Index: lib/Frontend/InitPreprocessor.cpp
===================================================================
--- lib/Frontend/InitPreprocessor.cpp
+++ lib/Frontend/InitPreprocessor.cpp
@@ -1099,6 +1099,44 @@
   TI.getTargetDefines(LangOpts, Builder);
 }
 
+/// Initialize macros based on AuxTargetInfo.
+static void InitializePredefinedAuxMacros(const TargetInfo &AuxTI,
+                                          const LangOptions &LangOpts,
+                                          MacroBuilder &Builder) {
+  auto AuxTriple = AuxTI.getTriple();
+
+  // Define basic target macros needed by at least bits/wordsize.h and
+  // bits/mathinline.h
+  switch (AuxTriple.getArch()) {
+  case llvm::Triple::x86_64:
+    Builder.defineMacro("__x86_64__");
+    break;
+  case llvm::Triple::ppc64:
+  case llvm::Triple::ppc64le:
+    Builder.defineMacro("__powerpc64__");
+    break;
+  default:
+    break;
+  }
+
+  // libc++ needs to find out the object file format and threading API.
+  if (AuxTriple.getOS() == llvm::Triple::Linux) {
+    Builder.defineMacro("__ELF__");
+    Builder.defineMacro("__linux__");
+    // Used in features.h. If this is omitted, math.h doesn't declare float
+    // versions of the functions in bits/mathcalls.h.
+    if (LangOpts.CPlusPlus)
+      Builder.defineMacro("_GNU_SOURCE");
+  } else if (AuxTriple.isOSDarwin()) {
+    Builder.defineMacro("__APPLE__");
+    Builder.defineMacro("__MACH__");
+  } else if (AuxTriple.isOSWindows()) {
+    Builder.defineMacro("_WIN32");
+    if (AuxTriple.isWindowsGNUEnvironment())
+      Builder.defineMacro("__MINGW32__");
+  }
+}
+
 /// InitializePreprocessor - Initialize the preprocessor getting it and the
 /// environment ready to process a single file. This returns true on error.
 ///
@@ -1120,13 +1158,9 @@
 
   // Install things like __POWERPC__, __GNUC__, etc into the macro table.
   if (InitOpts.UsePredefines) {
-    // FIXME: This will create multiple definitions for most of the predefined
-    // macros. This is not the right way to handle this.
-    if ((LangOpts.CUDA || LangOpts.OpenMPIsDevice) && PP.getAuxTargetInfo())
-      InitializePredefinedMacros(*PP.getAuxTargetInfo(), LangOpts, FEOpts,
-                                 Builder);
-
     InitializePredefinedMacros(PP.getTargetInfo(), LangOpts, FEOpts, Builder);
+    if ((LangOpts.CUDA || LangOpts.OpenMPIsDevice) && PP.getAuxTargetInfo())
+      InitializePredefinedAuxMacros(*PP.getAuxTargetInfo(), LangOpts, Builder);
 
     // Install definitions to make Objective-C++ ARC work well with various
     // C++ Standard Library implementations.
Index: test/Preprocessor/aux-triple.c
===================================================================
--- /dev/null
+++ test/Preprocessor/aux-triple.c
@@ -0,0 +1,62 @@
+// Ensure that Clang sets some very basic target defines based on -aux-triple.
+
+// RUN: %clang_cc1 -E -dM -ffreestanding < /dev/null \
+// RUN:     -triple nvptx64-none-none \
+// RUN:   | FileCheck -match-full-lines -check-prefixes NVPTX64,NONE %s
+// RUN: %clang_cc1 -x c++ -E -dM -ffreestanding < /dev/null \
+// RUN:     -triple nvptx64-none-none \
+// RUN:   | FileCheck -match-full-lines -check-prefixes NVPTX64,NONE %s
+// RUN: %clang_cc1 -x cuda -E -dM -ffreestanding < /dev/null \
+// RUN:     -triple nvptx64-none-none \
+// RUN:   | FileCheck -match-full-lines -check-prefixes NVPTX64,NONE %s
+
+// CUDA:
+// RUN: %clang_cc1 -x cuda -E -dM -ffreestanding < /dev/null \
+// RUN:     -triple nvptx64-none-none -aux-triple powerpc64le-unknown-linux-gnu \
+// RUN:   | FileCheck -match-full-lines %s \
+// RUN:     -check-prefixes NVPTX64,PPC64,LINUX,LINUX-CPP
+// RUN: %clang_cc1 -x cuda -E -dM -ffreestanding < /dev/null \
+// RUN:     -triple nvptx64-none-none -aux-triple x86_64-unknown-linux-gnu \
+// RUN:   | FileCheck -match-full-lines %s \
+// RUN:     -check-prefixes NVPTX64,X86_64,LINUX,LINUX-CPP
+
+// OpenMP:
+// RUN: %clang_cc1 -E -dM -ffreestanding < /dev/null \
+// RUN:     -fopenmp -fopenmp-is-device -triple nvptx64-none-none \
+// RUN:     -aux-triple powerpc64le-unknown-linux-gnu \
+// RUN:   | FileCheck -match-full-lines -check-prefixes NVPTX64,PPC64,LINUX %s
+// RUN: %clang_cc1 -E -dM -ffreestanding < /dev/null \
+// RUN:     -fopenmp -fopenmp-is-device -triple nvptx64-none-none \
+// RUN:     -aux-triple x86_64-unknown-linux-gnu \
+// RUN:   | FileCheck -match-full-lines -check-prefixes NVPTX64,X86_64,LINUX %s
+// RUN: %clang_cc1 -x c++ -E -dM -ffreestanding < /dev/null \
+// RUN:     -fopenmp -fopenmp-is-device -triple nvptx64-none-none \
+// RUN:     -aux-triple powerpc64le-unknown-linux-gnu \
+// RUN:   | FileCheck -match-full-lines %s \
+// RUN:     -check-prefixes NVPTX64,PPC64,LINUX,LINUX-CPP
+// RUN: %clang_cc1 -x c++ -E -dM -ffreestanding < /dev/null \
+// RUN:     -fopenmp -fopenmp-is-device -triple nvptx64-none-none \
+// RUN:     -aux-triple x86_64-unknown-linux-gnu \
+// RUN:   | FileCheck -match-full-lines %s \
+// RUN:     -check-prefixes NVPTX64,X86_64,LINUX,LINUX-CPP
+
+// NONE-NOT:#define _GNU_SOURCE
+// LINUX-CPP:#define _GNU_SOURCE 1
+
+// NVPTX64:#define _LP64 1
+
+// NONE-NOT:#define __ELF__
+// LINUX:#define __ELF__ 1
+
+// NVPTX64:#define __LP64__ 1
+// NVPTX64:#define __NVPTX__ 1
+// NVPTX64:#define __PTX__ 1
+
+// NONE-NOT:#define __linux__
+// LINUX:#define __linux__ 1
+
+// NONE-NOT:#define __powerpc64__
+// PPC64:#define __powerpc64__ 1
+
+// NONE-NOT:#define __x86_64__
+// X86_64:#define __x86_64__ 1
Index: test/SemaCUDA/builtins.cu
===================================================================
--- test/SemaCUDA/builtins.cu
+++ test/SemaCUDA/builtins.cu
@@ -12,8 +12,8 @@
 // RUN:     -aux-triple x86_64-unknown-unknown \
 // RUN:     -fsyntax-only -verify %s
 
-#if !(defined(__amd64__) && defined(__PTX__))
-#error "Expected to see preprocessor macros from both sides of compilation."
+#if !defined(__x86_64__)
+#error "Expected to see preprocessor macros from the host."
 #endif
 
 void hf() {