Driver: Prefer gcc toolchains with libgcc_s.so when not static linking libgcc

[tstellar]

Summary

Fedora ships cross-compilers on all platforms, so a user could end up
with a gcc x86_64 cross-compiler installed on an x86_64 system. clang
maintains a list of supported triples for each target and when all
else is equal will prefer toolchains with triples that appear earlier
in the list.

The cross-compiler triple on Fedora is x86_64-linux-gnu and this comes
before the Fedora system compiler's triple: x86_64-redhat-linux in
the triples list, so the cross compiler is always preferred. This
is a problem, because the cross compiler is missing libraries, like
libgcc_s.so, that clang expects to be there so linker invocations
will fail.

This patch fixes this by checking for the existence of libgcc_s.so
when it is required and taking that into account when selecting a
toolchain.

[jdenny]

This patch handles libgcc_s.so, but what about libatomic.so? It's also not provided by x86_64-linux-gnu, at least on CentOS Linux release 7.7.1908.

lib/Driver/ToolChains/Gnu.h

public:
   /// This is a class to find a viable GCC installation for Clang to
   /// use.
   ///
   /// This class tries to find a GCC installation on the system, and report
   /// information about it. It starts from the host information provided to the
   /// Driver, and has logic for fuzzing that where appropriate.
   class GCCInstallationDetector {
     bool IsValid;
+    bool HasLibGccShared;
     llvm::Triple GCCTriple;
     const Driver &D;
 
     // FIXME: These might be better as path objects.
     std::string GCCInstallPath;
     std::string GCCParentLibPath;
 
     /// The primary multilib appropriate for the given flags.
     Multilib SelectedMultilib;
     /// On Biarch systems, this corresponds to the default multilib when
     /// targeting the non-default multilib. Otherwise, it is empty.
     llvm::Optional<Multilib> BiarchSibling;
 
     GCCVersion Version;
 
     // We retain the list of install paths that were considered and rejected in
     // order to print out detailed information in verbose mode.
     std::set<std::string> CandidateGCCInstallPaths;
 
     /// The set of multilibs that the detected installation supports.
     MultilibSet Multilibs;
 
   public:
-    explicit GCCInstallationDetector(const Driver &D) : IsValid(false), D(D) {}
+    explicit GCCInstallationDetector(const Driver &D)
+        : IsValid(false), HasLibGccShared(false), D(D) {}
     void init(const llvm::Triple &TargetTriple, const llvm::opt::ArgList &Args,
               ArrayRef<std::string> ExtraTripleAliases = None);
 
     /// Check whether we detected a valid GCC install.
     bool isValid() const { return IsValid; }
 
     /// Get the GCC triple for the detected install.
     const llvm::Triple &getTriple() const { return GCCTriple; }

lib/Driver/ToolChains/Gnu.cpp

bool Active;
       // Deal with cases (on Ubuntu) where the system architecture could be i386
       // but the GCC target architecture could be (say) i686.
       // FIXME: It may be worthwhile to generalize this and look for a second
       // triple.
       {"i386-linux-gnu/gcc/" + CandidateTriple.str(), "../../..",
        (TargetArch == llvm::Triple::x86 &&
         TargetTriple.getOS() != llvm::Triple::Solaris)}};
 
+  bool NeedLibgccShared = !Args.hasArg(options::OPT_static_libgcc) &&
+                          !Args.hasArg(options::OPT_static);
   for (auto &Suffix : Suffixes) {
     if (!Suffix.Active)
       continue;
 
     StringRef LibSuffix = Suffix.LibSuffix;
     std::error_code EC;
     for (vfs::directory_iterator
              LI = D.getVFS().dir_begin(LibDir + "/" + LibSuffix, EC),
              LE;
          !EC && LI != LE; LI = LI.increment(EC)) {
       StringRef VersionText = llvm::sys::path::filename(LI->path());
       GCCVersion CandidateVersion = GCCVersion::Parse(VersionText);
       if (CandidateVersion.Major != -1) // Filter obviously bad entries.
         if (!CandidateGCCInstallPaths.insert(LI->path()).second)
           continue; // Saw this path before; no need to look at it again.
       if (CandidateVersion.isOlderThan(4, 1, 1))
         continue;
-      if (CandidateVersion <= Version)
+
+      bool CandidateHasLibGccShared = false;
+      if (CandidateVersion <= Version) {
+        if (NeedLibgccShared && !HasLibGccShared) {
+          CandidateHasLibGccShared =
+                D.getVFS().exists(LI->path() + "/libgcc_s.so");
+
+        }
+        if (HasLibGccShared || !CandidateHasLibGccShared)
           continue;
+      }
 
       if (!ScanGCCForMultilibs(TargetTriple, Args, LI->path(),
                                NeedsBiarchSuffix))
         continue;
 
       Version = CandidateVersion;
       GCCTriple.setTriple(CandidateTriple);
       // FIXME: We hack together the directory name here instead of
       // using LI to ensure stable path separators across Windows and
       // Linux.
       GCCInstallPath = (LibDir + "/" + LibSuffix + "/" + VersionText).str();
       GCCParentLibPath = (GCCInstallPath + "/../" + Suffix.ReversePath).str();
       IsValid = true;
+      HasLibGccShared = CandidateHasLibGccShared;
     }
   }
 }
 
 bool Generic_GCC::GCCInstallationDetector::ScanGentooConfigs(
     const llvm::Triple &TargetTriple, const ArgList &Args,
     const SmallVectorImpl<StringRef> &CandidateTriples,
     const SmallVectorImpl<StringRef> &CandidateBiarchTriples) {

test/Driver/linux-ld.c

 // CHECK-FEDORA-21-AARCH64: "{{.*}}/usr/lib/gcc/aarch64-redhat-linux/4.9.0/../../../../lib64{{/|\\\\}}crt1.o"
 // CHECK-FEDORA-21-AARCH64: "{{.*}}/usr/lib/gcc/aarch64-redhat-linux/4.9.0/../../../../lib64{{/|\\\\}}crti.o"
 // CHECK-FEDORA-21-AARCH64: "{{.*}}/usr/lib/gcc/aarch64-redhat-linux/4.9.0{{/|\\\\}}crtbegin.o"
 // CHECK-FEDORA-21-AARCH64: "-L[[SYSROOT]]/usr/lib/gcc/aarch64-redhat-linux/4.9.0"
 // CHECK-FEDORA-21-AARCH64: "-L[[SYSROOT]]/usr/lib/gcc/aarch64-redhat-linux/4.9.0/../../../../lib64"
 // CHECK-FEDORA-21-AARCH64: "{{.*}}/usr/lib/gcc/aarch64-redhat-linux/4.9.0{{/|\\\\}}crtend.o"
 // CHECK-FEDORA-21-AARCH64: "{{.*}}/usr/lib/gcc/aarch64-redhat-linux/4.9.0/../../../../lib64{{/|\\\\}}crtn.o"
 //
+// Check that clang does not select the cross compiler by default on Fedora 28.
+//
+// RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
+// RUN:     --target=x86_64-unknown-linux-gnu \
+// RUN:     --gcc-toolchain="" \
+// RUN:     --sysroot=%S/Inputs/fedora_28_tree \
+// RUN:   | FileCheck --check-prefix=CHECK-FEDORA-28-X86_64 %s
+//
+// CHECK-FEDORA-28-X86_64: "{{.*}}ld{{(.exe)?}}" "--sysroot=[[SYSROOT:[^"]+]]"
+// CHECK-FEDORA-28-X86_64: "[[SYSROOT]]/usr/lib/gcc/x86_64-redhat-linux/7/crtbegin.o"
+// CHECK-FEDORA-28-X86_64: "-L[[SYSROOT]]/usr/lib/gcc/x86_64-redhat-linux/7"
+//
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     --target=arm-unknown-linux-gnueabi \
 // RUN:     --gcc-toolchain="" \
 // RUN:     --sysroot=%S/Inputs/ubuntu_12.04_LTS_multiarch_tree \
 // RUN:   | FileCheck --check-prefix=CHECK-UBUNTU-12-04-ARM %s
 // CHECK-UBUNTU-12-04-ARM: "{{.*}}ld{{(.exe)?}}" "--sysroot=[[SYSROOT:[^"]+]]"
 // CHECK-UBUNTU-12-04-ARM: "{{.*}}/usr/lib/gcc/arm-linux-gnueabi/4.6.1/../../../arm-linux-gnueabi{{/|\\\\}}crt1.o"
 // CHECK-UBUNTU-12-04-ARM: "{{.*}}/usr/lib/gcc/arm-linux-gnueabi/4.6.1/../../../arm-linux-gnueabi{{/|\\\\}}crti.o"