Index: llvm/cmake/config-ix.cmake
===================================================================
--- llvm/cmake/config-ix.cmake
+++ llvm/cmake/config-ix.cmake
@@ -197,6 +197,11 @@
 check_symbol_exists(futimens sys/stat.h HAVE_FUTIMENS)
 check_symbol_exists(futimes sys/time.h HAVE_FUTIMES)
 check_symbol_exists(posix_fallocate fcntl.h HAVE_POSIX_FALLOCATE)
+
+set(CMAKE_REQUIRED_DEFINITIONS "-D_GNU_SOURCE")
+check_symbol_exists(fallocate fcntl.h HAVE_FALLOCATE)
+set(CMAKE_REQUIRED_DEFINITIONS "")
+
 # AddressSanitizer conflicts with lib/Support/Unix/Signals.inc
 # Avoid sigaltstack on Apple platforms, where backtrace() cannot handle it
 # (rdar://7089625) and _Unwind_Backtrace is unusable because it cannot unwind
Index: llvm/include/llvm/Config/config.h.cmake
===================================================================
--- llvm/include/llvm/Config/config.h.cmake
+++ llvm/include/llvm/Config/config.h.cmake
@@ -59,6 +59,9 @@
 /* Define to 1 if you have the <errno.h> header file. */
 #cmakedefine HAVE_ERRNO_H ${HAVE_ERRNO_H}
 
+/* Define to 1 if you have the `fallocate' function. */
+#cmakedefine HAVE_FALLOCATE ${HAVE_FALLOCATE}
+
 /* Define to 1 if you have the <fcntl.h> header file. */
 #cmakedefine HAVE_FCNTL_H ${HAVE_FCNTL_H}
 
Index: llvm/include/llvm/Support/FileSystem.h
===================================================================
--- llvm/include/llvm/Support/FileSystem.h
+++ llvm/include/llvm/Support/FileSystem.h
@@ -391,6 +391,14 @@
 ///          platform-specific error_code.
 std::error_code resize_file(int FD, uint64_t Size);
 
+/// @brief Preallocate disk blocks to a given file.
+///
+/// @param FD Input file descriptor.
+/// @param Size Size to resize to.
+/// @returns errc::success if \a path has been resized to \a size, otherwise a
+///          platform-specific error_code.
+std::error_code allocate_file(int FD, uint64_t Size);
+
 /// @brief Compute an MD5 hash of a file's contents.
 ///
 /// @param FD Input file descriptor.
Index: llvm/lib/Support/Unix/Path.inc
===================================================================
--- llvm/lib/Support/Unix/Path.inc
+++ llvm/lib/Support/Unix/Path.inc
@@ -438,6 +438,23 @@
   return std::error_code();
 }
 
+std::error_code allocate_file(int FD, uint64_t Size) {
+#ifdef HAVE_FALLOCATE
+  if (fallocate(FD, 0, 0, Size) == -1)
+    return std::error_code(errno, std::generic_category());
+  return std::error_code();
+#endif
+
+#ifdef __APPLE__
+  fstore_t Store = {F_ALLOCATEALL, F_PEOFPOSMODE, 0, Size};
+  if (fcntl(fd, F_PREALLOCATE, &Store) == -1)
+    return std::error_code(errno, std::generic_category());
+  return std::error_code();
+#endif
+
+  return make_error_code(errc::function_not_supported);
+}
+
 static int convertAccessMode(AccessMode Mode) {
   switch (Mode) {
   case AccessMode::Exist:
Index: llvm/lib/Support/Windows/Path.inc
===================================================================
--- llvm/lib/Support/Windows/Path.inc
+++ llvm/lib/Support/Windows/Path.inc
@@ -509,6 +509,8 @@
   return std::error_code(error, std::generic_category());
 }
 
+std::error_code allocate_file(int FD, uint64_t Size) { resize_file(FD, Size); }
+
 std::error_code access(const Twine &Path, AccessMode Mode) {
   SmallVector<wchar_t, 128> PathUtf16;