diff --git a/clang/include/clang/Serialization/ASTReader.h b/clang/include/clang/Serialization/ASTReader.h
--- a/clang/include/clang/Serialization/ASTReader.h
+++ b/clang/include/clang/Serialization/ASTReader.h
@@ -32,6 +32,7 @@
 #include "clang/Serialization/ModuleFile.h"
 #include "clang/Serialization/ModuleFileExtension.h"
 #include "clang/Serialization/ModuleManager.h"
+#include "clang/Serialization/SourceLocationEncoding.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
@@ -396,6 +397,8 @@
   using ModuleReverseIterator = ModuleManager::ModuleReverseIterator;
 
 private:
+  using LocSeq = SourceLocationSequence;
+
   /// The receiver of some callbacks invoked by ASTReader.
   std::unique_ptr<ASTReaderListener> Listener;
 
@@ -2155,16 +2158,16 @@
 
   /// Read a source location from raw form and return it in its
   /// originating module file's source location space.
-  SourceLocation
-  ReadUntranslatedSourceLocation(SourceLocation::UIntTy Raw) const {
-    return SourceLocation::getFromRawEncoding((Raw >> 1) |
-                                              (Raw << (8 * sizeof(Raw) - 1)));
+  SourceLocation ReadUntranslatedSourceLocation(SourceLocation::UIntTy Raw,
+                                                LocSeq *Seq = nullptr) const {
+    return SourceLocationEncoding::decode(Raw, Seq);
   }
 
   /// Read a source location from raw form.
   SourceLocation ReadSourceLocation(ModuleFile &ModuleFile,
-                                    SourceLocation::UIntTy Raw) const {
-    SourceLocation Loc = ReadUntranslatedSourceLocation(Raw);
+                                    SourceLocation::UIntTy Raw,
+                                    LocSeq *Seq = nullptr) const {
+    SourceLocation Loc = ReadUntranslatedSourceLocation(Raw, Seq);
     return TranslateSourceLocation(ModuleFile, Loc);
   }
 
@@ -2184,14 +2187,14 @@
 
   /// Read a source location.
   SourceLocation ReadSourceLocation(ModuleFile &ModuleFile,
-                                    const RecordDataImpl &Record,
-                                    unsigned &Idx) {
-    return ReadSourceLocation(ModuleFile, Record[Idx++]);
+                                    const RecordDataImpl &Record, unsigned &Idx,
+                                    LocSeq *Seq = nullptr) {
+    return ReadSourceLocation(ModuleFile, Record[Idx++], Seq);
   }
 
   /// Read a source range.
-  SourceRange ReadSourceRange(ModuleFile &F,
-                              const RecordData &Record, unsigned &Idx);
+  SourceRange ReadSourceRange(ModuleFile &F, const RecordData &Record,
+                              unsigned &Idx, LocSeq *Seq = nullptr);
 
   // Read a string
   static std::string ReadString(const RecordData &Record, unsigned &Idx);
diff --git a/clang/include/clang/Serialization/ASTRecordReader.h b/clang/include/clang/Serialization/ASTRecordReader.h
--- a/clang/include/clang/Serialization/ASTRecordReader.h
+++ b/clang/include/clang/Serialization/ASTRecordReader.h
@@ -18,6 +18,7 @@
 #include "clang/AST/AbstractBasicReader.h"
 #include "clang/Lex/Token.h"
 #include "clang/Serialization/ASTReader.h"
+#include "clang/Serialization/SourceLocationEncoding.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/APSInt.h"
@@ -30,6 +31,7 @@
 class ASTRecordReader
     : public serialization::DataStreamBasicReader<ASTRecordReader> {
   using ModuleFile = serialization::ModuleFile;
+  using LocSeq = SourceLocationSequence;
 
   ASTReader *Reader;
   ModuleFile *F;
@@ -160,8 +162,7 @@
   TypeSourceInfo *readTypeSourceInfo();
 
   /// Reads the location information for a type.
-  void readTypeLoc(TypeLoc TL);
-
+  void readTypeLoc(TypeLoc TL, LocSeq *Seq = nullptr);
 
   /// Map a local type ID within a given AST file to a global type ID.
   serialization::TypeID getGlobalTypeID(unsigned LocalID) const {
@@ -271,13 +272,13 @@
   void readOMPChildren(OMPChildren *Data);
 
   /// Read a source location, advancing Idx.
-  SourceLocation readSourceLocation() {
-    return Reader->ReadSourceLocation(*F, Record, Idx);
+  SourceLocation readSourceLocation(LocSeq *Seq = nullptr) {
+    return Reader->ReadSourceLocation(*F, Record, Idx, Seq);
   }
 
   /// Read a source range, advancing Idx.
-  SourceRange readSourceRange() {
-    return Reader->ReadSourceRange(*F, Record, Idx);
+  SourceRange readSourceRange(LocSeq *Seq = nullptr) {
+    return Reader->ReadSourceRange(*F, Record, Idx, Seq);
   }
 
   /// Read an arbitrary constant value, advancing Idx.
diff --git a/clang/include/clang/Serialization/ASTRecordWriter.h b/clang/include/clang/Serialization/ASTRecordWriter.h
--- a/clang/include/clang/Serialization/ASTRecordWriter.h
+++ b/clang/include/clang/Serialization/ASTRecordWriter.h
@@ -17,6 +17,7 @@
 #include "clang/AST/AbstractBasicWriter.h"
 #include "clang/AST/OpenMPClause.h"
 #include "clang/Serialization/ASTWriter.h"
+#include "clang/Serialization/SourceLocationEncoding.h"
 
 namespace clang {
 
@@ -25,6 +26,8 @@
 /// An object for streaming information to a record.
 class ASTRecordWriter
     : public serialization::DataStreamBasicWriter<ASTRecordWriter> {
+  using LocSeq = SourceLocationSequence;
+
   ASTWriter *Writer;
   ASTWriter::RecordDataImpl *Record;
 
@@ -131,16 +134,16 @@
   void AddFunctionDefinition(const FunctionDecl *FD);
 
   /// Emit a source location.
-  void AddSourceLocation(SourceLocation Loc) {
-    return Writer->AddSourceLocation(Loc, *Record);
+  void AddSourceLocation(SourceLocation Loc, LocSeq *Seq = nullptr) {
+    return Writer->AddSourceLocation(Loc, *Record, Seq);
   }
   void writeSourceLocation(SourceLocation Loc) {
     AddSourceLocation(Loc);
   }
 
   /// Emit a source range.
-  void AddSourceRange(SourceRange Range) {
-    return Writer->AddSourceRange(Range, *Record);
+  void AddSourceRange(SourceRange Range, LocSeq *Seq = nullptr) {
+    return Writer->AddSourceRange(Range, *Record, Seq);
   }
 
   void writeBool(bool Value) {
@@ -206,7 +209,7 @@
   void AddTypeSourceInfo(TypeSourceInfo *TInfo);
 
   /// Emits source location information for a type. Does not emit the type.
-  void AddTypeLoc(TypeLoc TL);
+  void AddTypeLoc(TypeLoc TL, LocSeq *Seq = nullptr);
 
   /// Emits a template argument location info.
   void AddTemplateArgumentLocInfo(TemplateArgument::ArgKind Kind,
diff --git a/clang/include/clang/Serialization/ASTWriter.h b/clang/include/clang/Serialization/ASTWriter.h
--- a/clang/include/clang/Serialization/ASTWriter.h
+++ b/clang/include/clang/Serialization/ASTWriter.h
@@ -25,6 +25,7 @@
 #include "clang/Serialization/ASTBitCodes.h"
 #include "clang/Serialization/ASTDeserializationListener.h"
 #include "clang/Serialization/PCHContainerOperations.h"
+#include "clang/Serialization/SourceLocationEncoding.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
@@ -104,6 +105,8 @@
   using TypeIdxMap = llvm::DenseMap<QualType, serialization::TypeIdx,
                                     serialization::UnsafeQualTypeDenseMapInfo>;
 
+  using LocSeq = SourceLocationSequence;
+
   /// The bitstream writer used to emit this precompiled header.
   llvm::BitstreamWriter &Stream;
 
@@ -581,10 +584,12 @@
                         RecordDataImpl &Record);
 
   /// Emit a source location.
-  void AddSourceLocation(SourceLocation Loc, RecordDataImpl &Record);
+  void AddSourceLocation(SourceLocation Loc, RecordDataImpl &Record,
+                         LocSeq *Seq = nullptr);
 
   /// Emit a source range.
-  void AddSourceRange(SourceRange Range, RecordDataImpl &Record);
+  void AddSourceRange(SourceRange Range, RecordDataImpl &Record,
+                      LocSeq *Seq = nullptr);
 
   /// Emit a reference to an identifier.
   void AddIdentifierRef(const IdentifierInfo *II, RecordDataImpl &Record);
diff --git a/clang/include/clang/Serialization/SourceLocationEncoding.h b/clang/include/clang/Serialization/SourceLocationEncoding.h
new file mode 100644
--- /dev/null
+++ b/clang/include/clang/Serialization/SourceLocationEncoding.h
@@ -0,0 +1,162 @@
+//===--- SourceLocationEncoding.h - Small serialized locations --*- 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
+//
+//===----------------------------------------------------------------------===//
+//
+// Source locations are stored pervasively in the AST, making up a third of
+// the size of typical serialized files. Storing them efficiently is important.
+//
+// We use integers optimized by VBR-encoding, because:
+//  - when abbrevations cannot be used, VBR6 encoding is our only choice
+//  - in the worst case a SourceLocation can be ~any 32-bit number, but in
+//    practice they are highly predictable
+//
+// We encode the integer so that likely values encode as small numbers that
+// turn into few VBR chunks:
+//  - the invalid sentinel location is a very common value: it encodes as 0
+//  - the "macro or not" bit is stored at the bottom of the integer
+//    (rather than at the top, as in memory), so macro locations can have
+//    small representations.
+//  - related locations (e.g. of a left and right paren pair) are usually
+//    similar, so when encoding a sequence of locations we store only
+//    differences between successive elements.
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Basic/SourceLocation.h"
+
+#ifndef LLVM_CLANG_SERIALIZATION_SOURCELOCATIONENCODING_H
+#define LLVM_CLANG_SERIALIZATION_SOURCELOCATIONENCODING_H
+
+namespace clang {
+class SourceLocationSequence;
+
+/// Serialized encoding of SourceLocations without context.
+/// Optimized to have small unsigned values (=> small after VBR encoding).
+///
+// Macro locations have the top bit set, we rotate by one so it is the low bit.
+class SourceLocationEncoding {
+  using UIntTy = SourceLocation::UIntTy;
+  constexpr static unsigned UIntBits = CHAR_BIT * sizeof(UIntTy);
+
+  static UIntTy encodeRaw(UIntTy Raw) {
+    return (Raw << 1) | (Raw >> (UIntBits - 1));
+  }
+  static UIntTy decodeRaw(UIntTy Raw) {
+    return (Raw >> 1) | (Raw << (UIntBits - 1));
+  }
+  friend SourceLocationSequence;
+
+public:
+  static uint64_t encode(SourceLocation Loc,
+                         SourceLocationSequence * = nullptr);
+  static SourceLocation decode(uint64_t, SourceLocationSequence * = nullptr);
+};
+
+/// Serialized encoding of a sequence of SourceLocations.
+///
+/// Optimized to produce small values when locations with the sequence are
+/// similar. Each element can be delta-encoded against the last nonzero element.
+///
+/// Sequences should be started by creating a SourceLocationSequence::State,
+/// and then passed around as SourceLocationSequence*. Example:
+///
+///   // establishes a sequence
+///   void EmitTopLevelThing() {
+///     SourceLocationSequence::State Seq;
+///     EmitContainedThing(Seq);
+///     EmitRecursiveThing(Seq);
+///   }
+///
+///   // optionally part of a sequence
+///   void EmitContainedThing(SourceLocationSequence *Seq = nullptr) {
+///     Record.push_back(SourceLocationEncoding::encode(SomeLoc, Seq));
+///   }
+///
+///   // establishes a sequence if there isn't one already
+///   void EmitRecursiveThing(SourceLocationSequence *ParentSeq = nullptr) {
+///     SourceLocationSequence::State Seq(ParentSeq);
+///     Record.push_back(SourceLocationEncoding::encode(SomeLoc, Seq));
+///     EmitRecursiveThing(Seq);
+///   }
+///
+class SourceLocationSequence {
+  using UIntTy = SourceLocation::UIntTy;
+  using EncodedTy = uint64_t;
+  constexpr static auto UIntBits = SourceLocationEncoding::UIntBits;
+  static_assert(sizeof(EncodedTy) > sizeof(UIntTy), "Need one extra bit!");
+
+  // Prev stores the rotated last nonzero location.
+  UIntTy &Prev;
+
+  // Zig-zag encoding turns small signed integers into small unsigned integers.
+  // 0 => 0, -1 => 1, 1 => 2, -2 => 3, ...
+  static UIntTy zigZag(UIntTy V) {
+    UIntTy Sign = (V & (1 << (UIntBits - 1))) ? UIntTy(-1) : UIntTy(0);
+    return Sign ^ (V << 1);
+  }
+  static UIntTy zagZig(UIntTy V) { return (V >> 1) ^ -(V & 1); }
+
+  SourceLocationSequence(UIntTy &Prev) : Prev(Prev) {}
+
+  EncodedTy encodeRaw(UIntTy Raw) {
+    if (Raw == 0)
+      return 0;
+    UIntTy Rotated = SourceLocationEncoding::encodeRaw(Raw);
+    if (Prev == 0)
+      return Prev = Rotated;
+    UIntTy Delta = Rotated - Prev;
+    Prev = Rotated;
+    // Exactly one 33 bit value is possible! (1 << 32).
+    // This is because we have two representations of zero: trivial & relative.
+    return 1 + EncodedTy{zigZag(Delta)};
+  }
+  UIntTy decodeRaw(EncodedTy Encoded) {
+    if (Encoded == 0)
+      return 0;
+    if (Prev == 0)
+      return SourceLocationEncoding::decodeRaw(Prev = Encoded);
+    return SourceLocationEncoding::decodeRaw(Prev += zagZig(Encoded - 1));
+  }
+
+public:
+  SourceLocation decode(EncodedTy Encoded) {
+    return SourceLocation::getFromRawEncoding(decodeRaw(Encoded));
+  }
+  EncodedTy encode(SourceLocation Loc) {
+    return encodeRaw(Loc.getRawEncoding());
+  }
+
+  class State;
+};
+
+/// This object establishes a SourceLocationSequence.
+class SourceLocationSequence::State {
+  UIntTy Prev = 0;
+  SourceLocationSequence Seq;
+
+public:
+  // If Parent is provided and non-null, then this root becomes part of that
+  // enclosing sequence instead of establishing a new one.
+  State(SourceLocationSequence *Parent = nullptr)
+      : Seq(Parent ? Parent->Prev : Prev) {}
+
+  // Implicit conversion for uniform use of roots vs propagated sequences.
+  operator SourceLocationSequence *() { return &Seq; }
+};
+
+inline uint64_t SourceLocationEncoding::encode(SourceLocation Loc,
+                                               SourceLocationSequence *Seq) {
+  return Seq ? Seq->encode(Loc) : encodeRaw(Loc.getRawEncoding());
+}
+inline SourceLocation
+SourceLocationEncoding::decode(uint64_t Encoded, SourceLocationSequence *Seq) {
+  return Seq ? Seq->decode(Encoded)
+             : SourceLocation::getFromRawEncoding(decodeRaw(Encoded));
+}
+
+} // namespace clang
+#endif
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -6466,11 +6466,13 @@
 namespace clang {
 
 class TypeLocReader : public TypeLocVisitor<TypeLocReader> {
+  using LocSeq = SourceLocationSequence;
+
   ASTRecordReader &Reader;
+  LocSeq *Seq;
 
-  SourceLocation readSourceLocation() {
-    return Reader.readSourceLocation();
-  }
+  SourceLocation readSourceLocation() { return Reader.readSourceLocation(Seq); }
+  SourceRange readSourceRange() { return Reader.readSourceRange(Seq); }
 
   TypeSourceInfo *GetTypeSourceInfo() {
     return Reader.readTypeSourceInfo();
@@ -6485,7 +6487,8 @@
   }
 
 public:
-  TypeLocReader(ASTRecordReader &Reader) : Reader(Reader) {}
+  TypeLocReader(ASTRecordReader &Reader, LocSeq *Seq)
+      : Reader(Reader), Seq(Seq) {}
 
   // We want compile-time assurance that we've enumerated all of
   // these, so unfortunately we have to declare them first, then
@@ -6582,7 +6585,7 @@
     DependentAddressSpaceTypeLoc TL) {
 
     TL.setAttrNameLoc(readSourceLocation());
-    TL.setAttrOperandParensRange(Reader.readSourceRange());
+    TL.setAttrOperandParensRange(readSourceRange());
     TL.setAttrExprOperand(Reader.readExpr());
 }
 
@@ -6606,7 +6609,7 @@
 
 void TypeLocReader::VisitConstantMatrixTypeLoc(ConstantMatrixTypeLoc TL) {
   TL.setAttrNameLoc(readSourceLocation());
-  TL.setAttrOperandParensRange(Reader.readSourceRange());
+  TL.setAttrOperandParensRange(readSourceRange());
   TL.setAttrRowOperand(Reader.readExpr());
   TL.setAttrColumnOperand(Reader.readExpr());
 }
@@ -6614,7 +6617,7 @@
 void TypeLocReader::VisitDependentSizedMatrixTypeLoc(
     DependentSizedMatrixTypeLoc TL) {
   TL.setAttrNameLoc(readSourceLocation());
-  TL.setAttrOperandParensRange(Reader.readSourceRange());
+  TL.setAttrOperandParensRange(readSourceRange());
   TL.setAttrRowOperand(Reader.readExpr());
   TL.setAttrColumnOperand(Reader.readExpr());
 }
@@ -6623,7 +6626,7 @@
   TL.setLocalRangeBegin(readSourceLocation());
   TL.setLParenLoc(readSourceLocation());
   TL.setRParenLoc(readSourceLocation());
-  TL.setExceptionSpecRange(Reader.readSourceRange());
+  TL.setExceptionSpecRange(readSourceRange());
   TL.setLocalRangeEnd(readSourceLocation());
   for (unsigned i = 0, e = TL.getNumParams(); i != e; ++i) {
     TL.setParam(i, Reader.readDeclAs<ParmVarDecl>());
@@ -6826,9 +6829,9 @@
   TL.setNameLoc(readSourceLocation());
 }
 
-
-void ASTRecordReader::readTypeLoc(TypeLoc TL) {
-  TypeLocReader TLR(*this);
+void ASTRecordReader::readTypeLoc(TypeLoc TL, LocSeq *ParentSeq) {
+  LocSeq::State Seq(ParentSeq);
+  TypeLocReader TLR(*this, Seq);
   for (; !TL.isNull(); TL = TL.getNextTypeLoc())
     TLR.Visit(TL);
 }
@@ -8995,11 +8998,10 @@
   return Builder.getWithLocInContext(Context);
 }
 
-SourceRange
-ASTReader::ReadSourceRange(ModuleFile &F, const RecordData &Record,
-                           unsigned &Idx) {
-  SourceLocation beg = ReadSourceLocation(F, Record, Idx);
-  SourceLocation end = ReadSourceLocation(F, Record, Idx);
+SourceRange ASTReader::ReadSourceRange(ModuleFile &F, const RecordData &Record,
+                                       unsigned &Idx, LocSeq *Seq) {
+  SourceLocation beg = ReadSourceLocation(F, Record, Idx, Seq);
+  SourceLocation end = ReadSourceLocation(F, Record, Idx, Seq);
   return SourceRange(beg, end);
 }
 
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -245,10 +245,19 @@
 };
 
 class TypeLocWriter : public TypeLocVisitor<TypeLocWriter> {
+  using LocSeq = SourceLocationSequence;
+
   ASTRecordWriter &Record;
+  LocSeq *Seq;
+
+  void addSourceLocation(SourceLocation Loc) {
+    Record.AddSourceLocation(Loc, Seq);
+  }
+  void addSourceRange(SourceRange Range) { Record.AddSourceRange(Range, Seq); }
 
 public:
-  TypeLocWriter(ASTRecordWriter &Record) : Record(Record) {}
+  TypeLocWriter(ASTRecordWriter &Record, LocSeq *Seq)
+      : Record(Record), Seq(Seq) {}
 
 #define ABSTRACT_TYPELOC(CLASS, PARENT)
 #define TYPELOC(CLASS, PARENT) \
@@ -266,7 +275,7 @@
 }
 
 void TypeLocWriter::VisitBuiltinTypeLoc(BuiltinTypeLoc TL) {
-  Record.AddSourceLocation(TL.getBuiltinLoc());
+  addSourceLocation(TL.getBuiltinLoc());
   if (TL.needsExtraLocalData()) {
     Record.push_back(TL.getWrittenTypeSpec());
     Record.push_back(static_cast<uint64_t>(TL.getWrittenSignSpec()));
@@ -276,11 +285,11 @@
 }
 
 void TypeLocWriter::VisitComplexTypeLoc(ComplexTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitPointerTypeLoc(PointerTypeLoc TL) {
-  Record.AddSourceLocation(TL.getStarLoc());
+  addSourceLocation(TL.getStarLoc());
 }
 
 void TypeLocWriter::VisitDecayedTypeLoc(DecayedTypeLoc TL) {
@@ -292,25 +301,25 @@
 }
 
 void TypeLocWriter::VisitBlockPointerTypeLoc(BlockPointerTypeLoc TL) {
-  Record.AddSourceLocation(TL.getCaretLoc());
+  addSourceLocation(TL.getCaretLoc());
 }
 
 void TypeLocWriter::VisitLValueReferenceTypeLoc(LValueReferenceTypeLoc TL) {
-  Record.AddSourceLocation(TL.getAmpLoc());
+  addSourceLocation(TL.getAmpLoc());
 }
 
 void TypeLocWriter::VisitRValueReferenceTypeLoc(RValueReferenceTypeLoc TL) {
-  Record.AddSourceLocation(TL.getAmpAmpLoc());
+  addSourceLocation(TL.getAmpAmpLoc());
 }
 
 void TypeLocWriter::VisitMemberPointerTypeLoc(MemberPointerTypeLoc TL) {
-  Record.AddSourceLocation(TL.getStarLoc());
+  addSourceLocation(TL.getStarLoc());
   Record.AddTypeSourceInfo(TL.getClassTInfo());
 }
 
 void TypeLocWriter::VisitArrayTypeLoc(ArrayTypeLoc TL) {
-  Record.AddSourceLocation(TL.getLBracketLoc());
-  Record.AddSourceLocation(TL.getRBracketLoc());
+  addSourceLocation(TL.getLBracketLoc());
+  addSourceLocation(TL.getRBracketLoc());
   Record.push_back(TL.getSizeExpr() ? 1 : 0);
   if (TL.getSizeExpr())
     Record.AddStmt(TL.getSizeExpr());
@@ -335,56 +344,56 @@
 
 void TypeLocWriter::VisitDependentAddressSpaceTypeLoc(
     DependentAddressSpaceTypeLoc TL) {
-  Record.AddSourceLocation(TL.getAttrNameLoc());
+  addSourceLocation(TL.getAttrNameLoc());
   SourceRange range = TL.getAttrOperandParensRange();
-  Record.AddSourceLocation(range.getBegin());
-  Record.AddSourceLocation(range.getEnd());
+  addSourceLocation(range.getBegin());
+  addSourceLocation(range.getEnd());
   Record.AddStmt(TL.getAttrExprOperand());
 }
 
 void TypeLocWriter::VisitDependentSizedExtVectorTypeLoc(
                                         DependentSizedExtVectorTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitVectorTypeLoc(VectorTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitDependentVectorTypeLoc(
     DependentVectorTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitExtVectorTypeLoc(ExtVectorTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitConstantMatrixTypeLoc(ConstantMatrixTypeLoc TL) {
-  Record.AddSourceLocation(TL.getAttrNameLoc());
+  addSourceLocation(TL.getAttrNameLoc());
   SourceRange range = TL.getAttrOperandParensRange();
-  Record.AddSourceLocation(range.getBegin());
-  Record.AddSourceLocation(range.getEnd());
+  addSourceLocation(range.getBegin());
+  addSourceLocation(range.getEnd());
   Record.AddStmt(TL.getAttrRowOperand());
   Record.AddStmt(TL.getAttrColumnOperand());
 }
 
 void TypeLocWriter::VisitDependentSizedMatrixTypeLoc(
     DependentSizedMatrixTypeLoc TL) {
-  Record.AddSourceLocation(TL.getAttrNameLoc());
+  addSourceLocation(TL.getAttrNameLoc());
   SourceRange range = TL.getAttrOperandParensRange();
-  Record.AddSourceLocation(range.getBegin());
-  Record.AddSourceLocation(range.getEnd());
+  addSourceLocation(range.getBegin());
+  addSourceLocation(range.getEnd());
   Record.AddStmt(TL.getAttrRowOperand());
   Record.AddStmt(TL.getAttrColumnOperand());
 }
 
 void TypeLocWriter::VisitFunctionTypeLoc(FunctionTypeLoc TL) {
-  Record.AddSourceLocation(TL.getLocalRangeBegin());
-  Record.AddSourceLocation(TL.getLParenLoc());
-  Record.AddSourceLocation(TL.getRParenLoc());
-  Record.AddSourceRange(TL.getExceptionSpecRange());
-  Record.AddSourceLocation(TL.getLocalRangeEnd());
+  addSourceLocation(TL.getLocalRangeBegin());
+  addSourceLocation(TL.getLParenLoc());
+  addSourceLocation(TL.getRParenLoc());
+  addSourceRange(TL.getExceptionSpecRange());
+  addSourceLocation(TL.getLocalRangeEnd());
   for (unsigned i = 0, e = TL.getNumParams(); i != e; ++i)
     Record.AddDeclRef(TL.getParam(i));
 }
@@ -398,81 +407,81 @@
 }
 
 void TypeLocWriter::VisitUnresolvedUsingTypeLoc(UnresolvedUsingTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitUsingTypeLoc(UsingTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitTypedefTypeLoc(TypedefTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitObjCTypeParamTypeLoc(ObjCTypeParamTypeLoc TL) {
   if (TL.getNumProtocols()) {
-    Record.AddSourceLocation(TL.getProtocolLAngleLoc());
-    Record.AddSourceLocation(TL.getProtocolRAngleLoc());
+    addSourceLocation(TL.getProtocolLAngleLoc());
+    addSourceLocation(TL.getProtocolRAngleLoc());
   }
   for (unsigned i = 0, e = TL.getNumProtocols(); i != e; ++i)
-    Record.AddSourceLocation(TL.getProtocolLoc(i));
+    addSourceLocation(TL.getProtocolLoc(i));
 }
 
 void TypeLocWriter::VisitTypeOfExprTypeLoc(TypeOfExprTypeLoc TL) {
-  Record.AddSourceLocation(TL.getTypeofLoc());
-  Record.AddSourceLocation(TL.getLParenLoc());
-  Record.AddSourceLocation(TL.getRParenLoc());
+  addSourceLocation(TL.getTypeofLoc());
+  addSourceLocation(TL.getLParenLoc());
+  addSourceLocation(TL.getRParenLoc());
 }
 
 void TypeLocWriter::VisitTypeOfTypeLoc(TypeOfTypeLoc TL) {
-  Record.AddSourceLocation(TL.getTypeofLoc());
-  Record.AddSourceLocation(TL.getLParenLoc());
-  Record.AddSourceLocation(TL.getRParenLoc());
+  addSourceLocation(TL.getTypeofLoc());
+  addSourceLocation(TL.getLParenLoc());
+  addSourceLocation(TL.getRParenLoc());
   Record.AddTypeSourceInfo(TL.getUnderlyingTInfo());
 }
 
 void TypeLocWriter::VisitDecltypeTypeLoc(DecltypeTypeLoc TL) {
-  Record.AddSourceLocation(TL.getDecltypeLoc());
-  Record.AddSourceLocation(TL.getRParenLoc());
+  addSourceLocation(TL.getDecltypeLoc());
+  addSourceLocation(TL.getRParenLoc());
 }
 
 void TypeLocWriter::VisitUnaryTransformTypeLoc(UnaryTransformTypeLoc TL) {
-  Record.AddSourceLocation(TL.getKWLoc());
-  Record.AddSourceLocation(TL.getLParenLoc());
-  Record.AddSourceLocation(TL.getRParenLoc());
+  addSourceLocation(TL.getKWLoc());
+  addSourceLocation(TL.getLParenLoc());
+  addSourceLocation(TL.getRParenLoc());
   Record.AddTypeSourceInfo(TL.getUnderlyingTInfo());
 }
 
 void TypeLocWriter::VisitAutoTypeLoc(AutoTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
   Record.push_back(TL.isConstrained());
   if (TL.isConstrained()) {
     Record.AddNestedNameSpecifierLoc(TL.getNestedNameSpecifierLoc());
-    Record.AddSourceLocation(TL.getTemplateKWLoc());
-    Record.AddSourceLocation(TL.getConceptNameLoc());
+    addSourceLocation(TL.getTemplateKWLoc());
+    addSourceLocation(TL.getConceptNameLoc());
     Record.AddDeclRef(TL.getFoundDecl());
-    Record.AddSourceLocation(TL.getLAngleLoc());
-    Record.AddSourceLocation(TL.getRAngleLoc());
+    addSourceLocation(TL.getLAngleLoc());
+    addSourceLocation(TL.getRAngleLoc());
     for (unsigned I = 0; I < TL.getNumArgs(); ++I)
       Record.AddTemplateArgumentLocInfo(TL.getTypePtr()->getArg(I).getKind(),
                                         TL.getArgLocInfo(I));
   }
   Record.push_back(TL.isDecltypeAuto());
   if (TL.isDecltypeAuto())
-    Record.AddSourceLocation(TL.getRParenLoc());
+    addSourceLocation(TL.getRParenLoc());
 }
 
 void TypeLocWriter::VisitDeducedTemplateSpecializationTypeLoc(
     DeducedTemplateSpecializationTypeLoc TL) {
-  Record.AddSourceLocation(TL.getTemplateNameLoc());
+  addSourceLocation(TL.getTemplateNameLoc());
 }
 
 void TypeLocWriter::VisitRecordTypeLoc(RecordTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitEnumTypeLoc(EnumTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitAttributedTypeLoc(AttributedTypeLoc TL) {
@@ -484,107 +493,107 @@
 }
 
 void TypeLocWriter::VisitTemplateTypeParmTypeLoc(TemplateTypeParmTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitSubstTemplateTypeParmTypeLoc(
                                             SubstTemplateTypeParmTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitSubstTemplateTypeParmPackTypeLoc(
                                           SubstTemplateTypeParmPackTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitTemplateSpecializationTypeLoc(
                                            TemplateSpecializationTypeLoc TL) {
-  Record.AddSourceLocation(TL.getTemplateKeywordLoc());
-  Record.AddSourceLocation(TL.getTemplateNameLoc());
-  Record.AddSourceLocation(TL.getLAngleLoc());
-  Record.AddSourceLocation(TL.getRAngleLoc());
+  addSourceLocation(TL.getTemplateKeywordLoc());
+  addSourceLocation(TL.getTemplateNameLoc());
+  addSourceLocation(TL.getLAngleLoc());
+  addSourceLocation(TL.getRAngleLoc());
   for (unsigned i = 0, e = TL.getNumArgs(); i != e; ++i)
     Record.AddTemplateArgumentLocInfo(TL.getArgLoc(i).getArgument().getKind(),
                                       TL.getArgLoc(i).getLocInfo());
 }
 
 void TypeLocWriter::VisitParenTypeLoc(ParenTypeLoc TL) {
-  Record.AddSourceLocation(TL.getLParenLoc());
-  Record.AddSourceLocation(TL.getRParenLoc());
+  addSourceLocation(TL.getLParenLoc());
+  addSourceLocation(TL.getRParenLoc());
 }
 
 void TypeLocWriter::VisitMacroQualifiedTypeLoc(MacroQualifiedTypeLoc TL) {
-  Record.AddSourceLocation(TL.getExpansionLoc());
+  addSourceLocation(TL.getExpansionLoc());
 }
 
 void TypeLocWriter::VisitElaboratedTypeLoc(ElaboratedTypeLoc TL) {
-  Record.AddSourceLocation(TL.getElaboratedKeywordLoc());
+  addSourceLocation(TL.getElaboratedKeywordLoc());
   Record.AddNestedNameSpecifierLoc(TL.getQualifierLoc());
 }
 
 void TypeLocWriter::VisitInjectedClassNameTypeLoc(InjectedClassNameTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitDependentNameTypeLoc(DependentNameTypeLoc TL) {
-  Record.AddSourceLocation(TL.getElaboratedKeywordLoc());
+  addSourceLocation(TL.getElaboratedKeywordLoc());
   Record.AddNestedNameSpecifierLoc(TL.getQualifierLoc());
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitDependentTemplateSpecializationTypeLoc(
        DependentTemplateSpecializationTypeLoc TL) {
-  Record.AddSourceLocation(TL.getElaboratedKeywordLoc());
+  addSourceLocation(TL.getElaboratedKeywordLoc());
   Record.AddNestedNameSpecifierLoc(TL.getQualifierLoc());
-  Record.AddSourceLocation(TL.getTemplateKeywordLoc());
-  Record.AddSourceLocation(TL.getTemplateNameLoc());
-  Record.AddSourceLocation(TL.getLAngleLoc());
-  Record.AddSourceLocation(TL.getRAngleLoc());
+  addSourceLocation(TL.getTemplateKeywordLoc());
+  addSourceLocation(TL.getTemplateNameLoc());
+  addSourceLocation(TL.getLAngleLoc());
+  addSourceLocation(TL.getRAngleLoc());
   for (unsigned I = 0, E = TL.getNumArgs(); I != E; ++I)
     Record.AddTemplateArgumentLocInfo(TL.getArgLoc(I).getArgument().getKind(),
                                       TL.getArgLoc(I).getLocInfo());
 }
 
 void TypeLocWriter::VisitPackExpansionTypeLoc(PackExpansionTypeLoc TL) {
-  Record.AddSourceLocation(TL.getEllipsisLoc());
+  addSourceLocation(TL.getEllipsisLoc());
 }
 
 void TypeLocWriter::VisitObjCInterfaceTypeLoc(ObjCInterfaceTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void TypeLocWriter::VisitObjCObjectTypeLoc(ObjCObjectTypeLoc TL) {
   Record.push_back(TL.hasBaseTypeAsWritten());
-  Record.AddSourceLocation(TL.getTypeArgsLAngleLoc());
-  Record.AddSourceLocation(TL.getTypeArgsRAngleLoc());
+  addSourceLocation(TL.getTypeArgsLAngleLoc());
+  addSourceLocation(TL.getTypeArgsRAngleLoc());
   for (unsigned i = 0, e = TL.getNumTypeArgs(); i != e; ++i)
     Record.AddTypeSourceInfo(TL.getTypeArgTInfo(i));
-  Record.AddSourceLocation(TL.getProtocolLAngleLoc());
-  Record.AddSourceLocation(TL.getProtocolRAngleLoc());
+  addSourceLocation(TL.getProtocolLAngleLoc());
+  addSourceLocation(TL.getProtocolRAngleLoc());
   for (unsigned i = 0, e = TL.getNumProtocols(); i != e; ++i)
-    Record.AddSourceLocation(TL.getProtocolLoc(i));
+    addSourceLocation(TL.getProtocolLoc(i));
 }
 
 void TypeLocWriter::VisitObjCObjectPointerTypeLoc(ObjCObjectPointerTypeLoc TL) {
-  Record.AddSourceLocation(TL.getStarLoc());
+  addSourceLocation(TL.getStarLoc());
 }
 
 void TypeLocWriter::VisitAtomicTypeLoc(AtomicTypeLoc TL) {
-  Record.AddSourceLocation(TL.getKWLoc());
-  Record.AddSourceLocation(TL.getLParenLoc());
-  Record.AddSourceLocation(TL.getRParenLoc());
+  addSourceLocation(TL.getKWLoc());
+  addSourceLocation(TL.getLParenLoc());
+  addSourceLocation(TL.getRParenLoc());
 }
 
 void TypeLocWriter::VisitPipeTypeLoc(PipeTypeLoc TL) {
-  Record.AddSourceLocation(TL.getKWLoc());
+  addSourceLocation(TL.getKWLoc());
 }
 
 void TypeLocWriter::VisitBitIntTypeLoc(clang::BitIntTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 void TypeLocWriter::VisitDependentBitIntTypeLoc(
     clang::DependentBitIntTypeLoc TL) {
-  Record.AddSourceLocation(TL.getNameLoc());
+  addSourceLocation(TL.getNameLoc());
 }
 
 void ASTWriter::WriteTypeAbbrevs() {
@@ -5214,14 +5223,15 @@
   Record.push_back(Raw);
 }
 
-void ASTWriter::AddSourceLocation(SourceLocation Loc, RecordDataImpl &Record) {
-  SourceLocation::UIntTy Raw = Loc.getRawEncoding();
-  Record.push_back((Raw << 1) | (Raw >> (8 * sizeof(Raw) - 1)));
+void ASTWriter::AddSourceLocation(SourceLocation Loc, RecordDataImpl &Record,
+                                  SourceLocationSequence *Seq) {
+  Record.push_back(SourceLocationEncoding::encode(Loc, Seq));
 }
 
-void ASTWriter::AddSourceRange(SourceRange Range, RecordDataImpl &Record) {
-  AddSourceLocation(Range.getBegin(), Record);
-  AddSourceLocation(Range.getEnd(), Record);
+void ASTWriter::AddSourceRange(SourceRange Range, RecordDataImpl &Record,
+                               SourceLocationSequence *Seq) {
+  AddSourceLocation(Range.getBegin(), Record, Seq);
+  AddSourceLocation(Range.getEnd(), Record, Seq);
 }
 
 void ASTRecordWriter::AddAPFloat(const llvm::APFloat &Value) {
@@ -5348,8 +5358,9 @@
   AddTypeLoc(TInfo->getTypeLoc());
 }
 
-void ASTRecordWriter::AddTypeLoc(TypeLoc TL) {
-  TypeLocWriter TLW(*this);
+void ASTRecordWriter::AddTypeLoc(TypeLoc TL, LocSeq *OuterSeq) {
+  LocSeq::State Seq(OuterSeq);
+  TypeLocWriter TLW(*this, Seq);
   for (; !TL.isNull(); TL = TL.getNextTypeLoc())
     TLW.Visit(TL);
 }
diff --git a/clang/unittests/Serialization/CMakeLists.txt b/clang/unittests/Serialization/CMakeLists.txt
--- a/clang/unittests/Serialization/CMakeLists.txt
+++ b/clang/unittests/Serialization/CMakeLists.txt
@@ -7,6 +7,7 @@
 add_clang_unittest(SerializationTests
   InMemoryModuleCacheTest.cpp
   ModuleCacheTest.cpp
+  SourceLocationEncodingTest.cpp
   )
 
 clang_target_link_libraries(SerializationTests
diff --git a/clang/unittests/Serialization/SourceLocationEncodingTest.cpp b/clang/unittests/Serialization/SourceLocationEncodingTest.cpp
new file mode 100644
--- /dev/null
+++ b/clang/unittests/Serialization/SourceLocationEncodingTest.cpp
@@ -0,0 +1,103 @@
+//===- unittests/Serialization/SourceLocationEncodingTests.cpp ------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/Serialization/SourceLocationEncoding.h"
+
+#include "gtest/gtest.h"
+
+using namespace llvm;
+using namespace clang;
+
+namespace {
+using LocSeq = SourceLocationSequence;
+
+// Convert a single source location into encoded form and back.
+// If ExpectedEncoded is provided, verify the encoded value too.
+// Loc is the raw (in-memory) form of SourceLocation.
+void roundTrip(SourceLocation::UIntTy Loc,
+               llvm::Optional<uint64_t> ExpectedEncoded = llvm::None) {
+  uint64_t ActualEncoded =
+      SourceLocationEncoding::encode(SourceLocation::getFromRawEncoding(Loc));
+  if (ExpectedEncoded)
+    ASSERT_EQ(ActualEncoded, *ExpectedEncoded) << "Encoding " << Loc;
+  SourceLocation::UIntTy DecodedEncoded =
+      SourceLocationEncoding::decode(ActualEncoded).getRawEncoding();
+  ASSERT_EQ(DecodedEncoded, Loc) << "Decoding " << ActualEncoded;
+}
+
+// As above, but use sequence encoding for a series of locations.
+void roundTrip(std::vector<SourceLocation::UIntTy> Locs,
+               std::vector<uint64_t> ExpectedEncoded = {}) {
+  std::vector<uint64_t> ActualEncoded;
+  {
+    LocSeq::State Seq;
+    for (auto L : Locs)
+      ActualEncoded.push_back(SourceLocationEncoding::encode(
+          SourceLocation::getFromRawEncoding(L), Seq));
+    if (!ExpectedEncoded.empty())
+      ASSERT_EQ(ActualEncoded, ExpectedEncoded)
+          << "Encoding " << testing::PrintToString(Locs);
+  }
+  std::vector<SourceLocation::UIntTy> DecodedEncoded;
+  {
+    LocSeq::State Seq;
+    for (auto L : ActualEncoded) {
+      SourceLocation Loc = SourceLocationEncoding::decode(L, Seq);
+      DecodedEncoded.push_back(Loc.getRawEncoding());
+    }
+    ASSERT_EQ(DecodedEncoded, Locs)
+        << "Decoding " << testing::PrintToString(ActualEncoded);
+  }
+}
+
+constexpr SourceLocation::UIntTy MacroBit =
+    1 << (sizeof(SourceLocation::UIntTy) * CHAR_BIT - 1);
+constexpr SourceLocation::UIntTy Big = MacroBit >> 1;
+constexpr SourceLocation::UIntTy Biggest = -1;
+
+TEST(SourceLocationEncoding, Individual) {
+  roundTrip(1, 2);
+  roundTrip(100, 200);
+  roundTrip(MacroBit, 1);
+  roundTrip(MacroBit | 5, 11);
+  roundTrip(Big);
+  roundTrip(Big + 1);
+  roundTrip(MacroBit | Big);
+  roundTrip(MacroBit | Big + 1);
+}
+
+TEST(SourceLocationEncoding, Sequence) {
+  roundTrip({1, 2, 3, 3, 2, 1},
+            {2, // 1
+             5, // +2 (+1 of non-raw)
+             5, // +2
+             1, // +0
+             4, // -2
+             4} // -2
+  );
+  roundTrip({100, 0, 100},
+            {200, // 100
+             0,   // 0
+             1}   // +0
+  );
+
+  roundTrip({1, Big}, {2, ((Big - 1) << 2) + 1});
+  roundTrip({2, MacroBit | Big}, {4, ((Big - 1) << 2) - 1});
+
+  roundTrip({3, MacroBit | 5, MacroBit | 4, 3},
+            {6,  // 3
+             11, // +5 (+2 of non-raw + set macro bit)
+             4,  // -2
+             6}  // -3 (-2 of non-raw, clear macro bit)
+  );
+
+  roundTrip(
+      {123 | MacroBit, 1, 9, Biggest, Big, Big + 1, 0, MacroBit | Big, 0});
+}
+
+} // namespace