LFCS 2012: LLVM and Linux

LFCS 2012: LLVM and Linux

For what it's worth, I've found two severe issues in my attempts at experimenting with Clang at the company I work for. (Our production compiler for Linux is currently GCC 4.2.4)

Issue 1 is that it doesn't support OpenMP pragmas for parallelization, which is an absolute blocker to any possibility of our using it for production code. We are currently using it only for its compiler warnings as a static analyzer.

Issue 2 is that if one has installed a version of GCC in /usr/local, it seems to be impossible to build Clang such that by default it will use that GCC version's object files (crtbegin.o, etc.) and libstdc++ headers without actually hacking the portions of the Clang source code where these paths are defined. Perhaps this has improved since the version of Clang I'm at (SVN trunk from Sept. 30, 2011), but I experienced so much pain in getting this to work that I'm very reluctant to attempt to upgrade from this: it looks like all of the relevant Clang code has changed since then, which will cause serious merge conflicts with my local hacks.

At least as of that version of Clang, rather than support different GCC installation locations and default linker arguments with arguments to the configure script, the authors have actually defined the header and library search paths by putting them into the C++ source code, with various if statements based on what Linux distribution and version thereof, as well as what GCC version, that the program appears to be being compiled on.

This makes things easier for distributors, I suppose, although it means Clang will forever be playing catch-up with new versions. For anyone else building Clang, code like the following snippets is so incredibly ill-thought-out that it gives me misgivings about the rest of the project.

  static const char* GccVersions[] = {"4.6.1", "4.6.0", "4.6",
                                      "4.5.3", "4.5.2", "4.5.1", "4.5",
                                      "4.4.6", "4.4.5", "4.4.4", "4.4.3", "4.4",
                                      "4.3.4", "4.3.3", "4.3.2", "4.3",
                                      "4.2.4", "4.2.3", "4.2.2", "4.2.1",
                                      "4.2", "4.1.1"};

...

        if (GccTriple == "i386-linux-gnu") {
          // Ubuntu 11.04 uses an unusual path.
          std::string t4 =
              std::string(*it + "lib/i386-linux-gnu/gcc/i686-linux-gnu/") +
              GccVersions[i];
          if (!llvm::sys::fs::exists(t4 + "/crtbegin.o", Exists) && Exists)
            return t4;
        }

...

    else if (!llvm::sys::fs::exists("/usr/lib/gcc/x86_64-redhat-linux",
             Exists) && Exists)
      GccTriple = "x86_64-redhat-linux";
    else if (!llvm::sys::fs::exists("/usr/lib64/gcc/x86_64-suse-linux",
             Exists) && Exists)
      GccTriple = "x86_64-suse-linux";

...

  LinuxDistro Distro = DetectLinuxDistro(Arch);

  if (IsOpenSuse(Distro) || IsUbuntu(Distro)) {
    ExtraOpts.push_back("-z");
    ExtraOpts.push_back("relro");
  }

...

  if (IsDebian(Distro) || IsOpenSuse(Distro) || Distro == UbuntuLucid ||
      Distro == UbuntuJaunty || Distro == UbuntuKarmic)
    ExtraOpts.push_back("--hash-style=both");

[All excerpts above are from lib/Driver/ToolChains.cpp at SVN rev 140874]