Index: lib/Passes/PassBuilder.cpp
===================================================================
--- lib/Passes/PassBuilder.cpp
+++ lib/Passes/PassBuilder.cpp
@@ -211,6 +211,8 @@
 
 extern cl::opt<bool> EnableHotColdSplit;
 
+extern cl::opt<bool> FlattenedProfileUsed;
+
 static bool isOptimizingForSize(PassBuilder::OptimizationLevel Level) {
   switch (Level) {
   case PassBuilder::O0:
@@ -615,9 +617,13 @@
   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));
+    // In ThinLTO mode, when flattened profile is used, all the available
+    // profile information will be annotated in PreLink phase so there is
+    // no need to load the profile again in PostLink.
+    if (!(FlattenedProfileUsed && Phase == ThinLTOPhase::PostLink))
+      MPM.addPass(SampleProfileLoaderPass(PGOOpt->SampleProfileFile,
+                                          PGOOpt->ProfileRemappingFile,
+                                          Phase == ThinLTOPhase::PreLink));
     // 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/PassManagerBuilder.cpp
===================================================================
--- lib/Transforms/IPO/PassManagerBuilder.cpp
+++ lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -104,6 +104,10 @@
     EnablePrepareForThinLTO("prepare-for-thinlto", cl::init(false), cl::Hidden,
                             cl::desc("Enable preparation for ThinLTO."));
 
+static cl::opt<bool>
+    EnablePerformThinLTO("perform-thinlto", cl::init(false), cl::Hidden,
+                         cl::desc("Enable performing ThinLTO."));
+
 cl::opt<bool> EnableHotColdSplit("hot-cold-split", cl::init(false), cl::Hidden,
     cl::desc("Enable hot-cold splitting pass"));
 
@@ -146,6 +150,11 @@
     EnableCHR("enable-chr", cl::init(true), cl::Hidden,
               cl::desc("Enable control height reduction optimization (CHR)"));
 
+cl::opt<bool> FlattenedProfileUsed(
+    "flattened-profile-used", cl::init(false), cl::Hidden,
+    cl::desc("Indicate the sample profile being used is flattened, i.e., "
+             "no inline hierachy exists in the profile. "));
+
 PassManagerBuilder::PassManagerBuilder() {
     OptLevel = 2;
     SizeLevel = 0;
@@ -166,7 +175,7 @@
     PGOInstrUse = "";
     PGOSampleUse = "";
     PrepareForThinLTO = EnablePrepareForThinLTO;
-    PerformThinLTO = false;
+    PerformThinLTO = EnablePerformThinLTO;
     DivergentTarget = false;
 }
 
@@ -414,7 +423,11 @@
     legacy::PassManagerBase &MPM) {
   if (!PGOSampleUse.empty()) {
     MPM.add(createPruneEHPass());
-    MPM.add(createSampleProfileLoaderPass(PGOSampleUse));
+    // In ThinLTO mode, when flattened profile is used, all the available
+    // profile information will be annotated in PreLink phase so there is
+    // no need to load the profile again in PostLink.
+    if (!(FlattenedProfileUsed && PerformThinLTO))
+      MPM.add(createSampleProfileLoaderPass(PGOSampleUse));
   }
 
   // Allow forcing function attributes as a debugging and tuning aid.
Index: test/Transforms/SampleProfile/Inputs/flattened.prof
===================================================================
--- test/Transforms/SampleProfile/Inputs/flattened.prof
+++ test/Transforms/SampleProfile/Inputs/flattened.prof
@@ -0,0 +1,2 @@
+foo:100:100
+ 1: 100
Index: test/Transforms/SampleProfile/flattened.ll
===================================================================
--- test/Transforms/SampleProfile/flattened.ll
+++ test/Transforms/SampleProfile/flattened.ll
@@ -0,0 +1,39 @@
+; Check flattened profile will not be read in thinlto postlink.
+; RUN: opt < %s -O2 -flattened-profile-used -pgo-kind=pgo-sample-use-pipeline -profile-file=%S/Inputs/flattened.prof -enable-chr=false -perform-thinlto=true -S | FileCheck %s
+; RUN: opt < %s -passes='thinlto<O2>' -pgo-kind=pgo-sample-use-pipeline -profile-file=%S/Inputs/flattened.prof -flattened-profile-used -S | FileCheck %s
+;
+; Check flattened profile will be read in thinlto prelink.
+; RUN: opt < %s -O2 -flattened-profile-used -pgo-kind=pgo-sample-use-pipeline -profile-file=%S/Inputs/flattened.prof -enable-chr=false -prepare-for-thinlto=true -S | FileCheck %s --check-prefix=PRELINK
+; RUN: opt < %s -passes='thinlto-pre-link<O2>' -pgo-kind=pgo-sample-use-pipeline -profile-file=%S/Inputs/flattened.prof -flattened-profile-used -S | FileCheck %s --check-prefix=PRELINK
+;
+; Check flattened profile will be read in non-thinlto mode.
+; RUN: opt < %s -O2 -flattened-profile-used -pgo-kind=pgo-sample-use-pipeline -profile-file=%S/Inputs/flattened.prof -enable-chr=false -S | FileCheck %s --check-prefix=NOTHINLTO
+; RUN: opt < %s -passes='default<O2>' -pgo-kind=pgo-sample-use-pipeline -profile-file=%S/Inputs/flattened.prof -flattened-profile-used -S | FileCheck %s --check-prefix=NOTHINLTO
+;
+; CHECK-NOT: !{!"ProfileFormat", !"SampleProfile"}
+; PRELINK:   !{!"ProfileFormat", !"SampleProfile"}
+; NOTHINLTO: !{!"ProfileFormat", !"SampleProfile"}
+
+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)