Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -208,6 +208,7 @@
 #================================
 include(CheckIncludeFile)
 check_include_file(unwind.h HAVE_UNWIND_H)
+check_include_file(mpi.h HAVE_MPI_H)
 
 include(config-ix)
 
Index: lib/profile/CMakeLists.txt
===================================================================
--- lib/profile/CMakeLists.txt
+++ lib/profile/CMakeLists.txt
@@ -36,6 +36,7 @@
   InstrProfilingPlatformDarwin.c
   InstrProfilingPlatformLinux.c
   InstrProfilingPlatformOther.c
+  InstrProfilingMPI.c
   InstrProfilingRuntime.cc
   InstrProfilingUtil.c)
 
@@ -55,6 +56,12 @@
      -DCOMPILER_RT_HAS_ATOMICS=1)
 endif() 
 
+if(HAVE_MPI_H)
+ set(EXTRA_FLAGS
+     ${EXTRA_FLAGS}
+     -DHAVE_MPI_H=1)
+endif() 
+
 if(APPLE)
   add_compiler_rt_runtime(clang_rt.profile
     STATIC
Index: lib/profile/InstrProfiling.h
===================================================================
--- lib/profile/InstrProfiling.h
+++ lib/profile/InstrProfiling.h
@@ -13,6 +13,8 @@
 #include "InstrProfilingPort.h"
 #include "InstrProfData.inc"
 
+COMPILER_RT_VISIBILITY extern int __llvm_write_prof_data;
+
 enum ValueKind {
 #define VALUE_PROF_KIND(Enumerator, Value) Enumerator = Value,
 #include "InstrProfData.inc"
Index: lib/profile/InstrProfilingFile.c
===================================================================
--- lib/profile/InstrProfilingFile.c
+++ lib/profile/InstrProfilingFile.c
@@ -14,6 +14,15 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+
+#ifdef _MSC_VER
+#include <fcntl.h>
+#include <io.h>
+#else
+#include <unistd.h>
+#endif
 
 #define UNCONST(ptr) ((void *)(uintptr_t)(ptr))
 
@@ -72,7 +81,6 @@
 
 static void truncateCurrentFile(void) {
   const char *Filename;
-  FILE *File;
 
   Filename = __llvm_profile_CurrentFilename;
   if (!Filename || !Filename[0])
@@ -86,11 +94,16 @@
     free(Copy);
   }
 
-  /* Truncate the file.  Later we'll reopen and append. */
-  File = fopen(Filename, "w");
-  if (!File)
+  /* truncate, or open without O_CREAT, is used to avoid creating empty files.
+   * In MPI applications, a file will only be created by one process */
+#ifdef _MSC_VER
+  int File = _open(Filename, _O_WRONLY | _O_TRUNC);
+  if (File == -1)
     return;
-  fclose(File);
+  _close(File);
+#else
+  (void) truncate(Filename, 0);
+#endif
 }
 
 static void setFilename(const char *Filename, int OwnsFilename) {
@@ -215,7 +228,10 @@
 }
 
 COMPILER_RT_VISIBILITY
-int __llvm_profile_write_file(void) {
+int __llvm_profile_write_file(void) {  
+  if (!__llvm_write_prof_data)
+      return 0;
+
   int rc;
 
   GetEnvHook = &getenv;
Index: lib/profile/InstrProfilingMPI.c
===================================================================
--- /dev/null
+++ lib/profile/InstrProfilingMPI.c
@@ -0,0 +1,50 @@
+/*===-- InstrProfilingMPI.c - Support library for applications using MPI --===*\
+|*
+|*                     The LLVM Compiler Infrastructure
+|*
+|* This file is distributed under the University of Illinois Open Source
+|* License. See LICENSE.TXT for details.
+|*
+\*===----------------------------------------------------------------------===*/
+
+#include <stdlib.h>
+#include <InstrProfiling.h>
+#ifdef HAVE_MPI_H
+#include <mpi.h>
+#endif
+
+/* Defined here to make sure that the linker brings in the MPI_Finalize
+ * definition provided in this file */
+COMPILER_RT_VISIBILITY int __llvm_write_prof_data = 1;
+
+COMPILER_RT_WEAK int PMPI_Finalize() { abort(); }
+
+#ifdef HAVE_MPI_H
+COMPILER_RT_VISIBILITY int MPI_Finalize() {
+   int Rank;
+   uint64_t *CountersBegin = __llvm_profile_begin_counters();
+   uint64_t *CountersEnd = __llvm_profile_end_counters();
+   uint64_t CountersSize = CountersEnd - CountersBegin;
+
+   PMPI_Comm_rank(MPI_COMM_WORLD, &Rank);
+
+   /* Only counters are reduced here, not value profile data */
+   if (Rank) {
+      PMPI_Reduce(CountersBegin, NULL, CountersSize, MPI_UINT64_T, MPI_SUM,
+              0, MPI_COMM_WORLD);
+      __llvm_write_prof_data = 0;
+   } else {
+      PMPI_Reduce(MPI_IN_PLACE, CountersBegin, CountersSize, MPI_UINT64_T,
+                 MPI_SUM, 0, MPI_COMM_WORLD);
+   }
+   return PMPI_Finalize();
+}
+
+COMPILER_RT_WEAK int PMPI_Comm_rank(MPI_Comm comm, int *rank) { abort(); }
+COMPILER_RT_WEAK int PMPI_Reduce(const void *sendbuf, void *recvbuf, int count,
+   MPI_Datatype datatype, MPI_Op op, int root, MPI_Comm comm) { abort(); }
+#else
+/* Included here so regression test has a definition to use */
+COMPILER_RT_WEAK int MPI_Finalize() { return PMPI_Finalize(); }
+#endif
+
Index: test/profile/instrprof-reduce.c
===================================================================
--- /dev/null
+++ test/profile/instrprof-reduce.c
@@ -0,0 +1,23 @@
+// RUN: %clang_profgen -o %t -O3 %s
+// RUN: rm -f %t.profraw
+// RUN: env LLVM_PROFILE_FILE=%t.profraw %run %t 0
+// RUN: test -e %t.profraw
+// RUN: rm -f %t.profraw
+// RUN: env LLVM_PROFILE_FILE=%t.profraw %run %t 1
+// RUN: test ! -e %t.profraw
+// REQUIRES: shell
+
+#include <stdlib.h>
+#include <stdio.h>
+int ThisRank;
+int PMPI_Comm_rank(void *Comm, int *Rank) { *Rank = ThisRank; return 0; }
+int PMPI_Reduce() { return 0; }
+int PMPI_Finalize() { return 0; }
+int MPI_Finalize();
+
+int main(int argc, const char *argv[]) {
+  ThisRank = atoi(argv[1]);
+  MPI_Finalize();
+  return 0;
+}
+