Index: include/clang/Driver/Options.td
===================================================================
--- include/clang/Driver/Options.td
+++ include/clang/Driver/Options.td
@@ -1332,6 +1332,12 @@
 def frandom_seed_EQ : Joined<["-"], "frandom-seed=">, Group<clang_ignored_f_Group>;
 def freg_struct_return : Flag<["-"], "freg-struct-return">, Group<f_Group>, Flags<[CC1Option]>,
   HelpText<"Override the default ABI to return small structs in registers">;
+def freorder_functions : Flag <["-"], "freorder-functions">,
+  Group<f_Group>, Flags<[CC1Option]>,
+  HelpText<"Using special subsections for hot/cold functions to improve code "
+           "locality">;
+def fno_reorder_functions : Flag <["-"], "fno-reorder-functions">,
+  Group<f_Group>, Flags<[CC1Option]>;
 def frtti : Flag<["-"], "frtti">, Group<f_Group>;
 def : Flag<["-"], "fsched-interblock">, Group<clang_ignored_f_Group>;
 def fshort_enums : Flag<["-"], "fshort-enums">, Group<f_Group>, Flags<[CC1Option]>,
Index: include/clang/Frontend/CodeGenOptions.def
===================================================================
--- include/clang/Frontend/CodeGenOptions.def
+++ include/clang/Frontend/CodeGenOptions.def
@@ -174,6 +174,7 @@
 CODEGENOPT(TimePasses        , 1, 0) ///< Set when -ftime-report is enabled.
 CODEGENOPT(UnrollLoops       , 1, 0) ///< Control whether loops are unrolled.
 CODEGENOPT(RerollLoops       , 1, 0) ///< Control whether loops are rerolled.
+CODEGENOPT(ReorderFunctions  , 1, 1) ///< Set when -freorder-functions is enabled.
 CODEGENOPT(NoUseJumpTables   , 1, 0) ///< Set when -fno-jump-tables is enabled.
 CODEGENOPT(UnsafeFPMath      , 1, 0) ///< Allow unsafe floating point optzns.
 CODEGENOPT(UnwindTables      , 1, 0) ///< Emit unwind tables.
Index: lib/CodeGen/BackendUtil.cpp
===================================================================
--- lib/CodeGen/BackendUtil.cpp
+++ lib/CodeGen/BackendUtil.cpp
@@ -431,6 +431,7 @@
   Options.FunctionSections = CodeGenOpts.FunctionSections;
   Options.DataSections = CodeGenOpts.DataSections;
   Options.UniqueSectionNames = CodeGenOpts.UniqueSectionNames;
+  Options.ReorderFunctions = CodeGenOpts.ReorderFunctions;
   Options.EmulatedTLS = CodeGenOpts.EmulatedTLS;
   Options.DebuggerTuning = CodeGenOpts.getDebuggerTuning();
 
Index: lib/Driver/ToolChains/Clang.cpp
===================================================================
--- lib/Driver/ToolChains/Clang.cpp
+++ lib/Driver/ToolChains/Clang.cpp
@@ -2861,6 +2861,10 @@
                     options::OPT_fno_unique_section_names, true))
     CmdArgs.push_back("-fno-unique-section-names");
 
+  if (!Args.hasFlag(options::OPT_freorder_functions,
+                    options::OPT_fno_reorder_functions, true))
+    CmdArgs.push_back("-fno-reorder-functions");
+
   Args.AddAllArgs(CmdArgs, options::OPT_finstrument_functions);
 
   addPGOAndCoverageFlags(C, D, Output, Args, CmdArgs);
Index: lib/Frontend/CompilerInvocation.cpp
===================================================================
--- lib/Frontend/CompilerInvocation.cpp
+++ lib/Frontend/CompilerInvocation.cpp
@@ -643,6 +643,8 @@
                                    OPT_fno_data_sections, false);
   Opts.UniqueSectionNames = Args.hasFlag(OPT_funique_section_names,
                                          OPT_fno_unique_section_names, true);
+  Opts.ReorderFunctions =
+      Args.hasFlag(OPT_freorder_functions, OPT_fno_reorder_functions, true);
 
   Opts.MergeFunctions = Args.hasArg(OPT_fmerge_functions);
 
Index: test/CodeGen/Inputs/freorder-functions.prof
===================================================================
--- /dev/null
+++ test/CodeGen/Inputs/freorder-functions.prof
@@ -0,0 +1,5 @@
+hot_func:1000:0
+ 1: 0
+cold_func:0:0
+ 1: 1
+ 2: 1
Index: test/CodeGen/freorder-functions.c
===================================================================
--- /dev/null
+++ test/CodeGen/freorder-functions.c
@@ -0,0 +1,22 @@
+// REQUIRES: x86-registered-target
+
+// RUN: %clang_cc1 -triple x86_64-pc-linux -S -O3 -ffunction-sections -fprofile-sample-use=%S/Inputs/freorder-functions.prof -o - < %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-pc-linux -S -O3 -ffunction-sections -fprofile-sample-use=%S/Inputs/freorder-functions.prof -fno-reorder-functions -o - < %s | FileCheck --check-prefix=CHECK-NOPREFIX %s
+
+// opt tool option precedes driver option.
+// RUN: %clang_cc1 -triple x86_64-pc-linux -S -O3 -ffunction-sections -fprofile-sample-use=%S/Inputs/freorder-functions.prof -fno-reorder-functions -mllvm -profile-guided-section-prefix=true -o - < %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-pc-linux -S -O3 -ffunction-sections -fprofile-sample-use=%S/Inputs/freorder-functions.prof -freorder-functions -mllvm -profile-guided-section-prefix=false -o - < %s | FileCheck --check-prefix=CHECK-NOPREFIX %s
+
+void hot_func() {
+  return;
+}
+
+void cold_func() {
+  hot_func();
+  return;
+}
+
+// CHECK: .section .text.hot.hot_func,"ax",@progbits
+// CHECK: .section .text.unlikely.cold_func,"ax",@progbits
+// CHECK-NOPREFIX: .section .text.hot_func,"ax",@progbits
+// CHECK-NOPREFIX: .section .text.cold_func,"ax",@progbits
Index: test/Driver/function-sections.c
===================================================================
--- test/Driver/function-sections.c
+++ test/Driver/function-sections.c
@@ -6,6 +6,8 @@
 // CHECK-NODS-NOT: -fdata-sections
 // CHECK-US-NOT: -fno-unique-section-names
 // CHECK-NOUS: -fno-unique-section-names
+// CHECK-RF-NOT: -fno-reorder-functions
+// CHECK-NORF: -fno-reorder-functions
 
 // RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1       \
 // RUN:     -target i386-unknown-linux \
@@ -72,3 +74,13 @@
 // RUN:     -target i386-unknown-linux \
 // RUN:     -fno-unique-section-names \
 // RUN:   | FileCheck --check-prefix=CHECK-NOUS %s
+
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1        \
+// RUN:     -target i386-unknown-linux \
+// RUN:     -freorder-functions \
+// RUN:   | FileCheck --check-prefix=CHECK-RF %s
+
+// RUN: %clang -no-canonical-prefixes %s -### -fsyntax-only 2>&1        \
+// RUN:     -target i386-unknown-linux \
+// RUN:     -fno-reorder-functions \
+// RUN:   | FileCheck --check-prefix=CHECK-NORF %s