Index: clang/lib/CodeGen/CodeGenPGO.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenPGO.cpp
+++ clang/lib/CodeGen/CodeGenPGO.cpp
@@ -747,13 +747,21 @@
     return Working;
 
   // Check for remaining work in Working.
-  if (Working)
-    MD5.update(Working);
+  if (Working) {
+    // Keep the buggy behavior from v1 for backward-compatibility. This is buggy
+    // because it implicitly converts a uint64_t into an array of uint8_t.
+    if (HashVersion == PGO_HASH_V1) {
+      MD5.update(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();
 }
 
Index: clang/test/Profile/Inputs/c-counter-overflows.proftext
===================================================================
--- clang/test/Profile/Inputs/c-counter-overflows.proftext
+++ clang/test/Profile/Inputs/c-counter-overflows.proftext
@@ -1,5 +1,5 @@
 main
-10111551811706059223
+7779561829442898616
 8
 1
 68719476720
Index: clang/test/Profile/Inputs/c-general.proftext
===================================================================
--- clang/test/Profile/Inputs/c-general.proftext
+++ 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
Index: clang/test/Profile/Inputs/c-unprofiled-blocks.proftext
===================================================================
--- clang/test/Profile/Inputs/c-unprofiled-blocks.proftext
+++ 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
Index: clang/test/Profile/Inputs/cxx-rangefor.proftext
===================================================================
--- clang/test/Profile/Inputs/cxx-rangefor.proftext
+++ clang/test/Profile/Inputs/cxx-rangefor.proftext
@@ -1,5 +1,5 @@
 _Z9range_forv
-6169071350249721981
+8789831523895825398
 5
 1
 4
Index: clang/test/Profile/Inputs/cxx-throws.proftext
===================================================================
--- clang/test/Profile/Inputs/cxx-throws.proftext
+++ clang/test/Profile/Inputs/cxx-throws.proftext
@@ -1,5 +1,5 @@
 _Z6throwsv
-340120998528097520
+18172607911962830854
 9
 1
 100
Index: clang/test/Profile/Inputs/misexpect-switch-default.proftext
===================================================================
--- clang/test/Profile/Inputs/misexpect-switch-default.proftext
+++ clang/test/Profile/Inputs/misexpect-switch-default.proftext
@@ -1,6 +1,6 @@
 main
 # Func Hash:
-8712453512413296413
+8734802134600123338
 # Num Counters:
 9
 # Counter Values:
Index: clang/test/Profile/Inputs/misexpect-switch-nonconst.proftext
===================================================================
--- clang/test/Profile/Inputs/misexpect-switch-nonconst.proftext
+++ clang/test/Profile/Inputs/misexpect-switch-nonconst.proftext
@@ -1,6 +1,6 @@
 main
 # Func Hash:
-1965403898329309329
+3721743393642630379
 # Num Counters:
 10
 # Counter Values:
Index: clang/test/Profile/Inputs/misexpect-switch.proftext
===================================================================
--- clang/test/Profile/Inputs/misexpect-switch.proftext
+++ clang/test/Profile/Inputs/misexpect-switch.proftext
@@ -1,6 +1,6 @@
 main
 # Func Hash:
-1965403898329309329
+872687477373597607
 # Num Counters:
 9
 # Counter Values:
Index: clang/test/Profile/c-collision.c
===================================================================
--- /dev/null
+++ 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
+    }
+  }
+}