Clang: GCCInstallationDetector follows --target value first

[wzssyqa]

Summary

If we have --target option, we should detect GCC installation
with this triple first.

So, If we call clang with --target=i386-linux-gnu -m64, we will try
the i386-linux-gnu env with multilib, instead of try x86_64-linux-gnu
first.

[wzssyqa]

I think that this is a step forward to drop *MIPSELTriples[].

[MaskRay]

-target has been deprecated since 3.4. Please don't make new uses as I have pointed out in other reviews.
I need to do more analysis whether this can be simplified.

[MaskRay]

Thank you for the patch. I think it would actually be nice that we probe the m32 multilib of x86_64-unknown-linux-gnu multilib first for --target=x86_64-unknown-linux-gnu -m32.

Unfortunately, due to various setArch code in computeTargetTriple (e.g. D54214), this change may change what lib they use.
And this well-intended direction is (currently) not feasible without adding more computeTargetTriple like code to Gnu.cpp.
Hopefully that your user can live with the current deficiency.

[wzssyqa]

In D158509#4611696, @MaskRay wrote:

Thank you for the patch. I think it would actually be nice that we probe the m32 multilib of x86_64-unknown-linux-gnu multilib first for --target=x86_64-unknown-linux-gnu -m32.

Unfortunately, due to various setArch code in computeTargetTriple (e.g. D54214), this change may change what lib they use.

In fact, it won't affect, since the internal triple is used after the arg of --target is used.

And this well-intended direction is (currently) not feasible without adding more computeTargetTriple like code to Gnu.cpp.
Hopefully that your user can live with the current deficiency.

computeTargetTriple should continue be there.

[wzssyqa]

ping

[wzssyqa]

For this case (--target is given), maybe we have 2 choice:

Group A, which is quite straight forward, while may broken some use case.

1. --target=T is given, and no multilib option is given, and we can find the LIBPATH with the same name of T. Let's just use it.
2. --target=T is given, and no multilib option is given, and we can *NOT* find the LIBPATH with the same name of T. Let's just trigger an error.
3. --target=T is given, and multilib option IS given, and we can find the LIBPATH with the same name of T. If T has multilib support, let's use T. If T has no multilib support, let's trigger error.
4. --target=T is given, and multilib option IS given, and we can *NOT* find the LIBPATH with the same name of T. Let's just trigger an error.

---

Group B:

1. --target=T is given, and no multilib option is given, and we can find the LIBPATH with the same name of T. Let's just use it.
2. --target=T is given, and no multilib option is given, and we can *NOT* find the LIBPATH with the same name of T. We will need try to alter triple, aka "x86_64-linux-gnu" -> "i686-linux-gnu -m64"
3. --target=T is given, and multilib option IS given, and we can find the LIBPATH with the same name of T. If T has multilib support, let's use T. If T has no multilib support, we will need try to alter triple.
4. --target=T is given, and multilib option IS given, and we can *NOT* find the LIBPATH with the same name of T. We will need try to alter triple.

---

I prefer Group A.

clang/lib/Driver/ToolChains/Gnu.cpp

void Generic_GCC::GCCInstallationDetector::init(
   SmallVector<StringRef, 16> CandidateBiarchTripleAliases;
   CollectLibDirsAndTriples(TargetTriple, BiarchVariantTriple, CandidateLibDirs,
                            CandidateTripleAliases, CandidateBiarchLibDirs,
                            CandidateBiarchTripleAliases);
 
   TripleNoVendor = TargetTriple.getArchName().str() + "-" +
                    TargetTriple.getOSAndEnvironmentName().str();
   StringRef TripleNoVendorRef(TripleNoVendor);
+  StringRef TripleArg;
 
   // If --gcc-install-dir= is specified, skip filesystem detection.
   if (const Arg *A =
           Args.getLastArg(clang::driver::options::OPT_gcc_install_dir_EQ);
       A && A->getValue()[0]) {
     StringRef InstallDir = A->getValue();
     if (!ScanGCCForMultilibs(TargetTriple, Args, InstallDir, false)) {
       D.Diag(diag::err_drv_invalid_gcc_install_dir) << InstallDir;
GentooTestTriples.append(ExtraTripleAliases.begin(),
                              ExtraTripleAliases.end());
     GentooTestTriples.append(CandidateTripleAliases.begin(),
                              CandidateTripleAliases.end());
     if (ScanGentooConfigs(TargetTriple, Args, GentooTestTriples,
                           CandidateBiarchTripleAliases))
       return;
   }
 
+  if (const Arg *A = Args.getLastArg(options::OPT_target))
+    TripleArg = A->getValue();
+
   // Loop over the various components which exist and select the best GCC
   // installation available. GCC installs are ranked by version number.
   const GCCVersion VersionZero = GCCVersion::Parse("0.0.0");
   Version = VersionZero;
   for (const std::string &Prefix : Prefixes) {
     auto &VFS = D.getVFS();
     if (!VFS.exists(Prefix))
       continue;
     for (StringRef Suffix : CandidateLibDirs) {
       const std::string LibDir = concat(Prefix, Suffix);
       if (!VFS.exists(LibDir))
         continue;
       // Maybe filter out <libdir>/gcc and <libdir>/gcc-cross.
       bool GCCDirExists = VFS.exists(LibDir + "/gcc");
       bool GCCCrossDirExists = VFS.exists(LibDir + "/gcc-cross");
+      // Try to match the exact target triple passed by -target option.
+      if (TripleArg.size() > 1)
+        ScanLibDirForGCCTriple(TargetTriple, Args, LibDir, TripleArg, false,
+                               GCCDirExists, GCCCrossDirExists);
       // Try to match the exact target triple first.
       ScanLibDirForGCCTriple(TargetTriple, Args, LibDir, TargetTriple.str(),
                              false, GCCDirExists, GCCCrossDirExists);
       // If vendor is unknown, let's try triple without vendor.
       if (TargetTriple.getVendor() == llvm::Triple::UnknownVendor)
         ScanLibDirForGCCTriple(TargetTriple, Args, LibDir, TripleNoVendorRef,
                                false, GCCDirExists, GCCCrossDirExists);
       // Try rest of possible triples.