[dsymutil] Apply relocations present in Swift reflection sections

[augusto2112]

Summary

The strippable Swift reflection sections contain subtractor relocations
that need to be applied. There are two situations we need to support.

1. Both symbols used in the relocation come from the .o file (for example, one symbol lives in swift5_fieldmd and the second in swift5_reflstr).
2. One symbol comes from th .o file and the second from the main binary (for example, swift5_fieldmd and swift5_typeref).

[rastogishubham]

I noticed that there is no test for this change. Can you please add one?

[aprantl]

llvm/tools/dsymutil/DwarfLinkerForBinary.cpp

342–500

We might want to sink this into MachO.h if it doesn't exist there already. You might want to check if there is code for it that we could upstream.

llvm/tools/dsymutil/DwarfLinkerForBinary.h

210

The keys in this map will be integers from 0..~10. Should this just be a std::vector instead?

llvm/tools/dsymutil/MachOUtils.cpp

618–637

Can you add a comment that explains that we are applying relocations here?

[augusto2112]

Updated reflection-dump test, moved isMachOPairedReloc to MachO.h, changed SectionToOffsetInDwarf from a map to a vector.

[augusto2112]

In D120574#3346115, @rastogishubham wrote:

I noticed that there is no test for this change. Can you please add one?

Updated the existing test to account for the relocations.

[aprantl]

llvm/include/llvm/Object/MachO.h

394

Doxygen Comment?

395

I know this would be a little redundant, but this function should probably have a unit test, since it's entirely self-contained.

llvm/include/llvm/Object/MachO.h

public:
   };
   using LoadCommandList = SmallVector<LoadCommandInfo, 4>;
   using load_command_iterator = LoadCommandList::const_iterator;
 
   static Expected<std::unique_ptr<MachOObjectFile>>
   create(MemoryBufferRef Object, bool IsLittleEndian, bool Is64Bits,
          uint32_t UniversalCputype = 0, uint32_t UniversalIndex = 0);
 
+  static bool isMachOPairedReloc(uint64_t RelocType, uint64_t Arch);
+
   void moveSymbolNext(DataRefImpl &Symb) const override;
 
   uint64_t getNValue(DataRefImpl Sym) const;
   Expected<StringRef> getSymbolName(DataRefImpl Symb) const override;
 
   // MachO specific.
   Error checkSymbolTable() const;
 

llvm/include/llvm/Object/ObjectFile.h

public:
   Triple makeTriple() const;
 
   /// Maps a debug section name to a standard DWARF section name.
   virtual StringRef mapDebugSectionName(StringRef Name) const { return Name; }
 
   /// True if this is a relocatable object (.o/.obj).
   virtual bool isRelocatableObject() const = 0;
 
+  /// True if the reflection section can be stripped by the linker.
+  bool isReflectionSectionStrippable(
+      llvm::binaryformat::Swift5ReflectionSectionKind ReflectionSectionKind)
+      const;
+
   /// @returns Pointer to ObjectFile subclass to handle this type of object.
   /// @param ObjectPath The path to the object file. ObjectPath.isObject must
   ///        return true.
   /// Create ObjectFile from path.
   static Expected<OwningBinary<ObjectFile>>
   createObjectFile(StringRef ObjectPath);
 
   static Expected<std::unique_ptr<ObjectFile>>

llvm/lib/Object/MachOObjectFile.cpp

 #define HANDLE_SWIFT_SECTION(KIND, MACHO, ELF, COFF)                           \
   .Case(MACHO, llvm::binaryformat::Swift5ReflectionSectionKind::KIND)
   return StringSwitch<llvm::binaryformat::Swift5ReflectionSectionKind>(
              SectionName)
 #include "llvm/BinaryFormat/Swift.def"
       .Default(llvm::binaryformat::Swift5ReflectionSectionKind::unknown);
 #undef HANDLE_SWIFT_SECTION
 }
+
+bool MachOObjectFile::isMachOPairedReloc(uint64_t RelocType, uint64_t Arch) {
+  switch (Arch) {
+  case Triple::x86:
+    return RelocType == MachO::GENERIC_RELOC_SECTDIFF ||
+           RelocType == MachO::GENERIC_RELOC_LOCAL_SECTDIFF;
+  case Triple::x86_64:
+    return RelocType == MachO::X86_64_RELOC_SUBTRACTOR;
+  case Triple::arm:
+  case Triple::thumb:
+    return RelocType == MachO::ARM_RELOC_SECTDIFF ||
+           RelocType == MachO::ARM_RELOC_LOCAL_SECTDIFF ||
+           RelocType == MachO::ARM_RELOC_HALF ||
+           RelocType == MachO::ARM_RELOC_HALF_SECTDIFF;
+  case Triple::aarch64:
+    return RelocType == MachO::ARM64_RELOC_SUBTRACTOR;
+  default:
+    return false;
+  }
+}

llvm/lib/Object/ObjectFile.cpp

ObjectFile::createObjectFile(StringRef ObjectPath) {
   Expected<std::unique_ptr<ObjectFile>> ObjOrErr =
       createObjectFile(Buffer->getMemBufferRef());
   if (Error Err = ObjOrErr.takeError())
     return std::move(Err);
   std::unique_ptr<ObjectFile> Obj = std::move(ObjOrErr.get());
 
   return OwningBinary<ObjectFile>(std::move(Obj), std::move(Buffer));
 }
+
+bool ObjectFile::isReflectionSectionStrippable(
+    llvm::binaryformat::Swift5ReflectionSectionKind ReflectionSectionKind)
+    const {
+  using llvm::binaryformat::Swift5ReflectionSectionKind;
+  return ReflectionSectionKind == Swift5ReflectionSectionKind::fieldmd ||
+         ReflectionSectionKind == Swift5ReflectionSectionKind::reflstr ||
+         ReflectionSectionKind == Swift5ReflectionSectionKind::assocty;
+}

llvm/test/tools/dsymutil/X86/reflection-dump.test

 RUN: rm -rf %t.dir && mkdir -p %t.dir/tmp
 RUN: cp %p/../Inputs/main.yaml %t.dir
 RUN: cp %p/../Inputs/test.yaml %t.dir
 RUN: cp %p/../Inputs/reflection_metadata.yaml %t.dir
 RUN: yaml2obj %p/../Inputs/main.yaml -o %t.dir/main
 RUN: yaml2obj %p/../Inputs/test.yaml -o %t.dir/tmp/test-1.o
 RUN: yaml2obj %p/../Inputs/reflection_metadata.yaml -o %t.dir/tmp/reflection_metadata-1.o
 
 RUN: dsymutil -oso-prepend-path=%t.dir %t.dir/main -o %t.dir/main.dSYM
 RUN: llvm-objdump -s %t.dir/main.dSYM/Contents/Resources/DWARF/main | FileCheck %s
 
 REQUIRES: host-byteorder-little-endian
 
-CHECK: Contents of section __DWARF,__swift5_typeref:
-CHECK-NEXT:  10000e000 53690000 01ffffff ff002473 346d6169  Si........$s4mai
-CHECK-NEXT:  10000e010 6e31304d 7950726f 746f636f 6c500000  n10MyProtocolP..
-CHECK-NEXT:  10000e020 01ffffff ff007800 42300000 53690000  ......x.B0..Si..
-CHECK-NEXT:  10000e030 01ffffff ff002473 346d6169 6e31304d  ......$s4main10M
-CHECK-NEXT:  10000e040 7950726f 746f636f 6c500000 01ffffff  yProtocolP......
-
-CHECK: Contents of section __DWARF,__swift5_reflstr:
-CHECK-NEXT:  10000e09b 496e6e65 7200696e 6e657200 496e6e65  Inner.inner.Inne
-CHECK-NEXT:  10000e0ab 72006900 6d73006d 6500696e 6e657200  r.i.ms.me.inner.
-CHECK-NEXT:  10000e0bb 43004900 74006d67 73006d67 65004743  C.I.t.mgs.mge.GC
-CHECK-NEXT:  10000e0cb 00
 
 CHECK: Contents of section __DWARF,__swift5_assocty:
-CHECK-NEXT:  10000e0cc 00000000 fcffffff 01000000 08000000  ................
-CHECK-NEXT:  10000e0dc f0ffffff ecffffff 00000000 fcffffff  ................
-CHECK-NEXT:  10000e0ec 01000000 08000000 f0ffffff ecffffff  ................
+CHECK-NEXT: 10000e000 00000000 fcffffff 01000000 08000000  ................
+CHECK-NEXT: 10000e010 f0ffffff ecffffff 00000000 fcffffff  ................
+CHECK-NEXT: 10000e020 01000000 08000000 f0ffffff ecffffff  ................
 
 CHECK: Contents of section __DWARF,__swift5_fieldmd:
-CHECK-NEXT:  10000e0fc 00000000 00000000 00000c00 01000000  ................
-CHECK-NEXT:  10000e10c 02000000 ecffffff e8ffffff 00000000  ................
-CHECK-NEXT:  10000e11c 00000000 00000c00 00000000 00000000  ................
-CHECK-NEXT:  10000e12c 00000000 04000c00 00000000 00000000  ................
-
-CHECK: Contents of section __DWARF,__swift5_capture:
-CHECK-NEXT:  10000e22c 01000000 01000000 02000000 f4ffffff  ................
-CHECK-NEXT:  10000e23c f0ffffff ecffffff                    ........
-
-CHECK: Contents of section __DWARF,__swift5_builtin:
-CHECK-NEXT:  10000e244 00000000 09000000 08000100 10000000  ................
-CHECK-NEXT:  10000e254 fe000000                             ....
-
-CHECK: Contents of section __DWARF,__swift5_proto:
-CHECK-NEXT: 10000e258 41424344 45464748 4950               ABCDEFGHIP
-
-CHECK: Contents of section __DWARF,__swift5_protos:
-CHECK-NEXT: 10000e264 51525354 55565758 5960               QRSTUVWXY`
+CHECK-NEXT: 10000e030 00000000 00000000 00000c00 01000000  ................
+CHECK-NEXT: 10000e040 02000000 ecffffff e8ffffff 00000000  ................
+CHECK-NEXT: 10000e050 00000000 00000c00 00000000 00000000  ................
+CHECK-NEXT: 10000e060 00000000 04000c00 00000000 00000000  ................
+CHECK-NEXT: 10000e070 00000000 01000c00 03000000 00000000  ................
+CHECK-NEXT: 10000e080 ecffffff e8ffffff 00000000 e0ffffff  ................
+CHECK-NEXT: 10000e090 dcffffff 00000000 d4ffffff d0ffffff  ................
+CHECK-NEXT: 10000e0a0 00000000 00000000 00000c00 01000000  ................
+CHECK-NEXT: 10000e0b0 02000000 ecffffff e8ffffff 00000000  ................
+CHECK-NEXT: 10000e0c0 00000000 00000c00 00000000 00000000  ................
+CHECK-NEXT: 10000e0d0 00000000 03000c00 02000000 00000000  ................
+CHECK-NEXT: 10000e0e0 ecffffff e8ffffff 00000000 e0ffffff  ................
+CHECK-NEXT: 10000e0f0 dcffffff 00000000 00000000 01000c00  ................
+CHECK-NEXT: 10000e100 04000000 00000000 ecffffff e8ffffff  ................
+CHECK-NEXT: 10000e110 00000000 e0ffffff dcffffff 00000000  ................
+CHECK-NEXT: 10000e120 d4ffffff d0ffffff 00000000 c8ffffff  ................
+CHECK-NEXT: 10000e130 c4ffffff 00000000 00000000 00000c00  ................
+CHECK-NEXT: 10000e140 00000000 00000000 00000000 02000c00  ................
+CHECK-NEXT: 10000e150 01000000 00000000 ecffffff e8ffffff  ................
 
-CHECK: Contents of section __DWARF,__swift5_acfuncs:
-CHECK-NEXT: 10000e270 61626364 65666768 6970               abcdefghip
-
-CHECK: Contents of section __DWARF,__swift5_mpenum:
-CHECK-NEXT: 10000e27c 71727374 75767778 7980               qrstuvwxy.
+CHECK: Contents of section __DWARF,__swift5_reflstr:
+CHECK-NEXT: 10000e160 496e6e65 7200696e 6e657200 496e6e65  Inner.inner.Inne
+CHECK-NEXT: 10000e170 72006900 6d73006d 6500696e 6e657200  r.i.ms.me.inner.
+CHECK-NEXT: 10000e180 43004900 74006d67 73006d67 65004743  C.I.t.mgs.mge.GC 

llvm/tools/dsymutil/DwarfLinkerForBinary.h

 //===- tools/dsymutil/DwarfLinkerForBinary.h --------------------*- 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
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_TOOLS_DSYMUTIL_DWARFLINKER_H
 #define LLVM_TOOLS_DSYMUTIL_DWARFLINKER_H
 
 #include "BinaryHolder.h"
 #include "DebugMap.h"
 #include "LinkUtils.h"
+#include "MachOUtils.h"
 #include "llvm/DWARFLinker/DWARFLinker.h"
 #include "llvm/DWARFLinker/DWARFLinkerCompileUnit.h"
 #include "llvm/DWARFLinker/DWARFLinkerDeclContext.h"
 #include "llvm/DWARFLinker/DWARFStreamer.h"
 #include "llvm/DebugInfo/DWARF/DWARFContext.h"
 #include "llvm/Remarks/RemarkFormat.h"
 #include "llvm/Remarks/RemarkLinker.h"
 
private:
 
   /// Attempt to load a debug object from disk.
   ErrorOr<const object::ObjectFile &> loadObject(const DebugMapObject &Obj,
                                                  const Triple &triple);
   ErrorOr<DWARFFile &> loadObject(const DebugMapObject &Obj,
                                   const DebugMap &DebugMap,
                                   remarks::RemarkLinker &RL);
 
+  void collectRelocationsToApplyToSwiftReflectionSections(
+      const object::SectionRef &Section, StringRef &Contents,
+      const llvm::object::MachOObjectFile *MO,
+      const std::vector<uint64_t> &SectionToOffsetInDwarf,
+      const llvm::dsymutil::DebugMapObject *Obj,
+      std::vector<MachOUtils::DwarfRelocationApplicationInfo>
+          &RelocationsToApply) const;
+
+  void copySwiftReflectionMetadata(
+      const llvm::dsymutil::DebugMapObject *Obj, DwarfStreamer *Streamer,
+      const std::vector<uint64_t> &SectionToOffsetInDwarf,
+      std::vector<MachOUtils::DwarfRelocationApplicationInfo>
+          &RelocationsToApply);
+
   raw_fd_ostream &OutFile;
   BinaryHolder &BinHolder;
   LinkOptions Options;
   std::unique_ptr<DwarfStreamer> Streamer;
   std::vector<std::unique_ptr<DWARFFile>> ObjectsForLinking;
   std::vector<std::unique_ptr<DWARFContext>> ContextForLinking;
   std::vector<std::unique_ptr<AddressManager>> AddressMapForLinking;
   std::vector<std::string> EmptyWarnings;

llvm/tools/dsymutil/DwarfLinkerForBinary.cpp

if (Error NewE = handleErrors(
       return errorToErrorCode(std::move(NewE));
 
     return *ObjectsForLinking.back();
   }
 
   return ErrorOrObj.getError();
 }
 
-static bool binaryHasSwiftReflectionSections(const DebugMap &Map,
-                                             const LinkOptions &Options,
-                                             BinaryHolder &BinHolder) {
-  // If the input binary has swift5 reflection sections, there is no need to
-  // copy them to the .dSYM. Only copy them for binaries where the linker
-  // omitted the reflection metadata.
+static bool binaryHasStrippableSwiftReflectionSections(
+    const DebugMap &Map, const LinkOptions &Options, BinaryHolder &BinHolder) {
+  // If the input binary has strippable swift5 reflection sections, there is no
+  // need to copy them to the .dSYM. Only copy them for binaries where the
+  // linker omitted the reflection metadata.
   if (!Map.getBinaryPath().empty() &&
       Options.FileType == OutputFileType::Object) {
 
     auto ObjectEntry = BinHolder.getObjectEntry(Map.getBinaryPath());
     // If ObjectEntry or Object has an error, no binary exists, therefore no
     // reflection sections exist.
     if (!ObjectEntry) {
       // Any errors will be diagnosed later in the main loop, ignore them here.
if (!Map.getBinaryPath().empty() &&
     for (auto &Section : Object->sections()) {
       llvm::Expected<llvm::StringRef> NameOrErr =
           Object->getSectionName(Section.getRawDataRefImpl());
       if (!NameOrErr) {
         llvm::consumeError(NameOrErr.takeError());
         continue;
       }
       NameOrErr->consume_back("__TEXT");
-      if (Object->mapReflectionSectionNameToEnumValue(*NameOrErr) !=
-          llvm::binaryformat::Swift5ReflectionSectionKind::unknown) {
+      auto ReflectionSectionKind =
+          Object->mapReflectionSectionNameToEnumValue(*NameOrErr);
+      if (Object->isReflectionSectionStrippable(ReflectionSectionKind)) {
         return true;
       }
     }
   }
   return false;
 }
 
-static void
-copySwiftReflectionMetadata(const llvm::dsymutil::DebugMapObject *Obj,
-                            DwarfStreamer *Streamer) {
+/// Calculate the start of the strippable swift reflection sections in Dwarf.
+/// Note that there's an assumption that the reflection sections will appear
+/// in alphabetic order.
+static std::vector<uint64_t>
+calculateStartOfStrippableReflectionSections(const DebugMap &Map) {
+  using llvm::binaryformat::Swift5ReflectionSectionKind;
+  uint64_t AssocTySize = 0;
+  uint64_t FieldMdSize = 0;
+  for (const auto &Obj : Map.objects()) {
     auto OF =
         llvm::object::ObjectFile::createObjectFile(Obj->getObjectFilename());
     if (!OF) {
       llvm::consumeError(OF.takeError());
-    return;
-  } else if (auto *MO =
-                 dyn_cast<llvm::object::MachOObjectFile>(OF->getBinary())) {
-    for (auto &Section : OF->getBinary()->sections()) {
+      continue;
+    }
+    if (auto *MO = dyn_cast<llvm::object::MachOObjectFile>(OF->getBinary())) {
+      for (auto &Section : MO->sections()) {
         llvm::Expected<llvm::StringRef> NameOrErr =
             MO->getSectionName(Section.getRawDataRefImpl());
         if (!NameOrErr) {
           llvm::consumeError(NameOrErr.takeError());
           continue;
         }
-      llvm::Expected<llvm::StringRef> SectionContents = Section.getContents();
-      if (SectionContents) {
         NameOrErr->consume_back("__TEXT");
-        Streamer->emitSwiftReflectionSection(
-            MO->mapReflectionSectionNameToEnumValue(*NameOrErr),
-            *SectionContents, Section.getAlignment(), Section.getSize());
-      }
-    }
+        auto ReflSectionKind =
+            MO->mapReflectionSectionNameToEnumValue(*NameOrErr);
+        switch (ReflSectionKind) {
+        case Swift5ReflectionSectionKind::assocty:
+          AssocTySize += Section.getSize();
+          break;
+        case Swift5ReflectionSectionKind::fieldmd:
+          FieldMdSize += Section.getSize();
+          break;
+        default:
+          break;
+        }
+      }
+    }
+  }
+  // Initialize the vector with enough space to fit every reflection section
+  // kind.
+  std::vector<uint64_t> SectionToOffset(Swift5ReflectionSectionKind::last, 0);
+  SectionToOffset[Swift5ReflectionSectionKind::assocty] = 0;
+  SectionToOffset[Swift5ReflectionSectionKind::fieldmd] =
+      llvm::alignTo(AssocTySize, 4);
+  SectionToOffset[Swift5ReflectionSectionKind::reflstr] = llvm::alignTo(
+      SectionToOffset[Swift5ReflectionSectionKind::fieldmd] + FieldMdSize, 4);
+
+  return SectionToOffset;
+}
+
+void DwarfLinkerForBinary::collectRelocationsToApplyToSwiftReflectionSections(
+    const object::SectionRef &Section, StringRef &Contents,
+    const llvm::object::MachOObjectFile *MO,
+    const std::vector<uint64_t> &SectionToOffsetInDwarf,
+    const llvm::dsymutil::DebugMapObject *Obj,
+    std::vector<MachOUtils::DwarfRelocationApplicationInfo> &RelocationsToApply)
+    const {
+  for (auto It = Section.relocation_begin(); It != Section.relocation_end();
+       ++It) {
+    object::DataRefImpl RelocDataRef = It->getRawDataRefImpl();
+    MachO::any_relocation_info MachOReloc = MO->getRelocation(RelocDataRef);
+
+    if (!object::MachOObjectFile::isMachOPairedReloc(
+            MO->getAnyRelocationType(MachOReloc), MO->getArch())) {
+      reportWarning(
+          "Unimplemented relocation type in strippable reflection section ",
+          Obj->getObjectFilename());
+      continue;
+    }
+
+    auto CalculateAddressOfSymbolInDwarfSegment =
+        [&]() -> llvm::Optional<int64_t> {
+      auto Symbol = It->getSymbol();
+      auto SymbolAbsoluteAddress = Symbol->getAddress();
+      if (!SymbolAbsoluteAddress)
+        return {};
+      auto Section = Symbol->getSection();
+      if (!Section) {
+        llvm::consumeError(Section.takeError());
+        return {};
+      }
+
+      if ((*Section)->getObject()->section_end() == *Section)
+        return {};
+
+      auto SectionStart = (*Section)->getAddress();
+      auto SymbolAddressInSection = *SymbolAbsoluteAddress - SectionStart;
+      auto SectionName = (*Section)->getName();
+      if (!SectionName)
+        return {};
+      auto ReflSectionKind =
+          MO->mapReflectionSectionNameToEnumValue(*SectionName);
+
+      int64_t SectionStartInLinkedBinary =
+          SectionToOffsetInDwarf[ReflSectionKind];
+
+      auto Addr = SectionStartInLinkedBinary + SymbolAddressInSection;
+      return Addr;
+    };
+
+    // The first symbol should always be in the section we're currently
+    // iterating over.
+    auto FirstSymbolAddress = CalculateAddressOfSymbolInDwarfSegment();
+    ++It;
+
+    bool ShouldSubtractDwarfVM = false;
+    // For the second symbol there are two possibilities.
+    llvm::Optional<int64_t> SecondSymbolAddress;
+    auto Sym = It->getSymbol();
+    if (Sym != MO->symbol_end()) {
+      Expected<StringRef> SymbolName = Sym->getName();
+      if (SymbolName) {
+        if (const auto *Mapping = Obj->lookupSymbol(*SymbolName)) {
+          // First possibility: the symbol exists in the binary, and exists in a
+          // non-strippable section (for example, typeref, or __TEXT,__const),
+          // in which case we look up its address in the  binary, which dsymutil
+          // will copy verbatim.
+          SecondSymbolAddress = Mapping->getValue().BinaryAddress;
+          // Since the symbols live in different segments, we have to substract
+          // the start of the Dwarf's vmaddr so the value calculated points to
+          // the correct place.
+          ShouldSubtractDwarfVM = true;
+        }
+      }
+    }
+
+    if (!SecondSymbolAddress) {
+      // Second possibility, this symbol is not present in the main binary, and
+      // must be in one of the strippable sections (for example, reflstr).
+      // Calculate its address in the same way as we did the first one.
+      SecondSymbolAddress = CalculateAddressOfSymbolInDwarfSegment();
+    }
+
+    if (!FirstSymbolAddress || !SecondSymbolAddress)
+      continue;
+
+    auto SectionName = Section.getName();
+    if (!SectionName)
+      continue;
+
+    int32_t Addend;
+    memcpy(&Addend, Contents.data() + It->getOffset(), sizeof(int32_t));
+    int32_t Value = (*SecondSymbolAddress + Addend) - *FirstSymbolAddress;
+    auto ReflSectionKind =
+        MO->mapReflectionSectionNameToEnumValue(*SectionName);
+    uint64_t AddressFromDwarfVM =
+        SectionToOffsetInDwarf[ReflSectionKind] + It->getOffset();
+    RelocationsToApply.emplace_back(AddressFromDwarfVM, Value,
+                                    ShouldSubtractDwarfVM);
+
+void DwarfLinkerForBinary::copySwiftReflectionMetadata(
+    const llvm::dsymutil::DebugMapObject *Obj, DwarfStreamer *Streamer,
+    const std::vector<uint64_t> &SectionToOffsetInDwarf,
+    std::vector<MachOUtils::DwarfRelocationApplicationInfo>
+        &RelocationsToApply) {
+  using binaryformat::Swift5ReflectionSectionKind;
+  auto OF =
+      llvm::object::ObjectFile::createObjectFile(Obj->getObjectFilename());
+  if (!OF) {
+    llvm::consumeError(OF.takeError());
+    return;
+  }
+  if (auto *MO = dyn_cast<llvm::object::MachOObjectFile>(OF->getBinary())) {
+    // Collect the swift reflection sections before emitting them. This is
+    // done so we control the order they're emitted.
+    std::unordered_map<Swift5ReflectionSectionKind, object::SectionRef>
+        SwiftSections;
+    for (auto &Section : MO->sections()) {
+      llvm::Expected<llvm::StringRef> NameOrErr =
+          MO->getSectionName(Section.getRawDataRefImpl());
+      if (!NameOrErr) {
+        llvm::consumeError(NameOrErr.takeError());
+        continue;
+      }
+      NameOrErr->consume_back("__TEXT");
+      auto ReflSectionKind =
+          MO->mapReflectionSectionNameToEnumValue(*NameOrErr);
+      if (MO->isReflectionSectionStrippable(ReflSectionKind))
+        SwiftSections[ReflSectionKind] = Section;
+    }
+    // Make sure we copy the sections in alphabetic order.
+    auto SectionKindsToEmit = {Swift5ReflectionSectionKind::assocty,
+                               Swift5ReflectionSectionKind::fieldmd,
+                               Swift5ReflectionSectionKind::reflstr};
+    for (auto SectionKind : SectionKindsToEmit) {
+      if (!SwiftSections.count(SectionKind))
+        continue;
+      auto &Section = SwiftSections[SectionKind];
+      llvm::Expected<llvm::StringRef> SectionContents = Section.getContents();
+      if (!SectionContents)
+        continue;
+      const auto *MO =
+          llvm::cast<llvm::object::MachOObjectFile>(Section.getObject());
+      collectRelocationsToApplyToSwiftReflectionSections(
+          Section, *SectionContents, MO, SectionToOffsetInDwarf, Obj,
+          RelocationsToApply);
+      Streamer->emitSwiftReflectionSection(SectionKind, *SectionContents,
+                                           Section.getAlignment(),
+                                           Section.getSize());
+    }
   }
 }
 
 bool DwarfLinkerForBinary::link(const DebugMap &Map) {
   if (!createStreamer(Map.getTriple(), OutFile))
     return false;
 
   ObjectsForLinking.clear();
GeneralLinker.setObjFileLoader(
         llvm_unreachable("Unhandled DebugMap object");
       });
   GeneralLinker.setSwiftInterfacesMap(&ParseableSwiftInterfaces);
   bool ReflectionSectionsPresentInBinary = false;
   // If there is no output specified, no point in checking the binary for swift5
   // reflection sections.
   if (!Options.NoOutput) {
     ReflectionSectionsPresentInBinary =
-        binaryHasSwiftReflectionSections(Map, Options, BinHolder);
+        binaryHasStrippableSwiftReflectionSections(Map, Options, BinHolder);
   }
 
-  for (const auto &Obj : Map.objects()) {
-    // If there is no output specified or the reflection sections are present in
-    // the Input binary, there is no need to copy the Swift Reflection Metadata
-    if (!Options.NoOutput && !ReflectionSectionsPresentInBinary)
-      copySwiftReflectionMetadata(Obj.get(), Streamer.get());
+  std::vector<MachOUtils::DwarfRelocationApplicationInfo> RelocationsToApply;
+  if (!Options.NoOutput && !ReflectionSectionsPresentInBinary) {
+    auto SectionToOffsetInDwarf =
+        calculateStartOfStrippableReflectionSections(Map);
+    for (const auto &Obj : Map.objects()) 
+      copySwiftReflectionMetadata(Obj.get(), Streamer.get(),
+                                  SectionToOffsetInDwarf, RelocationsToApply);
+  }
 
+  for (const auto &Obj : Map.objects()) {
     // N_AST objects (swiftmodule files) should get dumped directly into the
     // appropriate DWARF section.
     if (Obj->getType() == MachO::N_AST) {
       if (Options.Verbose)
         outs() << "DEBUG MAP OBJECT: " << Obj->getObjectFilename() << "\n";
 
       StringRef File = Obj->getObjectFilename();
       auto ErrorOrMem = MemoryBuffer::getFile(File);
if (auto E =
             copySwiftInterfaces(ParseableSwiftInterfaces, ArchName, Options))
       return error(toString(std::move(E)));
   }
 
   if (Map.getTriple().isOSDarwin() && !Map.getBinaryPath().empty() &&
       Options.FileType == OutputFileType::Object)
     return MachOUtils::generateDsymCompanion(
         Options.VFS, Map, Options.Translator,
-        *Streamer->getAsmPrinter().OutStreamer, OutFile);
+        *Streamer->getAsmPrinter().OutStreamer, OutFile, RelocationsToApply);
 
   Streamer->finish();
   return true;
 }
 
-static bool isMachOPairedReloc(uint64_t RelocType, uint64_t Arch) {
-  switch (Arch) {
-  case Triple::x86:
-    return RelocType == MachO::GENERIC_RELOC_SECTDIFF ||
-           RelocType == MachO::GENERIC_RELOC_LOCAL_SECTDIFF;
-  case Triple::x86_64:
-    return RelocType == MachO::X86_64_RELOC_SUBTRACTOR;
-  case Triple::arm:
-  case Triple::thumb:
-    return RelocType == MachO::ARM_RELOC_SECTDIFF ||
-           RelocType == MachO::ARM_RELOC_LOCAL_SECTDIFF ||
-           RelocType == MachO::ARM_RELOC_HALF ||
-           RelocType == MachO::ARM_RELOC_HALF_SECTDIFF;
-  case Triple::aarch64:
-    return RelocType == MachO::ARM64_RELOC_SUBTRACTOR;
-  default:
-    return false;
-  }
-}
-
 /// Iterate over the relocations of the given \p Section and
 /// store the ones that correspond to debug map entries into the
 /// ValidRelocs array.
 void DwarfLinkerForBinary::AddressManager::findValidRelocsMachO(
     const object::SectionRef &Section, const object::MachOObjectFile &Obj,
     const DebugMapObject &DMO, std::vector<ValidReloc> &ValidRelocs) {
   Expected<StringRef> ContentsOrErr = Section.getContents();
   if (!ContentsOrErr) {
     consumeError(ContentsOrErr.takeError());
     Linker.reportWarning("error reading section", DMO.getObjectFilename());
     return;
   }
   DataExtractor Data(*ContentsOrErr, Obj.isLittleEndian(), 0);
   bool SkipNext = false;
 
   for (const object::RelocationRef &Reloc : Section.relocations()) {
     if (SkipNext) {
       SkipNext = false;
       continue;
     }
 
     object::DataRefImpl RelocDataRef = Reloc.getRawDataRefImpl();
     MachO::any_relocation_info MachOReloc = Obj.getRelocation(RelocDataRef);
 
-    if (isMachOPairedReloc(Obj.getAnyRelocationType(MachOReloc),
+    if (object::MachOObjectFile::isMachOPairedReloc(Obj.getAnyRelocationType(MachOReloc),
                            Obj.getArch())) {
       SkipNext = true;
       Linker.reportWarning("unsupported relocation in " + *Section.getName() +
                                " section.",
                            DMO.getObjectFilename());
       continue;
     }
 

llvm/tools/dsymutil/MachOUtils.h

struct ArchAndFile {
   llvm::StringRef path() const;
 
   ArchAndFile(StringRef Arch) : Arch(std::string(Arch)) {}
   ArchAndFile(ArchAndFile &&A) = default;
   ArchAndFile &operator=(ArchAndFile &&A) = default;
   ~ArchAndFile();
 };
 
+struct DwarfRelocationApplicationInfo {
+  // The position in the stream that should be patched, starting from the
+  // Dwarf's segment file address.
+  uint64_t AddressFromDwarfStart;
+  int32_t Value;
+  // If we should subtract the Dwarf segment's VM address from value before
+  // writing it.
+  bool ShouldSubtractDwarfVM;
+
+  DwarfRelocationApplicationInfo(uint64_t AddressFromDwarfVM, uint32_t Value,
+                                 bool ShouldSubtractDwarfVM)
+      : AddressFromDwarfStart(AddressFromDwarfVM), Value(Value),
+        ShouldSubtractDwarfVM(ShouldSubtractDwarfVM) {}
+};
+
 bool generateUniversalBinary(SmallVectorImpl<ArchAndFile> &ArchFiles,
                              StringRef OutputFileName, const LinkOptions &,
                              StringRef SDKPath);
-
-bool generateDsymCompanion(llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS,
-                           const DebugMap &DM, SymbolMapTranslator &Translator,
-                           MCStreamer &MS, raw_fd_ostream &OutFile);
+bool generateDsymCompanion(
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS, const DebugMap &DM,
+    SymbolMapTranslator &Translator, MCStreamer &MS, raw_fd_ostream &OutFile,
+    const std::vector<MachOUtils::DwarfRelocationApplicationInfo>
+        &RelocationsToApply);
 
 std::string getArchName(StringRef Arch);
 } // namespace MachOUtils
 } // namespace dsymutil
 } // namespace llvm
 #endif // LLVM_TOOLS_DSYMUTIL_MACHOUTILS_H

llvm/tools/dsymutil/MachOUtils.cpp

return sizeof(MachO::segment_command_64) +
            NumSections * sizeof(MachO::section_64);
 
   return sizeof(MachO::segment_command) + NumSections * sizeof(MachO::section);
 }
 
 // Stream a dSYM companion binary file corresponding to the binary referenced
 // by \a DM to \a OutFile. The passed \a MS MCStreamer is setup to write to
 // \a OutFile and it must be using a MachObjectWriter object to do so.
-bool generateDsymCompanion(llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS,
-                           const DebugMap &DM, SymbolMapTranslator &Translator,
-                           MCStreamer &MS, raw_fd_ostream &OutFile) {
+bool generateDsymCompanion(
+    llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS, const DebugMap &DM,
+    SymbolMapTranslator &Translator, MCStreamer &MS, raw_fd_ostream &OutFile,
+    const std::vector<MachOUtils::DwarfRelocationApplicationInfo>
+        &RelocationsToApply) {
   auto &ObjectStreamer = static_cast<MCObjectStreamer &>(MS);
   MCAssembler &MCAsm = ObjectStreamer.getAssembler();
   auto &Writer = static_cast<MachObjectWriter &>(MCAsm.getWriter());
 
   // Layout but don't emit.
   ObjectStreamer.flushPendingLabels();
   MCAsmLayout Layout(MCAsm);
   MCAsm.layout(Layout);
bool generateDsymCompanion(
   for (const MCSection &Sec : MCAsm) {
     if (Sec.begin() == Sec.end())
       continue;
 
     uint64_t Pos = OutFile.tell();
     OutFile.write_zeros(alignTo(Pos, Sec.getAlignment()) - Pos);
     MCAsm.writeSectionData(OutFile, &Sec, Layout);
   }
 
+  // Apply relocations to the contents of the DWARF segment.
+  // We do this here because the final value written depend on the DWARF vm
+  // addr, which is only calculated in this function.
+  if (!RelocationsToApply.empty()) {
+    if (!OutFile.supportsSeeking())
+      report_fatal_error(
+          "Cannot apply relocations to file that doesn't support seeking!");
+
+    uint64_t Pos = OutFile.tell();
+    for (auto &RelocationToApply : RelocationsToApply) {
+      OutFile.seek(DwarfSegmentStart + RelocationToApply.AddressFromDwarfStart);
+      int32_t Value = RelocationToApply.Value;
+      if (RelocationToApply.ShouldSubtractDwarfVM)
+        Value -= DwarfVMAddr;
+      OutFile.write((char *)&Value, sizeof(int32_t));
+    }
+    OutFile.seek(Pos);
+  }
+
   return true;
 }
 } // namespace MachOUtils
 } // namespace dsymutil
 } // namespace llvm