diff --git a/include/llvm/Support/DataTypes.h.cmake b/include/llvm/Support/DataTypes.h.cmake
index 7484abd3686..1dcec0588fa 100644
--- a/include/llvm/Support/DataTypes.h.cmake
+++ b/include/llvm/Support/DataTypes.h.cmake
@@ -101,7 +101,11 @@ typedef short int16_t;
 typedef unsigned short uint16_t;
 typedef signed char int8_t;
 typedef unsigned char uint8_t;
-typedef signed int ssize_t;
+#if defined(_WIN64)
+  typedef signed __int64 ssize_t;
+#else
+  typedef signed int ssize_t;
+#endif
 #ifndef INT8_MAX
 # define INT8_MAX 127
 #endif
diff --git a/include/llvm/Support/DataTypes.h.in b/include/llvm/Support/DataTypes.h.in
index b9fb48a1d49..d6c38169504 100644
--- a/include/llvm/Support/DataTypes.h.in
+++ b/include/llvm/Support/DataTypes.h.in
@@ -98,7 +98,12 @@ typedef short int16_t;
 typedef unsigned short uint16_t;
 typedef signed char int8_t;
 typedef unsigned char uint8_t;
-typedef signed int ssize_t;
+#if defined(_WIN64)
+  typedef signed __int64 ssize_t;
+#else
+  typedef signed long ssize_t;
+#endif
+
 #ifndef INT8_MAX
 # define INT8_MAX 127
 #endif