[SampleFDO] Add the support to split the function profiles with context into separate sections.

[wmi]

Summary

For ThinLTO, all the function profiles without context has been annotated to outline functions if possible in prelink phase. In postlink phase, profile annotation in postlink phase is only meaningful for function profile with context. If the profile is large, it is better to split the profile into two parts, one with context and one without, so the profile reading in postlink phase only has to read the part with context. To have the profile splitting, we extend the ExtBinary format to support different section arrangement. It will be flexible to add other section layout in the future without the need to create new class inheriting from ExtBinary class.

[hoy]

Thanks for working on this which will help the thinLTO throughput. I guess there will be a separate patch to turn on this work, i.e, setting CtxSplitLayout?

llvm/lib/Transforms/IPO/SampleProfile.cpp

457

I'm wondering if we can just use one field for the phase it's currently are at. We may also want to check it for fullLTO in the future.

2118

F may get a chance to update its entry count with a post-inline count during top-down processing with the default layout. Maybe put this under the check against SkipNoContextProf?

[wmi]

I guess there will be a separate patch to turn on this work, i.e, setting CtxSplitLayout?

I will use the resetSecLayout interface to select CtxSplitLayout. That will be done in our own branch of autofdo tool. I think for now most of the cases will still want to use the DefaultLayout.

llvm/lib/Transforms/IPO/SampleProfile.cpp

457

Good point. Will change it.

2118

I think for any case we don't want to reinitiate F's entrycount to initialEntryCount if it has already had a valid value.

Even if SkipNoContextProf is false, in LTO/ThinLTO postlink phase, it is possible that the emitAnnotations function doesn't change anything which could affect profile annotation (i.e., the variable Changed in emitAnnotations is false), so if a function getting a valid entry count in prelink phase is reinitialized to -1 before entering emitAnnotations in postlink, emitAnnotations may not be able to update it with a valid entry count again.

In addition, if F has a valid entrycount entering emitAnnotations, emitAnnotations can still update it without problem.

[wenlei]

Thanks for the change.

If the profile is large, it is better to split the profile into two parts, one with context and one without, so the profile reading in postlink phase only has to read the part with context.

I guess the speed up is more visible with merged partial profile with explicit profile flattening? or did you see it speed up FDO with regular full profile too? For a regular FDO profile, I'd expect most function profiles to have some context except for really small functions..

[hoy]

llvm/lib/Transforms/IPO/SampleProfile.cpp

2118

Oh yeah, entrycount can be set in emitAnnotations. Thanks for pointing it out.

I was wondering if F's profile can change due to the counts returning from its inliner in postlink phase, thus it may need an update in postlink though itself doesn't have context samples. That might happen with fullLTO but with thinLTO, the counts returning is already less accurate since it cannot be done cross-threads.

[wmi]

In D94435#2495298, @wenlei wrote:

Thanks for the change.

If the profile is large, it is better to split the profile into two parts, one with context and one without, so the profile reading in postlink phase only has to read the part with context.

I guess the speed up is more visible with merged partial profile with explicit profile flattening? or did you see it speed up FDO with regular full profile too? For a regular FDO profile, I'd expect most function profiles to have some context except for really small functions..

Right, it is more visible with merged partial profile. Currently the speedup with regular profile is minor. However, we saw in an experiment that by keeping context for the hottest functions and flattened the rest of the warm/cold functions in regular FDO profile can shrink profile size significantly without hurting performance. That is only an experiment for one target. We havn't done evaluation for other targets because regular full profile size is not a major issue for us for the moment. But if we can have the same conclusion for more targets, we can flatten more functions in the profile and the profile splitting will be more effective to reduce profile reading cost in LTO/ThinLTO postlink.

[wmi]

llvm/lib/Transforms/IPO/SampleProfile.cpp

457

Sent out a NFC change in https://reviews.llvm.org/D94613 as a preparation for the change here.

[wmi]

Address Hongtao's comment.

[hoy]

llvm/include/llvm/ProfileData/SampleProf.h

169

Nit: how about name this SecFlagNoCalleeContext or SecFlagFlat or something? Just want to be a bit more clear as CS profile also uses the term context.

llvm/include/llvm/ProfileData/SampleProfWriter.h

159

Nit: name it ExtBinaryHdrLayoutTable?

[wmi]

Address Hongtao's comment.

[wmi]

llvm/include/llvm/ProfileData/SampleProf.h

169

Make sense. I chose SecFlagFlat.

llvm/include/llvm/ProfileData/SampleProfWriter.h

159

That is better.

[hoy]

LGTM, thanks.

[wenlei]

llvm/lib/Transforms/IPO/SampleProfile.cpp

2118

I was wondering if F's profile can change due to the counts returning from its inliner in postlink phase, thus it may need an update in postlink though itself doesn't have context samples.

I think In this case, emitAnnotations should still be the right place where the update happens, though we don't capture such profile merge as "change" today and emitAnnotations won't be triggered.

llvm/test/Transforms/SampleProfile/ctxsplit.ll

4

I noticed the input is a binary profile. Is the new layout of extended binary profile still two-way convertible with text profile? Text profile works better for small test cases?

[wmi]

llvm/test/Transforms/SampleProfile/ctxsplit.ll

4

Yes, I can start with a text profile, convert it to a extbinary profile then run the test. Then we need another option in llvm-profdata to switch the default layout to ctxsplit Layout. Currently I didn't add the option because I felt there were few cases using ctxsplit and adding another option in llvm-profdata may not worth it considering it already has many options. Going forward, whether we need an option in llvm-profdata for every change in extbinary format is a question.

[wenlei]

llvm/test/Transforms/SampleProfile/ctxsplit.ll

4

Ok, makes sense if your create_llvm_prof only generates extbin profile today. This is not critical.

Then we need another option in llvm-profdata to switch the default layout to ctxsplit Layout.

This is one step further, if we just want to have cxt layout, you could let create_llvm_prof produce text profile directly, or use llvm-profgen to convert extbinary with ctx layout to text with cxt layout? llvm-profgen doesn't have to support the conversion between the two layouts, which would need extra option.

I guess what I missed is that there's not going to be a text representation of the ctx layout, i.e. there's not going to be 1:1 mapping and round trip conversion between text and extbin for ctx layout?

[wmi]

llvm/test/Transforms/SampleProfile/ctxsplit.ll

4

I guess what I missed is that there's not going to be a text representation of the ctx layout, i.e. there's not going to be 1:1 mapping and round trip conversion between text and extbin for ctx layout?

Right. It is not going to have 1:1 mapping and round trip conversion between extbin and text format.

We can convert ctxlayout extbinary profile to text profile using llvm-profdata, but we cannot do that the other way around without adding an option, because the default layout in extbinary format is not ctxlayout. It means currently if we want to create a test using ctxlayout extbinary profile, we need to change a little code and rebuild llvm-profdata before we can generate ctxlayout extbinary profile. That is something not convenient, but that is a tradeoff in order to have less options in llvm-profdata.

In the test, I want to use ctxlayout extbinary profile instead of text profile converted from ctxlayout in the profile because I want to verify in thinlto postlink phase compiler won't read the part without context. That cannot be achieved if I use text profile.

[wenlei]

llvm/test/Transforms/SampleProfile/ctxsplit.ll

4

Got it, thanks for explaining.

llvm/include/llvm/ProfileData/SampleProf.h

struct SecHdrTableEntry {
   uint32_t LayoutIndex;
 };
 
 // Flags common for all sections are defined here. In SecHdrTableEntry::Flags,
 // common flags will be saved in the lower 32bits and section specific flags
 // will be saved in the higher 32 bits.
 enum class SecCommonFlags : uint32_t {
   SecFlagInValid = 0,
-  SecFlagCompress = (1 << 0)
+  SecFlagCompress = (1 << 0),
+  // Indicate the section contains only profile without context.
+  SecFlagFlat = (1 << 1)
 };
 
 // Section specific flags are defined here.
 // !!!Note: Everytime a new enum class is created here, please add
 // a new check in verifySecFlag.
 enum class SecNameTableFlags : uint32_t {
   SecFlagInValid = 0,
   SecFlagMD5Name = (1 << 0),

llvm/include/llvm/ProfileData/SampleProfReader.h

public:
   /// It includes all the names that have samples either in outline instance
   /// or inline instance.
   virtual std::vector<StringRef> *getNameTable() { return nullptr; }
   virtual bool dumpSectionInfo(raw_ostream &OS = dbgs()) { return false; };
 
   /// Return whether names in the profile are all MD5 numbers.
   virtual bool useMD5() { return false; }
 
+  /// Don't read profile without context if the flag is set. This is only meaningful
+  /// for ExtBinary format.
+  virtual void setSkipFlatProf(bool Skip) {}
+
   SampleProfileReaderItaniumRemapper *getRemapper() { return Remapper.get(); }
 
 protected:
   /// Map every function to its associated profile.
   ///
   /// The profile of every function executed at runtime is collected
   /// in the structure FunctionSamples. This maps function objects
   /// to their corresponding profiles.
protected:
   /// The uint64_t data has to be converted to a string and then the string
   /// will be used to initialize StringRef in NameTable.
   /// Note NameTable contains StringRef so it needs another buffer to own
   /// the string data. MD5StringBuf serves as the string buffer that is
   /// referenced by NameTable (vector of StringRef). We make sure
   /// the lifetime of MD5StringBuf is not shorter than that of NameTable.
   std::unique_ptr<std::vector<std::string>> MD5StringBuf;
 
+  /// If SkipFlatProf is true, skip the sections with
+  /// SecFlagFlat flag.
+  bool SkipFlatProf = false;
+
 public:
   SampleProfileReaderExtBinaryBase(std::unique_ptr<MemoryBuffer> B,
                                    LLVMContext &C, SampleProfileFormat Format)
       : SampleProfileReaderBinary(std::move(B), C, Format) {}
 
   /// Read sample profiles in extensible format from the associated file.
   std::error_code readImpl() override;
 
   /// Get the total size of all \p Type sections.
   uint64_t getSectionSize(SecType Type);
   /// Get the total size of header and all sections.
   uint64_t getFileSize();
   virtual bool dumpSectionInfo(raw_ostream &OS = dbgs()) override;
 
   /// Collect functions with definitions in Module \p M.
   void collectFuncsFrom(const Module &M) override;
 
   /// Return whether names in the profile are all MD5 numbers.
   virtual bool useMD5() override { return MD5StringBuf.get(); }
 
   virtual std::unique_ptr<ProfileSymbolList> getProfileSymbolList() override {
     return std::move(ProfSymList);
   };
+
+  virtual void setSkipFlatProf(bool Skip) override { SkipFlatProf = Skip; }
 };
 
 class SampleProfileReaderExtBinary : public SampleProfileReaderExtBinaryBase {
 private:
   virtual std::error_code verifySPMagic(uint64_t Magic) override;
   virtual std::error_code
   readCustomSection(const SecHdrTableEntry &Entry) override {
     return sampleprof_error::success;

llvm/include/llvm/ProfileData/SampleProfWriter.h

 //
 //===----------------------------------------------------------------------===//
 #ifndef LLVM_PROFILEDATA_SAMPLEPROFWRITER_H
 #define LLVM_PROFILEDATA_SAMPLEPROFWRITER_H
 
 #include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/IR/ProfileSummary.h"
 #include "llvm/ProfileData/SampleProf.h"
 #include "llvm/Support/ErrorOr.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cstdint>
 #include <memory>
 #include <set>
 #include <system_error>
 
 namespace llvm {
 namespace sampleprof {
 
+enum SectionLayout {
+  DefaultLayout,
+  // The layout splits profile with context information from profile without
+  // context information. When Thinlto is enabled, ThinLTO postlink phase only
+  // has to load profile with context information and can skip the other part.
+  CtxSplitLayout,
+  NumOfLayout,
+};
+
 /// Sample-based profile writer. Base class.
 class SampleProfileWriter {
 public:
   virtual ~SampleProfileWriter() = default;
 
   /// Write sample profiles in \p S.
   ///
   /// \returns status code of the file update operation.
public:
   /// For testing.
   static ErrorOr<std::unique_ptr<SampleProfileWriter>>
   create(std::unique_ptr<raw_ostream> &OS, SampleProfileFormat Format);
 
   virtual void setProfileSymbolList(ProfileSymbolList *PSL) {}
   virtual void setToCompressAllSections() {}
   virtual void setUseMD5() {}
   virtual void setPartialProfile() {}
+  virtual void resetSecLayout(SectionLayout SL) {}
 
 protected:
   SampleProfileWriter(std::unique_ptr<raw_ostream> &OS)
       : OutputStream(std::move(OS)) {}
 
   /// Write a file header for the profile file.
   virtual std::error_code
   writeHeader(const StringMap<FunctionSamples> &ProfileMap) = 0;
private:
   SampleProfileWriter::create(std::unique_ptr<raw_ostream> &OS,
                               SampleProfileFormat Format);
 };
 
 class SampleProfileWriterRawBinary : public SampleProfileWriterBinary {
   using SampleProfileWriterBinary::SampleProfileWriterBinary;
 };
 
+const std::array<SmallVector<SecHdrTableEntry, 8>, NumOfLayout>
+    ExtBinaryHdrLayoutTable = {
+        // Note that SecFuncOffsetTable section is written after SecLBRProfile
+        // in the profile, but is put before SecLBRProfile in SectionHdrLayout.
+        // This is because sample reader follows the order in SectionHdrLayout
+        // to read each section. To read function profiles on demand, sample
+        // reader need to get the offset of each function profile first.
+        //
+        // DefaultLayout
+        SmallVector<SecHdrTableEntry, 8>({{SecProfSummary},
+                                          {SecNameTable},
+                                          {SecFuncOffsetTable},
+                                          {SecLBRProfile},
+                                          {SecProfileSymbolList},
+                                          {SecFuncMetadata}}),
+        // CtxSplitLayout
+        SmallVector<SecHdrTableEntry, 8>({{SecProfSummary},
+                                          {SecNameTable},
+                                          // profile with context
+                                          // for next two sections
+                                          {SecFuncOffsetTable},
+                                          {SecLBRProfile},
+                                          // profile without context
+                                          // for next two sections
+                                          {SecFuncOffsetTable},
+                                          {SecLBRProfile},
+                                          {SecProfileSymbolList},
+                                          {SecFuncMetadata}}),
+};
+
 class SampleProfileWriterExtBinaryBase : public SampleProfileWriterBinary {
   using SampleProfileWriterBinary::SampleProfileWriterBinary;
 public:
   virtual std::error_code
   write(const StringMap<FunctionSamples> &ProfileMap) override;
 
   virtual void setToCompressAllSections() override;
   void setToCompressSection(SecType Type);
public:
   virtual void setPartialProfile() override {
     addSectionFlag(SecProfSummary, SecProfSummaryFlags::SecFlagPartial);
   }
 
   virtual void setProfileSymbolList(ProfileSymbolList *PSL) override {
     ProfSymList = PSL;
   };
 
+  virtual void resetSecLayout(SectionLayout SL) override {
+    verifySecLayout(SL);
+#ifndef NDEBUG
+    // Make sure resetSecLayout is called before any flag setting.
+    for (auto &Entry : SectionHdrLayout) {
+      assert(Entry.Flags == 0 &&
+             "resetSecLayout has to be called before any flag setting");
+    }
+#endif
+    SecLayout = SL;
+    SectionHdrLayout = ExtBinaryHdrLayoutTable[SL];
+  }
+
 protected:
   uint64_t markSectionStart(SecType Type, uint32_t LayoutIdx);
   std::error_code addNewSection(SecType Sec, uint32_t LayoutIdx,
                                 uint64_t SectionStart);
   template <class SecFlagType>
   void addSectionFlag(SecType Type, SecFlagType Flag) {
     for (auto &Entry : SectionHdrLayout) {
       if (Entry.Type == Type)
         addSecFlag(Entry, Flag);
     }
   }
+  template <class SecFlagType>
+  void addSectionFlag(uint32_t SectionIdx, SecFlagType Flag) {
+    addSecFlag(SectionHdrLayout[SectionIdx], Flag);
+  }
 
   // placeholder for subclasses to dispatch their own section writers.
   virtual std::error_code writeCustomSection(SecType Type) = 0;
+  // Verify the SecLayout is supported by the format.
+  virtual void verifySecLayout(SectionLayout SL) = 0;
 
-  virtual void initSectionHdrLayout() = 0;
   // specify the order to write sections.
   virtual std::error_code
   writeSections(const StringMap<FunctionSamples> &ProfileMap) = 0;
 
   // Dispatch section writer for each section. \p LayoutIdx is the sequence
   // number indicating where the section is located in SectionHdrLayout.
   virtual std::error_code
   writeOneSection(SecType Type, uint32_t LayoutIdx,
                   const StringMap<FunctionSamples> &ProfileMap);
 
   // Helper function to write name table.
   virtual std::error_code writeNameTable() override;
 
   std::error_code writeFuncMetadata(const StringMap<FunctionSamples> &Profiles);
 
   // Functions to write various kinds of sections.
   std::error_code
   writeNameTableSection(const StringMap<FunctionSamples> &ProfileMap);
   std::error_code writeFuncOffsetTable();
   std::error_code writeProfileSymbolListSection();
 
+  SectionLayout SecLayout = DefaultLayout;
   // Specifiy the order of sections in section header table. Note
   // the order of sections in SecHdrTable may be different that the
   // order in SectionHdrLayout. sample Reader will follow the order
   // in SectionHdrLayout to read each section.
-  SmallVector<SecHdrTableEntry, 8> SectionHdrLayout;
+  SmallVector<SecHdrTableEntry, 8> SectionHdrLayout =
+      ExtBinaryHdrLayoutTable[DefaultLayout];
 
   // Save the start of SecLBRProfile so we can compute the offset to the
   // start of SecLBRProfile for each Function's Profile and will keep it
   // in FuncOffsetTable.
   uint64_t SecLBRProfileStart = 0;
 
 private:
   void allocSecHdrTable();
private:
   bool UseMD5 = false;
 
   ProfileSymbolList *ProfSymList = nullptr;
 };
 
 class SampleProfileWriterExtBinary : public SampleProfileWriterExtBinaryBase {
 public:
   SampleProfileWriterExtBinary(std::unique_ptr<raw_ostream> &OS)
-      : SampleProfileWriterExtBinaryBase(OS) {
-    initSectionHdrLayout();
-  }
+      : SampleProfileWriterExtBinaryBase(OS) {}
 
 private:
-  virtual void initSectionHdrLayout() override {
-    // Note that SecFuncOffsetTable section is written after SecLBRProfile
-    // in the profile, but is put before SecLBRProfile in SectionHdrLayout.
-    //
-    // This is because sample reader follows the order of SectionHdrLayout to
-    // read each section, to read function profiles on demand sample reader
-    // need to get the offset of each function profile first.
-    //
-    // SecFuncOffsetTable section is written after SecLBRProfile in the
-    // profile because FuncOffsetTable needs to be populated while section
-    // SecLBRProfile is written.
-    SectionHdrLayout = {
-        {SecProfSummary, 0, 0, 0, 0},       {SecNameTable, 0, 0, 0, 0},
-        {SecFuncOffsetTable, 0, 0, 0, 0},   {SecLBRProfile, 0, 0, 0, 0},
-        {SecProfileSymbolList, 0, 0, 0, 0}, {SecFuncMetadata, 0, 0, 0, 0}};
-  };
+  std::error_code
+  writeDefaultLayout(const StringMap<FunctionSamples> &ProfileMap);
+  std::error_code
+  writeCtxSplitLayout(const StringMap<FunctionSamples> &ProfileMap);
+
   virtual std::error_code
   writeSections(const StringMap<FunctionSamples> &ProfileMap) override;
 
   virtual std::error_code writeCustomSection(SecType Type) override {
     return sampleprof_error::success;
   };
+
+  virtual void verifySecLayout(SectionLayout SL) override {
+    assert((SL == DefaultLayout || SL == CtxSplitLayout) &&
+           "Unsupported layout");
+  }
 };
 
 // CompactBinary is a compact format of binary profile which both reduces
 // the profile size and the load time needed when compiling. It has two
 // major difference with Binary format.
 // 1. It represents all the strings in name table using md5 hash.
 // 2. It saves a function offset table which maps function name index to
 // the offset of its function profile to the start of the binary profile,

llvm/lib/ProfileData/SampleProfReader.cpp

std::error_code SampleProfileReaderExtBinaryBase::readImpl() {
   const uint8_t *BufStart =
       reinterpret_cast<const uint8_t *>(Buffer->getBufferStart());
 
   for (auto &Entry : SecHdrTable) {
     // Skip empty section.
     if (!Entry.Size)
       continue;
 
+    // Skip sections without context when SkipFlatProf is true.
+    if (SkipFlatProf && hasSecFlag(Entry, SecCommonFlags::SecFlagFlat))
+      continue;
+
     const uint8_t *SecStart = BufStart + Entry.Offset;
     uint64_t SecSize = Entry.Size;
 
     // If the section is compressed, decompress it into a buffer
     // DecompressBuf before reading the actual data. The pointee of
     // 'Data' will be changed to buffer hold by DecompressBuf
     // temporarily when reading the actual data.
     bool isCompressed = hasSecFlag(Entry, SecCommonFlags::SecFlagCompress);

 
 static std::string getSecFlagsStr(const SecHdrTableEntry &Entry) {
   std::string Flags;
   if (hasSecFlag(Entry, SecCommonFlags::SecFlagCompress))
     Flags.append("{compressed,");
   else
     Flags.append("{");
 
+  if (hasSecFlag(Entry, SecCommonFlags::SecFlagFlat))
+    Flags.append("flat,");
+
   switch (Entry.Type) {
   case SecNameTable:
     if (hasSecFlag(Entry, SecNameTableFlags::SecFlagFixedLengthMD5))
       Flags.append("fixlenmd5,");
     else if (hasSecFlag(Entry, SecNameTableFlags::SecFlagMD5Name))
       Flags.append("md5,");
     break;
   case SecProfSummary:

llvm/lib/ProfileData/SampleProfWriter.cpp

 //
 // See lib/ProfileData/SampleProfReader.cpp for documentation on each of the
 // supported formats.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ProfileData/SampleProfWriter.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/ProfileData/ProfileCommon.h"
 #include "llvm/ProfileData/SampleProf.h"
 #include "llvm/Support/Compression.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/EndianStream.h"
 #include "llvm/Support/ErrorOr.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/LEB128.h"
if (auto EC = writeCustomSection(Type))
       return EC;
     break;
   }
   if (std::error_code EC = addNewSection(Type, LayoutIdx, SectionStart))
     return EC;
   return sampleprof_error::success;
 }
 
-std::error_code SampleProfileWriterExtBinary::writeSections(
+std::error_code SampleProfileWriterExtBinary::writeDefaultLayout(
     const StringMap<FunctionSamples> &ProfileMap) {
   // The const indices passed to writeOneSection below are specifying the
   // positions of the sections in SectionHdrLayout. Look at
   // initSectionHdrLayout to find out where each section is located in
   // SectionHdrLayout.
   if (auto EC = writeOneSection(SecProfSummary, 0, ProfileMap))
     return EC;
   if (auto EC = writeOneSection(SecNameTable, 1, ProfileMap))
     return EC;
   if (auto EC = writeOneSection(SecLBRProfile, 3, ProfileMap))
     return EC;
   if (auto EC = writeOneSection(SecProfileSymbolList, 4, ProfileMap))
     return EC;
   if (auto EC = writeOneSection(SecFuncOffsetTable, 2, ProfileMap))
     return EC;
   if (auto EC = writeOneSection(SecFuncMetadata, 5, ProfileMap))
     return EC;
   return sampleprof_error::success;
 }
 
+static void
+splitProfileMapToTwo(const StringMap<FunctionSamples> &ProfileMap,
+                     StringMap<FunctionSamples> &ContextProfileMap,
+                     StringMap<FunctionSamples> &NoContextProfileMap) {
+  for (const auto &I : ProfileMap) {
+    if (I.second.getCallsiteSamples().size())
+      ContextProfileMap.insert({I.first(), I.second});
+    else
+      NoContextProfileMap.insert({I.first(), I.second});
+  }
+}
+
+std::error_code SampleProfileWriterExtBinary::writeCtxSplitLayout(
+    const StringMap<FunctionSamples> &ProfileMap) {
+  StringMap<FunctionSamples> ContextProfileMap, NoContextProfileMap;
+  splitProfileMapToTwo(ProfileMap, ContextProfileMap, NoContextProfileMap);
+
+  if (auto EC = writeOneSection(SecProfSummary, 0, ProfileMap))
+    return EC;
+  if (auto EC = writeOneSection(SecNameTable, 1, ProfileMap))
+    return EC;
+  if (auto EC = writeOneSection(SecLBRProfile, 3, ContextProfileMap))
+    return EC;
+  if (auto EC = writeOneSection(SecFuncOffsetTable, 2, ContextProfileMap))
+    return EC;
+  // Mark the section to have no context. Note section flag needs to be set
+  // before writing the section.
+  addSectionFlag(5, SecCommonFlags::SecFlagFlat);
+  if (auto EC = writeOneSection(SecLBRProfile, 5, NoContextProfileMap))
+    return EC;
+  // Mark the section to have no context. Note section flag needs to be set
+  // before writing the section.
+  addSectionFlag(4, SecCommonFlags::SecFlagFlat);
+  if (auto EC = writeOneSection(SecFuncOffsetTable, 4, NoContextProfileMap))
+    return EC;
+  if (auto EC = writeOneSection(SecProfileSymbolList, 6, ProfileMap))
+    return EC;
+  if (auto EC = writeOneSection(SecFuncMetadata, 7, ProfileMap))
+    return EC;
+
+  return sampleprof_error::success;
+}
+
+std::error_code SampleProfileWriterExtBinary::writeSections(
+    const StringMap<FunctionSamples> &ProfileMap) {
+  std::error_code EC;
+  if (SecLayout == DefaultLayout)
+    EC = writeDefaultLayout(ProfileMap);
+  else if (SecLayout == CtxSplitLayout)
+    EC = writeCtxSplitLayout(ProfileMap);
+  else
+    llvm_unreachable("Unsupported layout");
+  return EC;
+}
+
 std::error_code SampleProfileWriterCompactBinary::write(
     const StringMap<FunctionSamples> &ProfileMap) {
   if (std::error_code EC = SampleProfileWriter::write(ProfileMap))
     return EC;
   if (std::error_code EC = writeFuncOffsetTable())
     return EC;
   return sampleprof_error::success;
 }

llvm/lib/Transforms/IPO/SampleProfile.cpp

protected:
 
   /// Flag indicating which LTO/ThinLTO phase the pass is invoked in.
   ///
   /// We need to know the LTO phase because for example in ThinLTOPrelink
   /// phase, in annotation, we should not promote indirect calls. Instead,
   /// we will mark GUIDs that needs to be annotated to the function.
   ThinOrFullLTOPhase LTOPhase;
 
   /// Profile Summary Info computed from sample profile.
   ProfileSummaryInfo *PSI = nullptr;
 
   /// Profle Symbol list tells whether a function name appears in the binary
   /// used to generate the current profile.
   std::unique_ptr<ProfileSymbolList> PSL;
 
   /// Total number of samples collected in this profile.
   ///
bool SampleProfileLoader::doInitialization(Module &M,
   auto ReaderOrErr =
       SampleProfileReader::create(Filename, Ctx, RemappingFilename);
   if (std::error_code EC = ReaderOrErr.getError()) {
     std::string Msg = "Could not open profile: " + EC.message();
     Ctx.diagnose(DiagnosticInfoSampleProfile(Filename, Msg));
     return false;
   }
   Reader = std::move(ReaderOrErr.get());
+  Reader->setSkipFlatProf(LTOPhase == ThinOrFullLTOPhase::ThinLTOPostLink);
   Reader->collectFuncsFrom(M);
   ProfileIsValid = (Reader->read() == sampleprof_error::success);
   PSL = Reader->getProfileSymbolList();
 
   // While profile-sample-accurate is on, ignore symbol list.
   ProfAccForSymsInList =
       ProfileAccurateForSymsInList && PSL && !ProfileSampleAccurate;
   if (ProfAccForSymsInList) {
if (ProfAccForSymsInList) {
     // imprecise debug information, or the callsites are all cold individually
     // but not cold accumulatively...), so the outline function showing up as
     // cold in sampled binary will actually not be cold after current build.
     StringRef CanonName = FunctionSamples::getCanonicalFnName(F);
     if (NamesInProfile.count(CanonName))
       initialEntryCount = -1;
   }
 
+  // Initialize entry count when the function has no existing entry
+  // count value.
+  if (!F.getEntryCount().hasValue())
     F.setEntryCount(ProfileCount(initialEntryCount, Function::PCT_Real));
   std::unique_ptr<OptimizationRemarkEmitter> OwnedORE;
   if (AM) {
     auto &FAM =
         AM->getResult<FunctionAnalysisManagerModuleProxy>(*F.getParent())
             .getManager();
     ORE = &FAM.getResult<OptimizationRemarkEmitterAnalysis>(F);
   } else {
     OwnedORE = std::make_unique<OptimizationRemarkEmitter>(&F);

llvm/test/Transforms/SampleProfile/ctxsplit.ll

+; Check the nonflattened part of the ctxsplit profile will be read in thinlto
+; postlink phase while flattened part of the ctxsplit profile will not be read.
+; RUN: opt < %s -passes='thinlto<O2>' -pgo-kind=pgo-sample-use-pipeline -profile-file=%S/Inputs/ctxsplit.extbinary.afdo -S | FileCheck %s --check-prefix=POSTLINK
+;
+; Check both the flattened and nonflattened parts of the ctxsplit profile will
+; be read in thinlto prelink phase.
+; RUN: opt < %s -passes='thinlto-pre-link<O2>' -pgo-kind=pgo-sample-use-pipeline -profile-file=%S/Inputs/ctxsplit.extbinary.afdo -S | FileCheck %s --check-prefix=PRELINK
+;
+; Check both the flattened and nonflattened parts of the ctxsplit profile will
+; be read in non-thinlto mode.
+; RUN: opt < %s -passes='default<O2>' -pgo-kind=pgo-sample-use-pipeline -profile-file=%S/Inputs/ctxsplit.extbinary.afdo -S | FileCheck %s --check-prefix=NOTHINLTO
+
+; POSTLINK: define dso_local i32 @goo() {{.*}} !prof ![[ENTRY1:[0-9]+]] {
+; POSTLINK: define dso_local i32 @foo() {{.*}} !prof ![[ENTRY2:[0-9]+]] {
+; POSTLINK: ![[ENTRY1]] = !{!"function_entry_count", i64 1001}
+; POSTLINK: ![[ENTRY2]] = !{!"function_entry_count", i64 -1}
+; PRELINK: define dso_local i32 @goo() {{.*}} !prof ![[ENTRY1:[0-9]+]] {
+; PRELINK: define dso_local i32 @foo() {{.*}} !prof ![[ENTRY2:[0-9]+]] {
+; PRELINK: ![[ENTRY1]] = !{!"function_entry_count", i64 1001}
+; PRELINK: ![[ENTRY2]] = !{!"function_entry_count", i64 3001}
+; NOTHINLTO: define dso_local i32 @goo() {{.*}} !prof ![[ENTRY1:[0-9]+]] {
+; NOTHINLTO: define dso_local i32 @foo() {{.*}} !prof ![[ENTRY2:[0-9]+]] {
+; NOTHINLTO: ![[ENTRY1]] = !{!"function_entry_count", i64 1001}
+; NOTHINLTO: ![[ENTRY2]] = !{!"function_entry_count", i64 3001}
+
+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 @goo() #0 !dbg !10 {
+entry:
+  ret i32 -1, !dbg !11
+}
+
+; Function Attrs: norecurse nounwind readnone uwtable
+define dso_local i32 @foo() #0 !dbg !7 {
+entry:
+  ret i32 -1, !dbg !9
+}
+
+attributes #0 = { "use-sample-profile" }
+
+!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)
+!10 = distinct !DISubprogram(name: "goo", scope: !1, file: !1, line: 8, type: !8, isLocal: false, isDefinition: true, scopeLine: 8, isOptimized: true, unit: !0, retainedNodes: !2)
+!11 = !DILocation(line: 10, column: 3, scope: !10)
+