diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -72,6 +72,13 @@
 -----------------------
 
 - -Wshadow now also checks for shadowed structured bindings
+- ``-B <prefix>`` (when ``<prefix>`` is a directory) was overloaded to additionally
+  detect GCC installations under ``<prefix>`` (``lib{,32,64}/gcc{,-cross}/$triple``).
+  This behavior was incompatible with GCC, caused interop issues with
+  ``--gcc-toolchain``, and was thus dropped. Specify ``--gcc-toolchain=<dir>``
+  instead. ``-B``'s other GCC-compatible semantics are preserved:
+  ``$prefix/$triple-$file`` and ``$prefix$file`` are searched for executables,
+  libraries, includes, and data files used by the compiler.
 
 Removed Compiler Flags
 -------------------------
diff --git a/clang/lib/Driver/ToolChains/Gnu.cpp b/clang/lib/Driver/ToolChains/Gnu.cpp
--- a/clang/lib/Driver/ToolChains/Gnu.cpp
+++ b/clang/lib/Driver/ToolChains/Gnu.cpp
@@ -1909,9 +1909,7 @@
                            CandidateBiarchTripleAliases);
 
   // Compute the set of prefixes for our search.
-  SmallVector<std::string, 8> Prefixes(D.PrefixDirs.begin(),
-                                       D.PrefixDirs.end());
-
+  SmallVector<std::string, 8> Prefixes;
   StringRef GCCToolchainDir = getGCCToolchainDir(Args, D.SysRoot);
   if (GCCToolchainDir != "") {
     if (GCCToolchainDir.back() == '/')
diff --git a/clang/test/Driver/android-ndk-standalone.cpp b/clang/test/Driver/android-ndk-standalone.cpp
--- a/clang/test/Driver/android-ndk-standalone.cpp
+++ b/clang/test/Driver/android-ndk-standalone.cpp
@@ -3,7 +3,7 @@
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm-linux-androideabi21 \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck  %s
 // CHECK: {{.*}}clang{{.*}}" "-cc1"
@@ -34,7 +34,7 @@
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm-linux-androideabi14 \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-14 %s
 // CHECK-14: "-L{{.*}}/sysroot/usr/lib/arm-linux-androideabi/14"
@@ -42,7 +42,7 @@
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm-linux-androideabi21 -stdlib=libstdc++ \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-STDCXX %s
 // CHECK-STDCXX: {{.*}}clang{{.*}}" "-cc1"
@@ -76,7 +76,7 @@
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target armv7a-none-linux-androideabi21 \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck  --check-prefix=CHECK-ARMV7 %s
 // CHECK-ARMV7: {{.*}}clang{{.*}}" "-cc1"
@@ -109,19 +109,19 @@
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm-linux-androideabi21 \
 // RUN:     -march=armv7 \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck  --check-prefix=CHECK-ARMV7 %s
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm-linux-androideabi21 \
 // RUN:     -march=armv7a \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck  --check-prefix=CHECK-ARMV7 %s
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm-linux-androideabi21 \
 // RUN:     -march=armv7-a \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck  --check-prefix=CHECK-ARMV7 %s
 //
@@ -129,7 +129,7 @@
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm-linux-androideabi21 \
 // RUN:     -mthumb \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck  --check-prefix=CHECK-THUMB %s
 // CHECK-THUMB: {{.*}}clang{{.*}}" "-cc1"
@@ -163,7 +163,7 @@
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm-linux-androideabi21 \
 // RUN:     -march=armv7-a -mthumb \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck  --check-prefix=CHECK-ARMV7THUMB %s
 // CHECK-ARMV7THUMB: {{.*}}clang{{.*}}" "-cc1"
@@ -195,7 +195,7 @@
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm-linux-androideabi21 \
 // RUN:     -march=armv7-a -mthumb \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:     -print-multi-lib \
 // RUN:   | FileCheck  --check-prefix=CHECK-ARM-MULTILIBS %s
@@ -209,13 +209,13 @@
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target armv7a-none-linux-androideabi21 \
 // RUN:     -mthumb \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck  --check-prefix=CHECK-ARMV7THUMB %s
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target aarch64-linux-android21 \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-AARCH64 %s
 // CHECK-AARCH64: {{.*}}clang{{.*}}" "-cc1"
@@ -231,7 +231,7 @@
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm64-linux-android21 \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-ARM64 %s
 // CHECK-ARM64: {{.*}}clang{{.*}}" "-cc1"
@@ -248,7 +248,7 @@
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target mipsel-linux-android21 \
 // RUN:     -mips32 \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-MIPS %s
 // CHECK-MIPS: {{.*}}clang{{.*}}" "-cc1"
@@ -263,7 +263,7 @@
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target i686-linux-android21 \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-I686 %s
 // CHECK-I686: {{.*}}clang{{.*}}" "-cc1"
@@ -279,7 +279,7 @@
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target x86_64-linux-android21 \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-X86_64 %s
 // CHECK-X86_64: {{.*}}clang{{.*}}" "-cc1"
diff --git a/clang/test/Driver/android-standalone.cpp b/clang/test/Driver/android-standalone.cpp
--- a/clang/test/Driver/android-standalone.cpp
+++ b/clang/test/Driver/android-standalone.cpp
@@ -3,7 +3,7 @@
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm-linux-androideabi -stdlib=libstdc++ \
-// RUN:     -B%S/Inputs/basic_android_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_tree/sysroot \
 // RUN:   | FileCheck  %s
 // CHECK: {{.*}}clang{{.*}}" "-cc1"
@@ -18,7 +18,7 @@
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target aarch64-linux-android -stdlib=libstdc++ \
-// RUN:     -B%S/Inputs/basic_android_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-AARCH64 %s
 // CHECK-AARCH64: {{.*}}clang{{.*}}" "-cc1"
@@ -33,7 +33,7 @@
 //
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target arm64-linux-android -stdlib=libstdc++ \
-// RUN:     -B%S/Inputs/basic_android_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-ARM64 %s
 // CHECK-ARM64: {{.*}}clang{{.*}}" "-cc1"
@@ -49,7 +49,7 @@
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target mipsel-linux-android \
 // RUN:     -mips32 -stdlib=libstdc++ \
-// RUN:     -B%S/Inputs/basic_android_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-MIPS %s
 // CHECK-MIPS: {{.*}}clang{{.*}}" "-cc1"
@@ -65,7 +65,7 @@
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target mipsel-linux-android \
 // RUN:     -march=mips32 -mips32r2 -stdlib=libstdc++ \
-// RUN:     -B%S/Inputs/basic_android_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-MIPSR2 %s
 // CHECK-MIPSR2: {{.*}}clang{{.*}}" "-cc1"
@@ -81,7 +81,7 @@
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>&1 \
 // RUN:     -target mipsel-linux-android \
 // RUN:     -mips32 -march=mips32r2 -stdlib=libstdc++ \
-// RUN:     -B%S/Inputs/basic_android_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_tree/sysroot \
 // RUN:   | FileCheck --check-prefix=CHECK-MIPSR2-A %s
 // CHECK-MIPSR2-A: {{.*}}clang{{.*}}" "-cc1"
diff --git a/clang/test/Driver/gcc-toolchain.cpp b/clang/test/Driver/gcc-toolchain.cpp
--- a/clang/test/Driver/gcc-toolchain.cpp
+++ b/clang/test/Driver/gcc-toolchain.cpp
@@ -29,3 +29,14 @@
 // CHECK: "{{[^"]*}}/usr/lib/i386-linux-gnu/gcc/i686-linux-gnu/4.5{{/|\\\\}}crtbegin.o"
 // CHECK: "-L[[TOOLCHAIN]]/usr/lib/i386-linux-gnu/gcc/i686-linux-gnu/4.5"
 // CHECK: "-L[[TOOLCHAIN]]/usr/lib/i386-linux-gnu/gcc/i686-linux-gnu/4.5/../../../.."
+
+/// Test we don't detect GCC installation under -B.
+// RUN: %clangxx -no-canonical-prefixes %s -### -o %t 2>&1 \
+// RUN:   --target=aarch64-suse-linux --gcc-toolchain=%S/Inputs/opensuse_42.2_aarch64_tree/usr | \
+// RUN:   FileCheck %s --check-prefix=AARCH64
+// RUN: %clangxx -no-canonical-prefixes %s -### -o %t 2>&1 \
+// RUN:   --target=aarch64-suse-linux -B%S/Inputs/opensuse_42.2_aarch64_tree/usr | \
+// RUN:   FileCheck %s --check-prefix=NO_AARCH64
+
+// AARCH64:        Inputs{{[^"]+}}aarch64-suse-linux/{{[^"]+}}crt1.o"
+// NO_AARCH64-NOT: Inputs{{[^"]+}}aarch64-suse-linux/{{[^"]+}}crt1.o"
diff --git a/clang/test/Driver/print-multi-directory.c b/clang/test/Driver/print-multi-directory.c
--- a/clang/test/Driver/print-multi-directory.c
+++ b/clang/test/Driver/print-multi-directory.c
@@ -19,7 +19,7 @@
 // RUN: %clang -no-canonical-prefixes %s -### -o %t.o 2>/dev/null \
 // RUN:     -target arm-linux-androideabi21 \
 // RUN:     -mthumb \
-// RUN:     -B%S/Inputs/basic_android_ndk_tree \
+// RUN:     --gcc-toolchain=%S/Inputs/basic_android_ndk_tree \
 // RUN:     --sysroot=%S/Inputs/basic_android_ndk_tree/sysroot \
 // RUN:     -print-multi-directory \
 // RUN:   | FileCheck --match-full-lines --check-prefix=CHECK-ARM-MULTILIBS %s