Index: include/llvm/Transforms/IPO/SampleProfile.h
===================================================================
--- include/llvm/Transforms/IPO/SampleProfile.h
+++ include/llvm/Transforms/IPO/SampleProfile.h
@@ -26,9 +26,11 @@
 class SampleProfileLoaderPass : public PassInfoMixin<SampleProfileLoaderPass> {
 public:
   SampleProfileLoaderPass(std::string File = "", std::string RemappingFile = "",
-                          bool IsThinLTOPreLink = false)
+                          bool IsThinLTOPreLink = false,
+                          bool IsThinLTOPostLink = false)
       : ProfileFileName(File), ProfileRemappingFileName(RemappingFile),
-        IsThinLTOPreLink(IsThinLTOPreLink) {}
+        IsThinLTOPreLink(IsThinLTOPreLink),
+        IsThinLTOPostLink(IsThinLTOPostLink) {}
 
   PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
 
@@ -36,6 +38,7 @@
   std::string ProfileFileName;
   std::string ProfileRemappingFileName;
   bool IsThinLTOPreLink;
+  bool IsThinLTOPostLink;
 };
 
 } // end namespace llvm
Index: lib/Passes/PassBuilder.cpp
===================================================================
--- lib/Passes/PassBuilder.cpp
+++ lib/Passes/PassBuilder.cpp
@@ -607,9 +607,9 @@
   if (PGOOpt && !PGOOpt->SampleProfileFile.empty()) {
     // Annotate sample profile right after early FPM to ensure freshness of
     // the debug info.
-    MPM.addPass(SampleProfileLoaderPass(PGOOpt->SampleProfileFile,
-                                        PGOOpt->ProfileRemappingFile,
-                                        Phase == ThinLTOPhase::PreLink));
+    MPM.addPass(SampleProfileLoaderPass(
+        PGOOpt->SampleProfileFile, PGOOpt->ProfileRemappingFile,
+        Phase == ThinLTOPhase::PreLink, Phase == ThinLTOPhase::PostLink));
     // Do not invoke ICP in the ThinLTOPrelink phase as it makes it hard
     // for the profile annotation to be accurate in the ThinLTO backend.
     if (Phase != ThinLTOPhase::PreLink)
Index: lib/Transforms/IPO/SampleProfile.cpp
===================================================================
--- lib/Transforms/IPO/SampleProfile.cpp
+++ lib/Transforms/IPO/SampleProfile.cpp
@@ -123,6 +123,11 @@
     cl::desc("Use this option to turn off/on warnings about function with "
              "samples but without debug information to use those samples. "));
 
+static cl::opt<bool> FlattenProfileUsed(
+    "flatten-profile-used", cl::init(false), cl::Hidden,
+    cl::desc("Indicate the sample profile being used is flatten, i.e., "
+             "no inline hierachy exists in the profile. "));
+
 namespace {
 
 using BlockWeightMap = DenseMap<const BasicBlock *, uint64_t>;
@@ -191,11 +196,13 @@
 public:
   SampleProfileLoader(
       StringRef Name, StringRef RemapName, bool IsThinLTOPreLink,
+      bool IsThinLTOPostLink,
       std::function<AssumptionCache &(Function &)> GetAssumptionCache,
       std::function<TargetTransformInfo &(Function &)> GetTargetTransformInfo)
       : GetAC(std::move(GetAssumptionCache)),
         GetTTI(std::move(GetTargetTransformInfo)), Filename(Name),
-        RemappingFilename(RemapName), IsThinLTOPreLink(IsThinLTOPreLink) {}
+        RemappingFilename(RemapName), IsThinLTOPreLink(IsThinLTOPreLink),
+        IsThinLTOPostLink(IsThinLTOPostLink) {}
 
   bool doInitialization(Module &M);
   bool runOnModule(Module &M, ModuleAnalysisManager *AM,
@@ -300,6 +307,8 @@
   /// In this phase, in annotation, we should not promote indirect calls.
   /// Instead, we will mark GUIDs that needs to be annotated to the function.
   bool IsThinLTOPreLink;
+  /// Flag indicating if the pass is invoked in ThinLTO backend compile phase.
+  bool IsThinLTOPostLink;
 
   /// Profile Summary Info computed from sample profile.
   ProfileSummaryInfo *PSI = nullptr;
@@ -320,15 +329,17 @@
   static char ID;
 
   SampleProfileLoaderLegacyPass(StringRef Name = SampleProfileFile,
-                                bool IsThinLTOPreLink = false)
-      : ModulePass(ID),
-        SampleLoader(Name, SampleProfileRemappingFile, IsThinLTOPreLink,
-                     [&](Function &F) -> AssumptionCache & {
-                       return ACT->getAssumptionCache(F);
-                     },
-                     [&](Function &F) -> TargetTransformInfo & {
-                       return TTIWP->getTTI(F);
-                     }) {
+                                bool IsThinLTOPreLink = false,
+                                bool IsThinLTOPostLink = false)
+      : ModulePass(ID), SampleLoader(
+                            Name, SampleProfileRemappingFile, IsThinLTOPreLink,
+                            IsThinLTOPostLink,
+                            [&](Function &F) -> AssumptionCache & {
+                              return ACT->getAssumptionCache(F);
+                            },
+                            [&](Function &F) -> TargetTransformInfo & {
+                              return TTIWP->getTTI(F);
+                            }) {
     initializeSampleProfileLoaderLegacyPassPass(
         *PassRegistry::getPassRegistry());
   }
@@ -1516,6 +1527,14 @@
                     "Sample Profile loader", false, false)
 
 bool SampleProfileLoader::doInitialization(Module &M) {
+  // In ThinLTO mode, when flatten profile is used, all the available
+  // profile information will be annotated in Prelink phase so there is
+  // no need to read the profile again.
+  if (IsThinLTOPostLink && FlattenProfileUsed)
+    return false;
+
+  LLVM_DEBUG(dbgs() << "SampleProfileLoader initialize for Module: " << M.getName() << "\n");
+
   auto &Ctx = M.getContext();
   auto ReaderOrErr = SampleProfileReader::create(Filename, Ctx);
   if (std::error_code EC = ReaderOrErr.getError()) {
@@ -1640,7 +1659,7 @@
       ProfileFileName.empty() ? SampleProfileFile : ProfileFileName,
       ProfileRemappingFileName.empty() ? SampleProfileRemappingFile
                                        : ProfileRemappingFileName,
-      IsThinLTOPreLink, GetAssumptionCache, GetTTI);
+      IsThinLTOPreLink, IsThinLTOPostLink, GetAssumptionCache, GetTTI);
 
   SampleLoader.doInitialization(M);
 
Index: test/Transforms/SampleProfile/Inputs/flatten.prof
===================================================================
--- test/Transforms/SampleProfile/Inputs/flatten.prof
+++ test/Transforms/SampleProfile/Inputs/flatten.prof
@@ -0,0 +1,2 @@
+foo:100:100
+ 1: 100
Index: test/Transforms/SampleProfile/flatten.ll
===================================================================
--- test/Transforms/SampleProfile/flatten.ll
+++ test/Transforms/SampleProfile/flatten.ll
@@ -0,0 +1,30 @@
+; REQUIRES: asserts
+; RUN: opt < %s -passes='thinlto<O2>' -pgo-kind=new-pm-pgo-sample-use-pipeline -profile-file=%S/Inputs/flatten.prof -debug-only=sample-profile -flatten-profile-used -S 2>&1| FileCheck %s
+;
+; Check SampleProfileLoader doesn't read the profile in ThinLTO postlink stage
+; when flatten sample profile is used.
+; CHECK-NOT: SampleProfileLoader initialize for Module:
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: norecurse nounwind readnone uwtable
+define dso_local i32 @foo() local_unnamed_addr !dbg !7 {
+entry:
+  ret i32 -1, !dbg !9
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4, !5}
+!llvm.ident = !{!6}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 8.0.0 (trunk 345241)", isOptimized: true, runtimeVersion: 0, emissionKind: LineTablesOnly, enums: !2, nameTableKind: None)
+!1 = !DIFile(filename: "a.c", directory: "")
+!2 = !{}
+!3 = !{i32 2, !"Dwarf Version", i32 4}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!5 = !{i32 1, !"wchar_size", i32 4}
+!6 = !{!"clang version 8.0.0 (trunk 345241)"}
+!7 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !8, isLocal: false, isDefinition: true, scopeLine: 1, isOptimized: true, unit: !0, retainedNodes: !2)
+!8 = !DISubroutineType(types: !2)
+!9 = !DILocation(line: 2, column: 3, scope: !7)