[clang] Avoid defining duplicate header search paths for libc++ on Darwin

[ldionne]

Summary

Before this patch, we would add both <toolchain>/usr/include/c++/v1
and /usr/include/c++/v1 to the header search paths for C++. The
toolchain path is added by the Driver and the /usr/include path
was added in CC1, a remnant of the non-refactoring of search paths
on Darwin. This patch removes /usr/include/c++/v1, since we should
be only looking for the libc++ headers beside the toolchain on Darwin.

The patch also adds basic tests validating that the right -internal-isystem
is passed from the Driver to CC1 for libc++. Once we refactor the rest of
the header search logic to be passed as -internal-isystem from the Driver
to CC1, we can add more tests to check that e.g. C headers are taken from
the SDK, etc..

[ldionne]

I will build a significant codebase with the change and report on the results, since we lack good tests for the removal of /usr/include/c++/v1.

[ldionne]

I decided to use a different approach and first refactor the header search logic from CC1 to the driver. A patch should come eventually when I've tested it properly.

clang/lib/Driver/ToolChains/Darwin.cpp

if (DriverArgs.hasArg(options::OPT_nostdlibinc) ||
       DriverArgs.hasArg(options::OPT_nostdincxx))
     return;
 
   switch (GetCXXStdlibType(DriverArgs)) {
   case ToolChain::CST_Libcxx: {
     llvm::StringRef InstallDir = getDriver().getInstalledDir();
     if (InstallDir.empty())
       break;
-    // On Darwin, libc++ may be installed alongside the compiler in
+    // On Darwin, libc++ is installed alongside the compiler in
     // include/c++/v1.
     // Get from 'foo/bin' to 'foo/include/c++/v1'.
     SmallString<128> P = InstallDir;
     // Note that InstallDir can be relative, so we have to '..' and not
     // parent_path.
     llvm::sys::path::append(P, "..", "include", "c++", "v1");
     addSystemInclude(DriverArgs, CC1Args, P);
     break;

clang/lib/Frontend/InitHeaderSearch.cpp

if (triple.getEnvironment() != llvm::Triple::Cygnus ||
         triple.isOSBinFormatMachO())
       return;
     break;
   }
 
   if (Lang.CPlusPlus && !Lang.AsmPreprocessor &&
       HSOpts.UseStandardCXXIncludes && HSOpts.UseStandardSystemIncludes) {
     if (HSOpts.UseLibcxx) {
+      // On Darwin, all libc++ header search paths are handled in the driver.
+      if (!triple.isOSDarwin()) {
         AddPath("/usr/include/c++/v1", CXXSystem, false);
+      }
     } else {
       AddDefaultCPlusPlusIncludePaths(Lang, triple, HSOpts);
     }
   }
 
   AddDefaultCIncludePaths(triple, HSOpts);
 
   // Add the default framework include paths on Darwin.

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

+// General tests that the header search paths detected by the driver and passed
+// to CC1 are sane on Darwin platforms.
+
+// Check a basic invocation (the headers alongside the toolchain should be used).
+// 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:   | FileCheck -DINPUTS=%S/Inputs --check-prefix=CHECK-BASIC-LIBCXX-ISYSTEM %s
+// CHECK-BASIC-LIBCXX-ISYSTEM: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
+// CHECK-BASIC-LIBCXX-ISYSTEM: "-internal-isystem" "[[INPUTS]]/basic_darwin_toolchain/usr/bin/../include/c++/v1"
+
+// Check with a custom -isysroot (the headers in the toolchain should still be used).
+// 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 \
+// RUN:   | FileCheck -DINPUTS=%S/Inputs --check-prefix=CHECK-BASIC-LIBCXX-ISYSTEM-WITH-SYSROOT %s
+// CHECK-BASIC-LIBCXX-ISYSTEM-WITH-SYSROOT: "{{[^"]*}}clang{{[^"]*}}" "-cc1"
+// CHECK-BASIC-LIBCXX-ISYSTEM-WITH-SYSROOT: "-internal-isystem" "[[INPUTS]]/basic_darwin_toolchain/usr/bin/../include/c++/v1"