Fuse the two halves of the GCC installation detection. This is

significantly cleaner (IMO) and more principled. We now walk down each
layer of the directory hierarchy searching for the GCC install. This
change does in fact introduce a significant behavior change in theory,
although in practice I don't know of any distro that will be impacted by
it negatively, and Debian may (untested) get slightly better through it.

Specifically, the logic now looks exhaustively for patterns such as:

  /usr/lib/<triple>/gcc/<triple>

Previously, this would only be selected if there was *also*
a '/usr/lib/gcc/<triple>' directory, or if '<triple>' were the excat
DefaultHostTriple in the driver.

There is a 4-deep nested loop here, but it doesn't do terribly many
filesystem operations, as we skip at each layer of that layer's
directory doesn't exist.

There remains a significant FIXME in this logic: it would be much better
to first build up a set of candidate components for each of the four
layers with a bottom-up pruning such as this, but then select the final
installation using a top-down algorithm in order to find the newest GCC
installation available, regardless of which particular path leads to it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@141071 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Driver/ToolChains.cpp

@@ -1536,17 +1536,19 @@ class GCCInstallationDetector {
     }
 
     llvm::Triple::ArchType HostArch = llvm::Triple(GccTriple).getArch();
-    // The directories which may contain x86-64 triple GCC installations.
+    // The library directories which may contain GCC installations.
     SmallVector<StringRef, 2> CandidateLibDirs;
-    // The compatible GCC triples for an x86-64 Linux Clang.
+    // The compatible GCC triples for this particular architecture.
     SmallVector<StringRef, 10> CandidateTriples;
     if (HostArch == llvm::Triple::arm || HostArch == llvm::Triple::thumb) {
-      static const char *const LibDirs[] = { "/usr/lib/gcc" };
+      static const char *const LibDirs[] = { "/lib/gcc" };
       static const char *const Triples[] = { "arm-linux-gnueabi" };
       CandidateLibDirs.append(LibDirs, LibDirs + llvm::array_lengthof(LibDirs));
       CandidateTriples.append(Triples, Triples + llvm::array_lengthof(Triples));
     } else if (HostArch == llvm::Triple::x86_64) {
-      static const char *const LibDirs[] = { "/usr/lib64/gcc", "/usr/lib/gcc" };
+      static const char *const LibDirs[] = {
+        "/lib64/gcc", "/lib/gcc", "/lib64", "/lib"
+      };
       static const char *const Triples[] = {
         "x86_64-linux-gnu",
         "x86_64-unknown-linux-gnu",
@@ -1561,7 +1563,9 @@ class GCCInstallationDetector {
       CandidateLibDirs.append(LibDirs, LibDirs + llvm::array_lengthof(LibDirs));
       CandidateTriples.append(Triples, Triples + llvm::array_lengthof(Triples));
     } else if (HostArch == llvm::Triple::x86) {
-      static const char *const LibDirs[] = { "/usr/lib32/gcc", "/usr/lib/gcc" };
+      static const char *const LibDirs[] = {
+        "/lib32/gcc", "/lib/gcc", "/lib32", "/lib"
+      };
       static const char *const Triples[] = {
         "i686-linux-gnu",
         "i386-linux-gnu",
@@ -1574,35 +1578,27 @@ class GCCInstallationDetector {
       CandidateLibDirs.append(LibDirs, LibDirs + llvm::array_lengthof(LibDirs));
       CandidateTriples.append(Triples, Triples + llvm::array_lengthof(Triples));
     } else if (HostArch == llvm::Triple::ppc) {
-      static const char *const LibDirs[] = { "/usr/lib32/gcc", "/usr/lib/gcc" };
+      static const char *const LibDirs[] = {
+        "/lib32/gcc", "/lib/gcc", "/lib32", "/lib"
+      };
       static const char *const Triples[] = {
         "powerpc-linux-gnu",
         "powerpc-unknown-linux-gnu"
       };
       CandidateLibDirs.append(LibDirs, LibDirs + llvm::array_lengthof(LibDirs));
       CandidateTriples.append(Triples, Triples + llvm::array_lengthof(Triples));
     } else if (HostArch == llvm::Triple::ppc64) {
-      static const char *const LibDirs[] = { "/usr/lib64/gcc", "/usr/lib/gcc" };
+      static const char *const LibDirs[] = {
+        "/lib64/gcc", "/lib/gcc", "/lib64", "/lib"
+      };
       static const char *const Triples[] = { "powerpc64-unknown-linux-gnu" };
       CandidateLibDirs.append(LibDirs, LibDirs + llvm::array_lengthof(LibDirs));
       CandidateTriples.append(Triples, Triples + llvm::array_lengthof(Triples));
     }
 
-    // First finds the 'prefix' which exists beneath the system root. Second,
-    // finds the first triple which exists beneath that prefix.
-    StringRef DetectedGccPrefix, DetectedGccTriple;
-    for (unsigned i = 0, ie = CandidateLibDirs.size(); i < ie; ++i) {
-      if (!PathExists(CandidateLibDirs[i]))
-        continue;
-
-      for (unsigned j = 0, je = CandidateTriples.size(); j < je; ++j) {
-        if (PathExists(CandidateLibDirs[i].str() + "/" +
-                       CandidateTriples[j].str())) {
-          DetectedGccPrefix = CandidateLibDirs[i];
-          DetectedGccTriple = CandidateTriples[j];
-        }
-      }
-    }
+    // Always include the default host triple as the final fallback if no
+    // specific triple is detected.
+    CandidateTriples.push_back(D.DefaultHostTriple);
 
     static const char* GccVersions[] = {
       "4.6.1", "4.6.0", "4.6",
@@ -1611,40 +1607,64 @@ class GCCInstallationDetector {
       "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"};
-    SmallVector<std::string, 8> Paths(D.PrefixDirs.begin(),
-                                      D.PrefixDirs.end());
-    Paths.push_back(D.SysRoot + "/usr/");
-    const std::string Triples[] = {DetectedGccTriple.str(), D.DefaultHostTriple};
-    IsValid = true;  // In case we're able to find a GCC install.
-    for (SmallVector<std::string, 8>::const_iterator I = Paths.begin(),
-                                                     E = Paths.end();
-         I != E; ++I) {
-      for (unsigned i = 0; i < sizeof(GccVersions)/sizeof(char*); ++i) {
-        for (unsigned j = 0; j < sizeof(Triples)/sizeof(Triples[0]); ++j) {
-          GccTriple = Triples[j];
-          std::string Suffix = Triples[j] + "/" + GccVersions[i];
-          GccInstallPath = *I + "lib/gcc/" + Suffix;
-          GccParentLibPath = GccInstallPath + "/../../..";
-          if (PathExists(GccInstallPath + "/crtbegin.o"))
-            return;
-          GccInstallPath = *I + "lib64/gcc/" + Suffix;
-          GccParentLibPath = GccInstallPath + "/../../..";
-          if (PathExists(GccInstallPath + "/crtbegin.o"))
-            return;
-          GccInstallPath = *I + "lib/" + GccTriple + "/gcc/" + Suffix;
-          GccParentLibPath = GccInstallPath + "/../../../..";
-          if (PathExists(GccInstallPath + "/crtbegin.o"))
-            return;
-
-          if (GccTriple != "i386-linux-gnu")
+
+    // Set this as valid so we can early exit from the loops if we find
+    // a viable installation.
+    IsValid = true;
+
+    SmallVector<std::string, 8> Prefixes(D.PrefixDirs.begin(),
+                                         D.PrefixDirs.end());
+    Prefixes.push_back(D.SysRoot + "/usr");
+    // FIXME: This entire nested loop structure is broken. The structure is
+    // well suited to quickly pruning impossible (non-existent) sub-trees, but
+    // doesn't find all viable GCC installations and choose the best one. We
+    // should invert this, and prune the impossible subtrees bottom-up, but
+    // then select the best GCC installation top-down so that we prefer the
+    // newest GCC, the most accurate triple, and only at the end select the lib
+    // and prefix directories containing that installation.
+    for (SmallVectorImpl<std::string>::const_iterator PI = Prefixes.begin(),
+                                                      PE = Prefixes.end();
+         PI != PE; ++PI) {
+      if (!PathExists(*PI))
+        continue;
+
+      // First finds the 'prefix' which exists beneath the system root. Second,
+      // finds the first triple which exists beneath that prefix.
+      StringRef DetectedGccPrefix, DetectedGccTriple;
+      for (unsigned i = 0, ie = CandidateLibDirs.size(); i < ie; ++i) {
+        const std::string LibDir = *PI + CandidateLibDirs[i].str();
+        if (!PathExists(LibDir))
+          continue;
+
+        for (unsigned j = 0, je = CandidateTriples.size(); j < je; ++j) {
+          GccTriple = CandidateTriples[j];
+          const std::string TripleDir = LibDir + "/" + GccTriple;
+          if (!PathExists(TripleDir))
             continue;
 
-          // Ubuntu 11.04 uses an unusual path.
-          GccInstallPath = *I + "lib/i386-linux-gnu/gcc/i686-linux-gnu/" +
-            GccVersions[i];
-          GccParentLibPath = GccInstallPath + "/../../../..";
-          if (PathExists(GccInstallPath + "/crtbegin.o"))
-            return;
+          for (unsigned k = 0; k < llvm::array_lengthof(GccVersions); ++k) {
+            GccInstallPath = TripleDir + "/" + GccVersions[k];
+            GccParentLibPath = GccInstallPath + "/../../..";
+            if (PathExists(GccInstallPath + "/crtbegin.o"))
+              return;
+
+            // Try an install directory with an extra triple in it.
+            GccInstallPath =
+              TripleDir + "/gcc/" + GccTriple + "/" + GccVersions[k];
+            GccParentLibPath = GccInstallPath + "/../../../..";
+            if (PathExists(GccInstallPath + "/crtbegin.o"))
+              return;
+
+            if (GccTriple != "i386-linux-gnu")
+              continue;
+
+            // Ubuntu 11.04 uses an unusual path.
+            GccInstallPath =
+              TripleDir + "/gcc/i686-linux-gnu/" + GccVersions[k];
+            GccParentLibPath = GccInstallPath + "/../../../..";
+            if (PathExists(GccInstallPath + "/crtbegin.o"))
+              return;
+          }
         }
       }
     }