[clang] [ToolChains/NetBSD] Support relative libc++ header path

[mgorny]

Summary

Support locating the libc++ header files relatively to the clang
executable, in addition to the default system path. This is meant
to cover two use cases: running just-built clang from the install
directory, and running installed clang from non-standard location
(e.g. /usr/local).

This is the first step towards ensuring that tests of more LLVM projects
can work out-of-the-box within the build tree, and use the correct set
of headers (rather than e.g. mixing just-built clang+libcxx with system
install of libcxx). It avoids requiring the user to hack around missing
include paths, or LLVM build system to replicate system-specific C++
library defaults in order to append appropriate paths implicitly.

[krytarowski]

clang/lib/Driver/ToolChains/NetBSD.cpp

430 ↗

(On Diff #188079)

I propose to go for:

getDriver().SysRoot + "/usr/include/c++/v1", with a fallback for getDriver().SysRoot + "/usr/include/c++",

This innocent customization of paths triggers a lot of headache.

We should switch system location to /usr/include/c++/v1 for next version of LLVM (9.0 as 8.0 is branched and will be released soon).

This way we will keep compat with legacy paths and new clang.

[mgorny]

clang/lib/Driver/ToolChains/NetBSD.cpp

430 ↗

(On Diff #188079)

This is already handled by the relative path (when getDriver().Dir is /usr/bin).

[krytarowski]

clang/lib/Driver/ToolChains/NetBSD.cpp

430 ↗

(On Diff #188079)

We still can build newer Clang in pkgsrc on newer base with headers moved to /usr/include/v1/c++ and it will break.

[mgorny]

clang/lib/Driver/ToolChains/NetBSD.cpp

430 ↗

(On Diff #188079)

I don't understand. Why would it break? (besides the typo in the path)

[krytarowski]

clang/lib/Driver/ToolChains/NetBSD.cpp

430 ↗

(On Diff #188079)

clang will be installed into /usr/pkg/bin/clang and we want to reach the default headers in /usr/include/c++/v1 (in a setup without installing libc++ in pkgsrc)

[joerg]

I'm not in favor of this. It adds overhead for the system compiler and generally makes the logic more complicated. This seems to be another hack around the fact that the driver has no clear notion of "use system runtime" vs "use custom runtime".

[mgorny]

Shamelessly increased overhead by checking one more path as requested by Kamil.

[krytarowski]

This will make life much more easier now with this change.

cfe/trunk/lib/Driver/ToolChains/NetBSD.cpp

 #include "Arch/Mips.h"
 #include "Arch/Sparc.h"
 #include "CommonArgs.h"
 #include "clang/Driver/Compilation.h"
 #include "clang/Driver/Driver.h"
 #include "clang/Driver/Options.h"
 #include "clang/Driver/SanitizerArgs.h"
 #include "llvm/Option/ArgList.h"
+#include "llvm/Support/VirtualFileSystem.h"
 
 using namespace clang::driver;
 using namespace clang::driver::tools;
 using namespace clang::driver::toolchains;
 using namespace clang;
 using namespace llvm::opt;
 
 void netbsd::Assembler::ConstructJob(Compilation &C, const JobAction &JA,
default:
       break;
     }
   }
   return ToolChain::CST_Libstdcxx;
 }
 
 void NetBSD::addLibCxxIncludePaths(const llvm::opt::ArgList &DriverArgs,
                                    llvm::opt::ArgStringList &CC1Args) const {
-  addSystemInclude(DriverArgs, CC1Args,
-                   getDriver().SysRoot + "/usr/include/c++/");
+  const std::string Candidates[] = {
+    // directory relative to build tree
+    getDriver().Dir + "/../include/c++/v1",
+    // system install with full upstream path
+    getDriver().SysRoot + "/usr/include/c++/v1",
+    // system install from src
+    getDriver().SysRoot + "/usr/include/c++",
+  };
+
+  for (const auto &IncludePath : Candidates) {
+    if (!getVFS().exists(IncludePath + "/__config"))
+      continue;
+
+    // Use the first candidate that looks valid.
+    addSystemInclude(DriverArgs, CC1Args, IncludePath);
+    return;
+  }
 }
 
 void NetBSD::addLibStdCxxIncludePaths(const llvm::opt::ArgList &DriverArgs,
                                       llvm::opt::ArgStringList &CC1Args) const {
   addLibStdCXXIncludePaths(getDriver().SysRoot, "/usr/include/g++", "", "", "",
                            "", DriverArgs, CC1Args);
 }