Index: lib/Fuzzer/FuzzerDefs.h
===================================================================
--- lib/Fuzzer/FuzzerDefs.h
+++ lib/Fuzzer/FuzzerDefs.h
@@ -19,25 +19,6 @@
 #include <string>
 #include <vector>
 
-// Platform detection.
-#ifdef __linux__
-#define LIBFUZZER_APPLE 0
-#define LIBFUZZER_LINUX 1
-#define LIBFUZZER_WINDOWS 0
-#elif __APPLE__
-#define LIBFUZZER_APPLE 1
-#define LIBFUZZER_LINUX 0
-#define LIBFUZZER_WINDOWS 0
-#elif _WIN32
-#define LIBFUZZER_APPLE 0
-#define LIBFUZZER_LINUX 0
-#define LIBFUZZER_WINDOWS 1
-#else
-#error "Support for your platform has not been implemented"
-#endif
-
-#define LIBFUZZER_POSIX LIBFUZZER_APPLE || LIBFUZZER_LINUX
-
 #ifdef __x86_64
 #define ATTRIBUTE_TARGET_POPCNT __attribute__((target("popcnt")))
 #else
Index: lib/Fuzzer/FuzzerExtFunctionsDlsym.cpp
===================================================================
--- lib/Fuzzer/FuzzerExtFunctionsDlsym.cpp
+++ lib/Fuzzer/FuzzerExtFunctionsDlsym.cpp
@@ -11,7 +11,7 @@
 // requires that clients of LibFuzzer pass ``--export-dynamic`` to the linker.
 // That is a complication we don't wish to expose to clients right now.
 //===----------------------------------------------------------------------===//
-#include "FuzzerDefs.h"
+#include "Support/Platform.h"
 #if LIBFUZZER_APPLE
 
 #include "FuzzerExtFunctions.h"
Index: lib/Fuzzer/FuzzerExtFunctionsWeak.cpp
===================================================================
--- lib/Fuzzer/FuzzerExtFunctionsWeak.cpp
+++ lib/Fuzzer/FuzzerExtFunctionsWeak.cpp
@@ -12,7 +12,7 @@
 // weak symbols to be undefined. That is a complication we don't want to expose
 // to clients right now.
 //===----------------------------------------------------------------------===//
-#include "FuzzerDefs.h"
+#include "Support/Platform.h"
 #if LIBFUZZER_LINUX
 
 #include "FuzzerExtFunctions.h"
Index: lib/Fuzzer/FuzzerExtFunctionsWeakAlias.cpp
===================================================================
--- lib/Fuzzer/FuzzerExtFunctionsWeakAlias.cpp
+++ lib/Fuzzer/FuzzerExtFunctionsWeakAlias.cpp
@@ -8,7 +8,7 @@
 //===----------------------------------------------------------------------===//
 // Implementation using weak aliases. Works for Windows.
 //===----------------------------------------------------------------------===//
-#include "FuzzerDefs.h"
+#include "Support/Platform.h"
 #if LIBFUZZER_WINDOWS
 
 #include "FuzzerExtFunctions.h"
Index: lib/Fuzzer/FuzzerIOPosix.cpp
===================================================================
--- lib/Fuzzer/FuzzerIOPosix.cpp
+++ lib/Fuzzer/FuzzerIOPosix.cpp
@@ -8,7 +8,7 @@
 //===----------------------------------------------------------------------===//
 // IO functions implementation using Posix API.
 //===----------------------------------------------------------------------===//
-#include "FuzzerDefs.h"
+#include "Support/Platform.h"
 #if LIBFUZZER_POSIX
 
 #include "FuzzerExtFunctions.h"
Index: lib/Fuzzer/FuzzerIOWindows.cpp
===================================================================
--- lib/Fuzzer/FuzzerIOWindows.cpp
+++ lib/Fuzzer/FuzzerIOWindows.cpp
@@ -8,7 +8,7 @@
 //===----------------------------------------------------------------------===//
 // IO functions implementation for Windows.
 //===----------------------------------------------------------------------===//
-#include "FuzzerDefs.h"
+#include "Support/Platform.h"
 #if LIBFUZZER_WINDOWS
 
 #include "FuzzerExtFunctions.h"
Index: lib/Fuzzer/FuzzerUtilDarwin.cpp
===================================================================
--- lib/Fuzzer/FuzzerUtilDarwin.cpp
+++ lib/Fuzzer/FuzzerUtilDarwin.cpp
@@ -8,7 +8,7 @@
 //===----------------------------------------------------------------------===//
 // Misc utils for Darwin.
 //===----------------------------------------------------------------------===//
-#include "FuzzerDefs.h"
+#include "Support/Platform.h"
 #if LIBFUZZER_APPLE
 
 #include "FuzzerIO.h"
Index: lib/Fuzzer/FuzzerUtilLinux.cpp
===================================================================
--- lib/Fuzzer/FuzzerUtilLinux.cpp
+++ lib/Fuzzer/FuzzerUtilLinux.cpp
@@ -8,7 +8,7 @@
 //===----------------------------------------------------------------------===//
 // Misc utils for Linux.
 //===----------------------------------------------------------------------===//
-#include "FuzzerDefs.h"
+#include "Support/Platform.h"
 #if LIBFUZZER_LINUX
 
 #include <stdlib.h>
Index: lib/Fuzzer/FuzzerUtilPosix.cpp
===================================================================
--- lib/Fuzzer/FuzzerUtilPosix.cpp
+++ lib/Fuzzer/FuzzerUtilPosix.cpp
@@ -8,7 +8,7 @@
 //===----------------------------------------------------------------------===//
 // Misc utils implementation using Posix API.
 //===----------------------------------------------------------------------===//
-#include "FuzzerDefs.h"
+#include "Support/Platform.h"
 #if LIBFUZZER_POSIX
 #include "FuzzerIO.h"
 #include "FuzzerInternal.h"
Index: lib/Fuzzer/FuzzerUtilWindows.cpp
===================================================================
--- lib/Fuzzer/FuzzerUtilWindows.cpp
+++ lib/Fuzzer/FuzzerUtilWindows.cpp
@@ -8,7 +8,7 @@
 //===----------------------------------------------------------------------===//
 // Misc utils implementation for Windows.
 //===----------------------------------------------------------------------===//
-#include "FuzzerDefs.h"
+#include "Support/Platform.h"
 #if LIBFUZZER_WINDOWS
 #include "FuzzerIO.h"
 #include "FuzzerInternal.h"
Index: lib/Fuzzer/Support/Platform.h
===================================================================
--- /dev/null
+++ lib/Fuzzer/Support/Platform.h
@@ -0,0 +1,34 @@
+//===- Platform.h - Basic definitions for different platforms ---*- C++ -* ===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Basic macro definitions for different platforms.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_FUZZER_SUPPORT_PLATFORM_H
+#define LLVM_FUZZER_SUPPORT_PLATFORM_H
+
+// Platform detection.
+#ifdef __linux__
+#define LIBFUZZER_APPLE 0
+#define LIBFUZZER_LINUX 1
+#define LIBFUZZER_WINDOWS 0
+#elif __APPLE__
+#define LIBFUZZER_APPLE 1
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_WINDOWS 0
+#elif _WIN32
+#define LIBFUZZER_APPLE 0
+#define LIBFUZZER_LINUX 0
+#define LIBFUZZER_WINDOWS 1
+#else
+#error "Support for your platform has not been implemented"
+#endif
+
+#define LIBFUZZER_POSIX LIBFUZZER_APPLE || LIBFUZZER_LINUX
+
+#endif  // LLVM_FUZZER_SUPPORT_PLATFORM_H