Index: lib/profile/InstrProfiling.h
===================================================================
--- lib/profile/InstrProfiling.h
+++ lib/profile/InstrProfiling.h
@@ -62,7 +62,9 @@
  *
  * Writes to the file with the last name given to \a __llvm_profile_set_filename(),
  * or if it hasn't been called, the \c LLVM_PROFILE_FILE environment variable,
- * or if that's not set, \c "default.profdata".
+ * or if that's not set, the last name given to
+ * \a __llvm_profile_override_default_filename(), or if that's not set,
+ * \c "default.profdata".
  */
 int __llvm_profile_write_file(void);
 
@@ -77,6 +79,19 @@
  */
 void __llvm_profile_set_filename(const char *Name);
 
+/*!
+ * \brief Set the filename for writing instrumentation data, unless the
+ * \c LLVM_PROFILE_FILE environment variable was set.
+ *
+ * Unless overridden, sets the filename to be used for subsequent calls to
+ * \a __llvm_profile_write_file().
+ *
+ * \c Name is not copied, so it must remain valid.  Passing NULL resets the
+ * filename logic to the default behaviour (unless the \c LLVM_PROFILE_FILE
+ * was set in which case it has no effect).
+ */
+void __llvm_profile_override_default_filename(const char *Name);
+
 /*! \brief Register to write instrumentation data to file at exit. */
 int __llvm_profile_register_write_file_atexit(void);
 
Index: lib/profile/InstrProfilingFile.c
===================================================================
--- lib/profile/InstrProfilingFile.c
+++ lib/profile/InstrProfilingFile.c
@@ -76,14 +76,6 @@
 __attribute__((weak)) int __llvm_profile_OwnsFilename = 0;
 __attribute__((weak)) const char *__llvm_profile_CurrentFilename = NULL;
 
-static void setFilename(const char *Filename, int OwnsFilename) {
-  if (__llvm_profile_OwnsFilename)
-    free(UNCONST(__llvm_profile_CurrentFilename));
-
-  __llvm_profile_CurrentFilename = Filename;
-  __llvm_profile_OwnsFilename = OwnsFilename;
-}
-
 static void truncateCurrentFile(void) {
   const char *Filename;
   FILE *File;
@@ -99,19 +91,36 @@
   fclose(File);
 }
 
-static void setDefaultFilename(void) { setFilename("default.profraw", 0); }
+static void setFilename(const char *Filename, int OwnsFilename) {
+  /* Check if this is a new filename and therefore needs truncation. */
+  int NewFile = !__llvm_profile_CurrentFilename ||
+      (Filename && strcmp(Filename, __llvm_profile_CurrentFilename));
+  if (__llvm_profile_OwnsFilename)
+    free(UNCONST(__llvm_profile_CurrentFilename));
+
+  __llvm_profile_CurrentFilename = Filename;
+  __llvm_profile_OwnsFilename = OwnsFilename;
+
+  /* If not a new file, append to support profiling multiple shared objects. */
+  if (NewFile)
+    truncateCurrentFile();
+}
+
+static void resetFilenameToDefault(void) { setFilename("default.profraw", 0); }
 
 int getpid(void);
-static int setFilenameFromEnvironment(void) {
-  const char *Filename = getenv("LLVM_PROFILE_FILE");
+static int setFilenamePossiblyWithPid(const char *Filename) {
 #define MAX_PID_SIZE 16
   char PidChars[MAX_PID_SIZE] = {0};
   int NumPids = 0, PidLength = 0;
   char *Allocated;
   int I, J;
 
-  if (!Filename || !Filename[0])
-    return -1;
+  /* Reset filename on NULL, except with env var which is checked by caller. */
+  if (!Filename) {
+    resetFilenameToDefault();
+    return 0;
+  }
 
   /* Check the filename for "%p", which indicates a pid-substitution. */
   for (I = 0; Filename[I]; ++I)
@@ -148,11 +157,20 @@
   return 0;
 }
 
+static int setFilenameFromEnvironment(void) {
+  const char *Filename = getenv("LLVM_PROFILE_FILE");
+
+  if (!Filename || !Filename[0])
+    return -1;
+
+  return setFilenamePossiblyWithPid(Filename);
+}
+
 static void setFilenameAutomatically(void) {
   if (!setFilenameFromEnvironment())
     return;
 
-  setDefaultFilename();
+  resetFilenameToDefault();
 }
 
 __attribute__((visibility("hidden")))
@@ -163,13 +181,20 @@
 
   /* Detect the filename and truncate. */
   setFilenameAutomatically();
-  truncateCurrentFile();
 }
 
 __attribute__((visibility("hidden")))
 void __llvm_profile_set_filename(const char *Filename) {
-  setFilename(Filename, 0);
-  truncateCurrentFile();
+  setFilenamePossiblyWithPid(Filename);
+}
+
+__attribute__((visibility("hidden")))
+void __llvm_profile_override_default_filename(const char *Filename) {
+  /* If the env var is set, skip setting filename from argument. */
+  const char *Env_Filename = getenv("LLVM_PROFILE_FILE");
+  if (Env_Filename && Env_Filename[0])
+    return;
+  setFilenamePossiblyWithPid(Filename);
 }
 
 __attribute__((visibility("hidden")))
Index: test/profile/instrprof-override-filename-then-reset-default.c
===================================================================
--- /dev/null
+++ test/profile/instrprof-override-filename-then-reset-default.c
@@ -0,0 +1,15 @@
+// RUN: %clang_profgen -o %t -O3 %s
+// RUN: %run %t %t.profraw
+// RUN: llvm-profdata merge -o %t.profdata default.profraw
+// RUN: %clang_profuse=%t.profdata -o - -S -emit-llvm %s | FileCheck %s
+
+void __llvm_profile_override_default_filename(const char *);
+int main(int argc, const char *argv[]) {
+  // CHECK: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}, !prof ![[PD1:[0-9]+]]
+  if (argc < 2)
+    return 1;
+  __llvm_profile_override_default_filename(argv[1]);
+  __llvm_profile_override_default_filename(0);
+  return 0;
+}
+// CHECK: ![[PD1]] = !{!"branch_weights", i32 1, i32 2}
Index: test/profile/instrprof-override-filename-with-env.c
===================================================================
--- /dev/null
+++ test/profile/instrprof-override-filename-with-env.c
@@ -0,0 +1,14 @@
+// RUN: %clang_profgen -o %t -O3 %s
+// RUN: env LLVM_PROFILE_FILE=%t.good.profraw %run %t %t.bad.profraw
+// RUN: llvm-profdata merge -o %t.profdata %t.good.profraw
+// RUN: %clang_profuse=%t.profdata -o - -S -emit-llvm %s | FileCheck %s
+
+void __llvm_profile_override_default_filename(const char *);
+int main(int argc, const char *argv[]) {
+  // CHECK: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}, !prof ![[PD1:[0-9]+]]
+  if (argc < 2)
+    return 1;
+  __llvm_profile_override_default_filename(argv[1]);
+  return 0;
+}
+// CHECK: ![[PD1]] = !{!"branch_weights", i32 1, i32 2}
Index: test/profile/instrprof-override-filename.c
===================================================================
--- /dev/null
+++ test/profile/instrprof-override-filename.c
@@ -0,0 +1,14 @@
+// RUN: %clang_profgen -o %t -O3 %s
+// RUN: %run %t %t.profraw
+// RUN: llvm-profdata merge -o %t.profdata %t.profraw
+// RUN: %clang_profuse=%t.profdata -o - -S -emit-llvm %s | FileCheck %s
+
+void __llvm_profile_override_default_filename(const char *);
+int main(int argc, const char *argv[]) {
+  // CHECK: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}, !prof ![[PD1:[0-9]+]]
+  if (argc < 2)
+    return 1;
+  __llvm_profile_override_default_filename(argv[1]);
+  return 0;
+}
+// CHECK: ![[PD1]] = !{!"branch_weights", i32 1, i32 2}
Index: test/profile/instrprof-set-filename-then-reset-default.c
===================================================================
--- /dev/null
+++ test/profile/instrprof-set-filename-then-reset-default.c
@@ -0,0 +1,15 @@
+// RUN: %clang_profgen -o %t -O3 %s
+// RUN: %run %t %t.profraw
+// RUN: llvm-profdata merge -o %t.profdata default.profraw
+// RUN: %clang_profuse=%t.profdata -o - -S -emit-llvm %s | FileCheck %s
+
+void __llvm_profile_set_filename(const char *);
+int main(int argc, const char *argv[]) {
+  // CHECK: br i1 %{{.*}}, label %{{.*}}, label %{{.*}}, !prof ![[PD1:[0-9]+]]
+  if (argc < 2)
+    return 1;
+  __llvm_profile_set_filename(argv[1]);
+  __llvm_profile_set_filename(0);
+  return 0;
+}
+// CHECK: ![[PD1]] = !{!"branch_weights", i32 1, i32 2}