Add back --sysroot support for darwin header search.

[jyknight]

Summary

Before e97b5f5cf37e ([clang][Darwin] Refactor header search path logic
into the driver), both --sysroot and -isysroot worked to specify where
to look for system and C++ headers on Darwin. However, that change
caused clang to start ignoring --sysroot.

This fixes the regression, and adds tests.

(I also note that on all other platforms, clang seems to almost
completely ignore -isysroot, but that's another issue...)

[ldionne]

This LGTM.

When I did the refactor, all the code was only using -isysroot (and never accessing --sysroot), so I thought only -isysroot was relevant on Darwin. Seems like I was wrong.

[jyknight]

In D62268#1512672, @ldionne wrote:

This LGTM.

When I did the refactor, all the code was only using -isysroot (and never accessing --sysroot), so I thought only -isysroot was relevant on Darwin. Seems like I was wrong.

The isysroot argument to the cc1 compiler invocation got populated from both isysroot and --sysroot in the driver, so that's why the behavior changed here.

lib/Driver/ToolChains/Darwin.h

void AddLinkSanitizerLibArgs(const llvm::opt::ArgList &Args,
                                bool shared = true) const;
 
   bool AddGnuCPlusPlusIncludePaths(const llvm::opt::ArgList &DriverArgs,
                                    llvm::opt::ArgStringList &CC1Args,
                                    llvm::SmallString<128> Base,
                                    llvm::StringRef Version,
                                    llvm::StringRef ArchDir,
                                    llvm::StringRef BitDir) const;
+
+  llvm::StringRef GetHeaderSysroot(const llvm::opt::ArgList &DriverArgs) const;
 };
 
 } // end namespace toolchains
 } // end namespace driver
 } // end namespace clang
 
 #endif // LLVM_CLANG_LIB_DRIVER_TOOLCHAINS_DARWIN_H

lib/Driver/ToolChains/Darwin.cpp

if (SDK.size() > 0) {
       StringRef SDKName = SDK.slice(0, StartVer);
       if (!SDKName.startswith(getPlatformFamily()))
         getDriver().Diag(diag::warn_incompatible_sysroot)
             << SDKName << getPlatformFamily();
     }
   }
 }
 
+// Returns the effective header sysroot path to use. This comes either from
+// -isysroot or --sysroot.
+llvm::StringRef DarwinClang::GetHeaderSysroot(const llvm::opt::ArgList &DriverArgs) const {
+  if(DriverArgs.hasArg(options::OPT_isysroot))
+    return DriverArgs.getLastArgValue(options::OPT_isysroot);
+  if (!getDriver().SysRoot.empty())
+    return getDriver().SysRoot;
+  return "/";
+}
+
 void DarwinClang::AddClangSystemIncludeArgs(const llvm::opt::ArgList &DriverArgs,
                                             llvm::opt::ArgStringList &CC1Args) const {
   const Driver &D = getDriver();
 
-  llvm::StringRef Sysroot = "/";
-  if (const Arg *A = DriverArgs.getLastArg(options::OPT_isysroot))
-    Sysroot = A->getValue();
+  llvm::StringRef Sysroot = GetHeaderSysroot(DriverArgs);
 
   bool NoStdInc = DriverArgs.hasArg(options::OPT_nostdinc);
   bool NoStdlibInc = DriverArgs.hasArg(options::OPT_nostdlibinc);
   bool NoBuiltinInc = DriverArgs.hasArg(options::OPT_nobuiltininc);
 
   // Add <sysroot>/usr/local/include
   if (!NoStdInc && !NoStdlibInc) {
       SmallString<128> P(Sysroot);
void DarwinClang::AddClangCXXStdlibIncludeArgs(
   //        (e.g. HeaderSearchOptions::UseLibcxx) and don't pipe -stdlib.
   //        Also check whether this is used for setting library search paths.
   ToolChain::AddClangCXXStdlibIncludeArgs(DriverArgs, CC1Args);
 
   if (DriverArgs.hasArg(options::OPT_nostdlibinc) ||
       DriverArgs.hasArg(options::OPT_nostdincxx))
     return;
 
-  llvm::SmallString<128> Sysroot;
-  if (const Arg *A = DriverArgs.getLastArg(options::OPT_isysroot)) {
-    Sysroot = A->getValue();
-  } else {
-    Sysroot = "/";
-  }
+  llvm::StringRef Sysroot = GetHeaderSysroot(DriverArgs);
 
   switch (GetCXXStdlibType(DriverArgs)) {
   case ToolChain::CST_Libcxx: {
     // On Darwin, libc++ is installed alongside the compiler in
     // include/c++/v1, so get from '<install>/bin' to '<install>/include/c++/v1'.
     {
       llvm::SmallString<128> P = llvm::StringRef(getDriver().getInstalledDir());
       // Note that P can be relative, so we have to '..' and not parent_path.

test/Driver/darwin-header-search-libcxx.cpp

 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
 // RUN:     -isysroot %S/Inputs/basic_darwin_sdk_no_libcxx \
 // RUN:   | FileCheck -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain --check-prefix=CHECK-LIBCXX-TOOLCHAIN-2 %s
 // CHECK-LIBCXX-TOOLCHAIN-2: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-LIBCXX-TOOLCHAIN-2: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 
 // Check with both headers in the sysroot and headers alongside the installation
 // (the headers in <sysroot> should be added after the toolchain headers).
+// Ensure that both -isysroot and --sysroot work, and that isysroot has precedence.
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin \
 // RUN:     -stdlib=libc++ \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir \
 // RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr \
 // RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
 // RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
 // RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1 %s
+//
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -stdlib=libc++ \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     --sysroot %S/Inputs/basic_darwin_sdk_usr \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
+// RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1 %s
+//
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -stdlib=libc++ \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain/usr/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr \
+// RUN:     --sysroot %S/Inputs/basic_darwin_sdk_no_libcxx \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr \
+// RUN:               -DTOOLCHAIN=%S/Inputs/basic_darwin_toolchain \
+// RUN:               --check-prefix=CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1 %s
+//
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[TOOLCHAIN]]/usr/bin/../include/c++/v1"
 // CHECK-LIBCXX-SYSROOT_AND_TOOLCHAIN-1: "-internal-isystem" "[[SYSROOT]]/usr/include/c++/v1"
 
 // Make sure that using -nostdinc or -nostdlibinc will drop the non-toolchain
 // C++ library include paths (so all except <toolchain>/usr/bin/../include/c++/v1).
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \

test/Driver/darwin-header-search-system.cpp

 // UNSUPPORTED: system-windows
 
 // General tests that the system header search paths detected by the driver
 // and passed to CC1 are correct on Darwin platforms.
 
-// Check system headers (everything below <sysroot> and <resource-dir>)
+// Check system headers (everything below <sysroot> and <resource-dir>).  Ensure
+// that both sysroot and isysroot are checked, and that isysroot has precedence.
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target x86_64-apple-darwin \
 // RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
 // RUN:     -resource-dir=%S/Inputs/resource_dir \
 // RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_and_usr_local \
 // RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_and_usr_local \
 // RUN:               -DRESOURCE=%S/Inputs/resource_dir \
 // RUN:               --check-prefix=CHECK-SYSTEM %s
+//
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     --sysroot %S/Inputs/basic_darwin_sdk_usr_and_usr_local \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_and_usr_local \
+// RUN:               -DRESOURCE=%S/Inputs/resource_dir \
+// RUN:               --check-prefix=CHECK-SYSTEM %s
+//
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-apple-darwin \
+// RUN:     -ccc-install-dir %S/Inputs/basic_darwin_toolchain_no_libcxx/usr/bin \
+// RUN:     -resource-dir=%S/Inputs/resource_dir \
+// RUN:     -isysroot %S/Inputs/basic_darwin_sdk_usr_and_usr_local \
+// RUN:     --sysroot / \
+// RUN:   | FileCheck -DSYSROOT=%S/Inputs/basic_darwin_sdk_usr_and_usr_local \
+// RUN:               -DRESOURCE=%S/Inputs/resource_dir \
+// RUN:               --check-prefix=CHECK-SYSTEM %s
+//
 // CHECK-SYSTEM: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
 // CHECK-SYSTEM: "-internal-isystem" "[[SYSROOT]]/usr/local/include"
 // CHECK-SYSTEM: "-internal-isystem" "[[RESOURCE]]/include"
 // CHECK-SYSTEM: "-internal-externc-isystem" "[[SYSROOT]]/usr/include"
 
 // Make sure that using -nobuiltininc will drop resource headers
 //
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \