SmallVector<StringRef, 5> dirs;
++      CExtraIncludeDirs.split(dirs, ":");
++      for (StringRef dir : dirs) {
++          StringRef Prefix =
++              llvm::sys::path::is_absolute(dir) ? StringRef(SysRoot) : "";
++          addExternCSystemInclude(DriverArgs, CC1Args, Prefix + dir);
++      }
++  }
+ }
+ 
+ static std::string DetectLibcxxIncludePath(StringRef base) {
diff --git a/gnu/packages/patches/clang-7.0-libc-search-path.patch b/gnu/packages/patches/clang-7.0-libc-search-path.patch
index 07ff8c90bd..445b5d17ac 100644
--- a/gnu/packages/patches/clang-7.0-libc-search-path.patch
+++ b/gnu/packages/patches/clang-7.0-libc-search-path.patch
@@ -8,7 +8,7 @@ to make sure Clang also works on non-GuixSD systems.
 
 --- a/lib/Driver/ToolChains/Linux.cpp
 +++ b/lib/Driver/ToolChains/Linux.cpp
-@@ -225,7 +225,9 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+@@ -225,7 +225,9 @@
    PPaths.push_back(Twine(GCCInstallation.getParentLibPath() + "/../" +
                           GCCInstallation.getTriple().str() + "/bin")
                         .str());
@@ -19,7 +19,7 @@ to make sure Clang also works on non-GuixSD systems.
    Distro Distro(D.getVFS());
  
    if (Distro.IsAlpineLinux()) {
-@@ -284,6 +286,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
+@@ -284,6 +286,7 @@
  
    if (IsAndroid || Distro.IsOpenSUSE())
      ExtraOpts.push_back("--enable-new-dtags");
@@ -27,56 +27,49 @@ to make sure Clang also works on non-GuixSD systems.
  
    // 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.
-@@ -342,7 +345,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
-     // the cross. Note that GCC does include some of these directories in some
-     // configurations but this seems somewhere between questionable and simply
-     // a bug.
--    if (StringRef(LibPath).startswith(SysRoot)) {
-+    if (0) {
-       addPathIfExists(D, LibPath + "/" + MultiarchTriple, Paths);
-       addPathIfExists(D, LibPath + "/../" + OSLibDir, Paths);
-     }
-@@ -361,6 +364,8 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
-   addPathIfExists(D, SysRoot + "/lib/" + MultiarchTriple, Paths);
-   addPathIfExists(D, SysRoot + "/lib/../" + OSLibDir, Paths);
+@@ -431,6 +434,10 @@
  
-+  // This requires the commented distro tweaks above.
-+#if 0
-   if (IsAndroid) {
-     // Android sysroots contain a library directory for each supported OS
-     // version as well as some unversioned libraries in the usual multiarch
-@@ -389,10 +394,14 @@ 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
- 
-   // Try walking via the GCC triple path in case of biarch or multiarch GCC
-   // installations with strange symlinks.
-   if (GCCInstallation.isValid()) {
-+    // The following code would end up adding things like
-+    // "/usr/lib/x86_64-unknown-linux-gnu/../../lib64" to the search path.
-+#if 0
-     addPathIfExists(D,
-                     SysRoot + "/usr/lib/" + GCCInstallation.getTriple().str() +
-                         "/../../" + OSLibDir,
-@@ -405,6 +414,7 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
-                              BiarchSibling.gccSuffix(),
-                       Paths);
-     }
-+#endif
- 
-     // See comments above on the multilib variant for details of why this is
-     // included even from outside the sysroot.
-@@ -429,8 +439,9 @@ Linux::Linux(const Driver &D, const llvm::Triple &Triple, const ArgList &Args)
-   if (StringRef(D.Dir).startswith(SysRoot))
-     addPathIfExists(D, D.Dir + "/../lib", Paths);
- 
--  addPathIfExists(D, SysRoot + "/lib", Paths);
--  addPathIfExists(D, SysRoot + "/usr/lib", Paths);
+   addPathIfExists(D, SysRoot + "/lib", Paths);
+   addPathIfExists(D, SysRoot + "/usr/lib", Paths);
++
 +  // 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);
  }
  
  bool Linux::HasNativeLLVMSupport() const { return true; }
+@@ -794,6 +801,34 @@
+   addExternCSystemInclude(DriverArgs, CC1Args, SysRoot + "/include");
+ 
+   addExternCSystemInclude(DriverArgs, CC1Args, SysRoot + "/usr/include");
++
++  // Check for configure-time "extra" C include directories. When constructing a
++  // toolchain, @C_EXTRA_INCLUDE_DIRS@ should be replaced with something like a
++  // colon-separated string of the include dirs of libc and kernel headers.
++  //
++  // The reason why we use this mechanism instead of C_INCLUDE_DIRS above is
++  // because when a user supplies clang with --sysroot, the normal expectation
++  // is that clang will detect and add the proper $SYSROOT/$MULTIARCHINCL,
++  // $SYSROOT/include, and $SYSROOT/usr/include to its list of search paths.
++  // However, if C_INCLUDE_DIRS is not empty, this function will return early
++  // and not attempt to add the aforementioned search paths, which is not
++  // desirable.
++  //
++  // By adding our configure-time "extra" C include directories here, after
++  // we've added $SYSROOT/include and $SYSROOT/usr/include, we make sure that IF
++  // --sysroot is supplied on the command line, we pick up the expected search
++  // paths in the $SYSROOT, and that they come before our configure-time "extra"
++  // C include directories.
++  StringRef CExtraIncludeDirs("@C_EXTRA_INCLUDE_DIRS@");
++  if (CExtraIncludeDirs != "") {
++      SmallVector<StringRef, 5> dirs;
++      CExtraIncludeDirs.split(dirs, ":");
++      for (StringRef dir : dirs) {
++          StringRef Prefix =
++              llvm::sys::path::is_absolute(dir) ? StringRef(SysRoot) : "";
++          addExternCSystemInclude(DriverArgs, CC1Args, Prefix + dir);
++      }
++  }
+ }
+ 
+ static std::string DetectLibcxxIncludePath(StringRef base) {