[Coverage] Revise format to reduce binary size

[vsk]

Summary

Revise the coverage mapping format to reduce binary size by:

1. Naming function records and marking them linkonce_odr, and
2. Compressing filenames.

This shrinks the size of llc's coverage segment by 82% (334MB -> 62MB)
and speeds up end-to-end single-threaded report generation by 10%. For
reference the compressed name data in llc is 81MB (__llvm_prf_names).

Rationale for changes to the format:

• With the current format, most coverage function records are discarded. E.g., more than 97% of the records in llc are *duplicate* placeholders for functions visible-but-not-used in TUs. Placeholders *are* used to show under-covered functions, but duplicate placeholders waste space.

• We reached general consensus about giving (1) a try at the 2017 code coverage BoF [1]. The thinking was that using linkonce_odr to merge duplicates is simpler than alternatives like teaching build systems about a coverage-aware database/module/etc on the side.

• Revising the format is expensive due to the backwards compatibility requirement, so we might as well compress filenames while we're at it. This shrinks the encoded filenames in llc by 86% (12MB -> 1.6MB).

See CoverageMappingFormat.rst for the details on what exactly has
changed.

Fixes PR34533 [2], hopefully.

[1] http://lists.llvm.org/pipermail/llvm-dev/2017-October/118428.html
[2] https://bugs.llvm.org/show_bug.cgi?id=34533

[vsk]

• Improve wording in CoverageMappingFormat.rst.

[dblaikie]

clang/lib/CodeGen/CoverageMappingGen.h

56–65

I'd probably skip all these & let it be a plain struct (& use {} init to construct it, as good as the ctor provided I think).

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

75

Declaring things from other files without a common header's not ideal. (makes it easy to violate ODR - accidentally change one declaration without the other, etc)

[vsk]

Declare DoInstrProfNameCompression in a shared header, address remaining feedback and rebase.

[rnk]

I remember the direction from the BoF back in 2017. I'm glad you were able to find the time to work on this and that it ultimately worked out. Thanks so much!

I thought maybe you would need to do more to find .lcovfun on Windows, but I saw this comment:
https://github.com/llvm/llvm-project/blob/e73ae9a/compiler-rt/lib/profile/InstrProfilingPlatformWindows.c#L19

/* Do *NOT* merge .lprfn and .lcovmap into .rdata. llvm-cov must be able to find
 * after the fact.
 */

So, I guess it will work as is.

[vsk]

Would it help review to split the NFC portion of the change in the coverage::accessors namespace into its own patch?

[vsk]

Gentle ping. I'm not in a rush to land this, but it'd help to have some feedback about whether/how this should patch should be split up.

[rnk]

I'm not listed as a reviewer, but it looks good to me.

clang/lib/CodeGen/CoverageMappingGen.cpp

1331

I think this needs to be namespaced somehow, something like __covrec_0afeec...u. The meaning is more obvious, and there is a (admittedly small) risk of colliding with other random 64-bit hex symbols without a double-underscore prefix.

1372

I guess we really do want llvm.used, we really want these records to be retained in the binary by the linker even though nothing references them.

clang/test/Profile/def-assignop.cpp

21

I guess 6 was just stale/wrong.

llvm/docs/CoverageMappingFormat.rst

227–229

This section of the document isn't word wrapped to 80 cols, but the rest is.

247–259

Do you need to highlight "dummy" records? I thought all function records are linkonce_odr, so that any duplicate records can be deduplicated, not just the dummy ones for unused inline functions.

[hans]

Looks good as far as I can tell. I think it can be simplified by asserting that zlib::compress doesn't fail, though.

llvm/docs/CoverageMappingFormat.rst

302–304

missing 'the' before "function name's"?

llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h

725

s/consider/considered/

llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp

38–39

It seems the only way this can fail is if zlib::compress fails, which should not be possible except if it runs out of memory, which we generally don't diagnose.

I think we could assert that zlib::compress succeeds, simplify the interface of this function, and remove the need for the new err_code_coverage_mapping_failed diag, as well as the compression_failed enumerator.

[vsk]

Address review feedback.

clang/lib/CodeGen/CoverageMappingGen.cpp

1372

Yes, this ensures that the records can be read back when generating a report. The record names can be stripped out post-link to reduce bloat in the linkedit section.

llvm/docs/CoverageMappingFormat.rst

247–259

I think it's helpful to highlight "dummy" records, as these motivate the linkonce_odr idea. I'll point out that all function records are linkonce_odr.

llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp

38–39

Thanks, sgtm.

[efriedma]

llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp

55

llvm::report_bad_alloc_error?

[rnk]

I think it looks good, but I'd wait for @hans too.

BTW, after this we should remove -limited-coverage-experimental:
https://github.com/llvm/llvm-project/blob/bbc328c/clang/lib/CodeGen/CodeGenModule.cpp#L67

llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp

572–574

While working on CodeView, I think we came up with a good code pattern that could be used here to make this easier to read and less error prone. Check out llvm::codeview::consume:
https://github.com/llvm/llvm-project/blob/master/llvm/include/llvm/DebugInfo/CodeView/RecordSerialization.h#L64

Ignore the use of BinaryStreamReader, that exists to abstract over discontiguous inputs, and isn't relevant to this code here.

You could make a helper that looks like this and use it in various places to avoid all the reinterpret_casts:

template <typename T>
bool consumeObject(StringRef &Buf, const T *&Obj) {
  if (Buf.size() < sizeof(T))
    return false;
  Obj = reinterpret_cast<const T*>(Buf.data());
  Buf = Buf.drop_front(sizeof(T));
  return true;
}

And then instead of dealing in pointers and pointer arithmetic, you slice up StringRef buffers into sub buffers. It becomes much harder to create array OOB bugs.

This is just a suggestion for future work, it seems out of scope for this patch.

llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp

52–55

I think the shorter pattern for this is cantFail(zlib::compress(...)).

[hans]

lgtm

[vsk]

llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp

572–574

Thanks for the suggestion, I'll keep this in mind for future refactors.

llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp

52–55

Both sound reasonable, I'll pick the shorter spelling.

[efriedma]

llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp

52–55

cantFail and report_bad_alloc_error have substantially different semantics. In particular, for non-Asserts builds, cantFail is a no-op, which doesn't seem appropriate for an API that fails on out-of-memory.

[vsk]

I reverted this due to failures on Windows that I did not encounter in local testing. I suspect that there's an error in the coverage parsing logic, as the same binary coverage data parses successfully locally, but not on some armv7 bots.

llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp

52–55

I see, I'll revise this to use report_bad_alloc_error in the next update.

[vsk]

Sorry for the delay here. An update: I tested this patch with a sanitized build on a Linux install, but could not reproduce the "malformed coverage data" errors seen on the Windows bots. At this point, it seems likely that there is a bug in CoverageMappingReader (this would explain why the .covmapping testing bundles fail to parse), and it may be that there is a separate issue on the producer side (this could explain why the 'instrprof-merging.cpp' test from check-profile failed). I think I need a Windows set up to debug further, and will try to find one. Meanwhile, if anyone has the bandwidth to attempt to reproduce on Windows, and to share a backtrace from where the CoverageMapError constructor is called, I would greatly appreciate it.

[rnk]

I have a Windows build directory and am motivated to debug this. I'll try to do it tomorrow, but I have a couple of other deadlines so I can't make a very firm promise.

[rnk]

I haven't forgotten about this, even though it's been two months. Hopefully I get to it soon.

[vsk]

Rebase.

Sorry this hasn't seen any update: at this point in our release cycle I've had to put this on the back burner.

[rnk]

Compiling with -fdump-record-layouts revealed the problem:

*** Dumping AST Record Layout
         0 | struct llvm::coverage::CovMapFunctionRecordV3
         0 |   struct llvm::coverage::accessors::FuncHashAndDataSize<struct llvm::coverage::CovMapFunctionRecordV3> (base) (empty)
         1 |   struct llvm::coverage::accessors::HashedNameRef<struct llvm::coverage::CovMapFunctionRecordV3> (base) (empty)
         1 |   const int64_t NameRef
         9 |   const uint32_t DataSize
        13 |   const uint64_t FuncHash
        21 |   const uint64_t FilenamesRef
        29 |   const char CoverageMapping
           | [sizeof=30, align=1,
           |  nvsize=30, nvalign=1]

Everything is off-by-one because the empty bases are not zero sized. The MSVC record layout algorithm is just different in this area. =/

So, I think this patch would be fine if you refactor it to avoid the accessor classes. I took a stab at it, but it's not straightforward.

[dexonsmith]

In D69471#1883912, @rnk wrote:

Everything is off-by-one because the empty bases are not zero sized. The MSVC record layout algorithm is just different in this area. =/

Do all the MSVCs we support building with support __declspec(empty_bases)?
https://devblogs.microsoft.com/cppblog/optimizing-the-layout-of-empty-base-classes-in-vs2015-update-2-3/

So, I think this patch would be fine if you refactor it to avoid the accessor classes. I took a stab at it, but it's not straightforward.

Another option is to chain the accessor classes:

template <class T, class Base = void> struct MyAccessor1 : Base { /* ... */ };
template <class T> struct MyAccessor1<void> { /* ... */ };
template <class T, class Base = void> struct MyAccessor2 : Base { /* ... */ };
template <class T> struct MyAccessor2<void> { /* ... */ };

Then you can:

struct MyFormat : MyAccessor1<MyFormat, MyAccessor2<MyFormat>> { /* ... */ };

[rnk]

In D69471#1884043, @dexonsmith wrote:

In D69471#1883912, @rnk wrote:

Everything is off-by-one because the empty bases are not zero sized. The MSVC record layout algorithm is just different in this area. =/

Do all the MSVCs we support building with support __declspec(empty_bases)?
https://devblogs.microsoft.com/cppblog/optimizing-the-layout-of-empty-base-classes-in-vs2015-update-2-3/

Yes, we can use it. Clang supports it too.

---

However, you might want this class to follow the rules for a standard layout type:
http://www.cplusplus.com/reference/type_traits/is_standard_layout/
" ... has .... at most one base class with non-static data members, or has no base classes with non-static data members."
So the use of multiple inheritance makes a type non-standard layout. Nevermind that MSVC accepts the following program:

#include <type_traits>
struct EmptyA {};
struct EmptyB {};
struct Foo : EmptyA, EmptyB { int x, y; };
static_assert(std::is_standard_layout<Foo>::value, "asdf");

Why not use free function templates like these?

template <typename T> uint64_t getFoo(T *p) { return p->Foo; }
template <typename T> uint64_t getBar(T *p) { return p->Bar; }

These accessors don't seem like they have to be methods. It actually makes them a bit more awkward to use:

uint32_t DataSize = CFR->template getDataSize<Endian>();

[vsk]

@rnk Thanks for chasing this down. I'll update the function record structs to use free functions instead of multiple inheritance.

I don't plan on getting rid of the awkward method calls at this point. The coverage reader is still templated by CovMapFunctionRecordX via CovMapTraits, so we'd need to untangle that first.

[vsk]

Get rid of multiple inheritance in the coverage::accessors namespace. This is ninja check-{llvm, clang, profile} clean on macOS. I'll wait on this until next week and re-land it unless any objections / new issues pop up.

[rnk]

Sounds like a plan, thanks!

clang/lib/CodeGen/CoverageMappingGen.h

 
 namespace CodeGen {
 
 class CodeGenModule;
 
 /// Organizes the cross-function state that is used while generating
 /// code coverage mapping data.
 class CoverageMappingModuleGen {
+  /// Information needed to emit a coverage record for a function.
+  struct FunctionInfo {
+    uint64_t NameHash;
+    uint64_t FuncHash;
+    std::string CoverageMapping;
+    bool IsUsed;
+  };
+
   CodeGenModule &CGM;
   CoverageSourceInfo &SourceInfo;
   llvm::SmallDenseMap<const FileEntry *, unsigned, 8> FileEntries;
-  std::vector<llvm::Constant *> FunctionRecords;
   std::vector<llvm::Constant *> FunctionNames;
-  llvm::StructType *FunctionRecordTy;
-  std::vector<std::string> CoverageMappings;
+  std::vector<FunctionInfo> FunctionRecords;
+
+  /// Emit a function record.
+  void emitFunctionMappingRecord(const FunctionInfo &Info,
+                                 uint64_t FilenamesRef);
 
 public:
   CoverageMappingModuleGen(CodeGenModule &CGM, CoverageSourceInfo &SourceInfo)
-      : CGM(CGM), SourceInfo(SourceInfo), FunctionRecordTy(nullptr) {}
+      : CGM(CGM), SourceInfo(SourceInfo) {}
 
   CoverageSourceInfo &getSourceInfo() const {
     return SourceInfo;
   }
 
   /// Add a function's coverage mapping record to the collection of the
   /// function mapping records.
   void addFunctionMappingRecord(llvm::GlobalVariable *FunctionName,

clang/lib/CodeGen/CoverageMappingGen.cpp

 //===--- CoverageMappingGen.cpp - Coverage mapping generation ---*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // Instrumentation-based code coverage mapping generator
 //
 //===----------------------------------------------------------------------===//
 
 #include "CoverageMappingGen.h"
 #include "CodeGenFunction.h"
 #include "clang/AST/StmtVisitor.h"
+#include "clang/Basic/Diagnostic.h"
+#include "clang/Frontend/FrontendDiagnostic.h"
 #include "clang/Lex/Lexer.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ProfileData/Coverage/CoverageMapping.h"
 #include "llvm/ProfileData/Coverage/CoverageMappingReader.h"
 #include "llvm/ProfileData/Coverage/CoverageMappingWriter.h"
 #include "llvm/ProfileData/InstrProfReader.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 
+// This selects the coverage mapping format defined when `InstrProfData.inc`
+// is textually included.
+#define COVMAP_V3
+
 using namespace clang;
 using namespace CodeGen;
 using namespace llvm::coverage;
 
 void CoverageSourceInfo::SourceRangeSkipped(SourceRange Range, SourceLocation) {
   SkippedRanges.push_back(Range);
 }
 
struct CounterCoverageMappingBuilder
   }
 
   void VisitLambdaExpr(const LambdaExpr *LE) {
     // Lambdas are treated as their own functions for now, so we shouldn't
     // propagate counts into them.
   }
 };
 
-std::string getCoverageSection(const CodeGenModule &CGM) {
-  return llvm::getInstrProfSectionName(
-      llvm::IPSK_covmap,
-      CGM.getContext().getTargetInfo().getTriple().getObjectFormat());
-}
-
 std::string normalizeFilename(StringRef Filename) {
   llvm::SmallString<256> Path(Filename);
   llvm::sys::fs::make_absolute(Path);
   llvm::sys::path::remove_dots(Path, /*remove_dot_dot=*/true);
   return std::string(Path);
 }
 
 } // end anonymous namespace
OS << "File " << R.FileID << ", " << R.LineStart << ":" << R.ColumnStart
        << " -> " << R.LineEnd << ":" << R.ColumnEnd << " = ";
     Ctx.dump(R.Count, OS);
     if (R.Kind == CounterMappingRegion::ExpansionRegion)
       OS << " (Expanded file = " << R.ExpandedFileID << ")";
     OS << "\n";
   }
 }
 
-void CoverageMappingModuleGen::addFunctionMappingRecord(
-    llvm::GlobalVariable *NamePtr, StringRef NameValue, uint64_t FuncHash,
-    const std::string &CoverageMapping, bool IsUsed) {
+static std::string getInstrProfSection(const CodeGenModule &CGM,
+                                       llvm::InstrProfSectKind SK) {
+  return llvm::getInstrProfSectionName(
+      SK, CGM.getContext().getTargetInfo().getTriple().getObjectFormat());
+}
+
+void CoverageMappingModuleGen::emitFunctionMappingRecord(
+    const FunctionInfo &Info, uint64_t FilenamesRef) {
   llvm::LLVMContext &Ctx = CGM.getLLVMContext();
-  if (!FunctionRecordTy) {
+
+  // Assign a name to the function record. This is used to merge duplicates.
+  std::string FuncRecordName = "__covrec_" + llvm::utohexstr(Info.NameHash);
+
+  // A dummy description for a function included-but-not-used in a TU can be
+  // replaced by full description provided by a different TU. The two kinds of
+  // descriptions play distinct roles: therefore, assign them different names
+  // to prevent `linkonce_odr` merging.
+  if (Info.IsUsed)
+    FuncRecordName += "u";
+
+  // Create the function record type.
+  const uint64_t NameHash = Info.NameHash;
+  const uint64_t FuncHash = Info.FuncHash;
+  const std::string &CoverageMapping = Info.CoverageMapping;
 #define COVMAP_FUNC_RECORD(Type, LLVMType, Name, Init) LLVMType,
-    llvm::Type *FunctionRecordTypes[] = {
-      #include "llvm/ProfileData/InstrProfData.inc"
-    };
-    FunctionRecordTy =
-        llvm::StructType::get(Ctx, makeArrayRef(FunctionRecordTypes),
-                              /*isPacked=*/true);
-  }
+  auto *FunctionRecordTy =
 
-  #define COVMAP_FUNC_RECORD(Type, LLVMType, Name, Init) Init,
+  // Create the function record constant.
   llvm::Constant *FunctionRecordVals[] = {
       #include "llvm/ProfileData/InstrProfData.inc"
   };
-  FunctionRecords.push_back(llvm::ConstantStruct::get(
-      FunctionRecordTy, makeArrayRef(FunctionRecordVals)));
+  auto *FuncRecordConstant = llvm::ConstantStruct::get(
+      FunctionRecordTy, makeArrayRef(FunctionRecordVals));
+
+  // Create the function record global.
+  auto *FuncRecord = new llvm::GlobalVariable(
+      CGM.getModule(), FunctionRecordTy, /*isConstant=*/true,
+      llvm::GlobalValue::LinkOnceODRLinkage, FuncRecordConstant,
+      FuncRecordName);
+  FuncRecord->setVisibility(llvm::GlobalValue::HiddenVisibility);
+  FuncRecord->setSection(getInstrProfSection(CGM, llvm::IPSK_covfun));
+  FuncRecord->setAlignment(llvm::Align(8));
+  if (CGM.supportsCOMDAT())
+    FuncRecord->setComdat(CGM.getModule().getOrInsertComdat(FuncRecordName));
+
+  // Make sure the data doesn't get deleted.
+  CGM.addUsedGlobal(FuncRecord);
+}
+
+void CoverageMappingModuleGen::addFunctionMappingRecord(
+    llvm::GlobalVariable *NamePtr, StringRef NameValue, uint64_t FuncHash,
+    const std::string &CoverageMapping, bool IsUsed) {
+  llvm::LLVMContext &Ctx = CGM.getLLVMContext();
+  const uint64_t NameHash = llvm::IndexedInstrProf::ComputeHash(NameValue);
+  FunctionRecords.push_back({NameHash, FuncHash, CoverageMapping, IsUsed});
+
   if (!IsUsed)
     FunctionNames.push_back(
         llvm::ConstantExpr::getBitCast(NamePtr, llvm::Type::getInt8PtrTy(Ctx)));
-  CoverageMappings.push_back(CoverageMapping);
 
   if (CGM.getCodeGenOpts().DumpCoverageMapping) {
     // Dump the coverage mapping data for this function by decoding the
     // encoded data. This allows us to dump the mapping regions which were
     // also processed by the CoverageMappingWriter which performs
     // additional minimization operations such as reducing the number of
     // expressions.
     std::vector<StringRef> Filenames;
void CoverageMappingModuleGen::emit() {
   FilenameStrs.resize(FileEntries.size());
   FilenameRefs.resize(FileEntries.size());
   for (const auto &Entry : FileEntries) {
     auto I = Entry.second;
     FilenameStrs[I] = normalizeFilename(Entry.first->getName());
     FilenameRefs[I] = FilenameStrs[I];
   }
 
-  std::string FilenamesAndCoverageMappings;
-  llvm::raw_string_ostream OS(FilenamesAndCoverageMappings);
+  std::string Filenames;
+  {
+    llvm::raw_string_ostream OS(Filenames);
     CoverageFilenamesSectionWriter(FilenameRefs).write(OS);
-
-  // Stream the content of CoverageMappings to OS while keeping
-  // memory consumption under control.
-  size_t CoverageMappingSize = 0;
-  for (auto &S : CoverageMappings) {
-    CoverageMappingSize += S.size();
-    OS << S;
-    S.clear();
-    S.shrink_to_fit();
-  }
-  CoverageMappings.clear();
-  CoverageMappings.shrink_to_fit();
-
-  size_t FilenamesSize = OS.str().size() - CoverageMappingSize;
-  // Append extra zeroes if necessary to ensure that the size of the filenames
-  // and coverage mappings is a multiple of 8.
-  if (size_t Rem = OS.str().size() % 8) {
-    CoverageMappingSize += 8 - Rem;
-    OS.write_zeros(8 - Rem);
   }
-  auto *FilenamesAndMappingsVal =
-      llvm::ConstantDataArray::getString(Ctx, OS.str(), false);
-
-  // Create the deferred function records array
-  auto RecordsTy =
-      llvm::ArrayType::get(FunctionRecordTy, FunctionRecords.size());
-  auto RecordsVal = llvm::ConstantArray::get(RecordsTy, FunctionRecords);
-
+  auto *FilenamesVal =
+      llvm::ConstantDataArray::getString(Ctx, Filenames, false);
+  const int64_t FilenamesRef = llvm::IndexedInstrProf::ComputeHash(Filenames);
+
+  // Emit the function records.
+  for (const FunctionInfo &Info : FunctionRecords)
+    emitFunctionMappingRecord(Info, FilenamesRef);
+
+  const unsigned NRecords = 0;
+  const size_t FilenamesSize = Filenames.size();
+  const unsigned CoverageMappingSize = 0;
   llvm::Type *CovDataHeaderTypes[] = {
 #define COVMAP_HEADER(Type, LLVMType, Name, Init) LLVMType,
 #include "llvm/ProfileData/InstrProfData.inc"
   };
   auto CovDataHeaderTy =
       llvm::StructType::get(Ctx, makeArrayRef(CovDataHeaderTypes));
   llvm::Constant *CovDataHeaderVals[] = {
 #define COVMAP_HEADER(Type, LLVMType, Name, Init) Init,
 #include "llvm/ProfileData/InstrProfData.inc"
   };
   auto CovDataHeaderVal = llvm::ConstantStruct::get(
       CovDataHeaderTy, makeArrayRef(CovDataHeaderVals));
 
   // Create the coverage data record
-  llvm::Type *CovDataTypes[] = {CovDataHeaderTy, RecordsTy,
-                                FilenamesAndMappingsVal->getType()};
+  llvm::Type *CovDataTypes[] = {CovDataHeaderTy, FilenamesVal->getType()};
   auto CovDataTy = llvm::StructType::get(Ctx, makeArrayRef(CovDataTypes));
-  llvm::Constant *TUDataVals[] = {CovDataHeaderVal, RecordsVal,
-                                  FilenamesAndMappingsVal};
+  llvm::Constant *TUDataVals[] = {CovDataHeaderVal, FilenamesVal};
   auto CovDataVal =
       llvm::ConstantStruct::get(CovDataTy, makeArrayRef(TUDataVals));
   auto CovData = new llvm::GlobalVariable(
-      CGM.getModule(), CovDataTy, true, llvm::GlobalValue::InternalLinkage,
+      CGM.getModule(), CovDataTy, true, llvm::GlobalValue::PrivateLinkage,
       CovDataVal, llvm::getCoverageMappingVarName());
 
-  CovData->setSection(getCoverageSection(CGM));
+  CovData->setSection(getInstrProfSection(CGM, llvm::IPSK_covmap));
   CovData->setAlignment(llvm::Align(8));
 
   // Make sure the data doesn't get deleted.
   CGM.addUsedGlobal(CovData);
   // Create the deferred function records array
   if (!FunctionNames.empty()) {
     auto NamesArrTy = llvm::ArrayType::get(llvm::Type::getInt8PtrTy(Ctx),
                                            FunctionNames.size());

clang/test/CoverageMapping/abspath.cpp

-// RUN: %clang_cc1 -fprofile-instrument=clang -fcoverage-mapping -emit-llvm -main-file-name abspath.cpp %S/Inputs/../abspath.cpp -o - | FileCheck -check-prefix=RMDOTS %s
+// RUN: %clang_cc1 -fprofile-instrument=clang -fcoverage-mapping -mllvm -enable-name-compression=false -emit-llvm -main-file-name abspath.cpp %S/Inputs/../abspath.cpp -o - | FileCheck -check-prefix=RMDOTS %s
 
 // RMDOTS: @__llvm_coverage_mapping = {{.*}}"\01
 // RMDOTS-NOT: Inputs
 // RMDOTS: "
 
 // RUN: mkdir -p %t/test && cd %t/test
 // RUN: echo "void f1() {}" > f1.c
-// RUN: %clang_cc1 -fprofile-instrument=clang -fcoverage-mapping -emit-llvm -main-file-name abspath.cpp ../test/f1.c -o - | FileCheck -check-prefix=RELPATH %s
+// RUN: %clang_cc1 -fprofile-instrument=clang -fcoverage-mapping -mllvm -enable-name-compression=false -emit-llvm -main-file-name abspath.cpp ../test/f1.c -o - | FileCheck -check-prefix=RELPATH %s
 
 // RELPATH: @__llvm_coverage_mapping = {{.*}}"\01
 // RELPATH: {{[/\\].*(/|\\\\)test(/|\\\\)f1}}.c
 // RELPATH: "
 
 void f1() {}

clang/test/CoverageMapping/ir.c

 // Check the data structures emitted by coverage mapping
-// RUN: %clang_cc1 -triple x86_64-apple-macosx10.9 -main-file-name ir.c %s -o - -emit-llvm -fprofile-instrument=clang -fcoverage-mapping | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-apple-macosx10.9 -main-file-name ir.c %s -o - -emit-llvm -fprofile-instrument=clang -fcoverage-mapping -mllvm -enable-name-compression=false | FileCheck %s -check-prefixes=COMMON,DARWIN
+// RUN: %clang_cc1 -triple x86_64--windows-msvc -main-file-name ir.c %s -o - -emit-llvm -fprofile-instrument=clang -fcoverage-mapping -mllvm -enable-name-compression=false | FileCheck %s -check-prefixes=COMMON,WINDOWS
 
+static inline void unused() {}
 
-void foo(void) { }
+void foo(void) {}
 
 int main(void) {
   foo();
   return 0;
 }
 
-// CHECK: @__llvm_coverage_mapping = internal constant { { i32, i32, i32, i32 }, [2 x <{ i64, i32, i64 }>], [{{[0-9]+}} x i8] } { { i32, i32, i32, i32 } { i32 2, i32 {{[0-9]+}}, i32 {{[0-9]+}}, i32 {{[0-9]+}} }, [2 x <{ i64, i32, i64 }>] [<{{.*}}> <{{.*}}>, <{{.*}}> <{{.*}}>]
+// Check the function records. Two of the record names should come in the 'used'
+// flavor, and one should not.
+
+// DARWIN: [[FuncRecord1:@__covrec_[0-9A-F]+u]] = linkonce_odr hidden constant <{ i64, i32, i64, i64, [{{.*}} x i8] }> <{ {{.*}} }>, section "__LLVM_COV,__llvm_covfun", align 8
+// DARWIN: [[FuncRecord2:@__covrec_[0-9A-F]+u]] = linkonce_odr hidden constant <{ i64, i32, i64, i64, [{{.*}} x i8] }> <{ {{.*}} }>, section "__LLVM_COV,__llvm_covfun", align 8
+// DARWIN: [[FuncRecord3:@__covrec_[0-9A-F]+]] = linkonce_odr hidden constant <{ i64, i32, i64, i64, [{{.*}} x i8] }> <{ {{.*}} }>, section "__LLVM_COV,__llvm_covfun", align 8
+// DARWIN: @__llvm_coverage_mapping = private constant { { i32, i32, i32, i32 }, [{{.*}} x i8] } { {{.*}} }, section "__LLVM_COV,__llvm_covmap", align 8
+
+// WINDOWS: [[FuncRecord1:@__covrec_[0-9A-F]+u]] = linkonce_odr hidden constant <{ i64, i32, i64, i64, [{{.*}} x i8] }> <{ {{.*}} }>, section ".lcovfun$M", comdat, align 8
+// WINDOWS: [[FuncRecord2:@__covrec_[0-9A-F]+u]] = linkonce_odr hidden constant <{ i64, i32, i64, i64, [{{.*}} x i8] }> <{ {{.*}} }>, section ".lcovfun$M", comdat, align 8
+// WINDOWS: [[FuncRecord3:@__covrec_[0-9A-F]+]] = linkonce_odr hidden constant <{ i64, i32, i64, i64, [{{.*}} x i8] }> <{ {{.*}} }>, section ".lcovfun$M", comdat, align 8
+// WINDOWS: @__llvm_coverage_mapping = private constant { { i32, i32, i32, i32 }, [{{.*}} x i8] } { {{.*}} }, section ".lcovmap$M", align 8
+
+// COMMON: @llvm.used = appending global [{{.*}} x i8*]
+// COMMON-SAME: [[FuncRecord1]]
+// COMMON-SAME: [[FuncRecord2]]
+// COMMON-SAME: [[FuncRecord3]]
+// COMMON-SAME: @__llvm_coverage_mapping

clang/test/Profile/def-assignop.cpp

struct A {
   // PGOGEN: {{.*}}add{{.*}}%pgocount, 1
   // PGOGEN: store{{.*}}@__profc__ZN1AaSERKS_
   A &operator=(A &&) = default;
   // PGOGEN: define {{.*}}@_ZN1AaSEOS_
   // PGOGEN: %pgocount = load {{.*}} @__profc__ZN1AaSEOS_
   // PGOGEN: {{.*}}add{{.*}}%pgocount, 1
   // PGOGEN: store{{.*}}@__profc__ZN1AaSEOS_
 
-  // Check that coverage mapping includes 6 function records including the
+  // Check that coverage mapping includes 3 function records including the
   // defaulted copy and move operators: A::operator=
-  // COVMAP: @__llvm_coverage_mapping = {{.*}} { { i32, i32, i32, i32 }, [3 x <{{.*}}>],
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: @__llvm_coverage_mapping = {{.*}} { { i32, i32, i32, i32 }
   B b;
 };
 
 A a1, a2;
 void foo() {
   a1 = a2;
   a2 = static_cast<A &&>(a1);
 }

clang/test/Profile/def-ctors.cpp

struct Derived : public Base {
   // PGOGEN-DAG: {{.*}}add{{.*}}%pgocount, 1
   // PGOGEN-DAG: store{{.*}}@__profc__ZN7DerivedC2ERKS_
   Derived() = default;
   // PGOGEN-DAG: define {{.*}}@_ZN7DerivedC2Ev
   // PGOGEN-DAG: %pgocount = load {{.*}} @__profc__ZN7DerivedC2Ev
   // PGOGEN-DAG: {{.*}}add{{.*}}%pgocount, 1
   // PGOGEN-DAG: store{{.*}}@__profc__ZN7DerivedC2Ev
 
-  // Check that coverage mapping has 6 function records including
+  // Check that coverage mapping has 5 function records including
   // the defaulted Derived::Derived(const Derived), and Derived::Derived()
   // methds.
-  // COVMAP: @__llvm_coverage_mapping = {{.*}} { { i32, i32, i32, i32 }, [5 x
-  // <{{.*}}>],
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: @__llvm_coverage_mapping = {{.*}} { { i32, i32, i32, i32 }
 };
 
 Derived dd;
 int g;
 int main() {
   Derived dd2(dd);
   g = dd2.B;
   return 0;
 }

clang/test/Profile/def-dtors.cpp

 struct Derived : public Base {
   Derived(int K) : Base(K) {}
   ~Derived() = default;
   // PGOGEN-LABEL: define {{.*}}@_ZN7DerivedD2Ev
   // PGOGEN: %pgocount = load {{.*}} @__profc__ZN7DerivedD2Ev
   // PGOGEN: {{.*}}add{{.*}}%pgocount, 1
   // PGOGEN: store{{.*}}@__profc__ZN7DerivedD2Ev
 
-  // Check that coverage mapping has 6 function records including
+  // Check that coverage mapping has 5 function records including
   // the default destructor in the derived class.
-  // COVMAP: @__llvm_coverage_mapping = {{.*}} { { i32, i32, i32, i32 }, [5 x
-  // <{{.*}}>],
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: section "__llvm_covfun", comdat
+  // COVMAP: @__llvm_coverage_mapping = {{.*}} { { i32, i32, i32, i32 }
 };
 
 int main() {
   Derived dd2(10);
   if (dd2.B != 10)
     return 1;
   return 0;
 }

compiler-rt/include/profile/InstrProfData.inc

  * indexed with the kind value.
  */
 VALUE_PROF_KIND(IPVK_First, IPVK_IndirectCallTarget, "first")
 VALUE_PROF_KIND(IPVK_Last, IPVK_MemOPSize, "last")
 
 #undef VALUE_PROF_KIND
 /* VALUE_PROF_KIND end */
 
+#undef COVMAP_V2_OR_V3
+#ifdef COVMAP_V2
+#define COVMAP_V2_OR_V3
+#endif
+#ifdef COVMAP_V3
+#define COVMAP_V2_OR_V3
+#endif
+
 /* COVMAP_FUNC_RECORD start */
 /* Definition of member fields of the function record structure in coverage
  * map.
  */
 #ifndef COVMAP_FUNC_RECORD
 #define COVMAP_FUNC_RECORD(Type, LLVMType, Name, Initializer)
 #else
 #define INSTR_PROF_DATA_DEFINED
 #endif
 #ifdef COVMAP_V1
 COVMAP_FUNC_RECORD(const IntPtrT, llvm::Type::getInt8PtrTy(Ctx), \
                    NamePtr, llvm::ConstantExpr::getBitCast(NamePtr, \
                    llvm::Type::getInt8PtrTy(Ctx)))
 COVMAP_FUNC_RECORD(const uint32_t, llvm::Type::getInt32Ty(Ctx), NameSize, \
                    llvm::ConstantInt::get(llvm::Type::getInt32Ty(Ctx), \
                    NameValue.size()))
-#else
+#endif
+#ifdef COVMAP_V2_OR_V3
 COVMAP_FUNC_RECORD(const int64_t, llvm::Type::getInt64Ty(Ctx), NameRef, \
-                   llvm::ConstantInt::get(llvm::Type::getInt64Ty(Ctx), \
-                   llvm::IndexedInstrProf::ComputeHash(NameValue)))
+                   llvm::ConstantInt::get( \
+                     llvm::Type::getInt64Ty(Ctx), NameHash))
 #endif
 COVMAP_FUNC_RECORD(const uint32_t, llvm::Type::getInt32Ty(Ctx), DataSize, \
-                   llvm::ConstantInt::get(llvm::Type::getInt32Ty(Ctx),\
-                   CoverageMapping.size()))
+                   llvm::ConstantInt::get( \
+                     llvm::Type::getInt32Ty(Ctx), CoverageMapping.size()))
 COVMAP_FUNC_RECORD(const uint64_t, llvm::Type::getInt64Ty(Ctx), FuncHash, \
-                   llvm::ConstantInt::get(llvm::Type::getInt64Ty(Ctx), FuncHash))
+                   llvm::ConstantInt::get( \
+                     llvm::Type::getInt64Ty(Ctx), FuncHash))
+#ifdef COVMAP_V3
+COVMAP_FUNC_RECORD(const uint64_t, llvm::Type::getInt64Ty(Ctx), FilenamesRef, \
+                   llvm::ConstantInt::get( \
+                     llvm::Type::getInt64Ty(Ctx), FilenamesRef))
+COVMAP_FUNC_RECORD(const char, \
+                   llvm::ArrayType::get(llvm::Type::getInt8Ty(Ctx), \
+                                        CoverageMapping.size()), \
+                   CoverageMapping,
+                   llvm::ConstantDataArray::getRaw( \
+                     CoverageMapping, CoverageMapping.size(), \
+                     llvm::Type::getInt8Ty(Ctx)))
+#endif
 #undef COVMAP_FUNC_RECORD
 /* COVMAP_FUNC_RECORD end.  */
 
 /* COVMAP_HEADER start */
 /* Definition of member fields of coverage map header.
  */
 #ifndef COVMAP_HEADER
 #define COVMAP_HEADER(Type, LLVMType, Name, Initializer)
 #else
 #define INSTR_PROF_DATA_DEFINED
 #endif
 COVMAP_HEADER(uint32_t, Int32Ty, NRecords, \
-              llvm::ConstantInt::get(Int32Ty,  FunctionRecords.size()))
+              llvm::ConstantInt::get(Int32Ty, NRecords))
 COVMAP_HEADER(uint32_t, Int32Ty, FilenamesSize, \
               llvm::ConstantInt::get(Int32Ty, FilenamesSize))
 COVMAP_HEADER(uint32_t, Int32Ty, CoverageSize, \
               llvm::ConstantInt::get(Int32Ty, CoverageMappingSize))
 COVMAP_HEADER(uint32_t, Int32Ty, Version, \
               llvm::ConstantInt::get(Int32Ty, CovMapVersion::CurrentVersion))
 #undef COVMAP_HEADER
 /* COVMAP_HEADER end.  */
INSTR_PROF_SECT_ENTRY(IPSK_vals, \
                       INSTR_PROF_QUOTE(INSTR_PROF_VALS_COMMON), \
                       INSTR_PROF_VALS_COFF, "__DATA,")
 INSTR_PROF_SECT_ENTRY(IPSK_vnodes, \
                       INSTR_PROF_QUOTE(INSTR_PROF_VNODES_COMMON), \
                       INSTR_PROF_VNODES_COFF, "__DATA,")
 INSTR_PROF_SECT_ENTRY(IPSK_covmap, \
                       INSTR_PROF_QUOTE(INSTR_PROF_COVMAP_COMMON), \
                       INSTR_PROF_COVMAP_COFF, "__LLVM_COV,")
+INSTR_PROF_SECT_ENTRY(IPSK_covfun, \
+                      INSTR_PROF_QUOTE(INSTR_PROF_COVFUN_COMMON), \
+                      INSTR_PROF_COVFUN_COFF, "__LLVM_COV,")
 INSTR_PROF_SECT_ENTRY(IPSK_orderfile, \
                       INSTR_PROF_QUOTE(INSTR_PROF_ORDERFILE_COMMON), \
                       INSTR_PROF_QUOTE(INSTR_PROF_ORDERFILE_COFF), "__DATA,")
 
 #undef INSTR_PROF_SECT_ENTRY
 #endif
 
 

 #define INSTR_PROF_RAW_MAGIC_32 (uint64_t)255 << 56 | (uint64_t)'l' << 48 | \
        (uint64_t)'p' << 40 | (uint64_t)'r' << 32 | (uint64_t)'o' << 24 |  \
         (uint64_t)'f' << 16 | (uint64_t)'R' << 8 | (uint64_t)129
 
 /* Raw profile format version (start from 1). */
 #define INSTR_PROF_RAW_VERSION 5
 /* Indexed profile format version (start from 1). */
 #define INSTR_PROF_INDEX_VERSION 5
-/* Coverage mapping format vresion (start from 0). */
-#define INSTR_PROF_COVMAP_VERSION 2
+/* Coverage mapping format version (start from 0). */
+#define INSTR_PROF_COVMAP_VERSION 3
 
 /* Profile version is always of type uint64_t. Reserve the upper 8 bits in the
  * version for other variants of profile. We set the lowest bit of the upper 8
  * bits (i.e. bit 56) to 1 to indicate if this is an IR-level instrumentaiton
  * generated profile, and 0 if this is a Clang FE generated profile.
  * 1 in bit 57 indicates there are context-sensitive records in the profile.
  */
 #define VARIANT_MASKS_ALL 0xff00000000000000ULL

 /* section name strings common to all targets other
    than WIN32 */
 #define INSTR_PROF_DATA_COMMON __llvm_prf_data
 #define INSTR_PROF_NAME_COMMON __llvm_prf_names
 #define INSTR_PROF_CNTS_COMMON __llvm_prf_cnts
 #define INSTR_PROF_VALS_COMMON __llvm_prf_vals
 #define INSTR_PROF_VNODES_COMMON __llvm_prf_vnds
 #define INSTR_PROF_COVMAP_COMMON __llvm_covmap
+#define INSTR_PROF_COVFUN_COMMON __llvm_covfun
 #define INSTR_PROF_ORDERFILE_COMMON __llvm_orderfile
 /* Windows section names. Because these section names contain dollar characters,
  * they must be quoted.
  */
 #define INSTR_PROF_DATA_COFF ".lprfd$M"
 #define INSTR_PROF_NAME_COFF ".lprfn$M"
 #define INSTR_PROF_CNTS_COFF ".lprfc$M"
 #define INSTR_PROF_VALS_COFF ".lprfv$M"
 #define INSTR_PROF_VNODES_COFF ".lprfnd$M"
 #define INSTR_PROF_COVMAP_COFF ".lcovmap$M"
+#define INSTR_PROF_COVFUN_COFF ".lcovfun$M"
 #define INSTR_PROF_ORDERFILE_COFF ".lorderfile$M"
 
 #ifdef _WIN32
 /* Runtime section names and name strings.  */
 #define INSTR_PROF_DATA_SECT_NAME INSTR_PROF_DATA_COFF
 #define INSTR_PROF_NAME_SECT_NAME INSTR_PROF_NAME_COFF
 #define INSTR_PROF_CNTS_SECT_NAME INSTR_PROF_CNTS_COFF
 /* Array of pointers. Each pointer points to a list
  * of value nodes associated with one value site.
  */
 #define INSTR_PROF_VALS_SECT_NAME INSTR_PROF_VALS_COFF
 /* Value profile nodes section. */
 #define INSTR_PROF_VNODES_SECT_NAME INSTR_PROF_VNODES_COFF
 #define INSTR_PROF_COVMAP_SECT_NAME INSTR_PROF_COVMAP_COFF
+#define INSTR_PROF_COVFUN_SECT_NAME INSTR_PROF_COVFUN_COFF
 #define INSTR_PROF_ORDERFILE_SECT_NAME INSTR_PROF_ORDERFILE_COFF
 #else
 /* Runtime section names and name strings.  */
 #define INSTR_PROF_DATA_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_DATA_COMMON)
 #define INSTR_PROF_NAME_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_NAME_COMMON)
 #define INSTR_PROF_CNTS_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_CNTS_COMMON)
 /* Array of pointers. Each pointer points to a list
  * of value nodes associated with one value site.
  */
 #define INSTR_PROF_VALS_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_VALS_COMMON)
 /* Value profile nodes section. */
 #define INSTR_PROF_VNODES_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_VNODES_COMMON)
 #define INSTR_PROF_COVMAP_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_COVMAP_COMMON)
+#define INSTR_PROF_COVFUN_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_COVFUN_COMMON)
 /* Order file instrumentation. */
 #define INSTR_PROF_ORDERFILE_SECT_NAME                                         \
   INSTR_PROF_QUOTE(INSTR_PROF_ORDERFILE_COMMON)
 #endif
 
 #define INSTR_PROF_ORDERFILE_BUFFER_NAME _llvm_order_file_buffer
 #define INSTR_PROF_ORDERFILE_BUFFER_NAME_STR                                   \
   INSTR_PROF_QUOTE(INSTR_PROF_ORDERFILE_BUFFER_NAME)

 /* The maximal # of functions: 128*1024 (the buffer size will be 128*4 KB). */
 #define INSTR_ORDER_FILE_BUFFER_SIZE 131072
 #define INSTR_ORDER_FILE_BUFFER_BITS 17
 #define INSTR_ORDER_FILE_BUFFER_MASK 0x1ffff
 #endif /* INSTR_ORDER_FILE_INC */
 #else
 #undef INSTR_PROF_DATA_DEFINED
 #endif
+
+#undef COVMAP_V2_OR_V3

llvm/docs/CoverageMappingFormat.rst

 
 Introduction
 ============
 
 LLVM's code coverage mapping format is used to provide code coverage
 analysis using LLVM's and Clang's instrumentation based profiling
 (Clang's ``-fprofile-instr-generate`` option).
 
-This document is aimed at those who use LLVM's code coverage mapping to provide
-code coverage analysis for their own programs, and for those who would like
-to know how it works under the hood. A prior knowledge of how Clang's profile
-guided optimization works is useful, but not required.
+This document is aimed at those who would like to know how LLVM's code coverage
+mapping works under the hood. A prior knowledge of how Clang's profile guided
+optimization works is useful, but not required. For those interested in using
+LLVM to provide code coverage analysis for their own programs, see the `Clang
+documentation <https://clang.llvm.org/docs/SourceBasedCodeCoverage.html>`.
 
-We start by showing how to use LLVM and Clang for code coverage analysis,
-then we briefly describe LLVM's code coverage mapping format and the
+We start by briefly describing LLVM's code coverage mapping format and the
 way that Clang and LLVM's code coverage tool work with this format. After
 the basics are down, more advanced features of the coverage mapping format
 are discussed - such as the data structures, LLVM IR representation and
 the binary encoding.
 
-Quick Start
-===========
-
-Here's a short story that describes how to generate code coverage overview
-for a sample source file called *test.c*.
-
-* First, compile an instrumented version of your program using Clang's
-  ``-fprofile-instr-generate`` option with the additional ``-fcoverage-mapping``
-  option:
-
-  ``clang -o test -fprofile-instr-generate -fcoverage-mapping test.c``
-* Then, run the instrumented binary. The runtime will produce a file called
-  *default.profraw* containing the raw profile instrumentation data:
-
-  ``./test``
-* After that, merge the profile data using the *llvm-profdata* tool:
-
-  ``llvm-profdata merge -o test.profdata default.profraw``
-* Finally, run LLVM's code coverage tool (*llvm-cov*) to produce the code
-  coverage overview for the sample source file:
-
-  ``llvm-cov show ./test -instr-profile=test.profdata test.c``
-
 High Level Overview
 ===================
 
 LLVM's code coverage mapping format is designed to be a self contained
-data format, that can be embedded into the LLVM IR and object files.
+data format that can be embedded into the LLVM IR and into object files.
 It's described in this document as a **mapping** format because its goal is
 to store the data that is required for a code coverage tool to map between
 the specific source ranges in a file and the execution counts obtained
 after running the instrumented version of the program.
 
 The mapping data is used in two places in the code coverage process:
 
 1. When clang compiles a source file with ``-fcoverage-mapping``, it

 The zero counters allow the code coverage tool to display proper line execution
 counts for the unreachable lines and highlight the unreachable code.
 Without them, the tool would think that those lines and regions were still
 executed, as it doesn't possess the frontend's knowledge.
 
 LLVM IR Representation
 ======================
 
-The coverage mapping data is stored in the LLVM IR using a single global
-constant structure variable called *__llvm_coverage_mapping*
-with the *__llvm_covmap* section specifier.
+The coverage mapping data is stored in the LLVM IR using a global constant
+structure variable called *__llvm_coverage_mapping* with the *IPSK_covmap*
+section specifier (i.e. ".lcovmap$M" on Windows and "__llvm_covmap" elsewhere).
 
 For example, let’s consider a C file and how it gets compiled to LLVM:
 
 .. _coverage mapping sample:
 
 .. code-block:: c
 
   int foo() {
     return 42;
   }
   int bar() {
     return 13;
   }
 
-The coverage mapping variable generated by Clang has 3 fields:
+The coverage mapping variable generated by Clang has 2 fields:
 
 * Coverage mapping header.
 
-* An array of function records.
+* An optionally compressed list of filenames present in the translation unit.
 
-* Coverage mapping data which is an array of bytes. Zero paddings are added at the end to force 8 byte alignment.
+The variable has 8-byte alignment because ld64 cannot always pack symbols from
+different object files tightly (the word-level alignment assumption is baked in
+too deeply).
 
 .. code-block:: llvm
 
-  @__llvm_coverage_mapping = internal constant { { i32, i32, i32, i32 }, [2 x { i64, i32, i64 }], [40 x i8] }
-  { 
+  @__llvm_coverage_mapping = internal constant { { i32, i32, i32, i32 }, [32 x i8] }
+  {
     { i32, i32, i32, i32 } ; Coverage map header
     {
-      i32 2,  ; The number of function records
-      i32 20, ; The length of the string that contains the encoded translation unit filenames
-      i32 20, ; The length of the string that contains the encoded coverage mapping data
-      i32 2,  ; Coverage mapping format version
-    },
-    [2 x { i64, i32, i64 }] [ ; Function records
-     { i64, i32, i64 } {
-       i64 0x5cf8c24cdb18bdac, ; Function's name MD5
-       i32 9, ; Function's encoded coverage mapping data string length
-       i64 0  ; Function's structural hash
-     },
-     { i64, i32, i64 } { 
-       i64 0xe413754a191db537, ; Function's name MD5
-       i32 9, ; Function's encoded coverage mapping data string length
-       i64 0  ; Function's structural hash
-     }],
-   [40 x i8] c"..." ; Encoded data (dissected later)
+      i32 0,  ; Always 0. In prior versions, the number of affixed function records
+      i32 32, ; The length of the string that contains the encoded translation unit filenames
+      i32 0,  ; Always 0. In prior versions, the length of the affixed string that contains the encoded coverage mapping data
+      i32 3,  ; Coverage mapping format version
+   [32 x i8] c"..." ; Encoded data (dissected later)
   }, section "__llvm_covmap", align 8
 
-The current version of the format is version 3. The only difference from version 2 is that a special encoding for column end locations was introduced to indicate gap regions.
+The current version of the format is version 4. There are two differences from version 3:
+
+* Function records are now named symbols, and are marked *linkonce_odr*. This
+  allows linkers to merge duplicate function records. Merging of duplicate
+  *dummy* records (emitted for functions included-but-not-used in a translation
+  unit) reduces size bloat in the coverage mapping data. As part of this
+  change, region mapping information for a function is now included within the
+  function record, instead of being affixed to the coverage header.
+
+* The filename list for a translation unit may optionally be zlib-compressed.
+
+The only difference between versions 3 and 2 is that a special encoding for
+column end locations was introduced to indicate gap regions.
 
-The function record layout has evolved since version 1. In version 1, the function record for *foo* is defined as follows:
+In version 1, the function record for *foo* was defined as follows:
 
 .. code-block:: llvm
 
      { i8*, i32, i32, i64 } { i8* getelementptr inbounds ([3 x i8]* @__profn_foo, i32 0, i32 0), ; Function's name
        i32 3, ; Function's name length
        i32 9, ; Function's encoded coverage mapping data string length
        i64 0  ; Function's structural hash
      }
 
+In version 2, the function record for *foo* was defined as follows:
+
+.. code-block:: llvm
+
+     { i64, i32, i64 } {
+       i64 0x5cf8c24cdb18bdac, ; Function's name MD5
+       i32 9, ; Function's encoded coverage mapping data string length
+       i64 0  ; Function's structural hash
 
 Coverage Mapping Header:
 ------------------------
 
 The coverage mapping header has the following fields:
 
-* The number of function records.
+* The number of function records affixed to the coverage header. Always 0, but present for backwards compatibility.
 
 * The length of the string in the third field of *__llvm_coverage_mapping* that contains the encoded translation unit filenames.
 
-* The length of the string in the third field of *__llvm_coverage_mapping* that contains the encoded coverage mapping data.
+* The length of the string in the third field of *__llvm_coverage_mapping* that contains any encoded coverage mapping data affixed to the coverage header. Always 0, but present for backwards compatibility.
 
-* The format version. The current version is 3 (encoded as a 2).
+* The format version. The current version is 4 (encoded as a 3).
 
 .. _function records:
 
 Function record:
 ----------------
 
 A function record is a structure of the following type:
 
 .. code-block:: llvm
 
-  { i64, i32, i64 }
-
-It contains function name's MD5, the length of the encoded mapping data for that function, and function's 
-structural hash value.
-
-Encoded data:
--------------
+  { i64, i32, i64, i64, [? x i8] }
 
-The encoded data is stored in a single string that contains
-the encoded filenames used by this translation unit and the encoded coverage
-mapping data for each function in this translation unit.
-
-The encoded data has the following structure:
-
-``[filenames, coverageMappingDataForFunctionRecord0, coverageMappingDataForFunctionRecord1, ..., padding]``
-
-If necessary, the encoded data is padded with zeroes so that the size
-of the data string is rounded up to the nearest multiple of 8 bytes.
+It contains the function name's MD5, the length of the encoded mapping data for
+that function, the function's structural hash value, the hash of the filenames
+in the function's translation unit, and the encoded mapping data.
 
 Dissecting the sample:
 ^^^^^^^^^^^^^^^^^^^^^^
 
 Here's an overview of the encoded data that was stored in the
 IR for the `coverage mapping sample`_ that was shown earlier:
 
 * The IR contains the following string constant that represents the encoded
   coverage mapping data for the sample translation unit:
 
   .. code-block:: llvm
 
-    c"\01\12/Users/alex/test.c\01\00\00\01\01\01\0C\02\02\01\00\00\01\01\04\0C\02\02\00\00"
+    c"\01\15\1Dx\DA\13\D1\0F-N-*\D6/+\CE\D6/\C9-\D0O\CB\CF\D7K\06\00N+\07]"
 
 * The string contains values that are encoded in the LEB128 format, which is
-  used throughout for storing integers. It also contains a string value.
+  used throughout for storing integers. It also contains a compressed payload.
 
-* The length of the substring that contains the encoded translation unit
-  filenames is the value of the second field in the *__llvm_coverage_mapping*
-  structure, which is 20, thus the filenames are encoded in this string:
-
-  .. code-block:: llvm
+* The first three LEB128-encoded numbers in the sample specify the number of
+  filenames, the length of the uncompressed filenames, and the length of the
+  compressed payload (or 0 if compression is disabled). In this sample, there
+  is 1 filename that is 21 bytes in length (uncompressed), and stored in 29
+  bytes (compressed).
 
-    c"\01\12/Users/alex/test.c"
-
-  This string contains the following data:
-
-  * Its first byte has a value of ``0x01``. It stores the number of filenames
-    contained in this string.
-  * Its second byte stores the length of the first filename in this string.
-  * The remaining 18 bytes are used to store the first filename.
-
-* The length of the substring that contains the encoded coverage mapping data
-  for the first function is the value of the third field in the first
-  structure in an array of `function records`_ stored in the
-  third field of the *__llvm_coverage_mapping* structure, which is the 9.
-  Therefore, the coverage mapping for the first function record is encoded
-  in this string:
+* The coverage mapping from the first function record is encoded in this string:
 
   .. code-block:: llvm
 
     c"\01\00\00\01\01\01\0C\02\02"
 
   This string consists of the following bytes:
 
   +----------+-------------------------------------------------------------------------------------------------------------------------+

llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h

 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/iterator.h"
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/ProfileData/InstrProf.h"
+#include "llvm/Support/Alignment.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <cstdint>
 #include <iterator>

 struct CoverageMappingRecord;
 
 enum class coveragemap_error {
   success = 0,
   eof,
   no_data_found,
   unsupported_version,
   truncated,
-  malformed
+  malformed,
+  decompression_failed
 };
 
 const std::error_category &coveragemap_category();
 
 inline std::error_code make_error_code(coveragemap_error E) {
   return std::error_code(static_cast<int>(E), coveragemap_category());
 }
 

 /// Get a \c LineCoverageIterator range for the lines described by \p CD.
 static inline iterator_range<LineCoverageIterator>
 getLineCoverageStats(const coverage::CoverageData &CD) {
   auto Begin = LineCoverageIterator(CD);
   auto End = Begin.getEnd();
   return make_range(Begin, End);
 }
 
-// Profile coverage map has the following layout:
-// [CoverageMapFileHeader]
-// [ArrayStart]
-//  [CovMapFunctionRecord]
-//  [CovMapFunctionRecord]
-//  ...
-// [ArrayEnd]
-// [Encoded Region Mapping Data]
-LLVM_PACKED_START
-template <class IntPtrT> struct CovMapFunctionRecordV1 {
-#define COVMAP_V1
-#define COVMAP_FUNC_RECORD(Type, LLVMType, Name, Init) Type Name;
-#include "llvm/ProfileData/InstrProfData.inc"
-#undef COVMAP_V1
+// Coverage mappping data (V2) has the following layout:
+// IPSK_covmap:
+//   [CoverageMapFileHeader]
+//   [ArrayStart]
+//    [CovMapFunctionRecordV2]
+//    [CovMapFunctionRecordV2]
+//    ...
+//   [ArrayEnd]
+//   [Encoded Filenames and Region Mapping Data]
+//
+// Coverage mappping data (V3) has the following layout:
+// IPSK_covmap:
+//   [CoverageMapFileHeader]
+//   [Encoded Filenames]
+// IPSK_covfun:
+//   [ArrayStart]
+//     odr_name_1: [CovMapFunctionRecordV3]
+//     odr_name_2: [CovMapFunctionRecordV3]
+//     ...
+//   [ArrayEnd]
+//
+// Both versions of the coverage mapping format encode the same information,
+// but the V3 format does so more compactly by taking advantage of linkonce_odr
+// semantics (it allows exactly 1 function record per name reference).
+
+/// This namespace defines accessors (*not* data) shared by different versions
+/// of coverage mapping records. The functionality is CRTP'd in.
+namespace accessors {
+
+/// Accessors for a 64-bit function hash and a 64-bit coverage mapping data
+/// size.
+template <class FuncRecordTy> struct FuncHashAndDataSize {
+  using ThisTy = const FuncRecordTy *;
 
   // Return the structural hash associated with the function.
   template <support::endianness Endian> uint64_t getFuncHash() const {
-    return support::endian::byte_swap<uint64_t, Endian>(FuncHash);
+    return support::endian::byte_swap<uint64_t, Endian>(
+        static_cast<ThisTy>(this)->FuncHash);
   }
 
-  // Return the coverage map data size for the funciton.
+  // Return the coverage map data size for the function.
   template <support::endianness Endian> uint32_t getDataSize() const {
-    return support::endian::byte_swap<uint32_t, Endian>(DataSize);
+    return support::endian::byte_swap<uint32_t, Endian>(
+        static_cast<ThisTy>(this)->DataSize);
+  }
+};
+
+/// Accessors for a hashed function name.
+template <class FuncRecordTy> struct HashedNameRef {
+  using ThisTy = const FuncRecordTy *;
+
+  // Return the function lookup key. The value is considered opaque.
+  template <support::endianness Endian> uint64_t getFuncNameRef() const {
+    return support::endian::byte_swap<uint64_t, Endian>(
+        static_cast<ThisTy>(this)->NameRef);
   }
 
+  // Return the PGO name of the function.
+  template <support::endianness Endian>
+  Error getFuncName(InstrProfSymtab &ProfileNames, StringRef &FuncName) const {
+    uint64_t NameRef = getFuncNameRef<Endian>();
+    FuncName = ProfileNames.getFuncName(NameRef);
+    return Error::success();
+  }
+};
+
+/// Accessors for an out-of-line coverage mapping: this is affixed to the
+/// coverage map file header, instead of to the function record.
+template <class FuncRecordTy> struct OutOfLineCovMapping {
+  using ThisTy = const FuncRecordTy *;
+
+  // Return an invalid filename set reference. When an out-of-line coverage
+  // mapping string is expected, the function record doen't contain a filenames
+  // ref.
+  template <support::endianness Endian> uint64_t getFilenamesRef() const {
+    llvm_unreachable("Record does not contain a reference to filenames");
+  }
+
+  // Read coverage mapping out-of-line, from \p MappingBuf.
+  template <support::endianness Endian>
+  StringRef getCoverageMapping(const char *MappingBuf) const {
+    return {MappingBuf,
+            static_cast<ThisTy>(this)->template getDataSize<Endian>()};
+  }
+
+  // Advance to the next out-of-line coverage mapping and its associated
+  // function record.
+  template <support::endianness Endian>
+  std::pair<const char *, const FuncRecordTy *>
+  advanceByOne(const char *MappingBuf) const {
+    auto *CFR = static_cast<ThisTy>(this);
+    return {MappingBuf + CFR->template getDataSize<Endian>(), CFR + 1};
+  }
+};
+} // end namespace accessors
+
+LLVM_PACKED_START
+template <class IntPtrT>
+struct CovMapFunctionRecordV1
+    : accessors::FuncHashAndDataSize<CovMapFunctionRecordV1<IntPtrT>>,
+      accessors::OutOfLineCovMapping<CovMapFunctionRecordV1<IntPtrT>> {
+#define COVMAP_V1
+#define COVMAP_FUNC_RECORD(Type, LLVMType, Name, Init) Type Name;
+#include "llvm/ProfileData/InstrProfData.inc"
+#undef COVMAP_V1
+  CovMapFunctionRecordV1() = delete;
+
   // Return function lookup key. The value is consider opaque.
   template <support::endianness Endian> IntPtrT getFuncNameRef() const {
     return support::endian::byte_swap<IntPtrT, Endian>(NamePtr);
   }
 
-  // Return the PGO name of the function */
+  // Return the PGO name of the function.
   template <support::endianness Endian>
   Error getFuncName(InstrProfSymtab &ProfileNames, StringRef &FuncName) const {
     IntPtrT NameRef = getFuncNameRef<Endian>();
     uint32_t NameS = support::endian::byte_swap<uint32_t, Endian>(NameSize);
     FuncName = ProfileNames.getFuncName(NameRef, NameS);
     if (NameS && FuncName.empty())
       return make_error<CoverageMapError>(coveragemap_error::malformed);
     return Error::success();
   }
 };
 
-struct CovMapFunctionRecord {
+struct CovMapFunctionRecordV2
+    : accessors::FuncHashAndDataSize<CovMapFunctionRecordV2>,
+      accessors::HashedNameRef<CovMapFunctionRecordV2>,
+      accessors::OutOfLineCovMapping<CovMapFunctionRecordV2> {
+#define COVMAP_V2
 #define COVMAP_FUNC_RECORD(Type, LLVMType, Name, Init) Type Name;
 #include "llvm/ProfileData/InstrProfData.inc"
+#undef COVMAP_V2
+  CovMapFunctionRecordV2() = delete;
+};
 
-  // Return the structural hash associated with the function.
-  template <support::endianness Endian> uint64_t getFuncHash() const {
-    return support::endian::byte_swap<uint64_t, Endian>(FuncHash);
-  }
+struct CovMapFunctionRecordV3
+    : accessors::FuncHashAndDataSize<CovMapFunctionRecordV3>,
+      accessors::HashedNameRef<CovMapFunctionRecordV3> {
+#define COVMAP_V3
+#define COVMAP_FUNC_RECORD(Type, LLVMType, Name, Init) Type Name;
+#include "llvm/ProfileData/InstrProfData.inc"
+#undef COVMAP_V3
+  CovMapFunctionRecordV3() = delete;
 
-  // Return the coverage map data size for the funciton.
-  template <support::endianness Endian> uint32_t getDataSize() const {
-    return support::endian::byte_swap<uint32_t, Endian>(DataSize);
+  // Get the filename set reference.
+  template <support::endianness Endian> uint64_t getFilenamesRef() const {
+    return support::endian::byte_swap<uint64_t, Endian>(FilenamesRef);
   }
 
-  // Return function lookup key. The value is consider opaque.
-  template <support::endianness Endian> uint64_t getFuncNameRef() const {
-    return support::endian::byte_swap<uint64_t, Endian>(NameRef);
+  // Read the inline coverage mapping. Ignore the out-of-line coverage mapping
+  // buffer.
+  template <support::endianness Endian>
+  StringRef getCoverageMapping(const char *) const {
+    return StringRef(&CoverageMapping, getDataSize<Endian>());
   }
 
-  // Return the PGO name of the function */
+  // Advance to the next inline coverage mapping and its associated function
+  // record. Ignore the out-of-line coverage mapping buffer.
   template <support::endianness Endian>
-  Error getFuncName(InstrProfSymtab &ProfileNames, StringRef &FuncName) const {
-    uint64_t NameRef = getFuncNameRef<Endian>();
-    FuncName = ProfileNames.getFuncName(NameRef);
-    return Error::success();
+  std::pair<const char *, const CovMapFunctionRecordV3 *>
+  advanceByOne(const char *) const {
+    assert(isAddrAligned(Align(8), this) && "Function record not aligned");
+    const char *Next = ((const char *)this) + sizeof(CovMapFunctionRecordV3) -
+                       sizeof(char) + getDataSize<Endian>();
+    // Each function record has an alignment of 8, so we need to adjust
+    // alignment before reading the next record.
+    Next += offsetToAlignedAddr(Next, Align(8));
+    return {nullptr, reinterpret_cast<const CovMapFunctionRecordV3 *>(Next)};
   }
 };
 
 // Per module coverage mapping data header, i.e. CoverageMapFileHeader
 // documented above.
 struct CovMapHeader {
 #define COVMAP_HEADER(Type, LLVMType, Name, Init) Type Name;
 #include "llvm/ProfileData/InstrProfData.inc"
enum CovMapVersion {
   Version1 = 0,
   // Function's name reference from CovMapFuncRecord is changed from raw
   // name string pointer to MD5 to support name section compression. Name
   // section is also compressed.
   Version2 = 1,
   // A new interpretation of the columnEnd field is added in order to mark
   // regions as gap areas.
   Version3 = 2,
-  // The current version is Version3
+  // Function records are named, uniqued, and moved to a dedicated section.
+  Version4 = 3,
+  // The current version is Version4.
   CurrentVersion = INSTR_PROF_COVMAP_VERSION
 };
 
 template <int CovMapVersion, class IntPtrT> struct CovMapTraits {
-  using CovMapFuncRecordType = CovMapFunctionRecord;
+  using CovMapFuncRecordType = CovMapFunctionRecordV3;
+  using NameRefType = uint64_t;
+};
+
+template <class IntPtrT> struct CovMapTraits<CovMapVersion::Version3, IntPtrT> {
+  using CovMapFuncRecordType = CovMapFunctionRecordV2;
+  using NameRefType = uint64_t;
+};
+
+template <class IntPtrT> struct CovMapTraits<CovMapVersion::Version2, IntPtrT> {
+  using CovMapFuncRecordType = CovMapFunctionRecordV2;
   using NameRefType = uint64_t;
 };
 
 template <class IntPtrT> struct CovMapTraits<CovMapVersion::Version1, IntPtrT> {
   using CovMapFuncRecordType = CovMapFunctionRecordV1<IntPtrT>;
   using NameRefType = IntPtrT;
 };
 

llvm/include/llvm/ProfileData/Coverage/CoverageMappingReader.h

protected:
   RawCoverageReader(StringRef Data) : Data(Data) {}
 
   Error readULEB128(uint64_t &Result);
   Error readIntMax(uint64_t &Result, uint64_t MaxPlus1);
   Error readSize(uint64_t &Result);
   Error readString(StringRef &Result);
 };
 
-/// Reader for the raw coverage filenames.
-class RawCoverageFilenamesReader : public RawCoverageReader {
-  std::vector<StringRef> &Filenames;
-
-public:
-  RawCoverageFilenamesReader(StringRef Data, std::vector<StringRef> &Filenames)
-      : RawCoverageReader(Data), Filenames(Filenames) {}
-  RawCoverageFilenamesReader(const RawCoverageFilenamesReader &) = delete;
-  RawCoverageFilenamesReader &
-  operator=(const RawCoverageFilenamesReader &) = delete;
-
-  Error read();
-};
-
 /// Checks if the given coverage mapping data is exported for
 /// an unused function.
 class RawCoverageMappingDummyChecker : public RawCoverageReader {
 public:
   RawCoverageMappingDummyChecker(StringRef MappingData)
       : RawCoverageReader(MappingData) {}
 
   Expected<bool> isDummy();
struct ProfileMappingRecord {
     ProfileMappingRecord(CovMapVersion Version, StringRef FunctionName,
                          uint64_t FunctionHash, StringRef CoverageMapping,
                          size_t FilenamesBegin, size_t FilenamesSize)
         : Version(Version), FunctionName(FunctionName),
           FunctionHash(FunctionHash), CoverageMapping(CoverageMapping),
           FilenamesBegin(FilenamesBegin), FilenamesSize(FilenamesSize) {}
   };
 
+  using DecompressedData = std::vector<std::unique_ptr<SmallVector<char, 0>>>;
+
 private:
   std::vector<StringRef> Filenames;
   std::vector<ProfileMappingRecord> MappingRecords;
   InstrProfSymtab ProfileNames;
   size_t CurrentRecord = 0;
   std::vector<StringRef> FunctionsFilenames;
   std::vector<CounterExpression> Expressions;
   std::vector<CounterMappingRegion> MappingRegions;
 
+  // Used to tie the lifetimes of decompressed strings to the lifetime of this
+  // BinaryCoverageReader instance.
+  DecompressedData Decompressed;
+
   BinaryCoverageReader() = default;
 
 public:
   BinaryCoverageReader(const BinaryCoverageReader &) = delete;
   BinaryCoverageReader &operator=(const BinaryCoverageReader &) = delete;
 
   static Expected<std::vector<std::unique_ptr<BinaryCoverageReader>>>
   create(MemoryBufferRef ObjectBuffer, StringRef Arch,
          SmallVectorImpl<std::unique_ptr<MemoryBuffer>> &ObjectFileBuffers);
 
   static Expected<std::unique_ptr<BinaryCoverageReader>>
-  createCoverageReaderFromBuffer(StringRef Coverage,
+  createCoverageReaderFromBuffer(StringRef Coverage, StringRef FuncRecords,
                                  InstrProfSymtab &&ProfileNames,
                                  uint8_t BytesInAddress,
                                  support::endianness Endian);
 
   Error readNextRecord(CoverageMappingRecord &Record) override;
 };
 
+/// Reader for the raw coverage filenames.
+class RawCoverageFilenamesReader : public RawCoverageReader {
+  std::vector<StringRef> &Filenames;
+
+  // Read an uncompressed sequence of filenames.
+  Error readUncompressed(uint64_t NumFilenames);
+
+public:
+  RawCoverageFilenamesReader(StringRef Data, std::vector<StringRef> &Filenames)
+      : RawCoverageReader(Data), Filenames(Filenames) {}
+  RawCoverageFilenamesReader(const RawCoverageFilenamesReader &) = delete;
+  RawCoverageFilenamesReader &
+  operator=(const RawCoverageFilenamesReader &) = delete;
+
+  Error read(CovMapVersion Version,
+             BinaryCoverageReader::DecompressedData &Decompressed);
+};
+
 } // end namespace coverage
 } // end namespace llvm
 
 #endif // LLVM_PROFILEDATA_COVERAGE_COVERAGEMAPPINGREADER_H

llvm/include/llvm/ProfileData/Coverage/CoverageMappingWriter.h

 /// Writer of the filenames section for the instrumentation
 /// based code coverage.
 class CoverageFilenamesSectionWriter {
   ArrayRef<StringRef> Filenames;
 
 public:
   CoverageFilenamesSectionWriter(ArrayRef<StringRef> Filenames);
 
-  /// Write encoded filenames to the given output stream.
-  void write(raw_ostream &OS);
+  /// Write encoded filenames to the given output stream. If \p Compress is
+  /// true, attempt to compress the filenames.
+  void write(raw_ostream &OS, bool Compress = true);
 };
 
 /// Writer for instrumentation based coverage mapping data.
 class CoverageMappingWriter {
   ArrayRef<unsigned> VirtualFileMapping;
   ArrayRef<CounterExpression> Expressions;
   MutableArrayRef<CounterMappingRegion> MappingRegions;
 

llvm/include/llvm/ProfileData/InstrProf.h

 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/IR/GlobalValue.h"
 #include "llvm/IR/ProfileSummary.h"
 #include "llvm/ProfileData/InstrProfData.inc"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/Host.h"
 #include "llvm/Support/MD5.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"

 
 // Create a COMDAT variable INSTR_PROF_RAW_VERSION_VAR to make the runtime
 // aware this is an ir_level profile so it can set the version flag.
 void createIRLevelProfileFlagVar(Module &M, bool IsCS);
 
 // Create the variable for the profile file name.
 void createProfileFileNameVar(Module &M, StringRef InstrProfileOutput);
 
+// Whether to compress function names in profile records, and filenames in
+// code coverage mappings. Used by the Instrumentation library and unit tests.
+extern cl::opt<bool> DoInstrProfNameCompression;
+
 } // end namespace llvm
 #endif // LLVM_PROFILEDATA_INSTRPROF_H

llvm/include/llvm/ProfileData/InstrProfData.inc

  * indexed with the kind value.
  */
 VALUE_PROF_KIND(IPVK_First, IPVK_IndirectCallTarget, "first")
 VALUE_PROF_KIND(IPVK_Last, IPVK_MemOPSize, "last")
 
 #undef VALUE_PROF_KIND
 /* VALUE_PROF_KIND end */
 
+#undef COVMAP_V2_OR_V3
+#ifdef COVMAP_V2
+#define COVMAP_V2_OR_V3
+#endif
+#ifdef COVMAP_V3
+#define COVMAP_V2_OR_V3
+#endif
+
 /* COVMAP_FUNC_RECORD start */
 /* Definition of member fields of the function record structure in coverage
  * map.
  */
 #ifndef COVMAP_FUNC_RECORD
 #define COVMAP_FUNC_RECORD(Type, LLVMType, Name, Initializer)
 #else
 #define INSTR_PROF_DATA_DEFINED
 #endif
 #ifdef COVMAP_V1
 COVMAP_FUNC_RECORD(const IntPtrT, llvm::Type::getInt8PtrTy(Ctx), \
                    NamePtr, llvm::ConstantExpr::getBitCast(NamePtr, \
                    llvm::Type::getInt8PtrTy(Ctx)))
 COVMAP_FUNC_RECORD(const uint32_t, llvm::Type::getInt32Ty(Ctx), NameSize, \
                    llvm::ConstantInt::get(llvm::Type::getInt32Ty(Ctx), \
                    NameValue.size()))
-#else
+#endif
+#ifdef COVMAP_V2_OR_V3
 COVMAP_FUNC_RECORD(const int64_t, llvm::Type::getInt64Ty(Ctx), NameRef, \
-                   llvm::ConstantInt::get(llvm::Type::getInt64Ty(Ctx), \
-                   llvm::IndexedInstrProf::ComputeHash(NameValue)))
+                   llvm::ConstantInt::get( \
+                     llvm::Type::getInt64Ty(Ctx), NameHash))
 #endif
 COVMAP_FUNC_RECORD(const uint32_t, llvm::Type::getInt32Ty(Ctx), DataSize, \
-                   llvm::ConstantInt::get(llvm::Type::getInt32Ty(Ctx),\
-                   CoverageMapping.size()))
+                   llvm::ConstantInt::get( \
+                     llvm::Type::getInt32Ty(Ctx), CoverageMapping.size()))
 COVMAP_FUNC_RECORD(const uint64_t, llvm::Type::getInt64Ty(Ctx), FuncHash, \
-                   llvm::ConstantInt::get(llvm::Type::getInt64Ty(Ctx), FuncHash))
+                   llvm::ConstantInt::get( \
+                     llvm::Type::getInt64Ty(Ctx), FuncHash))
+#ifdef COVMAP_V3
+COVMAP_FUNC_RECORD(const uint64_t, llvm::Type::getInt64Ty(Ctx), FilenamesRef, \
+                   llvm::ConstantInt::get( \
+                     llvm::Type::getInt64Ty(Ctx), FilenamesRef))
+COVMAP_FUNC_RECORD(const char, \
+                   llvm::ArrayType::get(llvm::Type::getInt8Ty(Ctx), \
+                                        CoverageMapping.size()), \
+                   CoverageMapping,
+                   llvm::ConstantDataArray::getRaw( \
+                     CoverageMapping, CoverageMapping.size(), \
+                     llvm::Type::getInt8Ty(Ctx)))
+#endif
 #undef COVMAP_FUNC_RECORD
 /* COVMAP_FUNC_RECORD end.  */
 
 /* COVMAP_HEADER start */
 /* Definition of member fields of coverage map header.
  */
 #ifndef COVMAP_HEADER
 #define COVMAP_HEADER(Type, LLVMType, Name, Initializer)
 #else
 #define INSTR_PROF_DATA_DEFINED
 #endif
 COVMAP_HEADER(uint32_t, Int32Ty, NRecords, \
-              llvm::ConstantInt::get(Int32Ty,  FunctionRecords.size()))
+              llvm::ConstantInt::get(Int32Ty, NRecords))
 COVMAP_HEADER(uint32_t, Int32Ty, FilenamesSize, \
               llvm::ConstantInt::get(Int32Ty, FilenamesSize))
 COVMAP_HEADER(uint32_t, Int32Ty, CoverageSize, \
               llvm::ConstantInt::get(Int32Ty, CoverageMappingSize))
 COVMAP_HEADER(uint32_t, Int32Ty, Version, \
               llvm::ConstantInt::get(Int32Ty, CovMapVersion::CurrentVersion))
 #undef COVMAP_HEADER
 /* COVMAP_HEADER end.  */
INSTR_PROF_SECT_ENTRY(IPSK_vals, \
                       INSTR_PROF_QUOTE(INSTR_PROF_VALS_COMMON), \
                       INSTR_PROF_VALS_COFF, "__DATA,")
 INSTR_PROF_SECT_ENTRY(IPSK_vnodes, \
                       INSTR_PROF_QUOTE(INSTR_PROF_VNODES_COMMON), \
                       INSTR_PROF_VNODES_COFF, "__DATA,")
 INSTR_PROF_SECT_ENTRY(IPSK_covmap, \
                       INSTR_PROF_QUOTE(INSTR_PROF_COVMAP_COMMON), \
                       INSTR_PROF_COVMAP_COFF, "__LLVM_COV,")
+INSTR_PROF_SECT_ENTRY(IPSK_covfun, \
+                      INSTR_PROF_QUOTE(INSTR_PROF_COVFUN_COMMON), \
+                      INSTR_PROF_COVFUN_COFF, "__LLVM_COV,")
 INSTR_PROF_SECT_ENTRY(IPSK_orderfile, \
                       INSTR_PROF_QUOTE(INSTR_PROF_ORDERFILE_COMMON), \
                       INSTR_PROF_QUOTE(INSTR_PROF_ORDERFILE_COFF), "__DATA,")
 
 #undef INSTR_PROF_SECT_ENTRY
 #endif
 
 

 #define INSTR_PROF_RAW_MAGIC_32 (uint64_t)255 << 56 | (uint64_t)'l' << 48 | \
        (uint64_t)'p' << 40 | (uint64_t)'r' << 32 | (uint64_t)'o' << 24 |  \
         (uint64_t)'f' << 16 | (uint64_t)'R' << 8 | (uint64_t)129
 
 /* Raw profile format version (start from 1). */
 #define INSTR_PROF_RAW_VERSION 5
 /* Indexed profile format version (start from 1). */
 #define INSTR_PROF_INDEX_VERSION 5
-/* Coverage mapping format vresion (start from 0). */
-#define INSTR_PROF_COVMAP_VERSION 2
+/* Coverage mapping format version (start from 0). */
+#define INSTR_PROF_COVMAP_VERSION 3
 
 /* Profile version is always of type uint64_t. Reserve the upper 8 bits in the
  * version for other variants of profile. We set the lowest bit of the upper 8
  * bits (i.e. bit 56) to 1 to indicate if this is an IR-level instrumentaiton
  * generated profile, and 0 if this is a Clang FE generated profile.
  * 1 in bit 57 indicates there are context-sensitive records in the profile.
  */
 #define VARIANT_MASKS_ALL 0xff00000000000000ULL

 /* section name strings common to all targets other
    than WIN32 */
 #define INSTR_PROF_DATA_COMMON __llvm_prf_data
 #define INSTR_PROF_NAME_COMMON __llvm_prf_names
 #define INSTR_PROF_CNTS_COMMON __llvm_prf_cnts
 #define INSTR_PROF_VALS_COMMON __llvm_prf_vals
 #define INSTR_PROF_VNODES_COMMON __llvm_prf_vnds
 #define INSTR_PROF_COVMAP_COMMON __llvm_covmap
+#define INSTR_PROF_COVFUN_COMMON __llvm_covfun
 #define INSTR_PROF_ORDERFILE_COMMON __llvm_orderfile
 /* Windows section names. Because these section names contain dollar characters,
  * they must be quoted.
  */
 #define INSTR_PROF_DATA_COFF ".lprfd$M"
 #define INSTR_PROF_NAME_COFF ".lprfn$M"
 #define INSTR_PROF_CNTS_COFF ".lprfc$M"
 #define INSTR_PROF_VALS_COFF ".lprfv$M"
 #define INSTR_PROF_VNODES_COFF ".lprfnd$M"
 #define INSTR_PROF_COVMAP_COFF ".lcovmap$M"
+#define INSTR_PROF_COVFUN_COFF ".lcovfun$M"
 #define INSTR_PROF_ORDERFILE_COFF ".lorderfile$M"
 
 #ifdef _WIN32
 /* Runtime section names and name strings.  */
 #define INSTR_PROF_DATA_SECT_NAME INSTR_PROF_DATA_COFF
 #define INSTR_PROF_NAME_SECT_NAME INSTR_PROF_NAME_COFF
 #define INSTR_PROF_CNTS_SECT_NAME INSTR_PROF_CNTS_COFF
 /* Array of pointers. Each pointer points to a list
  * of value nodes associated with one value site.
  */
 #define INSTR_PROF_VALS_SECT_NAME INSTR_PROF_VALS_COFF
 /* Value profile nodes section. */
 #define INSTR_PROF_VNODES_SECT_NAME INSTR_PROF_VNODES_COFF
 #define INSTR_PROF_COVMAP_SECT_NAME INSTR_PROF_COVMAP_COFF
+#define INSTR_PROF_COVFUN_SECT_NAME INSTR_PROF_COVFUN_COFF
 #define INSTR_PROF_ORDERFILE_SECT_NAME INSTR_PROF_ORDERFILE_COFF
 #else
 /* Runtime section names and name strings.  */
 #define INSTR_PROF_DATA_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_DATA_COMMON)
 #define INSTR_PROF_NAME_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_NAME_COMMON)
 #define INSTR_PROF_CNTS_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_CNTS_COMMON)
 /* Array of pointers. Each pointer points to a list
  * of value nodes associated with one value site.
  */
 #define INSTR_PROF_VALS_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_VALS_COMMON)
 /* Value profile nodes section. */
 #define INSTR_PROF_VNODES_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_VNODES_COMMON)
 #define INSTR_PROF_COVMAP_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_COVMAP_COMMON)
+#define INSTR_PROF_COVFUN_SECT_NAME INSTR_PROF_QUOTE(INSTR_PROF_COVFUN_COMMON)
 /* Order file instrumentation. */
 #define INSTR_PROF_ORDERFILE_SECT_NAME                                         \
   INSTR_PROF_QUOTE(INSTR_PROF_ORDERFILE_COMMON)
 #endif
 
 #define INSTR_PROF_ORDERFILE_BUFFER_NAME _llvm_order_file_buffer
 #define INSTR_PROF_ORDERFILE_BUFFER_NAME_STR                                   \
   INSTR_PROF_QUOTE(INSTR_PROF_ORDERFILE_BUFFER_NAME)

 /* The maximal # of functions: 128*1024 (the buffer size will be 128*4 KB). */
 #define INSTR_ORDER_FILE_BUFFER_SIZE 131072
 #define INSTR_ORDER_FILE_BUFFER_BITS 17
 #define INSTR_ORDER_FILE_BUFFER_MASK 0x1ffff
 #endif /* INSTR_ORDER_FILE_INC */
 #else
 #undef INSTR_PROF_DATA_DEFINED
 #endif
+
+#undef COVMAP_V2_OR_V3

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

static SectionKind getELFKindForNamedSection(StringRef Name, SectionKind K) {
   // N.B.: The defaults used in here are not the same ones used in MC.
   // We follow gcc, MC follows gas. For example, given ".section .eh_frame",
   // both gas and MC will produce a section with no flags. Given
   // section(".eh_frame") gcc will produce:
   //
   //   .section   .eh_frame,"a",@progbits
 
   if (Name == getInstrProfSectionName(IPSK_covmap, Triple::ELF,
+                                      /*AddSegmentInfo=*/false) ||
+      Name == getInstrProfSectionName(IPSK_covfun, Triple::ELF,
                                       /*AddSegmentInfo=*/false))
     return SectionKind::getMetadata();
 
   if (Name.empty() || Name[0] != '.') return K;
 
   // Default implementation based on some magic section names.
   if (Name == ".bss" ||
       Name.startswith(".bss.") ||

llvm/lib/ProfileData/Coverage/CoverageMapping.cpp

static std::string getCoverageMapErrString(coveragemap_error Err) {
   case coveragemap_error::no_data_found:
     return "No coverage data found";
   case coveragemap_error::unsupported_version:
     return "Unsupported coverage format version";
   case coveragemap_error::truncated:
     return "Truncated coverage data";
   case coveragemap_error::malformed:
     return "Malformed coverage data";
+  case coveragemap_error::decompression_failed:
+    return "Failed to decompress coverage data (zlib)";
   }
   llvm_unreachable("A value of coveragemap_error has no message.");
 }
 
 namespace {
 
 // FIXME: This class is only here to support the transition to llvm::Error. It
 // will be removed once this transition is complete. Clients should prefer to

llvm/lib/ProfileData/Coverage/CoverageMappingReader.cpp

 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ProfileData/Coverage/CoverageMappingReader.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/Object/Binary.h"
 #include "llvm/Object/Error.h"
 #include "llvm/Object/MachOUniversal.h"
 #include "llvm/Object/ObjectFile.h"
 #include "llvm/Object/COFF.h"
 #include "llvm/ProfileData/InstrProf.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/Compression.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/LEB128.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"
 #include <vector>
 
 using namespace llvm;
 using namespace coverage;
 using namespace object;
 
 #define DEBUG_TYPE "coverage-mapping"
 
+STATISTIC(CovMapNumRecords, "The # of coverage function records");
+STATISTIC(CovMapNumUsedRecords, "The # of used coverage function records");
+
 void CoverageMappingIterator::increment() {
   if (ReadErr != coveragemap_error::success)
     return;
 
   // Check if all the records were read or if an error occurred while reading
   // the next record.
   if (auto E = Reader->readNextRecord(Record))
     handleAllErrors(std::move(E), [&](const CoverageMapError &CME) {
Error RawCoverageReader::readString(StringRef &Result) {
   uint64_t Length;
   if (auto Err = readSize(Length))
     return Err;
   Result = Data.substr(0, Length);
   Data = Data.substr(Length);
   return Error::success();
 }
 
-Error RawCoverageFilenamesReader::read() {
+Error RawCoverageFilenamesReader::read(
+    CovMapVersion Version,
+    BinaryCoverageReader::DecompressedData &Decompressed) {
   uint64_t NumFilenames;
   if (auto Err = readSize(NumFilenames))
     return Err;
+  if (!NumFilenames)
+    return make_error<CoverageMapError>(coveragemap_error::malformed);
+
+  if (Version < CovMapVersion::Version4)
+    return readUncompressed(NumFilenames);
+
+  // The uncompressed length may exceed the size of the encoded filenames.
+  // Skip size validation.
+  uint64_t UncompressedLen;
+  if (auto Err = readULEB128(UncompressedLen))
+    return Err;
+
+  uint64_t CompressedLen;
+  if (auto Err = readSize(CompressedLen))
+    return Err;
+
+  if (CompressedLen > 0) {
+    if (!zlib::isAvailable())
+      return make_error<CoverageMapError>(
+          coveragemap_error::decompression_failed);
+
+    // Allocate memory for the decompressed filenames. Transfer ownership of
+    // the memory to BinaryCoverageReader.
+    auto DecompressedStorage = std::make_unique<SmallVector<char, 0>>();
+    SmallVectorImpl<char> &StorageBuf = *DecompressedStorage.get();
+    Decompressed.push_back(std::move(DecompressedStorage));
+
+    // Read compressed filenames.
+    StringRef CompressedFilenames = Data.substr(0, CompressedLen);
+    Data = Data.substr(CompressedLen);
+    auto Err =
+        zlib::uncompress(CompressedFilenames, StorageBuf, UncompressedLen);
+    if (Err) {
+      consumeError(std::move(Err));
+      return make_error<CoverageMapError>(
+          coveragemap_error::decompression_failed);
+    }
+
+    StringRef UncompressedFilenames(StorageBuf.data(), StorageBuf.size());
+    RawCoverageFilenamesReader Delegate(UncompressedFilenames, Filenames);
+    return Delegate.readUncompressed(NumFilenames);
+  }
+
+  return readUncompressed(NumFilenames);
+}
+
+Error RawCoverageFilenamesReader::readUncompressed(uint64_t NumFilenames) {
+  // Read uncompressed filenames.
   for (size_t I = 0; I < NumFilenames; ++I) {
     StringRef Filename;
     if (auto Err = readString(Filename))
       return Err;
     Filenames.push_back(Filename);
   }
   return Error::success();
 }

 // Check if the mapping data is a dummy, i.e. is emitted for an unused function.
 static Expected<bool> isCoverageMappingDummy(uint64_t Hash, StringRef Mapping) {
   // The hash value of dummy mapping records is always zero.
   if (Hash)
     return false;
   return RawCoverageMappingDummyChecker(Mapping).isDummy();
 }
 
+/// A range of filename indices. Used to specify the location of a batch of
+/// filenames in a vector-like container.
+struct FilenameRange {
+  unsigned StartingIndex;
+  unsigned Length;
+
+  FilenameRange(unsigned StartingIndex, unsigned Length)
+      : StartingIndex(StartingIndex), Length(Length) {}
+
+  void markInvalid() { Length = 0; }
+  bool isInvalid() const { return Length == 0; }
+};
+
 namespace {
 
+/// The interface to read coverage mapping function records for a module.
 struct CovMapFuncRecordReader {
   virtual ~CovMapFuncRecordReader() = default;
 
-  // The interface to read coverage mapping function records for a module.
+  // Read a coverage header.
   //
-  // \p Buf points to the buffer containing the \c CovHeader of the coverage
+  // \p CovBuf points to the buffer containing the \c CovHeader of the coverage
   // mapping data associated with the module.
   //
-  // Returns a pointer to the next \c CovHeader if it exists, or a pointer
-  // greater than \p End if not.
-  virtual Expected<const char *> readFunctionRecords(const char *Buf,
-                                                     const char *End) = 0;
+  // Returns a pointer to the next \c CovHeader if it exists, or to an address
+  // greater than \p CovEnd if not.
+  virtual Expected<const char *>
+  readCoverageHeader(const char *CovBuf, const char *CovBufEnd,
+                     BinaryCoverageReader::DecompressedData &Decompressed) = 0;
+
+  // Read function records.
+  //
+  // \p FuncRecBuf points to the buffer containing a batch of function records.
+  // \p FuncRecBufEnd points past the end of the batch of records.
+  //
+  // Prior to Version4, \p OutOfLineFileRange points to a sequence of filenames
+  // associated with the function records. It is unused in Version4.
+  //
+  // Prior to Version4, \p OutOfLineMappingBuf points to a sequence of coverage
+  // mappings associated with the function records. It is unused in Version4.
+  virtual Error readFunctionRecords(const char *FuncRecBuf,
+                                    const char *FuncRecBufEnd,
+                                    Optional<FilenameRange> OutOfLineFileRange,
+                                    const char *OutOfLineMappingBuf,
+                                    const char *OutOfLineMappingBufEnd) = 0;
 
   template <class IntPtrT, support::endianness Endian>
   static Expected<std::unique_ptr<CovMapFuncRecordReader>>
   get(CovMapVersion Version, InstrProfSymtab &P,
       std::vector<BinaryCoverageReader::ProfileMappingRecord> &R,
       std::vector<StringRef> &F);
 };
 
 // A class for reading coverage mapping function records for a module.
 template <CovMapVersion Version, class IntPtrT, support::endianness Endian>
 class VersionedCovMapFuncRecordReader : public CovMapFuncRecordReader {
   using FuncRecordType =
       typename CovMapTraits<Version, IntPtrT>::CovMapFuncRecordType;
   using NameRefType = typename CovMapTraits<Version, IntPtrT>::NameRefType;
 
   // Maps function's name references to the indexes of their records
   // in \c Records.
   DenseMap<NameRefType, size_t> FunctionRecords;
   InstrProfSymtab &ProfileNames;
   std::vector<StringRef> &Filenames;
   std::vector<BinaryCoverageReader::ProfileMappingRecord> &Records;
 
+  // Maps a hash of the filenames in a TU to a \c FileRange. The range
+  // specifies the location of the hashed filenames in \c Filenames.
+  DenseMap<uint64_t, FilenameRange> FileRangeMap;
+
   // Add the record to the collection if we don't already have a record that
   // points to the same function name. This is useful to ignore the redundant
   // records for the functions with ODR linkage.
   // In addition, prefer records with real coverage mapping data to dummy
   // records, which were emitted for inline functions which were seen but
   // not used in the corresponding translation unit.
   Error insertFunctionRecordIfNeeded(const FuncRecordType *CFR,
-                                     StringRef Mapping, size_t FilenamesBegin) {
+                                     StringRef Mapping,
+                                     FilenameRange FileRange) {
+    ++CovMapNumRecords;
     uint64_t FuncHash = CFR->template getFuncHash<Endian>();
     NameRefType NameRef = CFR->template getFuncNameRef<Endian>();
     auto InsertResult =
         FunctionRecords.insert(std::make_pair(NameRef, Records.size()));
     if (InsertResult.second) {
       StringRef FuncName;
       if (Error Err = CFR->template getFuncName<Endian>(ProfileNames, FuncName))
         return Err;
       if (FuncName.empty())
         return make_error<InstrProfError>(instrprof_error::malformed);
-      Records.emplace_back(Version, FuncName, FuncHash, Mapping, FilenamesBegin,
-                           Filenames.size() - FilenamesBegin);
+      ++CovMapNumUsedRecords;
+      Records.emplace_back(Version, FuncName, FuncHash, Mapping,
+                           FileRange.StartingIndex, FileRange.Length);
       return Error::success();
     }
     // Update the existing record if it's a dummy and the new record is real.
     size_t OldRecordIndex = InsertResult.first->second;
     BinaryCoverageReader::ProfileMappingRecord &OldRecord =
         Records[OldRecordIndex];
     Expected<bool> OldIsDummyExpected = isCoverageMappingDummy(
         OldRecord.FunctionHash, OldRecord.CoverageMapping);
     if (Error Err = OldIsDummyExpected.takeError())
       return Err;
     if (!*OldIsDummyExpected)
       return Error::success();
     Expected<bool> NewIsDummyExpected =
         isCoverageMappingDummy(FuncHash, Mapping);
     if (Error Err = NewIsDummyExpected.takeError())
       return Err;
     if (*NewIsDummyExpected)
       return Error::success();
+    ++CovMapNumUsedRecords;
     OldRecord.FunctionHash = FuncHash;
     OldRecord.CoverageMapping = Mapping;
-    OldRecord.FilenamesBegin = FilenamesBegin;
-    OldRecord.FilenamesSize = Filenames.size() - FilenamesBegin;
+    OldRecord.FilenamesBegin = FileRange.StartingIndex;
+    OldRecord.FilenamesSize = FileRange.Length;
     return Error::success();
   }
 
 public:
   VersionedCovMapFuncRecordReader(
       InstrProfSymtab &P,
       std::vector<BinaryCoverageReader::ProfileMappingRecord> &R,
       std::vector<StringRef> &F)
       : ProfileNames(P), Filenames(F), Records(R) {}
 
   ~VersionedCovMapFuncRecordReader() override = default;
 
-  Expected<const char *> readFunctionRecords(const char *Buf,
-                                             const char *End) override {
+  Expected<const char *> readCoverageHeader(
+      const char *CovBuf, const char *CovBufEnd,
+      BinaryCoverageReader::DecompressedData &Decompressed) override {
     using namespace support;
 
-    if (Buf + sizeof(CovMapHeader) > End)
+    if (CovBuf + sizeof(CovMapHeader) > CovBufEnd)
       return make_error<CoverageMapError>(coveragemap_error::malformed);
-    auto CovHeader = reinterpret_cast<const CovMapHeader *>(Buf);
+    auto CovHeader = reinterpret_cast<const CovMapHeader *>(CovBuf);
     uint32_t NRecords = CovHeader->getNRecords<Endian>();
     uint32_t FilenamesSize = CovHeader->getFilenamesSize<Endian>();
     uint32_t CoverageSize = CovHeader->getCoverageSize<Endian>();
     assert((CovMapVersion)CovHeader->getVersion<Endian>() == Version);
-    Buf = reinterpret_cast<const char *>(CovHeader + 1);
+    CovBuf = reinterpret_cast<const char *>(CovHeader + 1);
 
     // Skip past the function records, saving the start and end for later.
-    const char *FunBuf = Buf;
-    Buf += NRecords * sizeof(FuncRecordType);
-    const char *FunEnd = Buf;
+    // This is a no-op in Version4 (function records are read after all headers
+    // are read).
+    const char *FuncRecBuf = nullptr;
+    const char *FuncRecBufEnd = nullptr;
+    if (Version < CovMapVersion::Version4)
+      FuncRecBuf = CovBuf;
+    CovBuf += NRecords * sizeof(FuncRecordType);
+    if (Version < CovMapVersion::Version4)
+      FuncRecBufEnd = CovBuf;
 
     // Get the filenames.
-    if (Buf + FilenamesSize > End)
+    if (CovBuf + FilenamesSize > CovBufEnd)
       return make_error<CoverageMapError>(coveragemap_error::malformed);
     size_t FilenamesBegin = Filenames.size();
-    RawCoverageFilenamesReader Reader(StringRef(Buf, FilenamesSize), Filenames);
-    if (auto Err = Reader.read())
+    StringRef FilenameRegion(CovBuf, FilenamesSize);
+    RawCoverageFilenamesReader Reader(FilenameRegion, Filenames);
+    if (auto Err = Reader.read(Version, Decompressed))
       return std::move(Err);
-    Buf += FilenamesSize;
+    CovBuf += FilenamesSize;
+    FilenameRange FileRange(FilenamesBegin, Filenames.size() - FilenamesBegin);
+
+    if (Version == CovMapVersion::Version4) {
+      // Map a hash of the filenames region to the filename range associated
+      // with this coverage header.
+      int64_t FilenamesRef =
+          llvm::IndexedInstrProf::ComputeHash(FilenameRegion);
+      auto Insert =
+          FileRangeMap.insert(std::make_pair(FilenamesRef, FileRange));
+      if (!Insert.second) {
+        // The same filenames ref was encountered twice. It's possible that
+        // the associated filenames are the same.
+        auto It = Filenames.begin();
+        FilenameRange &OrigRange = Insert.first->getSecond();
+        if (std::equal(It + OrigRange.StartingIndex,
+                       It + OrigRange.StartingIndex + OrigRange.Length,
+                       It + FileRange.StartingIndex,
+                       It + FileRange.StartingIndex + FileRange.Length))
+          // Map the new range to the original one.
+          FileRange = OrigRange;
+        else
+          // This is a hash collision. Mark the filenames ref invalid.
+          OrigRange.markInvalid();
+      }
+    }
 
     // We'll read the coverage mapping records in the loop below.
-    const char *CovBuf = Buf;
-    Buf += CoverageSize;
-    const char *CovEnd = Buf;
+    // This is a no-op in Version4 (coverage mappings are not affixed to the
+    // coverage header).
+    const char *MappingBuf = CovBuf;
+    if (Version == CovMapVersion::Version4 && CoverageSize != 0)
+      return make_error<CoverageMapError>(coveragemap_error::malformed);
+    CovBuf += CoverageSize;
+    const char *MappingEnd = CovBuf;
 
-    if (Buf > End)
+    if (CovBuf > CovBufEnd)
       return make_error<CoverageMapError>(coveragemap_error::malformed);
+
+    if (Version < CovMapVersion::Version4) {
+      // Read each function record.
+      if (Error E = readFunctionRecords(FuncRecBuf, FuncRecBufEnd, FileRange,
+                                        MappingBuf, MappingEnd))
+        return std::move(E);
+    }
+
     // Each coverage map has an alignment of 8, so we need to adjust alignment
     // before reading the next map.
-    Buf += offsetToAlignedAddr(Buf, Align(8));
+    CovBuf += offsetToAlignedAddr(CovBuf, Align(8));
 
-    auto CFR = reinterpret_cast<const FuncRecordType *>(FunBuf);
-    while ((const char *)CFR < FunEnd) {
-      // Read the function information
-      uint32_t DataSize = CFR->template getDataSize<Endian>();
+    return CovBuf;
+  }
 
-      // Now use that to read the coverage data.
-      if (CovBuf + DataSize > CovEnd)
+  Error readFunctionRecords(const char *FuncRecBuf, const char *FuncRecBufEnd,
+                            Optional<FilenameRange> OutOfLineFileRange,
+                            const char *OutOfLineMappingBuf,
+                            const char *OutOfLineMappingBufEnd) override {
+    auto CFR = reinterpret_cast<const FuncRecordType *>(FuncRecBuf);
+    while ((const char *)CFR < FuncRecBufEnd) {
+      // Validate the length of the coverage mapping for this function.
+      const char *NextMappingBuf;
+      const FuncRecordType *NextCFR;
+      std::tie(NextMappingBuf, NextCFR) =
+          CFR->template advanceByOne<Endian>(OutOfLineMappingBuf);
+      if (Version < CovMapVersion::Version4)
+        if (NextMappingBuf > OutOfLineMappingBufEnd)
           return make_error<CoverageMapError>(coveragemap_error::malformed);
-      auto Mapping = StringRef(CovBuf, DataSize);
-      CovBuf += DataSize;
 
-      if (Error Err =
-              insertFunctionRecordIfNeeded(CFR, Mapping, FilenamesBegin))
-        return std::move(Err);
-      CFR++;
+      // Look up the set of filenames associated with this function record.
+      Optional<FilenameRange> FileRange;
+      if (Version < CovMapVersion::Version4) {
+        FileRange = OutOfLineFileRange;
+      } else {
+        uint64_t FilenamesRef = CFR->template getFilenamesRef<Endian>();
+        auto It = FileRangeMap.find(FilenamesRef);
+        if (It == FileRangeMap.end())
+          return make_error<CoverageMapError>(coveragemap_error::malformed);
+        else
+          FileRange = It->getSecond();
+      }
+
+      // Now, read the coverage data.
+      if (FileRange && !FileRange->isInvalid()) {
+        StringRef Mapping =
+            CFR->template getCoverageMapping<Endian>(OutOfLineMappingBuf);
+        if (Version == CovMapVersion::Version4 &&
+            Mapping.data() + Mapping.size() > FuncRecBufEnd)
+          return make_error<CoverageMapError>(coveragemap_error::malformed);
+        if (Error Err = insertFunctionRecordIfNeeded(CFR, Mapping, *FileRange))
+          return Err;
       }
-    return Buf;
+
+      std::tie(OutOfLineMappingBuf, CFR) = std::tie(NextMappingBuf, NextCFR);
+    }
+    return Error::success();
   }
 };
 
 } // end anonymous namespace
 
 template <class IntPtrT, support::endianness Endian>
 Expected<std::unique_ptr<CovMapFuncRecordReader>> CovMapFuncRecordReader::get(
     CovMapVersion Version, InstrProfSymtab &P,
     std::vector<BinaryCoverageReader::ProfileMappingRecord> &R,
     std::vector<StringRef> &F) {
   using namespace coverage;
 
   switch (Version) {
   case CovMapVersion::Version1:
     return std::make_unique<VersionedCovMapFuncRecordReader<
         CovMapVersion::Version1, IntPtrT, Endian>>(P, R, F);
   case CovMapVersion::Version2:
   case CovMapVersion::Version3:
+  case CovMapVersion::Version4:
     // Decompress the name data.
     if (Error E = P.create(P.getNameData()))
       return std::move(E);
     if (Version == CovMapVersion::Version2)
       return std::make_unique<VersionedCovMapFuncRecordReader<
           CovMapVersion::Version2, IntPtrT, Endian>>(P, R, F);
-    else
+    else if (Version == CovMapVersion::Version3)
       return std::make_unique<VersionedCovMapFuncRecordReader<
           CovMapVersion::Version3, IntPtrT, Endian>>(P, R, F);
+    else if (Version == CovMapVersion::Version4)
+      return std::make_unique<VersionedCovMapFuncRecordReader<
+          CovMapVersion::Version4, IntPtrT, Endian>>(P, R, F);
   }
   llvm_unreachable("Unsupported version");
 }
 
 template <typename T, support::endianness Endian>
 static Error readCoverageMappingData(
-    InstrProfSymtab &ProfileNames, StringRef Data,
+    InstrProfSymtab &ProfileNames, StringRef CovMap, StringRef FuncRecords,
     std::vector<BinaryCoverageReader::ProfileMappingRecord> &Records,
-    std::vector<StringRef> &Filenames) {
+    std::vector<StringRef> &Filenames,
+    BinaryCoverageReader::DecompressedData &Decompressed) {
   using namespace coverage;
 
   // Read the records in the coverage data section.
   auto CovHeader =
-      reinterpret_cast<const CovMapHeader *>(Data.data());
+      reinterpret_cast<const CovMapHeader *>(CovMap.data());
   CovMapVersion Version = (CovMapVersion)CovHeader->getVersion<Endian>();
   if (Version > CovMapVersion::CurrentVersion)
     return make_error<CoverageMapError>(coveragemap_error::unsupported_version);
   Expected<std::unique_ptr<CovMapFuncRecordReader>> ReaderExpected =
       CovMapFuncRecordReader::get<T, Endian>(Version, ProfileNames, Records,
                                              Filenames);
   if (Error E = ReaderExpected.takeError())
     return E;
   auto Reader = std::move(ReaderExpected.get());
-  for (const char *Buf = Data.data(), *End = Buf + Data.size(); Buf < End;) {
-    auto NextHeaderOrErr = Reader->readFunctionRecords(Buf, End);
-    if (auto E = NextHeaderOrErr.takeError())
+  const char *CovBuf = CovMap.data();
+  const char *CovBufEnd = CovBuf + CovMap.size();
+  const char *FuncRecBuf = FuncRecords.data();
+  const char *FuncRecBufEnd = FuncRecords.data() + FuncRecords.size();
+  while (CovBuf < CovBufEnd) {
+    // Read the current coverage header & filename data.
+    //
+    // Prior to Version4, this also reads all function records affixed to the
+    // header.
+    //
+    // Return a pointer to the next coverage header.
+    auto NextOrErr =
+        Reader->readCoverageHeader(CovBuf, CovBufEnd, Decompressed);
+    if (auto E = NextOrErr.takeError())
       return E;
-    Buf = NextHeaderOrErr.get();
+    CovBuf = NextOrErr.get();
   }
+  // In Version4, function records are not affixed to coverage headers. Read
+  // the records from their dedicated section.
+  if (Version == CovMapVersion::Version4)
+    return Reader->readFunctionRecords(FuncRecBuf, FuncRecBufEnd, None, nullptr,
+                                       nullptr);
   return Error::success();
 }
 
 static const char *TestingFormatMagic = "llvmcovmtestdata";
 
 Expected<std::unique_ptr<BinaryCoverageReader>>
 BinaryCoverageReader::createCoverageReaderFromBuffer(
-    StringRef Coverage, InstrProfSymtab &&ProfileNames, uint8_t BytesInAddress,
-    support::endianness Endian) {
+    StringRef Coverage, StringRef FuncRecords, InstrProfSymtab &&ProfileNames,
+    uint8_t BytesInAddress, support::endianness Endian) {
   std::unique_ptr<BinaryCoverageReader> Reader(new BinaryCoverageReader());
   Reader->ProfileNames = std::move(ProfileNames);
   if (BytesInAddress == 4 && Endian == support::endianness::little) {
     if (Error E =
             readCoverageMappingData<uint32_t, support::endianness::little>(
-                Reader->ProfileNames, Coverage, Reader->MappingRecords,
-                Reader->Filenames))
+                Reader->ProfileNames, Coverage, FuncRecords,
+                Reader->MappingRecords, Reader->Filenames,
+                Reader->Decompressed))
       return std::move(E);
   } else if (BytesInAddress == 4 && Endian == support::endianness::big) {
     if (Error E = readCoverageMappingData<uint32_t, support::endianness::big>(
-            Reader->ProfileNames, Coverage, Reader->MappingRecords,
-            Reader->Filenames))
+            Reader->ProfileNames, Coverage, FuncRecords, Reader->MappingRecords,
+            Reader->Filenames, Reader->Decompressed))
       return std::move(E);
   } else if (BytesInAddress == 8 && Endian == support::endianness::little) {
     if (Error E =
             readCoverageMappingData<uint64_t, support::endianness::little>(
-                Reader->ProfileNames, Coverage, Reader->MappingRecords,
-                Reader->Filenames))
+                Reader->ProfileNames, Coverage, FuncRecords,
+                Reader->MappingRecords, Reader->Filenames,
+                Reader->Decompressed))
       return std::move(E);
   } else if (BytesInAddress == 8 && Endian == support::endianness::big) {
     if (Error E = readCoverageMappingData<uint64_t, support::endianness::big>(
-            Reader->ProfileNames, Coverage, Reader->MappingRecords,
-            Reader->Filenames))
+            Reader->ProfileNames, Coverage, FuncRecords, Reader->MappingRecords,
+            Reader->Filenames, Reader->Decompressed))
       return std::move(E);
   } else
     return make_error<CoverageMapError>(coveragemap_error::malformed);
   return std::move(Reader);
 }
 
 static Expected<std::unique_ptr<BinaryCoverageReader>>
 loadTestingFormat(StringRef Data) {
loadTestingFormat(StringRef Data) {
   // Skip the padding bytes because coverage map data has an alignment of 8.
   if (CoverageMapping.empty())
     return make_error<CoverageMapError>(coveragemap_error::truncated);
   size_t Pad = offsetToAlignedAddr(CoverageMapping.data(), Align(8));
   if (CoverageMapping.size() < Pad)
     return make_error<CoverageMapError>(coveragemap_error::malformed);
   CoverageMapping = CoverageMapping.substr(Pad);
   return BinaryCoverageReader::createCoverageReaderFromBuffer(
-      CoverageMapping, std::move(ProfileNames), BytesInAddress, Endian);
+      CoverageMapping, "", std::move(ProfileNames), BytesInAddress, Endian);
 }
 
 static Expected<SectionRef> lookupSection(ObjectFile &OF, StringRef Name) {
   // On COFF, the object file section name may end in "$M". This tells the
   // linker to sort these sections between "$A" and "$Z". The linker removes the
   // dollar and everything after it in the final binary. Do the same to match.
   bool IsCOFF = isa<COFFObjectFile>(OF);
   auto stripSuffix = [IsCOFF](StringRef N) {
auto NamesSection =
                                                  /*AddSegmentInfo=*/false));
   if (auto E = NamesSection.takeError())
     return std::move(E);
   auto CoverageSection =
       lookupSection(*OF, getInstrProfSectionName(IPSK_covmap, ObjFormat,
                                                  /*AddSegmentInfo=*/false));
   if (auto E = CoverageSection.takeError())
     return std::move(E);
-
-  // Get the contents of the given sections.
   auto CoverageMappingOrErr = CoverageSection->getContents();
   if (!CoverageMappingOrErr)
     return CoverageMappingOrErr.takeError();
+  StringRef CoverageMapping = CoverageMappingOrErr.get();
 
   InstrProfSymtab ProfileNames;
   if (Error E = ProfileNames.create(*NamesSection))
     return std::move(E);
 
+  // Look for the coverage records section (Version4 only).
+  StringRef FuncRecords;
+  auto CoverageRecordsSection =
+      lookupSection(*OF, getInstrProfSectionName(IPSK_covfun, ObjFormat,
+                                                 /*AddSegmentInfo=*/false));
+  if (auto E = CoverageRecordsSection.takeError())
+    consumeError(std::move(E));
+  else {
+    auto CoverageRecordsOrErr = CoverageRecordsSection->getContents();
+    if (!CoverageRecordsOrErr)
+      return CoverageRecordsOrErr.takeError();
+    FuncRecords = CoverageRecordsOrErr.get();
+  }
+
   return BinaryCoverageReader::createCoverageReaderFromBuffer(
-      CoverageMappingOrErr.get(), std::move(ProfileNames), BytesInAddress,
+      CoverageMapping, FuncRecords, std::move(ProfileNames), BytesInAddress,
       Endian);
 }
 
 Expected<std::vector<std::unique_ptr<BinaryCoverageReader>>>
 BinaryCoverageReader::create(
     MemoryBufferRef ObjectBuffer, StringRef Arch,
     SmallVectorImpl<std::unique_ptr<MemoryBuffer>> &ObjectFileBuffers) {
   std::vector<std::unique_ptr<BinaryCoverageReader>> Readers;

llvm/lib/ProfileData/Coverage/CoverageMappingWriter.cpp

 //===- CoverageMappingWriter.cpp - Code coverage mapping writer -----------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file contains support for writing coverage mapping data for
 // instrumentation based coverage.
 //
 //===----------------------------------------------------------------------===//
 
+#include "llvm/ProfileData/InstrProf.h"
 #include "llvm/ProfileData/Coverage/CoverageMappingWriter.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/Support/Compression.h"
 #include "llvm/Support/LEB128.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
 #include <limits>
 #include <vector>
 
 using namespace llvm;
 using namespace coverage;
 
 CoverageFilenamesSectionWriter::CoverageFilenamesSectionWriter(
     ArrayRef<StringRef> Filenames)
     : Filenames(Filenames) {
 #ifndef NDEBUG
   StringSet<> NameSet;
   for (StringRef Name : Filenames)
     assert(NameSet.insert(Name).second && "Duplicate filename");
 #endif
 }
 
-void CoverageFilenamesSectionWriter::write(raw_ostream &OS) {
-  encodeULEB128(Filenames.size(), OS);
+void CoverageFilenamesSectionWriter::write(raw_ostream &OS, bool Compress) {
+  std::string FilenamesStr;
+  {
+    raw_string_ostream FilenamesOS{FilenamesStr};
     for (const auto &Filename : Filenames) {
-    encodeULEB128(Filename.size(), OS);
-    OS << Filename;
+      encodeULEB128(Filename.size(), FilenamesOS);
+      FilenamesOS << Filename;
+    }
+  }
+
+  SmallString<128> CompressedStr;
+  bool doCompression =
+      Compress && zlib::isAvailable() && DoInstrProfNameCompression;
+  if (doCompression) {
+    auto E =
+        zlib::compress(FilenamesStr, CompressedStr, zlib::BestSizeCompression);
+    if (E)
+      report_bad_alloc_error("Failed to zlib compress coverage data");
   }
+
+  // ::= <num-filenames>
+  //     <uncompressed-len>
+  //     <compressed-len-or-zero>
+  //     (<compressed-filenames> | <uncompressed-filenames>)
+  encodeULEB128(Filenames.size(), OS);
+  encodeULEB128(FilenamesStr.size(), OS);
+  encodeULEB128(doCompression ? CompressedStr.size() : 0U, OS);
+  OS << (doCompression ? StringRef(CompressedStr) : StringRef(FilenamesStr));
 }
 
 namespace {
 
 /// Gather only the expressions that are used by the mapping
 /// regions in this function.
 class CounterExpressionsMinimizer {
   ArrayRef<CounterExpression> Expressions;

llvm/lib/ProfileData/InstrProf.cpp

#define INSTR_PROF_SECT_ENTRY(Kind, SectNameCommon, SectNameCoff, Prefix)      \
   Prefix,
 #include "llvm/ProfileData/InstrProfData.inc"
 };
 
 } // namespace
 
 namespace llvm {
 
+cl::opt<bool> DoInstrProfNameCompression(
+    "enable-name-compression",
+    cl::desc("Enable name/filename string compression"), cl::init(true));
+
 std::string getInstrProfSectionName(InstrProfSectKind IPSK,
                                     Triple::ObjectFormatType OF,
                                     bool AddSegmentInfo) {
   std::string SectName;
 
   if (OF == Triple::MachO && AddSegmentInfo)
     SectName = InstrProfSectNamePrefix[IPSK];
 

llvm/lib/Transforms/Instrumentation/InstrProfiling.cpp

 
 // The value that considered to be large value in  memory intrinsic.
 cl::opt<unsigned> MemOPSizeLarge(
     "memop-size-large",
     cl::desc("Set large value thresthold in memory intrinsic size profiling. "
              "Value of 0 disables the large value profiling."),
     cl::init(8192));
 
 namespace {
 
-cl::opt<bool> DoNameCompression("enable-name-compression",
-                                cl::desc("Enable name string compression"),
-                                cl::init(true));
-
 cl::opt<bool> DoHashBasedCounterSplit(
     "hash-based-counter-split",
     cl::desc("Rename counter variable of a comdat function based on cfg hash"),
     cl::init(true));
 
 cl::opt<bool> RuntimeCounterRelocation(
     "runtime-counter-relocation",
     cl::desc("Enable relocating counters at runtime."),

 void InstrProfiling::emitNameData() {
   std::string UncompressedData;
 
   if (ReferencedNames.empty())
     return;
 
   std::string CompressedNameStr;
   if (Error E = collectPGOFuncNameStrings(ReferencedNames, CompressedNameStr,
-                                          DoNameCompression)) {
+                                          DoInstrProfNameCompression)) {
     report_fatal_error(toString(std::move(E)), false);
   }
 
   auto &Ctx = M->getContext();
   auto *NamesVal = ConstantDataArray::getString(
       Ctx, StringRef(CompressedNameStr), false);
   NamesVar = new GlobalVariable(*M, NamesVal->getType(), true,
                                 GlobalValue::PrivateLinkage, NamesVal,

llvm/test/Instrumentation/InstrProfiling/X86/alloc.ll

 ;; Ensure that SHF_ALLOC section flag is not set for the __llvm_covmap section on Linux.
 ; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu | FileCheck %s
 
+@covfun = linkonce_odr hidden constant i32 0, section "__llvm_covfun"
 @__llvm_coverage_mapping = internal constant i32 0, section "__llvm_covmap"
 
+; CHECK-DAG: .section __llvm_covfun,""
 ; CHECK-DAG: .section __llvm_covmap,""

llvm/test/tools/llvm-cov/binary-formats.c

 // Checks for reading various formats.
 
 // CHECK: [[@LINE+1]]| 100|int main
 int main(int argc, const char *argv[]) {}
 
 // RUN: llvm-profdata merge %S/Inputs/binary-formats.proftext -o %t.profdata
 // RUN: llvm-cov show %S/Inputs/binary-formats.macho32l -instr-profile %t.profdata -path-equivalence=/tmp,%S %s | FileCheck %s
 // RUN: llvm-cov show %S/Inputs/binary-formats.macho64l -instr-profile %t.profdata -path-equivalence=/tmp,%S %s | FileCheck %s
 // RUN: llvm-cov show %S/Inputs/binary-formats.macho32b -instr-profile %t.profdata -path-equivalence=/tmp,%S %s | FileCheck %s
+// RUN: llvm-cov show %S/Inputs/binary-formats.v3.macho64l -instr-profile %t.profdata -path-equivalence=/tmp,%S %s | FileCheck %s
 
 // RUN: llvm-cov export %S/Inputs/binary-formats.macho64l -instr-profile %t.profdata | FileCheck %S/Inputs/binary-formats.canonical.json

llvm/unittests/ProfileData/CoverageMappingTest.cpp

 
 // FIXME: Use ::testing::Combine() when llvm updates its copy of googletest.
 INSTANTIATE_TEST_CASE_P(ParameterizedCovMapTest, CoverageMappingTest,
                         ::testing::Values(std::pair<bool, bool>({false, false}),
                                           std::pair<bool, bool>({false, true}),
                                           std::pair<bool, bool>({true, false}),
                                           std::pair<bool, bool>({true, true})),);
 
+TEST(CoverageMappingTest, filename_roundtrip) {
+  std::vector<StringRef> Paths({"a", "b", "c", "d", "e"});
+
+  for (bool Compress : {false, true}) {
+    std::string EncodedFilenames;
+    {
+      raw_string_ostream OS(EncodedFilenames);
+      CoverageFilenamesSectionWriter Writer(Paths);
+      Writer.write(OS, Compress);
+    }
+
+    std::vector<StringRef> ReadFilenames;
+    RawCoverageFilenamesReader Reader(EncodedFilenames, ReadFilenames);
+    BinaryCoverageReader::DecompressedData Decompressed;
+    EXPECT_THAT_ERROR(Reader.read(CovMapVersion::CurrentVersion, Decompressed),
+                      Succeeded());
+    if (!Compress)
+      ASSERT_EQ(Decompressed.size(), 0U);
+
+    ASSERT_EQ(ReadFilenames.size(), Paths.size());
+    for (unsigned I = 0; I < Paths.size(); ++I)
+      ASSERT_TRUE(ReadFilenames[I] == Paths[I]);
+  }
+}
+
 } // end anonymous namespace