Check whether Gentoo-specific configuration directory exists

[dmantipov]

Summary

Check whether /etc/env.d/gcc exists before trying to read from any
file from there. This saves a few OS calls on a non-Gentoo system.

[manojgupta]

clang/lib/Driver/ToolChains/Gnu.cpp

2538–2539

I think it should be D.SysRoot + GentooConfigDir. Otherwise, there is no way to test a Gentoo configuration tests on a non-Gentoo machine.

[mgorny]

clang/lib/Driver/ToolChains/Gnu.cpp

2538–2539

Probably correct, given that D.SysRoot is used below. However, this code is a bit above my pay grade and I was never sure whether it's doing right what it's supposed to do. I would really prefer if someone higher up reviewed this.

[dmantipov]

Add D.SysRoot as suggested during initial review.

[manojgupta]

Looks ok to me.

[MaskRay]

clang/lib/Driver/ToolChains/Gnu.cpp

2538

Thanks. This should reduce the number of stat syscalls.

clang/lib/Driver/ToolChains/Gnu.h

class GCCInstallationDetector {
 
     // 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;
 
+    // Gentoo-specific toolchain configurations are stored here.
+    const std::string GentooConfigDir = "/etc/env.d/gcc";
+
   public:
     explicit GCCInstallationDetector(const Driver &D) : IsValid(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; }
 

clang/lib/Driver/ToolChains/Gnu.cpp

for (auto &Suffix : Suffixes) {
     }
   }
 }
 
 bool Generic_GCC::GCCInstallationDetector::ScanGentooConfigs(
     const llvm::Triple &TargetTriple, const ArgList &Args,
     const SmallVectorImpl<StringRef> &CandidateTriples,
     const SmallVectorImpl<StringRef> &CandidateBiarchTriples) {
+  if (!D.getVFS().exists(D.SysRoot + GentooConfigDir))
+    return false;
+
   for (StringRef CandidateTriple : CandidateTriples) {
     if (ScanGentooGccConfig(TargetTriple, Args, CandidateTriple))
       return true;
   }
 
   for (StringRef CandidateTriple : CandidateBiarchTriples) {
     if (ScanGentooGccConfig(TargetTriple, Args, CandidateTriple, true))
       return true;
   }
   return false;
 }
 
 bool Generic_GCC::GCCInstallationDetector::ScanGentooGccConfig(
     const llvm::Triple &TargetTriple, const ArgList &Args,
     StringRef CandidateTriple, bool NeedsBiarchSuffix) {
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> File =
-      D.getVFS().getBufferForFile(D.SysRoot + "/etc/env.d/gcc/config-" +
+      D.getVFS().getBufferForFile(D.SysRoot + GentooConfigDir + "/config-" +
                                   CandidateTriple.str());
   if (File) {
     SmallVector<StringRef, 2> Lines;
     File.get()->getBuffer().split(Lines, "\n");
     for (StringRef Line : Lines) {
       Line = Line.trim();
       // CURRENT=triple-version
       if (!Line.consume_front("CURRENT="))
         continue;
       // Process the config file pointed to by CURRENT.
       llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> ConfigFile =
-          D.getVFS().getBufferForFile(D.SysRoot + "/etc/env.d/gcc/" +
+          D.getVFS().getBufferForFile(D.SysRoot + GentooConfigDir + "/" +
                                       Line.str());
       std::pair<StringRef, StringRef> ActiveVersion = Line.rsplit('-');
       // List of paths to scan for libraries.
       SmallVector<StringRef, 4> GentooScanPaths;
       // Scan the Config file to find installed GCC libraries path.
       // Typical content of the GCC config file:
       // LDPATH="/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.x:/usr/lib/gcc/
       // (continued from previous line) x86_64-pc-linux-gnu/4.9.x/32"