Index: lib/Driver/ToolChains.cpp
===================================================================
--- lib/Driver/ToolChains.cpp
+++ lib/Driver/ToolChains.cpp
@@ -1430,6 +1430,43 @@
     }
   }
 
+  // Try to respect gcc-config on Gentoo. However, do that only
+  // if --gcc-toolchain is not provided or equal to the Gentoo install
+  // in /usr. This avoids accidentally enforcing the system GCC version
+  // when using a custom toolchain.
+  if (GCCToolchainDir == "" || GCCToolchainDir == D.SysRoot + "/usr") {
+    for (StringRef CandidateTriple : CandidateTripleAliases) {
+      llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> File =
+          D.getVFS().getBufferForFile(D.SysRoot + "/etc/env.d/gcc/config-" +
+                                      CandidateTriple.str());
+      if (File) {
+        SmallVector<StringRef, 2> Lines;
+        File.get()->getBuffer().split(Lines, "\n");
+        for (StringRef Line : Lines) {
+          // CURRENT=triple-version
+          if (Line.consume_front("CURRENT=")) {
+            const std::pair<StringRef, StringRef> ActiveVersion =
+              Line.rsplit('-');
+            // Note: Strictly speaking, we should be reading
+            // /etc/env.d/gcc/${CURRENT} now. However, the file doesn't
+            // contain anything new or especially useful to us.
+            const std::string GentooPath = D.SysRoot + "/usr/lib/gcc/" +
+                                           ActiveVersion.first.str() + "/" +
+                                           ActiveVersion.second.str();
+            if (D.getVFS().exists(GentooPath + "/crtbegin.o")) {
+              Version = GCCVersion::Parse(ActiveVersion.second);
+              GCCInstallPath = GentooPath;
+              GCCParentLibPath = GentooPath + "/../../..";
+              GCCTriple.setTriple(ActiveVersion.first);
+              IsValid = true;
+              return;
+            }
+          }
+        }
+      }
+    }
+  }
+
   // Loop over the various components which exist and select the best GCC
   // installation available. GCC installs are ranked by version number.
   Version = GCCVersion::Parse("0.0.0");
Index: test/Driver/Inputs/gentoo_linux_gcc_multi_version_tree/etc/env.d/gcc/config-x86_64-pc-linux-gnu
===================================================================
--- /dev/null
+++ test/Driver/Inputs/gentoo_linux_gcc_multi_version_tree/etc/env.d/gcc/config-x86_64-pc-linux-gnu
@@ -0,0 +1 @@
+CURRENT=x86_64-pc-linux-gnu-4.9.3
Index: test/Driver/Inputs/gentoo_linux_gcc_multi_version_tree/etc/env.d/gcc/x86_64-pc-linux-gnu-4.9.3
===================================================================
--- /dev/null
+++ test/Driver/Inputs/gentoo_linux_gcc_multi_version_tree/etc/env.d/gcc/x86_64-pc-linux-gnu-4.9.3
@@ -0,0 +1,10 @@
+PATH="/usr/x86_64-pc-linux-gnu/gcc-bin/4.9.3"
+ROOTPATH="/usr/x86_64-pc-linux-gnu/gcc-bin/4.9.3"
+GCC_PATH="/usr/x86_64-pc-linux-gnu/gcc-bin/4.9.3"
+LDPATH="/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3:/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/32"
+MANPATH="/usr/share/gcc-data/x86_64-pc-linux-gnu/4.9.3/man"
+INFOPATH="/usr/share/gcc-data/x86_64-pc-linux-gnu/4.9.3/info"
+STDCXX_INCDIR="g++-v4"
+CTARGET="x86_64-pc-linux-gnu"
+GCC_SPECS=""
+MULTIOSDIRS="../lib64:../lib32"
Index: test/Driver/Inputs/gentoo_linux_gcc_multi_version_tree/etc/gentoo-release
===================================================================
--- /dev/null
+++ test/Driver/Inputs/gentoo_linux_gcc_multi_version_tree/etc/gentoo-release
@@ -0,0 +1 @@
+Gentoo Base System release 2.3
Index: test/Driver/linux-header-search.cpp
===================================================================
--- test/Driver/linux-header-search.cpp
+++ test/Driver/linux-header-search.cpp
@@ -301,6 +301,15 @@
 // CHECK-GENTOO-4-9-3: "-internal-externc-isystem" "[[SYSROOT]]/include"
 // CHECK-GENTOO-4-9-3: "-internal-externc-isystem" "[[SYSROOT]]/usr/include"
 //
+// Test support for Gentoo's gcc-config -- clang should prefer the older
+// (4.9.3) version over the newer (5.4.0) due to preference specified
+// in /etc/env.d/gcc/x86_64-pc-linux-gnu.
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
+// RUN:     -target x86_64-unknown-linux-gnu -stdlib=libstdc++ \
+// RUN:     --sysroot=%S/Inputs/gentoo_linux_gcc_multi_version_tree \
+// RUN:     --gcc-toolchain="" \
+// RUN:   | FileCheck --check-prefix=CHECK-GENTOO-4-9-3 %s
+//
 // Check header search on Debian 6 / MIPS64
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1 \
 // RUN:     -target mips64-unknown-linux-gnuabi64 -stdlib=libstdc++ \