Index: lib/profile/GCDAProfiling.c
===================================================================
--- lib/profile/GCDAProfiling.c
+++ lib/profile/GCDAProfiling.c
@@ -341,38 +341,26 @@
 COMPILER_RT_VISIBILITY
 void llvm_gcda_start_file(const char *orig_filename, const char version[4],
                           uint32_t checksum) {
-  const char *mode = "r+b";
   filename = mangle_filename(orig_filename);
-
-  /* Try just opening the file. */
   new_file = 0;
-  fd = open(filename, O_RDWR | O_BINARY);
+  fd = lprofOpenFile(filename, &new_file);
 
   if (fd == -1) {
-    /* Try opening the file, creating it if necessary. */
-    new_file = 1;
-    mode = "w+b";
-    fd = open(filename, O_RDWR | O_CREAT | O_BINARY, 0644);
-    if (fd == -1) {
-      /* Try creating the directories first then opening the file. */
-      __llvm_profile_recursive_mkdir(filename);
-      fd = open(filename, O_RDWR | O_CREAT | O_BINARY, 0644);
-      if (fd == -1) {
-        /* Bah! It's hopeless. */
-        int errnum = errno;
-        fprintf(stderr, "profiling: %s: cannot open: %s\n", filename,
-                strerror(errnum));
-        return;
-      }
-    }
+    /* Bah! It's hopeless. */
+    fprintf(stderr, "profiling: %s: cannot open: %s\n", filename,
+            strerror(errno));
+    return;
   }
 
   /* Try to flock the file to serialize concurrent processes writing out to the
    * same GCDA. This can fail if the filesystem doesn't support it, but in that
    * case we'll just carry on with the old racy behaviour and hope for the best.
    */
-  lprofLockFd(fd);
-  output_file = fdopen(fd, mode);
+  if (lprofLockFd(fd) != 0) {
+    fprintf(stderr, "profiling: %s: cannot lock: %s\n", filename,
+            strerror(errno));
+  }
+  output_file = fdopen(fd, new_file ? "w+b" : "r+b");
 
   /* Initialize the write buffer. */
   write_buffer = NULL;
Index: lib/profile/InstrProfilingUtil.h
===================================================================
--- lib/profile/InstrProfilingUtil.h
+++ lib/profile/InstrProfilingUtil.h
@@ -24,6 +24,8 @@
 
 int lprofLockFd(int fd);
 int lprofUnlockFd(int fd);
+int lprofOpenOrCreateFile(const char *filename, int *newfile);
+int lprofOpenFile(char *filename, int *newfile);
 
 /*! Open file \c Filename for read+write with write
  * lock for exclusive access. The caller will block
Index: lib/profile/InstrProfilingUtil.c
===================================================================
--- lib/profile/InstrProfilingUtil.c
+++ lib/profile/InstrProfilingUtil.c
@@ -7,6 +7,8 @@
 |*
 \*===----------------------------------------------------------------------===*/
 
+#include <fcntl.h>
+
 #ifdef _WIN32
 #include <direct.h>
 #include <process.h>
@@ -16,7 +18,6 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <unistd.h>
-#include <fcntl.h>
 #include <errno.h>
 #endif
 
@@ -125,8 +126,7 @@
   }
   return 0;
 #else
-  flock(fd, LOCK_EX);
-  return 0;
+  return flock(fd, LOCK_EX);
 #endif
 }
 
@@ -150,8 +150,7 @@
   }
   return 0;
 #else
-  flock(fd, LOCK_UN);
-  return 0;
+  return flock(fd, LOCK_UN);
 #endif
 }
 
@@ -288,3 +287,25 @@
   prctl(PR_SET_PDEATHSIG, SIGKILL);
 #endif
 }
+
+COMPILER_RT_VISIBILITY
+int lprofOpenOrCreateFile(const char *filename, int *newfile) {
+  int fd = open(filename, O_RDWR | O_CREAT | O_EXCL | O_BINARY, 0644);
+  if (fd == -1 && errno == EEXIST) {
+    return open(filename, O_RDWR | O_BINARY);
+  }
+  *newfile = 1;
+  return fd;
+}
+
+COMPILER_RT_VISIBILITY
+int lprofOpenFile(char *filename, int *newfile) {
+  int fd = lprofOpenOrCreateFile(filename, newfile);
+  if (fd == -1) {
+    /* Try creating the directories first then opening the file. */
+    __llvm_profile_recursive_mkdir(filename);
+    *newfile = 0;
+    return lprofOpenOrCreateFile(filename, newfile);
+  }
+  return fd;
+}
Index: test/profile/Inputs/instrprof-gcov-prefix.c
===================================================================
--- /dev/null
+++ test/profile/Inputs/instrprof-gcov-prefix.c
@@ -0,0 +1 @@
+int main() { return 0; }
Index: test/profile/Posix/instrprof-gcov-prefix.test
===================================================================
--- /dev/null
+++ test/profile/Posix/instrprof-gcov-prefix.test
@@ -0,0 +1,16 @@
+RUN: mkdir -p %t.d
+RUN: cd %t.d
+
+RUN: rm -f instrprof-gcov-prefix.gcno instrprof-gcov-prefix.gcda
+RUN: %clang --coverage -o %t %S/../Inputs/instrprof-gcov-prefix.c
+RUN: test -f instrprof-gcov-prefix.gcno
+RUN: %run %t
+RUN: test -f instrprof-gcov-prefix.gcda
+RUN: rm -f instrprof-gcov-prefix.gcno instrprof-gcov-prefix.gcda
+
+RUN: rm -fr %t.d/dir1
+RUN: GCOV_PREFIX=%t.d/dir1/dir2/dir3 %run %t
+RUN: GCOV_PREFIX=%t.d/dir1/dir2/dir3 %run %t
+RUN: test ! -f instrprof-gcov-prefix.gcda
+RUN: test -f %t.d/dir1/dir2/dir3/`pwd`/instrprof-gcov-prefix.gcda
+RUN: rm -fr %t.d/dir1