diff --git a/clang-tools-extra/clang-tidy/readability/FunctionSizeCheck.cpp b/clang-tools-extra/clang-tidy/readability/FunctionSizeCheck.cpp
--- a/clang-tools-extra/clang-tidy/readability/FunctionSizeCheck.cpp
+++ b/clang-tools-extra/clang-tidy/readability/FunctionSizeCheck.cpp
@@ -9,6 +9,7 @@
 #include "FunctionSizeCheck.h"
 #include "clang/AST/RecursiveASTVisitor.h"
 #include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "llvm/ADT/BitVector.h"
 
 using namespace clang::ast_matchers;
 
@@ -118,7 +119,7 @@
     std::vector<SourceLocation> NestingThresholders;
   };
   FunctionInfo Info;
-  std::vector<bool> TrackedParent;
+  llvm::BitVector TrackedParent;
   unsigned StructNesting = 0;
   unsigned CurrentNestingLevel = 0;
 };
diff --git a/clang-tools-extra/clangd/SourceCode.cpp b/clang-tools-extra/clangd/SourceCode.cpp
--- a/clang-tools-extra/clangd/SourceCode.cpp
+++ b/clang-tools-extra/clangd/SourceCode.cpp
@@ -27,6 +27,7 @@
 #include "clang/Tooling/Core/Replacement.h"
 #include "clang/Tooling/Syntax/Tokens.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringExtras.h"
@@ -663,7 +664,7 @@
   // Stack of enclosing namespaces, e.g. {"clang", "clangd"}
   std::vector<std::string> Enclosing; // Contains e.g. "clang", "clangd"
   // Stack counts open braces. true if the brace opened a namespace.
-  std::vector<bool> BraceStack;
+  llvm::BitVector BraceStack;
 
   enum {
     Default,
diff --git a/clang/lib/Format/UnwrappedLineParser.h b/clang/lib/Format/UnwrappedLineParser.h
--- a/clang/lib/Format/UnwrappedLineParser.h
+++ b/clang/lib/Format/UnwrappedLineParser.h
@@ -18,6 +18,7 @@
 #include "FormatToken.h"
 #include "clang/Basic/IdentifierTable.h"
 #include "clang/Format/Format.h"
+#include "llvm/ADT/BitVector.h"
 #include "llvm/Support/Regex.h"
 #include <stack>
 #include <vector>
@@ -231,7 +232,7 @@
 
   // We store for each line whether it must be a declaration depending on
   // whether we are in a compound statement or not.
-  std::vector<bool> DeclarationScopeStack;
+  llvm::BitVector DeclarationScopeStack;
 
   const FormatStyle &Style;
   const AdditionalKeywords &Keywords;
diff --git a/clang/lib/Format/UnwrappedLineParser.cpp b/clang/lib/Format/UnwrappedLineParser.cpp
--- a/clang/lib/Format/UnwrappedLineParser.cpp
+++ b/clang/lib/Format/UnwrappedLineParser.cpp
@@ -58,7 +58,7 @@
 
 class ScopedDeclarationState {
 public:
-  ScopedDeclarationState(UnwrappedLine &Line, std::vector<bool> &Stack,
+  ScopedDeclarationState(UnwrappedLine &Line, llvm::BitVector &Stack,
                          bool MustBeDeclaration)
       : Line(Line), Stack(Stack) {
     Line.MustBeDeclaration = MustBeDeclaration;
@@ -74,7 +74,7 @@
 
 private:
   UnwrappedLine &Line;
-  std::vector<bool> &Stack;
+  llvm::BitVector &Stack;
 };
 
 static bool isLineComment(const FormatToken &FormatTok) {
diff --git a/llvm/include/llvm/ADT/BitVector.h b/llvm/include/llvm/ADT/BitVector.h
--- a/llvm/include/llvm/ADT/BitVector.h
+++ b/llvm/include/llvm/ADT/BitVector.h
@@ -444,6 +444,12 @@
     return (Bits[Idx / BITWORD_SIZE] & Mask) != 0;
   }
 
+  /// Return the last element in the vector.
+  bool back() const {
+    assert(!empty() && "Getting last element of empty vector.");
+    return (*this)[size() - 1];
+  }
+
   bool test(unsigned Idx) const {
     return (*this)[Idx];
   }
@@ -465,6 +471,12 @@
       set(OldSize);
   }
 
+  /// Pop one bit from the end of the vector.
+  void pop_back() {
+    assert(!empty() && "Empty vector has no element to pop.");
+    resize(size() - 1);
+  }
+
   /// Test if any common bits are set.
   bool anyCommon(const BitVector &RHS) const {
     unsigned ThisWords = Bits.size();
diff --git a/llvm/include/llvm/ADT/SmallBitVector.h b/llvm/include/llvm/ADT/SmallBitVector.h
--- a/llvm/include/llvm/ADT/SmallBitVector.h
+++ b/llvm/include/llvm/ADT/SmallBitVector.h
@@ -462,6 +462,12 @@
     return getPointer()->operator[](Idx);
   }
 
+  /// Return the last element in the vector.
+  bool back() const {
+    assert(!empty() && "Getting last element of empty vector.");
+    return (*this)[size() - 1];
+  }
+
   bool test(unsigned Idx) const {
     return (*this)[Idx];
   }
@@ -471,6 +477,12 @@
     resize(size() + 1, Val);
   }
 
+  /// Pop one bit from the end of the vector.
+  void pop_back() {
+    assert(!empty() && "Empty vector has no element to pop.");
+    resize(size() - 1);
+  }
+
   /// Test if any common bits are set.
   bool anyCommon(const SmallBitVector &RHS) const {
     if (isSmall() && RHS.isSmall())
diff --git a/llvm/lib/MC/MCParser/MasmParser.cpp b/llvm/lib/MC/MCParser/MasmParser.cpp
--- a/llvm/lib/MC/MCParser/MasmParser.cpp
+++ b/llvm/lib/MC/MCParser/MasmParser.cpp
@@ -13,6 +13,7 @@
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/BitVector.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/Optional.h"
@@ -379,7 +380,7 @@
   /// time of assembly
   struct tm TM;
 
-  std::vector<bool> EndStatementAtEOFStack;
+  BitVector EndStatementAtEOFStack;
 
   AsmCond TheCondState;
   std::vector<AsmCond> TheCondStack;
diff --git a/llvm/unittests/ADT/BitVectorTest.cpp b/llvm/unittests/ADT/BitVectorTest.cpp
--- a/llvm/unittests/ADT/BitVectorTest.cpp
+++ b/llvm/unittests/ADT/BitVectorTest.cpp
@@ -1171,21 +1171,25 @@
   EXPECT_EQ(-1, Vec.find_first());
   EXPECT_EQ(10U, Vec.size());
   EXPECT_EQ(0U, Vec.count());
+  EXPECT_EQ(false, Vec.back());
 
   Vec.push_back(true);
   EXPECT_EQ(10, Vec.find_first());
   EXPECT_EQ(11U, Vec.size());
   EXPECT_EQ(1U, Vec.count());
+  EXPECT_EQ(true, Vec.back());
 
   Vec.push_back(false);
   EXPECT_EQ(10, Vec.find_first());
   EXPECT_EQ(12U, Vec.size());
   EXPECT_EQ(1U, Vec.count());
+  EXPECT_EQ(false, Vec.back());
 
   Vec.push_back(true);
   EXPECT_EQ(10, Vec.find_first());
   EXPECT_EQ(13U, Vec.size());
   EXPECT_EQ(2U, Vec.count());
+  EXPECT_EQ(true, Vec.back());
 
   // Add a lot of values to cause reallocation.
   for (int i = 0; i != 100; ++i) {
@@ -1197,6 +1201,28 @@
   EXPECT_EQ(102U, Vec.count());
 }
 
+TYPED_TEST(BitVectorTest, PopBack) {
+  TypeParam Vec(10, true);
+  EXPECT_EQ(10U, Vec.size());
+  EXPECT_EQ(10U, Vec.count());
+  EXPECT_EQ(true, Vec.back());
+
+  Vec.pop_back();
+  EXPECT_EQ(9U, Vec.size());
+  EXPECT_EQ(9U, Vec.count());
+  EXPECT_EQ(true, Vec.back());
+
+  Vec.push_back(false);
+  EXPECT_EQ(10U, Vec.size());
+  EXPECT_EQ(9U, Vec.count());
+  EXPECT_EQ(false, Vec.back());
+
+  Vec.pop_back();
+  EXPECT_EQ(9U, Vec.size());
+  EXPECT_EQ(9U, Vec.count());
+  EXPECT_EQ(true, Vec.back());
+}
+
 TYPED_TEST(BitVectorTest, DenseSet) {
   DenseSet<TypeParam> Set;
   TypeParam A(10, true);