Index: cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
===================================================================
--- cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
+++ cfe/trunk/include/clang/Basic/DiagnosticDriverKinds.td
@@ -26,6 +26,9 @@
 def err_drv_no_cuda_installation : Error<
   "cannot find CUDA installation.  Provide its path via --cuda-path, or pass "
   "-nocudainc to build without CUDA includes.">;
+def err_drv_no_cuda_libdevice : Error<
+  "cannot find libdevice for %0. Provide path to different CUDA installation "
+  "via --cuda-path, or pass -nocudalib to build without linking with libdevice.">;
 def err_drv_cuda_version_too_low : Error<
   "GPU arch %1 requires CUDA version at least %3, but installation at %0 is %2. "
   "Use --cuda-path to specify a different CUDA install, or pass "
Index: cfe/trunk/lib/Driver/ToolChains.cpp
===================================================================
--- cfe/trunk/lib/Driver/ToolChains.cpp
+++ cfe/trunk/lib/Driver/ToolChains.cpp
@@ -1791,22 +1791,32 @@
           LibDeviceName.size(), FileName.find('.', LibDeviceName.size()));
       LibDeviceMap[GpuArch] = FilePath.str();
       // Insert map entries for specifc devices with this compute capability.
+      // NVCC's choice of libdevice library version is rather peculiar:
+      // http://docs.nvidia.com/cuda/libdevice-users-guide/basic-usage.html#version-selection
+      // TODO: this will need to be updated once CUDA-8 is released.
       if (GpuArch == "compute_20") {
         LibDeviceMap["sm_20"] = FilePath;
         LibDeviceMap["sm_21"] = FilePath;
+        LibDeviceMap["sm_32"] = FilePath;
       } else if (GpuArch == "compute_30") {
         LibDeviceMap["sm_30"] = FilePath;
-        LibDeviceMap["sm_32"] = FilePath;
-      } else if (GpuArch == "compute_35") {
-        LibDeviceMap["sm_35"] = FilePath;
-        LibDeviceMap["sm_37"] = FilePath;
-      } else if (GpuArch == "compute_50") {
+        // compute_30 is the fallback libdevice variant for sm_30+,
+        // unless CUDA specifies different version for specific GPU
+        // arch.
         LibDeviceMap["sm_50"] = FilePath;
         LibDeviceMap["sm_52"] = FilePath;
         LibDeviceMap["sm_53"] = FilePath;
+        // sm_6? are currently all aliases for sm_53 in LLVM and
+        // should use compute_30.
         LibDeviceMap["sm_60"] = FilePath;
         LibDeviceMap["sm_61"] = FilePath;
         LibDeviceMap["sm_62"] = FilePath;
+      } else if (GpuArch == "compute_35") {
+        LibDeviceMap["sm_35"] = FilePath;
+        LibDeviceMap["sm_37"] = FilePath;
+      } else if (GpuArch == "compute_50") {
+        // NVCC does not use compute_50 libdevice at all at the moment.
+        // The version that's shipped with CUDA-7.5 is a copy of compute_30.
       }
     }
 
@@ -4759,18 +4769,23 @@
   if (DriverArgs.hasArg(options::OPT_nocudalib))
     return;
 
-  std::string LibDeviceFile = CudaInstallation.getLibDeviceFile(
-      DriverArgs.getLastArgValue(options::OPT_march_EQ));
-  if (!LibDeviceFile.empty()) {
-    CC1Args.push_back("-mlink-cuda-bitcode");
-    CC1Args.push_back(DriverArgs.MakeArgString(LibDeviceFile));
-
-    // Libdevice in CUDA-7.0 requires PTX version that's more recent
-    // than LLVM defaults to. Use PTX4.2 which is the PTX version that
-    // came with CUDA-7.0.
-    CC1Args.push_back("-target-feature");
-    CC1Args.push_back("+ptx42");
+  StringRef GpuArch = DriverArgs.getLastArgValue(options::OPT_march_EQ);
+  assert(!GpuArch.empty() && "Must have an explicit GPU arch.");
+  std::string LibDeviceFile = CudaInstallation.getLibDeviceFile(GpuArch);
+
+  if (LibDeviceFile.empty()) {
+    getDriver().Diag(diag::err_drv_no_cuda_libdevice) << GpuArch;
+    return;
   }
+
+  CC1Args.push_back("-mlink-cuda-bitcode");
+  CC1Args.push_back(DriverArgs.MakeArgString(LibDeviceFile));
+
+  // Libdevice in CUDA-7.0 requires PTX version that's more recent
+  // than LLVM defaults to. Use PTX4.2 which is the PTX version that
+  // came with CUDA-7.0.
+  CC1Args.push_back("-target-feature");
+  CC1Args.push_back("+ptx42");
 }
 
 void CudaToolChain::AddCudaIncludeArgs(const ArgList &DriverArgs,
Index: cfe/trunk/test/Driver/cuda-detect.cu
===================================================================
--- cfe/trunk/test/Driver/cuda-detect.cu
+++ cfe/trunk/test/Driver/cuda-detect.cu
@@ -10,15 +10,41 @@
 // RUN: %clang -v --target=i386-unknown-linux \
 // RUN:   --cuda-path=%S/Inputs/CUDA/usr/local/cuda 2>&1 | FileCheck %s
 
-// Make sure we map libdevice bitcode files to proper GPUs.
+// Make sure we map libdevice bitcode files to proper GPUs. These
+// tests use Inputs/CUDA_80 which has full set of libdevice files.
+// However, libdevice mapping only matches CUDA-7.x at the moment.
+// sm_2x, sm_32 -> compute_20
 // RUN: %clang -### -v --target=i386-unknown-linux --cuda-gpu-arch=sm_21 \
-// RUN:   --cuda-path=%S/Inputs/CUDA/usr/local/cuda %s 2>&1 \
+// RUN:   --cuda-path=%S/Inputs/CUDA_80/usr/local/cuda %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix COMMON \
+// RUN:     -check-prefix LIBDEVICE -check-prefix LIBDEVICE20
+// RUN: %clang -### -v --target=i386-unknown-linux --cuda-gpu-arch=sm_32 \
+// RUN:   --cuda-path=%S/Inputs/CUDA_80/usr/local/cuda %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix COMMON \
+// RUN:     -check-prefix LIBDEVICE -check-prefix LIBDEVICE20
+// sm_30, sm_5x and sm_6x map to compute_30
+// RUN: %clang -### -v --target=i386-unknown-linux --cuda-gpu-arch=sm_30 \
+// RUN:   --cuda-path=%S/Inputs/CUDA_80/usr/local/cuda %s 2>&1 \
 // RUN:   | FileCheck %s -check-prefix COMMON \
-// RUN:     -check-prefix LIBDEVICE -check-prefix LIBDEVICE21
+// RUN:     -check-prefix LIBDEVICE -check-prefix LIBDEVICE30
+// RUN: %clang -### -v --target=i386-unknown-linux --cuda-gpu-arch=sm_50 \
+// RUN:   --cuda-path=%S/Inputs/CUDA_80/usr/local/cuda %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix COMMON \
+// RUN:     -check-prefix LIBDEVICE -check-prefix LIBDEVICE30
+// RUN: %clang -### -v --target=i386-unknown-linux --cuda-gpu-arch=sm_60 \
+// RUN:   --cuda-path=%S/Inputs/CUDA_80/usr/local/cuda %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix COMMON \
+// RUN:     -check-prefix LIBDEVICE -check-prefix LIBDEVICE30
+// sm_35 and sm_37 -> compute_35
 // RUN: %clang -### -v --target=i386-unknown-linux --cuda-gpu-arch=sm_35 \
-// RUN:   --cuda-path=%S/Inputs/CUDA/usr/local/cuda %s 2>&1 \
+// RUN:   --cuda-path=%S/Inputs/CUDA_80/usr/local/cuda %s 2>&1 \
 // RUN:   | FileCheck %s -check-prefix COMMON -check-prefix CUDAINC \
 // RUN:     -check-prefix LIBDEVICE -check-prefix LIBDEVICE35
+// RUN: %clang -### -v --target=i386-unknown-linux --cuda-gpu-arch=sm_37 \
+// RUN:   --cuda-path=%S/Inputs/CUDA_80/usr/local/cuda %s 2>&1 \
+// RUN:   | FileCheck %s -check-prefix COMMON -check-prefix CUDAINC \
+// RUN:     -check-prefix LIBDEVICE -check-prefix LIBDEVICE35
+
 // Verify that -nocudainc prevents adding include path to CUDA headers.
 // RUN: %clang -### -v --target=i386-unknown-linux --cuda-gpu-arch=sm_35 \
 // RUN:   -nocudainc --cuda-path=%S/Inputs/CUDA/usr/local/cuda %s 2>&1 \
@@ -29,12 +55,13 @@
 // RUN:   --cuda-path=%S/no-cuda-there %s 2>&1 \
 // RUN:   | FileCheck %s -check-prefix COMMON -check-prefix NOCUDAINC
 
-// Verify that no options related to bitcode linking are passes if
-// there's no bitcode file.
+// Verify that we get an error if there's no libdevice library to link with.
+// NOTE: Inputs/CUDA deliberately does *not* have libdevice.compute_30  for this purpose.
 // RUN: %clang -### -v --target=i386-unknown-linux --cuda-gpu-arch=sm_30 \
 // RUN:   --cuda-path=%S/Inputs/CUDA/usr/local/cuda %s 2>&1 \
-// RUN:   | FileCheck %s -check-prefix COMMON -check-prefix NOLIBDEVICE
-// .. or if we explicitly passed -nocudalib
+// RUN:   | FileCheck %s -check-prefix COMMON -check-prefix MISSINGLIBDEVICE
+
+// Verify that  -nocudalib prevents linking libdevice bitcode in.
 // RUN: %clang -### -v --target=i386-unknown-linux --cuda-gpu-arch=sm_35 \
 // RUN:   -nocudalib --cuda-path=%S/Inputs/CUDA/usr/local/cuda %s 2>&1 \
 // RUN:   | FileCheck %s -check-prefix COMMON -check-prefix NOLIBDEVICE
@@ -48,16 +75,19 @@
 // CHECK: Found CUDA installation: {{.*}}/Inputs/CUDA/usr/local/cuda
 // NOCUDA-NOT: Found CUDA installation:
 
+// MISSINGLIBDEVICE: error: cannot find libdevice for sm_30.
+
 // COMMON: "-triple" "nvptx-nvidia-cuda"
 // COMMON-SAME: "-fcuda-is-device"
 // LIBDEVICE-SAME: "-mlink-cuda-bitcode"
 // NOLIBDEVICE-NOT: "-mlink-cuda-bitcode"
-// LIBDEVICE21-SAME: libdevice.compute_20.10.bc
+// LIBDEVICE20-SAME: libdevice.compute_20.10.bc
+// LIBDEVICE30-SAME: libdevice.compute_30.10.bc
 // LIBDEVICE35-SAME: libdevice.compute_35.10.bc
 // NOLIBDEVICE-NOT: libdevice.compute_{{.*}}.bc
 // LIBDEVICE-SAME: "-target-feature" "+ptx42"
 // NOLIBDEVICE-NOT: "-target-feature" "+ptx42"
-// CUDAINC-SAME: "-internal-isystem" "{{.*}}/Inputs/CUDA/usr/local/cuda/include"
+// CUDAINC-SAME: "-internal-isystem" "{{.*}}/Inputs/CUDA{{[_0-9]+}}/usr/local/cuda/include"
 // NOCUDAINC-NOT: "-internal-isystem" "{{.*}}/cuda/include"
 // CUDAINC-SAME: "-include" "__clang_cuda_runtime_wrapper.h"
 // NOCUDAINC-NOT: "-include" "__clang_cuda_runtime_wrapper.h"