Index: cfe/trunk/include/clang/Basic/LangOptions.def
===================================================================
--- cfe/trunk/include/clang/Basic/LangOptions.def
+++ cfe/trunk/include/clang/Basic/LangOptions.def
@@ -190,6 +190,7 @@
 LANGOPT(CUDAAllowVariadicFunctions, 1, 0, "allowing variadic functions in CUDA device code")
 LANGOPT(CUDAHostDeviceConstexpr, 1, 1, "treating unattributed constexpr functions as __host__ __device__")
 LANGOPT(CUDADeviceFlushDenormalsToZero, 1, 0, "flushing denormals to zero")
+LANGOPT(CUDADeviceApproxTranscendentals, 1, 0, "using approximate transcendental functions")
 
 LANGOPT(SizedDeallocation , 1, 0, "enable sized deallocation functions")
 LANGOPT(ConceptsTS , 1, 0, "enable C++ Extensions for Concepts")
Index: cfe/trunk/include/clang/Driver/Options.td
===================================================================
--- cfe/trunk/include/clang/Driver/Options.td
+++ cfe/trunk/include/clang/Driver/Options.td
@@ -395,6 +395,9 @@
 def fcuda_flush_denormals_to_zero : Flag<["-"], "fcuda-flush-denormals-to-zero">,
   Flags<[CC1Option]>, HelpText<"Flush denormal floating point values to zero in CUDA device mode.">;
 def fno_cuda_flush_denormals_to_zero : Flag<["-"], "fno-cuda-flush-denormals-to-zero">;
+def fcuda_approx_transcendentals : Flag<["-"], "fcuda-approx-transcendentals">,
+  Flags<[CC1Option]>, HelpText<"Use approximate transcendental functions">;
+def fno_cuda_approx_transcendentals : Flag<["-"], "fno-cuda-approx-transcendentals">;
 def dA : Flag<["-"], "dA">, Group<d_Group>;
 def dD : Flag<["-"], "dD">, Group<d_Group>, Flags<[CC1Option]>,
   HelpText<"Print macro definitions in -E mode in addition to normal output">;
Index: cfe/trunk/lib/Driver/ToolChains.cpp
===================================================================
--- cfe/trunk/lib/Driver/ToolChains.cpp
+++ cfe/trunk/lib/Driver/ToolChains.cpp
@@ -4502,6 +4502,10 @@
                          options::OPT_fno_cuda_flush_denormals_to_zero, false))
     CC1Args.push_back("-fcuda-flush-denormals-to-zero");
 
+  if (DriverArgs.hasFlag(options::OPT_fcuda_approx_transcendentals,
+                         options::OPT_fno_cuda_approx_transcendentals, false))
+    CC1Args.push_back("-fcuda-approx-transcendentals");
+
   if (DriverArgs.hasArg(options::OPT_nocudalib))
     return;
 
Index: cfe/trunk/lib/Frontend/CompilerInvocation.cpp
===================================================================
--- cfe/trunk/lib/Frontend/CompilerInvocation.cpp
+++ cfe/trunk/lib/Frontend/CompilerInvocation.cpp
@@ -1616,6 +1616,9 @@
   if (Opts.CUDAIsDevice && Args.hasArg(OPT_fcuda_flush_denormals_to_zero))
     Opts.CUDADeviceFlushDenormalsToZero = 1;
 
+  if (Opts.CUDAIsDevice && Args.hasArg(OPT_fcuda_approx_transcendentals))
+    Opts.CUDADeviceApproxTranscendentals = 1;
+
   if (Opts.ObjC1) {
     if (Arg *arg = Args.getLastArg(OPT_fobjc_runtime_EQ)) {
       StringRef value = arg->getValue();
Index: cfe/trunk/lib/Frontend/InitPreprocessor.cpp
===================================================================
--- cfe/trunk/lib/Frontend/InitPreprocessor.cpp
+++ cfe/trunk/lib/Frontend/InitPreprocessor.cpp
@@ -938,6 +938,12 @@
     Builder.defineMacro("__CUDA_ARCH__");
   }
 
+  // We need to communicate this to our CUDA header wrapper, which in turn
+  // informs the proper CUDA headers of this choice.
+  if (LangOpts.CUDADeviceApproxTranscendentals || LangOpts.FastMath) {
+    Builder.defineMacro("__CLANG_CUDA_APPROX_TRANSCENDENTALS__");
+  }
+
   // OpenCL definitions.
   if (LangOpts.OpenCL) {
 #define OPENCLEXT(Ext) \
Index: cfe/trunk/lib/Headers/__clang_cuda_runtime_wrapper.h
===================================================================
--- cfe/trunk/lib/Headers/__clang_cuda_runtime_wrapper.h
+++ cfe/trunk/lib/Headers/__clang_cuda_runtime_wrapper.h
@@ -142,7 +142,20 @@
 #pragma push_macro("__forceinline__")
 #define __forceinline__ __device__ __inline__ __attribute__((always_inline))
 #include "device_functions.hpp"
+
+// math_function.hpp uses the __USE_FAST_MATH__ macro to determine whether we
+// get the slow-but-accurate or fast-but-inaccurate versions of functions like
+// sin and exp.  This is controlled in clang by -fcuda-approx-transcendentals.
+//
+// device_functions.hpp uses __USE_FAST_MATH__ for a different purpose (fast vs.
+// slow divides), so we need to scope our define carefully here.
+#pragma push_macro("__USE_FAST_MATH__")
+#if defined(__CLANG_CUDA_APPROX_TRANSCENDENTALS__)
+#define __USE_FAST_MATH__
+#endif
 #include "math_functions.hpp"
+#pragma pop_macro("__USE_FAST_MATH__")
+
 #include "math_functions_dbl_ptx3.hpp"
 #pragma pop_macro("__forceinline__")
 
@@ -296,6 +309,7 @@
 #include "curand_mtgp32_kernel.h"
 #pragma pop_macro("dim3")
 #pragma pop_macro("uint3")
+#pragma pop_macro("__USE_FAST_MATH__")
 
 #endif // __CUDA__
 #endif // __CLANG_CUDA_RUNTIME_WRAPPER_H__
Index: cfe/trunk/test/Preprocessor/cuda-approx-transcendentals.cu
===================================================================
--- cfe/trunk/test/Preprocessor/cuda-approx-transcendentals.cu
+++ cfe/trunk/test/Preprocessor/cuda-approx-transcendentals.cu
@@ -0,0 +1,8 @@
+// RUN: %clang --cuda-host-only -nocudainc -target i386-unknown-linux-gnu -x cuda -E -dM -o - /dev/null | FileCheck --check-prefix HOST %s
+// RUN: %clang --cuda-device-only -nocudainc -target i386-unknown-linux-gnu -x cuda -E -dM -o - /dev/null | FileCheck --check-prefix DEVICE-NOFAST %s
+// RUN: %clang -fcuda-approx-transcendentals --cuda-device-only -nocudainc -target i386-unknown-linux-gnu -x cuda -E -dM -o - /dev/null | FileCheck --check-prefix DEVICE-FAST %s
+// RUN: %clang -ffast-math --cuda-device-only -nocudainc -target i386-unknown-linux-gnu -x cuda -E -dM -o - /dev/null | FileCheck --check-prefix DEVICE-FAST %s
+
+// HOST-NOT: __CLANG_CUDA_APPROX_TRANSCENDENTALS__
+// DEVICE-NOFAST-NOT: __CLANG_CUDA_APPROX_TRANSCENDENTALS__
+// DEVICE-FAST: __CLANG_CUDA_APPROX_TRANSCENDENTALS__