diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -82,6 +82,10 @@
   linker. If the user links the program with the ``clang`` or ``clang-cl``
   drivers, the driver will pass this flag for them.
 
+- Clang's profile files generated through ``-fprofile-instr-generate`` are using
+  a fixed hashing algorithm that prevents some collision when loading
+  out-of-date profile informations. Clang can still read old profile files.
+
 New Compiler Flags
 ------------------
 
diff --git a/clang/lib/CodeGen/CodeGenPGO.cpp b/clang/lib/CodeGen/CodeGenPGO.cpp
--- a/clang/lib/CodeGen/CodeGenPGO.cpp
+++ b/clang/lib/CodeGen/CodeGenPGO.cpp
@@ -52,9 +52,10 @@
 enum PGOHashVersion : unsigned {
   PGO_HASH_V1,
   PGO_HASH_V2,
+  PGO_HASH_V3,
 
   // Keep this set to the latest hash version.
-  PGO_HASH_LATEST = PGO_HASH_V2
+  PGO_HASH_LATEST = PGO_HASH_V3
 };
 
 namespace {
@@ -122,7 +123,7 @@
     BinaryOperatorGE,
     BinaryOperatorEQ,
     BinaryOperatorNE,
-    // The preceding values are available with PGO_HASH_V2.
+    // The preceding values are available since PGO_HASH_V2.
 
     // Keep this last.  It's for the static assert that follows.
     LastHashType
@@ -144,7 +145,9 @@
                                         CodeGenModule &CGM) {
   if (PGOReader->getVersion() <= 4)
     return PGO_HASH_V1;
-  return PGO_HASH_V2;
+  if (PGOReader->getVersion() <= 5)
+    return PGO_HASH_V2;
+  return PGO_HASH_V3;
 }
 
 /// A RecursiveASTVisitor that fills a map of statements to PGO counters.
@@ -288,7 +291,7 @@
         return PGOHash::BinaryOperatorLAnd;
       if (BO->getOpcode() == BO_LOr)
         return PGOHash::BinaryOperatorLOr;
-      if (HashVersion == PGO_HASH_V2) {
+      if (HashVersion >= PGO_HASH_V2) {
         switch (BO->getOpcode()) {
         default:
           break;
@@ -310,7 +313,7 @@
     }
     }
 
-    if (HashVersion == PGO_HASH_V2) {
+    if (HashVersion >= PGO_HASH_V2) {
       switch (S->getStmtClass()) {
       default:
         break;
@@ -747,13 +750,21 @@
     return Working;
 
   // Check for remaining work in Working.
-  if (Working)
-    MD5.update(Working);
+  if (Working) {
+    // Keep the buggy behavior from v1 and v2 for backward-compatibility. This
+    // is buggy because it converts a uint64_t into an array of uint8_t.
+    if (HashVersion < PGO_HASH_V3) {
+      MD5.update({(uint8_t)Working});
+    } else {
+      using namespace llvm::support;
+      uint64_t Swapped = endian::byte_swap<uint64_t, little>(Working);
+      MD5.update(llvm::makeArrayRef((uint8_t *)&Swapped, sizeof(Swapped)));
+    }
+  }
 
   // Finalize the MD5 and return the hash.
   llvm::MD5::MD5Result Result;
   MD5.final(Result);
-  using namespace llvm::support;
   return Result.low();
 }
 
diff --git a/clang/test/Profile/Inputs/c-counter-overflows.proftext b/clang/test/Profile/Inputs/c-counter-overflows.proftext
--- a/clang/test/Profile/Inputs/c-counter-overflows.proftext
+++ b/clang/test/Profile/Inputs/c-counter-overflows.proftext
@@ -1,5 +1,5 @@
 main
-10111551811706059223
+7779561829442898616
 8
 1
 68719476720
diff --git a/clang/test/Profile/Inputs/c-general.profdata.v5 b/clang/test/Profile/Inputs/c-general.profdata.v5
new file mode 100644
index 0000000000000000000000000000000000000000..0000000000000000000000000000000000000000
GIT binary patch
literal 0
Hc$@<O00001

literal 0
Hc$@<O00001

diff --git a/clang/test/Profile/Inputs/c-general.proftext b/clang/test/Profile/Inputs/c-general.proftext
--- a/clang/test/Profile/Inputs/c-general.proftext
+++ b/clang/test/Profile/Inputs/c-general.proftext
@@ -7,7 +7,7 @@
 75
 
 conditionals
-4190663230902537370
+4904767535850050386
 11
 1
 100
@@ -22,7 +22,7 @@
 100
 
 early_exits
-8265526549255474475
+2880354649761471549
 9
 1
 0
@@ -35,7 +35,7 @@
 0
 
 jumps
-15872630527555456493
+15051420506203462683
 22
 1
 1
@@ -61,7 +61,7 @@
 9
 
 switches
-11892326508727782373
+43242458792028222
 19
 1
 1
@@ -84,7 +84,7 @@
 0
 
 big_switch
-16933280399284440835
+13144136522122330070
 17
 1
 32
@@ -117,7 +117,7 @@
 50
 
 boolop_loops
-11270260636676715317
+12402604614320574815
 9
 1
 50
@@ -137,7 +137,7 @@
 1
 
 do_fallthrough
-6898770640283947069
+8714614136504380050
 4
 1
 10
diff --git a/clang/test/Profile/Inputs/c-unprofiled-blocks.proftext b/clang/test/Profile/Inputs/c-unprofiled-blocks.proftext
--- a/clang/test/Profile/Inputs/c-unprofiled-blocks.proftext
+++ b/clang/test/Profile/Inputs/c-unprofiled-blocks.proftext
@@ -1,5 +1,5 @@
 never_called
-5644096560937528444
+6820425066224770721
 9
 0
 0
@@ -17,7 +17,7 @@
 1
 
 dead_code
-9636018207904213947
+5254464978620792806
 10
 1
 0
diff --git a/clang/test/Profile/Inputs/cxx-rangefor.proftext b/clang/test/Profile/Inputs/cxx-rangefor.proftext
--- a/clang/test/Profile/Inputs/cxx-rangefor.proftext
+++ b/clang/test/Profile/Inputs/cxx-rangefor.proftext
@@ -1,5 +1,5 @@
 _Z9range_forv
-6169071350249721981
+8789831523895825398
 5
 1
 4
diff --git a/clang/test/Profile/Inputs/cxx-throws.proftext b/clang/test/Profile/Inputs/cxx-throws.proftext
--- a/clang/test/Profile/Inputs/cxx-throws.proftext
+++ b/clang/test/Profile/Inputs/cxx-throws.proftext
@@ -1,5 +1,5 @@
 _Z6throwsv
-340120998528097520
+18172607911962830854
 9
 1
 100
diff --git a/clang/test/Profile/Inputs/misexpect-switch-default.proftext b/clang/test/Profile/Inputs/misexpect-switch-default.proftext
--- a/clang/test/Profile/Inputs/misexpect-switch-default.proftext
+++ b/clang/test/Profile/Inputs/misexpect-switch-default.proftext
@@ -1,6 +1,6 @@
 main
 # Func Hash:
-8712453512413296413
+8734802134600123338
 # Num Counters:
 9
 # Counter Values:
diff --git a/clang/test/Profile/Inputs/misexpect-switch-nonconst.proftext b/clang/test/Profile/Inputs/misexpect-switch-nonconst.proftext
--- a/clang/test/Profile/Inputs/misexpect-switch-nonconst.proftext
+++ b/clang/test/Profile/Inputs/misexpect-switch-nonconst.proftext
@@ -1,6 +1,6 @@
 main
 # Func Hash:
-1965403898329309329
+3721743393642630379
 # Num Counters:
 10
 # Counter Values:
diff --git a/clang/test/Profile/Inputs/misexpect-switch.proftext b/clang/test/Profile/Inputs/misexpect-switch.proftext
--- a/clang/test/Profile/Inputs/misexpect-switch.proftext
+++ b/clang/test/Profile/Inputs/misexpect-switch.proftext
@@ -1,6 +1,6 @@
 main
 # Func Hash:
-1965403898329309329
+872687477373597607
 # Num Counters:
 9
 # Counter Values:
diff --git a/clang/test/Profile/c-collision.c b/clang/test/Profile/c-collision.c
new file mode 100644
--- /dev/null
+++ b/clang/test/Profile/c-collision.c
@@ -0,0 +1,22 @@
+// Test that a slight change in the code leads to a different hash.
+// RUN: %clang_cc1 -UEXTRA -triple x86_64-unknown-linux-gnu -main-file-name c-collision.c %s -o - -emit-llvm -fprofile-instrument=clang | FileCheck %s --check-prefix=CHECK-NOEXTRA
+// RUN: %clang_cc1 -DEXTRA -triple x86_64-unknown-linux-gnu -main-file-name c-collision.c %s -o - -emit-llvm -fprofile-instrument=clang | FileCheck %s --check-prefix=CHECK-EXTRA
+
+// CHECK-NOEXTRA: @__profd_foo = private global { {{.*}} } { i64 6699318081062747564, i64 7156072912471487002,
+// CHECK-EXTRA:   @__profd_foo = private global { {{.*}} } { i64 6699318081062747564, i64 -4383447408116050035,
+
+extern int bar;
+void foo() {
+  if (bar) {
+  }
+  if (bar) {
+  }
+  if (bar) {
+    if (bar) {
+#ifdef EXTRA
+      if (bar) {
+      }
+#endif
+    }
+  }
+}
diff --git a/clang/test/Profile/c-general.c b/clang/test/Profile/c-general.c
--- a/clang/test/Profile/c-general.c
+++ b/clang/test/Profile/c-general.c
@@ -4,6 +4,7 @@
 
 // RUN: llvm-profdata merge %S/Inputs/c-general.proftext -o %t.profdata
 // RUN: %clang_cc1 -triple x86_64-apple-macosx10.9 -main-file-name c-general.c %s -o - -emit-llvm -fprofile-instrument-use-path=%t.profdata | FileCheck -allow-deprecated-dag-overlap  -check-prefix=PGOUSE %s
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.9 -main-file-name c-general.c %s -o - -emit-llvm -fprofile-instrument-use-path=%S/Inputs/c-general.profdata.v5 | FileCheck -allow-deprecated-dag-overlap  -check-prefix=PGOUSE %s
 // RUN: %clang_cc1 -triple x86_64-apple-macosx10.9 -main-file-name c-general.c %s -o - -emit-llvm -fprofile-instrument-use-path=%S/Inputs/c-general.profdata.v3 | FileCheck -allow-deprecated-dag-overlap  -check-prefix=PGOUSE %s
 // Also check compatibility with older profiles.
 // RUN: %clang_cc1 -triple x86_64-apple-macosx10.9 -main-file-name c-general.c %s -o - -emit-llvm -fprofile-instrument-use-path=%S/Inputs/c-general.profdata.v1 | FileCheck -allow-deprecated-dag-overlap  -check-prefix=PGOUSE %s
diff --git a/llvm/include/llvm/ProfileData/InstrProf.h b/llvm/include/llvm/ProfileData/InstrProf.h
--- a/llvm/include/llvm/ProfileData/InstrProf.h
+++ b/llvm/include/llvm/ProfileData/InstrProf.h
@@ -979,6 +979,9 @@
   Version4 = 4,
   // In this version, the frontend PGO stable hash algorithm defaults to V2.
   Version5 = 5,
+  // In this version, the frontend PGO stable hash algorithm got fixed and
+  // may produce hashes different from Version5.
+  Version6 = 6,
   // The current version is 5.
   CurrentVersion = INSTR_PROF_INDEX_VERSION
 };
diff --git a/llvm/include/llvm/ProfileData/InstrProfData.inc b/llvm/include/llvm/ProfileData/InstrProfData.inc
--- a/llvm/include/llvm/ProfileData/InstrProfData.inc
+++ b/llvm/include/llvm/ProfileData/InstrProfData.inc
@@ -657,7 +657,7 @@
 /* Raw profile format version (start from 1). */
 #define INSTR_PROF_RAW_VERSION 5
 /* Indexed profile format version (start from 1). */
-#define INSTR_PROF_INDEX_VERSION 5
+#define INSTR_PROF_INDEX_VERSION 6
 /* Coverage mapping format version (start from 0). */
 #define INSTR_PROF_COVMAP_VERSION 3