Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -556,6 +556,8 @@
 def no_cuda_noopt_device_debug : Flag<["--"], "no-cuda-noopt-device-debug">;
 def cuda_path_EQ : Joined<["--"], "cuda-path=">, Group<i_Group>,
   HelpText<"CUDA installation path">;
+def cuda_path_ignore_env : Joined<["--"], "cuda-path-ignore-env">, Group<i_Group>,
+  HelpText<"Ignore environment variables to detect CUDA installation">;
 def ptxas_path_EQ : Joined<["--"], "ptxas-path=">, Group<i_Group>,
   HelpText<"Path to ptxas (used for compiling CUDA code)">;
 def fcuda_flush_denormals_to_zero : Flag<["-"], "fcuda-flush-denormals-to-zero">,
Index: lib/Driver/ToolChains/Cuda.cpp
===================================================================
--- lib/Driver/ToolChains/Cuda.cpp
+++ lib/Driver/ToolChains/Cuda.cpp
@@ -8,18 +8,19 @@
 //===----------------------------------------------------------------------===//
 
 #include "Cuda.h"
-#include "InputInfo.h"
 #include "CommonArgs.h"
+#include "InputInfo.h"
 #include "clang/Basic/Cuda.h"
-#include "clang/Config/config.h"
 #include "clang/Basic/VirtualFileSystem.h"
-#include "clang/Driver/Distro.h"
+#include "clang/Config/config.h"
 #include "clang/Driver/Compilation.h"
+#include "clang/Driver/Distro.h"
 #include "clang/Driver/Driver.h"
 #include "clang/Driver/DriverDiagnostic.h"
 #include "clang/Driver/Options.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/Program.h"
 #include <system_error>
 
 using namespace clang::driver;
@@ -73,6 +74,19 @@
           D.SysRoot + "/Program Files/NVIDIA GPU Computing Toolkit/CUDA/v" +
           Ver);
   } else {
+    if (!Args.hasArg(clang::driver::options::OPT_cuda_path_ignore_env)) {
+      // Try to find ptxas binary. If the executable is located in a directory
+      // called 'bin/', its parent directory might be a good guess for a valid
+      // CUDA installation.
+      if (llvm::ErrorOr<std::string> ptxas =
+              llvm::sys::findProgramByName("ptxas")) {
+        StringRef ptxasDir = llvm::sys::path::parent_path(*ptxas);
+        if (llvm::sys::path::filename(ptxasDir) == "bin") {
+          CudaPathCandidates.push_back(llvm::sys::path::parent_path(ptxasDir));
+        }
+      }
+    }
+
     CudaPathCandidates.push_back(D.SysRoot + "/usr/local/cuda");
     for (const char *Ver : Versions)
       CudaPathCandidates.push_back(D.SysRoot + "/usr/local/cuda-" + Ver);
Index: test/Driver/cuda-detect-path.cu
===================================================================
--- /dev/null
+++ test/Driver/cuda-detect-path.cu
@@ -0,0 +1,32 @@
+// REQUIRES: clang-driver
+// REQUIRES: x86-registered-target
+// REQUIRES: nvptx-registered-target
+// This tests uses the PATH environment variable.
+// REQUIRES: !system-windows
+
+// RUN: env PATH=%S/Inputs/CUDA/usr/local/cuda/bin \
+// RUN:    %clang -v --target=i386-unknown-linux 2>&1 | FileCheck %s
+// RUN: env PATH=%S/Inputs/CUDA/usr/local/cuda/bin \
+// RUN:    %clang -v --target=i386-apple-macosx 2>&1 | FileCheck %s
+// RUN: env PATH=%S/Inputs/CUDA/usr/local/cuda/bin \
+// RUN:    %clang -v --target=x86_64-unknown-linux 2>&1 | FileCheck %s
+// RUN: env PATH=%S/Inputs/CUDA/usr/local/cuda/bin \
+// RUN:    %clang -v --target=x86_64-apple-macosx 2>&1 | FileCheck %s
+
+// Check that the CUDA installation in PATH is not taken when passing
+// the option --cuda-path-ignore-env
+// RUN: env PATH=%S/Inputs/CUDA/usr/local/cuda/bin \
+// RUN:    %clang -v --target=i386-unknown-linux --cuda-path-ignore-env \
+// RUN:    2>&1 | FileCheck %s --check-prefix NOCUDA
+// RUN: env PATH=%S/Inputs/CUDA/usr/local/cuda/bin \
+// RUN:    %clang -v --target=i386-apple-macosx --cuda-path-ignore-env \
+// RUN:    2>&1 | FileCheck %s --check-prefix NOCUDA
+// RUN: env PATH=%S/Inputs/CUDA/usr/local/cuda/bin \
+// RUN:    %clang -v --target=x86_64-unknown-linux --cuda-path-ignore-env \
+// RUN:    2>&1 | FileCheck %s --check-prefix NOCUDA
+// RUN: env PATH=%S/Inputs/CUDA/usr/local/cuda/bin \
+// RUN:    %clang -v --target=x86_64-apple-macosx --cuda-path-ignore-env \
+// RUN:    2>&1 | FileCheck %s --check-prefix NOCUDA
+
+// CHECK: Found CUDA installation: {{.*}}/Inputs/CUDA/usr/local/cuda
+// NOCUDA-NOT: Found CUDA installation:
Index: test/Driver/cuda-detect.cu
===================================================================
--- test/Driver/cuda-detect.cu
+++ test/Driver/cuda-detect.cu
@@ -4,9 +4,14 @@
 //
 // Check that we properly detect CUDA installation.
 // RUN: %clang -v --target=i386-unknown-linux \
-// RUN:   --sysroot=%S/no-cuda-there 2>&1 | FileCheck %s -check-prefix NOCUDA
+// RUN:   --sysroot=%S/no-cuda-there --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NOCUDA
 // RUN: %clang -v --target=i386-apple-macosx \
-// RUN:   --sysroot=%S/no-cuda-there 2>&1 | FileCheck %s -check-prefix NOCUDA
+// RUN:   --sysroot=%S/no-cuda-there --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NOCUDA
+// RUN: %clang -v --target=x86_64-unknown-linux \
+// RUN:   --sysroot=%S/no-cuda-there --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NOCUDA
+// RUN: %clang -v --target=x86_64-apple-macosx \
+// RUN:   --sysroot=%S/no-cuda-there --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NOCUDA
+
 
 // RUN: %clang -v --target=i386-unknown-linux \
 // RUN:   --sysroot=%S/Inputs/CUDA 2>&1 | FileCheck %s
@@ -20,15 +25,23 @@
 
 // Check that we don't find a CUDA installation without libdevice ...
 // RUN: %clang -v --target=i386-unknown-linux \
-// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice 2>&1 | FileCheck %s -check-prefix NOCUDA
+// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NOCUDA
 // RUN: %clang -v --target=i386-apple-macosx \
-// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice 2>&1 | FileCheck %s -check-prefix NOCUDA
+// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NOCUDA
+// RUN: %clang -v --target=x86_64-unknown-linux \
+// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NOCUDA
+// RUN: %clang -v --target=x84_64-apple-macosx \
+// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NOCUDA
 
 // ... unless the user doesn't need libdevice
 // RUN: %clang -v --target=i386-unknown-linux -nocudalib \
-// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice 2>&1 | FileCheck %s -check-prefix NO-LIBDEVICE
+// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NO-LIBDEVICE
 // RUN: %clang -v --target=i386-apple-macosx -nocudalib \
-// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice 2>&1 | FileCheck %s -check-prefix NO-LIBDEVICE
+// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NO-LIBDEVICE
+// RUN: %clang -v --target=x86_64-unknown-linux -nocudalib \
+// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NO-LIBDEVICE
+// RUN: %clang -v --target=x86_64-apple-macosx -nocudalib \
+// RUN:   --sysroot=%S/Inputs/CUDA-nolibdevice --cuda-path-ignore-env 2>&1 | FileCheck %s -check-prefix NO-LIBDEVICE
 
 
 // Make sure we map libdevice bitcode files to proper GPUs. These
Index: test/Driver/cuda-not-found.cu
===================================================================
--- test/Driver/cuda-not-found.cu
+++ test/Driver/cuda-not-found.cu
@@ -3,10 +3,10 @@
 // Check that we raise an error if we're trying to compile CUDA code but can't
 // find a CUDA install, unless -nocudainc was passed.
 
-// RUN: %clang -### --sysroot=%s/no-cuda-there %s 2>&1 | FileCheck %s --check-prefix ERR
+// RUN: %clang -### --sysroot=%s/no-cuda-there --cuda-path-ignore-env %s 2>&1 | FileCheck %s --check-prefix ERR
 // RUN: %clang -### --cuda-path=%s/no-cuda-there %s 2>&1 | FileCheck %s --check-prefix ERR
 // ERR: cannot find CUDA installation
 
-// RUN: %clang -### -nocudainc --sysroot=%s/no-cuda-there %s 2>&1 | FileCheck %s --check-prefix OK
+// RUN: %clang -### -nocudainc --sysroot=%s/no-cuda-there --cuda-path-ignore-env %s 2>&1 | FileCheck %s --check-prefix OK
 // RUN: %clang -### -nocudainc --cuda-path=%s/no-cuda-there %s 2>&1 | FileCheck %s --check-prefix OK
 // OK-NOT: cannot find CUDA installation
Index: test/Driver/cuda-version-check.cu
===================================================================
--- test/Driver/cuda-version-check.cu
+++ test/Driver/cuda-version-check.cu
@@ -2,40 +2,40 @@
 // REQUIRES: x86-registered-target
 // REQUIRES: nvptx-registered-target
 
-// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_20 --sysroot=%S/Inputs/CUDA 2>&1 %s | \
+// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_20 --cuda-path=%S/Inputs/CUDA/usr/local/cuda 2>&1 %s | \
 // RUN:    FileCheck %s --check-prefix=OK
-// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_20 --sysroot=%S/Inputs/CUDA_80 2>&1 %s | \
+// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_20 --cuda-path=%S/Inputs/CUDA_80/usr/local/cuda 2>&1 %s | \
 // RUN:    FileCheck %s --check-prefix=OK
-// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --sysroot=%S/Inputs/CUDA_80 2>&1 %s | \
+// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --cuda-path=%S/Inputs/CUDA_80/usr/local/cuda 2>&1 %s | \
 // RUN:    FileCheck %s --check-prefix=OK
 
 // The installation at Inputs/CUDA is CUDA 7.0, which doesn't support sm_60.
-// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --sysroot=%S/Inputs/CUDA 2>&1 %s | \
+// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --cuda-path=%S/Inputs/CUDA/usr/local/cuda 2>&1 %s | \
 // RUN:    FileCheck %s --check-prefix=ERR_SM60
 
 // This should only complain about sm_60, not sm_35.
 // RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --cuda-gpu-arch=sm_35 \
-// RUN:    --sysroot=%S/Inputs/CUDA 2>&1 %s | \
+// RUN:    --cuda-path=%S/Inputs/CUDA/usr/local/cuda 2>&1 %s | \
 // RUN:    FileCheck %s --check-prefix=ERR_SM60 --check-prefix=OK_SM35
 
 // We should get two errors here, one for sm_60 and one for sm_61.
 // RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --cuda-gpu-arch=sm_61 \
-// RUN:    --sysroot=%S/Inputs/CUDA 2>&1 %s | \
+// RUN:    --cuda-path=%S/Inputs/CUDA/usr/local/cuda 2>&1 %s | \
 // RUN:    FileCheck %s --check-prefix=ERR_SM60 --check-prefix=ERR_SM61
 
 // We should still get an error if we pass -nocudainc, because this compilation
 // would invoke ptxas, and we do a version check on that, too.
-// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 -nocudainc --sysroot=%S/Inputs/CUDA 2>&1 %s | \
+// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 -nocudainc --cuda-path=%S/Inputs/CUDA/usr/local/cuda 2>&1 %s | \
 // RUN:    FileCheck %s --check-prefix=ERR_SM60
 
 // If with -nocudainc and -E, we don't touch the CUDA install, so we
 // shouldn't get an error.
 // RUN: %clang --target=x86_64-linux -v -### -E --cuda-device-only --cuda-gpu-arch=sm_60 -nocudainc \
-// RUN:    --sysroot=%S/Inputs/CUDA 2>&1 %s | \
+// RUN:    --cuda-path=%S/Inputs/CUDA/usr/local/cuda 2>&1 %s | \
 // RUN:    FileCheck %s --check-prefix=OK
 
 // --no-cuda-version-check should suppress all of these errors.
-// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --sysroot=%S/Inputs/CUDA 2>&1 \
+// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --cuda-path=%S/Inputs/CUDA/usr/local/cuda 2>&1 \
 // RUN:    --no-cuda-version-check %s | \
 // RUN:    FileCheck %s --check-prefix=OK
 
@@ -43,9 +43,9 @@
 // therefore we should not get an error in host-only mode. We use the -S here
 // to avoid the error being produced in case by the assembler tool, which does
 // the same check.
-// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --cuda-host-only --sysroot=%S/Inputs/CUDA -S 2>&1 %s | \
+// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --cuda-host-only --cuda-path=%S/Inputs/CUDA/usr/local/cuda -S 2>&1 %s | \
 // RUN:    FileCheck %s --check-prefix=OK
-// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --cuda-device-only --sysroot=%S/Inputs/CUDA -S 2>&1 %s | \
+// RUN: %clang --target=x86_64-linux -v -### --cuda-gpu-arch=sm_60 --cuda-device-only --cuda-path=%S/Inputs/CUDA/usr/local/cuda -S 2>&1 %s | \
 // RUN:    FileCheck %s --check-prefix=ERR_SM60
 
 // OK-NOT: error: GPU arch