From 55469625a6668d386f073fced28e1499d08d5ae8 Mon Sep 17 00:00:00 2001
From: Tim Kientzle <tkientzle@apple.com>
Date: Fri, 7 Jun 2024 16:10:42 -0700
Subject: [PATCH] [RemoteMirror] Remove compiler code to emit spare bit info in
 reflection metadata

This mostly reverts "[RemoteMirror] Get spare bit info from reflection records (#40906)"
I recently figured out that RemoteMirror can compute all the spare
bit data directly without requiring this information.  So let's
remove it to save some space in binaries.

This mostly reverts commit 4d91b45988e39f1ba2793aac8494ab4653120772.
---
 .../swift/RemoteInspection/DescriptorFinder.h |  23 ---
 include/swift/RemoteInspection/Records.h      | 105 -------------
 .../RemoteInspection/ReflectionContext.h      |  27 +---
 .../llvm/BinaryFormat/Swift.def               |   1 -
 .../swift/RemoteInspection/TypeRefBuilder.h   |  38 +----
 lib/IRGen/GenReflection.cpp                   |  87 +----------
 lib/IRGen/IRGenModule.h                       |   2 -
 .../public/RemoteInspection/TypeLowering.cpp  |   2 -
 .../RemoteInspection/TypeRefBuilder.cpp       | 142 ------------------
 .../SwiftRemoteMirror/SwiftRemoteMirror.cpp   |   2 -
 .../SwiftShims/swift/shims/MetadataSections.h |   1 -
 stdlib/public/runtime/SwiftRT-COFF.cpp        |   2 -
 stdlib/public/runtime/SwiftRT-ELF-WASM.cpp    |   2 -
 13 files changed, 10 insertions(+), 424 deletions(-)

diff --git a/include/swift/RemoteInspection/DescriptorFinder.h b/include/swift/RemoteInspection/DescriptorFinder.h
index 9335474be6fbb..8af44a4384656 100644
--- a/include/swift/RemoteInspection/DescriptorFinder.h
+++ b/include/swift/RemoteInspection/DescriptorFinder.h
@@ -93,26 +93,6 @@ struct FieldDescriptorBase {
   getFieldRecords() = 0;
 };
 
-struct MultiPayloadEnumDescriptorBase {
-  virtual ~MultiPayloadEnumDescriptorBase(){};
-
-  virtual llvm::StringRef getMangledTypeName() = 0;
-
-  virtual uint32_t getContentsSizeInWords() const = 0;
-
-  virtual size_t getSizeInBytes() const = 0;
-
-  virtual uint32_t getFlags() const = 0;
-
-  virtual bool usesPayloadSpareBits() const = 0;
-
-  virtual uint32_t getPayloadSpareBitMaskByteOffset() const = 0;
-
-  virtual uint32_t getPayloadSpareBitMaskByteCount() const = 0;
-
-  virtual const uint8_t *getPayloadSpareBits() const = 0;
-
-};
 /// Interface for finding type descriptors. Implementors may provide descriptors
 /// that live inside or outside reflection metadata.
 struct DescriptorFinder {
@@ -124,9 +104,6 @@ struct DescriptorFinder {
 
   virtual std::unique_ptr<FieldDescriptorBase>
   getFieldDescriptor(const TypeRef *TR) = 0;
-
-  virtual std::unique_ptr<MultiPayloadEnumDescriptorBase>
-  getMultiPayloadEnumDescriptor(const TypeRef *TR) = 0;
 };
 
 } // namespace reflection
diff --git a/include/swift/RemoteInspection/Records.h b/include/swift/RemoteInspection/Records.h
index aa08ce3ef5b6d..1c0ceeb976257 100644
--- a/include/swift/RemoteInspection/Records.h
+++ b/include/swift/RemoteInspection/Records.h
@@ -378,111 +378,6 @@ class BuiltinTypeDescriptor {
   }
 };
 
-class MultiPayloadEnumDescriptor {
-public:
-  const RelativeDirectPointer<const char> TypeName;
-
-private:
-  // This descriptor contains a series of 32-bit words
-  uint32_t contents[];
-
-  // Properties are stored in `contents` at particular indexes:
-
-  // uint32_t SizeFlags;
-  // Upper 16 bits are the size of the contents (in 32-bit words):
-  //  (This allows us to expand this structure in the future;
-  //  new fields should have accessors that test whether the
-  //  size is large enough and return "non-existent" if the
-  //  descriptor isn't large enough to have that field.)
-  // Lower 16 bits are flag bits
-
-  int getSizeFlagsIndex() const { return 0; }
-
-  // uint32_t PayloadSpareBitMaskByteOffsetCount;
-  // Number of bytes in "payload spare bits", and
-  // offset of them within the payload area
-  // Only present if `usePayloadSpareBits()`
-
-  int getPayloadSpareBitMaskByteCountIndex() const {
-    return getSizeFlagsIndex() + 1;
-  }
-
-  // uint8_t  *PayloadSpareBits;
-  // Variably-sized bitmask field (padded to a multiple of 4 bytes)
-  // Only present if `usePayloadSpareBits()`
-
-  int getPayloadSpareBitsIndex() const {
-    int PayloadSpareBitMaskByteCountFieldSize = usesPayloadSpareBits() ? 1 : 0;
-    return getPayloadSpareBitMaskByteCountIndex() + PayloadSpareBitMaskByteCountFieldSize;
-  }
-
-  // uint32_t foo;
-  // TODO: Some future field
-  // int getFooIndex() const {
-  //   int PayloadSpareBitMaskFieldSize = (getPayloadSpareBitMaskByteCount() + 3) / 4;
-  //   return getPayloadSpareBitsIndex() + PayloadSpareBitMaskFieldSize;
-  // }
-
-  // uint32_t getFoo() const {
-  //   if (getFooIndex() < getContentsSizeInWords()) {
-  //       return contents[getFooIndex()];
-  //   } else {
-  //       return 0; // Field isn't present
-  //   }
-  // }
-
-public:
-  //
-  // Data derived from the above...
-  //
-
-  uint32_t getContentsSizeInWords() const {
-    return contents[getSizeFlagsIndex()] >> 16;
-  }
-
-  size_t getSizeInBytes() const {
-//    assert(getContentsSizeInWords() > 0 && "Malformed MPEnum reflection record");
-    size_t sizeInBytes = sizeof(TypeName) + getContentsSizeInWords() * 4;
-    return sizeInBytes;
-  }
-
-  uint32_t getFlags() const {
-    assert(getContentsSizeInWords() > 0 && "Malformed MPEnum reflection record");
-    return contents[getSizeFlagsIndex()] & 0xffff;
-  }
-
-  bool usesPayloadSpareBits() const {
-    return getFlags() & 1;
-  }
-
-  uint32_t getPayloadSpareBitMaskByteOffset() const {
-    if (usesPayloadSpareBits()) {
-      return contents[getPayloadSpareBitMaskByteCountIndex()] >> 16;
-    } else {
-      return 0;
-    }
-  }
-
-  uint32_t getPayloadSpareBitMaskByteCount() const {
-    if (usesPayloadSpareBits()) {
-      auto byteCount = contents[getPayloadSpareBitMaskByteCountIndex()] & 0xffff;
-      assert(getContentsSizeInWords() >= 2 + (byteCount + 3) / 4
-            && "Malformed MPEnum reflection record: mask bigger than record");
-      return byteCount;
-    } else {
-      return 0;
-    }
-  }
-
-  const uint8_t *getPayloadSpareBits() const {
-    if (usesPayloadSpareBits()) {
-      return reinterpret_cast<const uint8_t *>(&contents[getPayloadSpareBitsIndex()]);
-    } else {
-      return nullptr;
-    }
-  }
-};
-
 class CaptureTypeRecord {
 public:
   const RelativeDirectPointer<const char> MangledTypeName;
diff --git a/include/swift/RemoteInspection/ReflectionContext.h b/include/swift/RemoteInspection/ReflectionContext.h
index e2384813ab4d7..8b0f109483251 100644
--- a/include/swift/RemoteInspection/ReflectionContext.h
+++ b/include/swift/RemoteInspection/ReflectionContext.h
@@ -335,8 +335,6 @@ class ReflectionContext
         ObjectFileFormat.getSectionName(ReflectionSectionKind::reflstr));
     auto ConformMdSec = findMachOSectionByName(
         ObjectFileFormat.getSectionName(ReflectionSectionKind::conform));
-    auto MPEnumMdSec = findMachOSectionByName(
-        ObjectFileFormat.getSectionName(ReflectionSectionKind::mpenum));
 
     if (FieldMdSec.first == nullptr &&
         AssocTySec.first == nullptr &&
@@ -344,8 +342,7 @@ class ReflectionContext
         CaptureSec.first == nullptr &&
         TypeRefMdSec.first == nullptr &&
         ReflStrMdSec.first == nullptr &&
-        ConformMdSec.first == nullptr &&
-        MPEnumMdSec.first == nullptr)
+        ConformMdSec.first == nullptr)
       return {};
 
     ReflectionInfo info = {{FieldMdSec.first, FieldMdSec.second},
@@ -355,7 +352,6 @@ class ReflectionContext
                            {TypeRefMdSec.first, TypeRefMdSec.second},
                            {ReflStrMdSec.first, ReflStrMdSec.second},
                            {ConformMdSec.first, ConformMdSec.second},
-                           {MPEnumMdSec.first, MPEnumMdSec.second},
                            PotentialModuleNames};
 
     auto InfoID = this->addReflectionInfo(info);
@@ -470,8 +466,6 @@ class ReflectionContext
         ObjectFileFormat.getSectionName(ReflectionSectionKind::reflstr));
     auto ConformMdSec = findCOFFSectionByName(
         ObjectFileFormat.getSectionName(ReflectionSectionKind::conform));
-    auto MPEnumMdSec = findCOFFSectionByName(
-        ObjectFileFormat.getSectionName(ReflectionSectionKind::mpenum));
 
     if (FieldMdSec.first == nullptr &&
         AssocTySec.first == nullptr &&
@@ -479,8 +473,7 @@ class ReflectionContext
         CaptureSec.first == nullptr &&
         TypeRefMdSec.first == nullptr &&
         ReflStrMdSec.first == nullptr &&
-        ConformMdSec.first == nullptr &&
-        MPEnumMdSec.first == nullptr)
+        ConformMdSec.first == nullptr)
       return {};
 
     ReflectionInfo Info = {{FieldMdSec.first, FieldMdSec.second},
@@ -490,7 +483,6 @@ class ReflectionContext
                            {TypeRefMdSec.first, TypeRefMdSec.second},
                            {ReflStrMdSec.first, ReflStrMdSec.second},
                            {ConformMdSec.first, ConformMdSec.second},
-                           {MPEnumMdSec.first, MPEnumMdSec.second},
                            PotentialModuleNames};
     return this->addReflectionInfo(Info);
   }
@@ -687,9 +679,6 @@ class ReflectionContext
         ObjectFileFormat.getSectionName(ReflectionSectionKind::reflstr), true);
     auto ConformMdSec = findELFSectionByName(
         ObjectFileFormat.getSectionName(ReflectionSectionKind::conform), true);
-    auto MPEnumMdSec = findELFSectionByName(
-        ObjectFileFormat.getSectionName(ReflectionSectionKind::mpenum), true);
-
     if (Error)
       return {};
 
@@ -699,7 +688,7 @@ class ReflectionContext
     // ELF executable.
     if (FieldMdSec.first || AssocTySec.first || BuiltinTySec.first ||
         CaptureSec.first || TypeRefMdSec.first || ReflStrMdSec.first ||
-        ConformMdSec.first || MPEnumMdSec.first) {
+        ConformMdSec.first) {
       ReflectionInfo info = {{FieldMdSec.first, FieldMdSec.second},
                              {AssocTySec.first, AssocTySec.second},
                              {BuiltinTySec.first, BuiltinTySec.second},
@@ -707,7 +696,6 @@ class ReflectionContext
                              {TypeRefMdSec.first, TypeRefMdSec.second},
                              {ReflStrMdSec.first, ReflStrMdSec.second},
                              {ConformMdSec.first, ConformMdSec.second},
-                             {MPEnumMdSec.first, MPEnumMdSec.second},
                              PotentialModuleNames};
       result = this->addReflectionInfo(info);
     }
@@ -730,15 +718,13 @@ class ReflectionContext
         ObjectFileFormat.getSectionName(ReflectionSectionKind::reflstr), false);
     ConformMdSec = findELFSectionByName(
         ObjectFileFormat.getSectionName(ReflectionSectionKind::conform), false);
-    MPEnumMdSec = findELFSectionByName(
-        ObjectFileFormat.getSectionName(ReflectionSectionKind::mpenum), false);
 
     if (Error)
       return {};
 
     if (FieldMdSec.first || AssocTySec.first || BuiltinTySec.first ||
         CaptureSec.first || TypeRefMdSec.first || ReflStrMdSec.first ||
-        ConformMdSec.first || MPEnumMdSec.first) {
+        ConformMdSec.first) {
       ReflectionInfo info = {{FieldMdSec.first, FieldMdSec.second},
                              {AssocTySec.first, AssocTySec.second},
                              {BuiltinTySec.first, BuiltinTySec.second},
@@ -746,7 +732,6 @@ class ReflectionContext
                              {TypeRefMdSec.first, TypeRefMdSec.second},
                              {ReflStrMdSec.first, ReflStrMdSec.second},
                              {ConformMdSec.first, ConformMdSec.second},
-                             {MPEnumMdSec.first, MPEnumMdSec.second},
                              PotentialModuleNames};
       auto rid = this->addReflectionInfo(info);
       if (!result)
@@ -861,7 +846,8 @@ class ReflectionContext
         ReflectionSectionKind::fieldmd, ReflectionSectionKind::assocty,
         ReflectionSectionKind::builtin, ReflectionSectionKind::capture,
         ReflectionSectionKind::typeref, ReflectionSectionKind::reflstr,
-        ReflectionSectionKind::conform, ReflectionSectionKind::mpenum};
+        ReflectionSectionKind::conform
+    };
 
     llvm::SmallVector<std::pair<RemoteRef<void>, uint64_t>, 6> Pairs;
     for (auto Section : Sections) {
@@ -887,7 +873,6 @@ class ReflectionContext
                            {Pairs[4].first, Pairs[4].second},
                            {Pairs[5].first, Pairs[5].second},
                            {Pairs[6].first, Pairs[6].second},
-                           {Pairs[7].first, Pairs[7].second},
                            PotentialModuleNames};
     return addReflectionInfo(Info);
   }
diff --git a/include/swift/RemoteInspection/RuntimeHeaders/llvm/BinaryFormat/Swift.def b/include/swift/RemoteInspection/RuntimeHeaders/llvm/BinaryFormat/Swift.def
index 05b60e40632cd..1ea0bc548b37e 100644
--- a/include/swift/RemoteInspection/RuntimeHeaders/llvm/BinaryFormat/Swift.def
+++ b/include/swift/RemoteInspection/RuntimeHeaders/llvm/BinaryFormat/Swift.def
@@ -30,4 +30,3 @@ HANDLE_SWIFT_SECTION(protocs, "__swift5_protos", "swift5_protocols",
                      ".sw5prt$B")
 HANDLE_SWIFT_SECTION(acfuncs, "__swift5_acfuncs", "swift5_accessible_functions",
                      ".sw5acfn$B")
-HANDLE_SWIFT_SECTION(mpenum, "__swift5_mpenum", "swift5_mpenum", ".sw5mpen$B")
diff --git a/include/swift/RemoteInspection/TypeRefBuilder.h b/include/swift/RemoteInspection/TypeRefBuilder.h
index 0a98b782d05c2..b787b8522ded5 100644
--- a/include/swift/RemoteInspection/TypeRefBuilder.h
+++ b/include/swift/RemoteInspection/TypeRefBuilder.h
@@ -231,21 +231,6 @@ class CaptureDescriptorIterator
 };
 using CaptureSection = ReflectionSection<CaptureDescriptorIterator>;
 
-class MultiPayloadEnumDescriptorIterator
-    : public ReflectionSectionIteratorBase<MultiPayloadEnumDescriptorIterator,
-                                           MultiPayloadEnumDescriptor> {
-public:
-  MultiPayloadEnumDescriptorIterator(RemoteRef<void> Cur, uint64_t Size)
-      : ReflectionSectionIteratorBase(Cur, Size, "MultiPayloadEnum") {}
-
-  static uint64_t
-  getCurrentRecordSize(RemoteRef<MultiPayloadEnumDescriptor> MPER) {
-    return MPER->getSizeInBytes();
-  }
-};
-using MultiPayloadEnumSection =
-    ReflectionSection<MultiPayloadEnumDescriptorIterator>;
-
 using GenericSection = ReflectionSection<const void *>;
 
 struct ReflectionInfo {
@@ -256,7 +241,6 @@ struct ReflectionInfo {
   GenericSection TypeReference;
   GenericSection ReflectionString;
   GenericSection Conformance;
-  MultiPayloadEnumSection MultiPayloadEnum;
   llvm::SmallVector<llvm::StringRef, 1> PotentialModuleNames;
 };
 
@@ -490,10 +474,6 @@ class TypeRefBuilder {
     /// Get the unsubstituted capture types for a closure context.
     ClosureContextInfo getClosureContextInfo(RemoteRef<CaptureDescriptor> CD);
 
-    /// Get the multipayload enum projection information for a given TR
-    std::unique_ptr<MultiPayloadEnumDescriptorBase>
-    getMultiPayloadEnumDescriptor(const TypeRef *TR) override;
-
     const TypeRef *lookupTypeWitness(const std::string &MangledTypeName,
                                      const std::string &Member,
                                      StringRef Protocol);
@@ -516,8 +496,6 @@ class TypeRefBuilder {
     /// Load unsubstituted field types for a nominal type.
     RemoteRef<FieldDescriptor> getFieldTypeInfo(const TypeRef *TR);
 
-    RemoteRef<MultiPayloadEnumDescriptor> getMultiPayloadEnumInfo(const TypeRef *TR);
-
     void populateFieldTypeInfoCacheWithReflectionAtIndex(size_t Index);
 
     std::optional<RemoteRef<FieldDescriptor>>
@@ -567,8 +545,7 @@ class TypeRefBuilder {
 
   public:
     ///
-    /// Dumping typerefs, field declarations, builtin types, captures,
-    /// multi-payload enums
+    /// Dumping typerefs, field declarations, builtin types, captures
     ///
     void dumpTypeRef(RemoteRef<char> MangledName, std::ostream &stream,
                      bool printTypeName = false);
@@ -577,7 +554,6 @@ class TypeRefBuilder {
     void dumpFieldSection(std::ostream &stream);
     void dumpBuiltinTypeSection(std::ostream &stream);
     void dumpCaptureSection(std::ostream &stream);
-    void dumpMultiPayloadEnumSection(std::ostream &stream);
 
     template <template <typename Runtime> class ObjCInteropKind,
               unsigned PointerSize>
@@ -817,10 +793,6 @@ class TypeRefBuilder {
       stream << "=============\n";
       dumpConformanceSection<ObjCInteropKind, PointerSize>(stream);
       stream << "\n";
-      stream << "MULTI-PAYLOAD ENUM DESCRIPTORS:\n";
-      stream << "===============================\n";
-      dumpMultiPayloadEnumSection(stream);
-      stream << "\n";
     }
   };
   friend struct ReflectionTypeDescriptorFinder;
@@ -1577,17 +1549,10 @@ class TypeRefBuilder {
     return RDF.getClosureContextInfo(CD);
   }
 
-  /// Get the multipayload enum projection information for a given TR
-  std::unique_ptr<MultiPayloadEnumDescriptorBase>
-  getMultiPayloadEnumDescriptor(const TypeRef *TR);
-
 private:
   /// Get the primitive type lowering for a builtin type.
   RemoteRef<BuiltinTypeDescriptor> getBuiltinTypeInfo(const TypeRef *TR);
 
-  RemoteRef<MultiPayloadEnumDescriptor>
-  getMultiPayloadEnumInfo(const TypeRef *TR);
-
   std::optional<uint64_t> multiPayloadEnumPointerMask;
 
 public:
@@ -1617,6 +1582,7 @@ class TypeRefBuilder {
     }
     return multiPayloadEnumPointerMask.value();
   }
+
   FieldTypeCollectionResult
   collectFieldTypes(std::optional<std::string> forMangledTypeName) {
     return RDF.collectFieldTypes(forMangledTypeName);
diff --git a/lib/IRGen/GenReflection.cpp b/lib/IRGen/GenReflection.cpp
index e3020f98e0b32..27d366c941ebb 100644
--- a/lib/IRGen/GenReflection.cpp
+++ b/lib/IRGen/GenReflection.cpp
@@ -37,7 +37,6 @@
 #include "GenMeta.h"
 #include "GenProto.h"
 #include "GenType.h"
-#include "GenValueWitness.h"
 #include "IRGenDebugInfo.h"
 #include "IRGenFunction.h"
 #include "IRGenMangler.h"
@@ -1131,74 +1130,6 @@ void IRGenModule::emitBuiltinTypeMetadataRecord(CanType builtinType) {
   builder.emit();
 }
 
-class MultiPayloadEnumDescriptorBuilder : public ReflectionMetadataBuilder {
-  CanType type;
-  CanType typeInContext;
-  const FixedTypeInfo *ti;
-
-public:
-  MultiPayloadEnumDescriptorBuilder(IRGenModule &IGM,
-                                    const NominalTypeDecl *nominalDecl)
-    : ReflectionMetadataBuilder(IGM) {
-    type = nominalDecl->getDeclaredType()->getCanonicalType();
-    typeInContext = nominalDecl->getDeclaredTypeInContext()->getCanonicalType();
-    ti = &cast<FixedTypeInfo>(IGM.getTypeInfoForUnlowered(typeInContext));
-  }
-
-  void layout() override {
-    auto &strategy = getEnumImplStrategy(IGM, typeInContext);
-    bool isMPE = strategy.getElementsWithPayload().size() > 1;
-    assert(isMPE && "Cannot emit Multi-Payload Enum data for an enum that "
-                    "doesn't have multiple payloads");
-
-    const TypeInfo &TI = strategy.getTypeInfo();
-    auto fixedTI = dyn_cast<FixedTypeInfo>(&TI);
-    assert(fixedTI != nullptr &&
-           "MPE reflection records can only be emitted for fixed-layout enums");
-
-    auto spareBitsMaskInfo = strategy.calculateSpareBitsMask();
-
-    // Never write an MPE descriptor bigger than 16k
-    // The runtime will fall back on its own internal
-    // spare bits calculation for this (very rare) case.
-    if (!spareBitsMaskInfo)
-      return;
-
-    auto bits = spareBitsMaskInfo->bits;
-
-    addTypeRef(type, CanGenericSignature());
-
-    bool usesPayloadSpareBits = spareBitsMaskInfo->bytesInMask > 0;
-
-    // MPE record contents are a multiple of 32-bits
-    uint32_t contentsSizeInWords = 1; /* Size + flags is mandatory */
-    if (usesPayloadSpareBits) {
-      contentsSizeInWords += 1 /* SpareBits byte count */
-                             + spareBitsMaskInfo->wordsInMask();
-    }
-
-    uint32_t flags = usesPayloadSpareBits ? 1 : 0;
-
-    B.addInt32((contentsSizeInWords << 16) | flags);
-
-    if (usesPayloadSpareBits) {
-      B.addInt32((spareBitsMaskInfo->byteOffset << 16) |
-                 spareBitsMaskInfo->bytesInMask);
-      // TODO: Endianness??
-      for (unsigned i = 0; i < spareBitsMaskInfo->wordsInMask(); ++i) {
-        uint32_t nextWord = bits.extractBitsAsZExtValue(32, 0);
-        B.addInt32(nextWord);
-        bits.lshrInPlace(32);
-      }
-    }
-  }
-
-  llvm::GlobalVariable *emit() {
-    auto section = IGM.getMultiPayloadEnumDescriptorSectionName();
-    return ReflectionMetadataBuilder::emit(std::nullopt, section);
-  }
-};
-
 /// Builds a constant LLVM struct describing the layout of a fixed-size
 /// SIL @box. These look like closure contexts, but without any necessary
 /// bindings or metadata sources, and only a single captured value.
@@ -1563,12 +1494,6 @@ const char *IRGenModule::getReflectionTypeRefSectionName() {
   return ReflectionTypeRefSection.c_str();
 }
 
-const char *IRGenModule::getMultiPayloadEnumDescriptorSectionName() {
-  if (MultiPayloadEnumDescriptorSection.empty())
-    MultiPayloadEnumDescriptorSection = getReflectionSectionName(*this, "mpenum", "mpen");
-  return MultiPayloadEnumDescriptorSection.c_str();
-}
-
 llvm::Constant *IRGenModule::getAddrOfFieldName(StringRef Name) {
   auto &entry = FieldNames[Name];
   if (entry.second)
@@ -1694,7 +1619,6 @@ void IRGenModule::emitFieldDescriptor(const NominalTypeDecl *D) {
   auto T = D->getDeclaredTypeInContext()->getCanonicalType();
 
   bool needsOpaqueDescriptor = false;
-  bool needsMPEDescriptor = false;
   bool needsFieldDescriptor = true;
 
   if (auto *ED = dyn_cast<EnumDecl>(D)) {
@@ -1708,13 +1632,11 @@ void IRGenModule::emitFieldDescriptor(const NominalTypeDecl *D) {
     }
 
     // If this is a fixed-size multi-payload enum, we have to emit a descriptor
-    // with the size and alignment of the type and another with the spare bit
-    // mask data, because the reflection library cannot consistently derive this
-    // information at runtime.
+    // with the size and alignment of the type, because the reflection library
+    // cannot derive this information at runtime.
     if (strategy.getElementsWithPayload().size() > 1 &&
         !strategy.needsPayloadSizeInMetadata()) {
       needsOpaqueDescriptor = true;
-      needsMPEDescriptor = true;
     }
   }
 
@@ -1761,11 +1683,6 @@ void IRGenModule::emitFieldDescriptor(const NominalTypeDecl *D) {
     builder.emit();
   }
 
-  if (needsMPEDescriptor) {
-    MultiPayloadEnumDescriptorBuilder builder(*this, D);
-    builder.emit();
-  }
-
   if (needsFieldDescriptor) {
     FieldTypeMetadataBuilder builder(*this, D);
     builder.emit();
diff --git a/lib/IRGen/IRGenModule.h b/lib/IRGen/IRGenModule.h
index 9b1e8bf8386d8..4bfaaf5f294e5 100644
--- a/lib/IRGen/IRGenModule.h
+++ b/lib/IRGen/IRGenModule.h
@@ -1396,7 +1396,6 @@ class IRGenModule {
   std::string CaptureDescriptorSection;
   std::string ReflectionStringsSection;
   std::string ReflectionTypeRefSection;
-  std::string MultiPayloadEnumDescriptorSection;
 
   /// Builtin types referenced by types in this module when emitting
   /// reflection metadata.
@@ -1476,7 +1475,6 @@ class IRGenModule {
   const char *getCaptureDescriptorMetadataSectionName();
   const char *getReflectionStringsSectionName();
   const char *getReflectionTypeRefSectionName();
-  const char *getMultiPayloadEnumDescriptorSectionName();
 
   /// Returns the special builtin types that should be emitted in the stdlib
   /// module.
diff --git a/stdlib/public/RemoteInspection/TypeLowering.cpp b/stdlib/public/RemoteInspection/TypeLowering.cpp
index 2bdb4c85b85b2..a74889afb2450 100644
--- a/stdlib/public/RemoteInspection/TypeLowering.cpp
+++ b/stdlib/public/RemoteInspection/TypeLowering.cpp
@@ -29,8 +29,6 @@
 #include "swift/RemoteInspection/TypeRefBuilder.h"
 #include "swift/Basic/Unreachable.h"
 #include <iostream>
-#include <sstream>
-#include <limits>
 
 #ifdef DEBUG_TYPE_LOWERING
   #define DEBUG_LOG(expr) expr;
diff --git a/stdlib/public/RemoteInspection/TypeRefBuilder.cpp b/stdlib/public/RemoteInspection/TypeRefBuilder.cpp
index 9e2a02871f541..7eef905e149c9 100644
--- a/stdlib/public/RemoteInspection/TypeRefBuilder.cpp
+++ b/stdlib/public/RemoteInspection/TypeRefBuilder.cpp
@@ -574,113 +574,6 @@ TypeRefBuilder::getBuiltinTypeDescriptor(const TypeRef *TR) {
   return nullptr;
 }
 
-namespace {
-/// A builtin type descriptor implementation that wraps a reflection builtin
-/// type descriptor.
-class MultiPayloadEnumDescriptorImpl : public MultiPayloadEnumDescriptorBase {
-  RemoteRef<MultiPayloadEnumDescriptor> MPED;
-  TypeRefBuilder &Builder;
-
-public:
-  MultiPayloadEnumDescriptorImpl(RemoteRef<MultiPayloadEnumDescriptor> MPED,
-                                 TypeRefBuilder &Builder)
-      : MultiPayloadEnumDescriptorBase(), MPED(MPED), Builder(Builder) {}
-
-  ~MultiPayloadEnumDescriptorImpl() override {}
-
-  StringRef getMangledTypeName() override {
-    return Builder.getTypeRefString(Builder.readTypeRef(MPED, MPED->TypeName));
-  };
-
-  uint32_t getContentsSizeInWords() const override {
-    return MPED->getContentsSizeInWords();
-  }
-
-  size_t getSizeInBytes() const override { return MPED->getSizeInBytes(); }
-
-  uint32_t getFlags() const override { return MPED->getFlags(); }
-
-  bool usesPayloadSpareBits() const override {
-    return MPED->usesPayloadSpareBits();
-  }
-
-  uint32_t getPayloadSpareBitMaskByteOffset() const override {
-    return MPED->getPayloadSpareBitMaskByteOffset();
-  }
-
-  uint32_t getPayloadSpareBitMaskByteCount() const override {
-    return MPED->getPayloadSpareBitMaskByteCount();
-  }
-
-  const uint8_t *getPayloadSpareBits() const override {
-    return MPED->getPayloadSpareBits();
-  }
-};
-} // namespace
-  
-RemoteRef<MultiPayloadEnumDescriptor>
-TypeRefBuilder::ReflectionTypeDescriptorFinder::getMultiPayloadEnumInfo(
-    const TypeRef *TR) {
-  std::string MangledName;
-  if (auto B = dyn_cast<BuiltinTypeRef>(TR))
-    MangledName = B->getMangledName();
-  else if (auto N = dyn_cast<NominalTypeRef>(TR))
-    MangledName = N->getMangledName();
-  else if (auto B = dyn_cast<BoundGenericTypeRef>(TR))
-    MangledName = B->getMangledName();
-  else
-    return nullptr;
-
-  for (auto Info : ReflectionInfos) {
-    for (auto MultiPayloadEnumDescriptor : Info.MultiPayloadEnum) {
-
-      // Assert that descriptor size is sound...
-      assert(MultiPayloadEnumDescriptor->getContentsSizeInWords() >= 1);
-      // We're limited to 64k of spare bits mask...
-      assert(MultiPayloadEnumDescriptor->getContentsSizeInWords() < 16384);
-      assert(MultiPayloadEnumDescriptor->getSizeInBytes() ==
-             4 + MultiPayloadEnumDescriptor->getContentsSizeInWords() * 4);
-      // Must have a non-empty spare bits mask iff spare bits are used...
-      assert(
-          MultiPayloadEnumDescriptor->usesPayloadSpareBits() ==
-          (MultiPayloadEnumDescriptor->getPayloadSpareBitMaskByteCount() != 0));
-      // BitMask must fit within the advertised size...
-      if (MultiPayloadEnumDescriptor->usesPayloadSpareBits()) {
-        assert(
-            MultiPayloadEnumDescriptor->getContentsSizeInWords() >=
-            2 + (MultiPayloadEnumDescriptor->getPayloadSpareBitMaskByteCount() +
-                 3) /
-                    4);
-      }
-
-      auto CandidateMangledName = readTypeRef(
-          MultiPayloadEnumDescriptor, MultiPayloadEnumDescriptor->TypeName);
-      if (!reflectionNameMatches(CandidateMangledName, MangledName))
-        continue;
-      return MultiPayloadEnumDescriptor;
-    }
-  }
-
-  return nullptr;
-}
-
-std::unique_ptr<MultiPayloadEnumDescriptorBase>
-TypeRefBuilder::ReflectionTypeDescriptorFinder::getMultiPayloadEnumDescriptor(
-    const TypeRef *TR) {
-  if (auto BTI = getMultiPayloadEnumInfo(TR))
-    return std::make_unique<MultiPayloadEnumDescriptorImpl>(BTI, Builder);
-  return nullptr;
-}
-
-std::unique_ptr<MultiPayloadEnumDescriptorBase>
-TypeRefBuilder::getMultiPayloadEnumDescriptor(const TypeRef *TR) {
-  for (auto *DF : getDescriptorFinders())
-    if (auto descriptor = DF->getMultiPayloadEnumDescriptor(TR))
-      return descriptor;
-
-  return nullptr;
-}
-
 RemoteRef<CaptureDescriptor>
 TypeRefBuilder::ReflectionTypeDescriptorFinder::getCaptureDescriptor(
     uint64_t RemoteAddress) {
@@ -895,39 +788,4 @@ void TypeRefBuilder::ReflectionTypeDescriptorFinder::dumpCaptureSection(
   }
 }
 
-void TypeRefBuilder::ReflectionTypeDescriptorFinder::
-    dumpMultiPayloadEnumSection(std::ostream &stream) {
-  for (const auto &sections : ReflectionInfos) {
-    for (const auto descriptor : sections.MultiPayloadEnum) {
-      TypeRefBuilder::ScopedNodeFactoryCheckpoint checkpoint(&Builder);
-      auto typeNode = Builder.demangleTypeRef(
-          readTypeRef(descriptor, descriptor->TypeName));
-      auto typeName = nodeToString(typeNode);
-
-      stream << "\n- " << typeName << ":\n";
-      stream << "  Descriptor Size: " << descriptor->getSizeInBytes() << "\n";
-      stream << "  Flags: " << std::hex << descriptor->getFlags() << std::dec;
-      if (descriptor->usesPayloadSpareBits()) {
-        stream << " usesPayloadSpareBits";
-      }
-      stream << "\n";
-      auto maskBytes = descriptor->getPayloadSpareBitMaskByteCount();
-      auto maskOffset = descriptor->getPayloadSpareBitMaskByteOffset();
-      if (maskBytes > 0) {
-        if (maskOffset > 0) {
-          stream << "  Spare bit mask: (offset " << maskOffset << " bytes) 0x";
-        } else {
-          stream << "  Spare bit mask: 0x";
-        }
-        const uint8_t *p = descriptor->getPayloadSpareBits();
-        for (unsigned i = 0; i < maskBytes; i++) {
-          stream << std::hex << std::setw(2) << std::setfill('0') << (int)p[i];
-        }
-        stream << std::dec << "\n";
-      }
-      stream << "\n";
-    }
-  }
-}
-
 #endif
diff --git a/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp b/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp
index 23d5ced5d0b22..4957abedfd378 100644
--- a/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp
+++ b/stdlib/public/SwiftRemoteMirror/SwiftRemoteMirror.cpp
@@ -271,7 +271,6 @@ swift_reflection_addReflectionInfo(SwiftReflectionContextRef ContextRef,
         sectionFromInfo<const void *>(Info, Info.type_references),
         sectionFromInfo<const void *>(Info, Info.reflection_strings),
         ReflectionSection<const void *>(nullptr, 0),
-        ReflectionSection<MultiPayloadEnumDescriptorIterator>(0, 0),
         {}};
 
     Context->addReflectionInfo(ContextInfo);
@@ -295,7 +294,6 @@ void swift_reflection_addReflectionMappingInfo(
         reflectionSectionFromLocalAndRemote<const void *>(
             Info.reflection_strings),
         ReflectionSection<const void *>(nullptr, 0),
-        MultiPayloadEnumSection(0, 0),
         {}};
 
     Context->addReflectionInfo(ContextInfo);
diff --git a/stdlib/public/SwiftShims/swift/shims/MetadataSections.h b/stdlib/public/SwiftShims/swift/shims/MetadataSections.h
index 1e419e4b8f763..e7baba432d4bb 100644
--- a/stdlib/public/SwiftShims/swift/shims/MetadataSections.h
+++ b/stdlib/public/SwiftShims/swift/shims/MetadataSections.h
@@ -95,7 +95,6 @@ struct MetadataSections {
   MetadataSectionRange swift5_replac2;
   MetadataSectionRange swift5_builtin;
   MetadataSectionRange swift5_capture;
-  MetadataSectionRange swift5_mpenum;
   MetadataSectionRange swift5_accessible_functions;
   MetadataSectionRange swift5_runtime_attributes;
   MetadataSectionRange swift5_tests;
diff --git a/stdlib/public/runtime/SwiftRT-COFF.cpp b/stdlib/public/runtime/SwiftRT-COFF.cpp
index 2e64204d0ab1a..f581f53dca76d 100644
--- a/stdlib/public/runtime/SwiftRT-COFF.cpp
+++ b/stdlib/public/runtime/SwiftRT-COFF.cpp
@@ -52,7 +52,6 @@ DECLARE_SWIFT_SECTION(sw5repl)
 DECLARE_SWIFT_SECTION(sw5reps)
 DECLARE_SWIFT_SECTION(sw5bltn)
 DECLARE_SWIFT_SECTION(sw5cptr)
-DECLARE_SWIFT_SECTION(sw5mpen)
 DECLARE_SWIFT_SECTION(sw5acfn)
 DECLARE_SWIFT_SECTION(sw5ratt)
 DECLARE_SWIFT_SECTION(sw5test)
@@ -86,7 +85,6 @@ static void swift_image_constructor() {
       SWIFT_SECTION_RANGE(sw5reps),
       SWIFT_SECTION_RANGE(sw5bltn),
       SWIFT_SECTION_RANGE(sw5cptr),
-      SWIFT_SECTION_RANGE(sw5mpen),
       SWIFT_SECTION_RANGE(sw5acfn),
       SWIFT_SECTION_RANGE(sw5ratt),
       SWIFT_SECTION_RANGE(sw5test),
diff --git a/stdlib/public/runtime/SwiftRT-ELF-WASM.cpp b/stdlib/public/runtime/SwiftRT-ELF-WASM.cpp
index 032a97cd442e4..53006d036b5ac 100644
--- a/stdlib/public/runtime/SwiftRT-ELF-WASM.cpp
+++ b/stdlib/public/runtime/SwiftRT-ELF-WASM.cpp
@@ -68,7 +68,6 @@ DECLARE_SWIFT_REFLECTION_SECTION(swift5_assocty)
 DECLARE_SWIFT_REFLECTION_SECTION(swift5_capture)
 DECLARE_SWIFT_REFLECTION_SECTION(swift5_reflstr)
 DECLARE_SWIFT_REFLECTION_SECTION(swift5_typeref)
-DECLARE_SWIFT_REFLECTION_SECTION(swift5_mpenum)
 
 DECLARE_SWIFT_SECTION(swift5_replace)
 DECLARE_SWIFT_SECTION(swift5_replac2)
@@ -109,7 +108,6 @@ static void swift_image_constructor() {
       SWIFT_SECTION_RANGE(swift5_replac2),
       SWIFT_SECTION_RANGE(swift5_builtin),
       SWIFT_SECTION_RANGE(swift5_capture),
-      SWIFT_SECTION_RANGE(swift5_mpenum),
       SWIFT_SECTION_RANGE(swift5_accessible_functions),
       SWIFT_SECTION_RANGE(swift5_runtime_attributes),
       SWIFT_SECTION_RANGE(swift5_tests),