diff --git a/llvm/include/llvm/ADT/SmallVector.h b/llvm/include/llvm/ADT/SmallVector.h
--- a/llvm/include/llvm/ADT/SmallVector.h
+++ b/llvm/include/llvm/ADT/SmallVector.h
@@ -16,10 +16,10 @@
 #include "llvm/ADT/iterator_range.h"
 #include "llvm/Support/AlignOf.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/MemAlloc.h"
 #include "llvm/Support/type_traits.h"
-#include "llvm/Support/ErrorHandling.h"
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
@@ -27,6 +27,7 @@
 #include <cstring>
 #include <initializer_list>
 #include <iterator>
+#include <limits>
 #include <memory>
 #include <new>
 #include <type_traits>
@@ -34,11 +35,23 @@
 
 namespace llvm {
 
-/// This is all the non-templated stuff common to all SmallVectors.
-class SmallVectorBase {
+/// This is all the stuff common to all SmallVectors.
+///
+/// The template parameter specifies the type which should be used to hold the
+/// Size and Capacity of the SmallVector, so it can be adjusted.
+/// Using 32 bit size is desirable to shink the size of the SmallVector.
+/// Using 64 bit size is desirable for cases like SmallVector<char>, where a
+/// 32 bit size would limit the vector to ~4GB. SmallVectors are used for
+/// buffering bitcode output - which can exceed 4GB.
+template <class Size_T> class SmallVectorBase {
 protected:
   void *BeginX;
-  unsigned Size = 0, Capacity;
+  Size_T Size = 0, Capacity;
+
+  /// The maximum value of the Size_T used.
+  static constexpr size_t SizeTypeMax() {
+    return std::numeric_limits<Size_T>::max();
+  }
 
   SmallVectorBase() = delete;
   SmallVectorBase(void *FirstEl, size_t TotalCapacity)
@@ -70,9 +83,14 @@
   }
 };
 
+template <class T>
+using SmallVectorSizeType =
+    typename std::conditional<sizeof(T) < 4 && sizeof(void *) >= 8, uint64_t,
+                              uint32_t>::type;
+
 /// Figure out the offset of the first element.
 template <class T, typename = void> struct SmallVectorAlignmentAndSize {
-  AlignedCharArrayUnion<SmallVectorBase> Base;
+  AlignedCharArrayUnion<SmallVectorBase<SmallVectorSizeType<T>>> Base;
   AlignedCharArrayUnion<T> FirstEl;
 };
 
@@ -80,7 +98,10 @@
 /// the type T is a POD. The extra dummy template argument is used by ArrayRef
 /// to avoid unnecessarily requiring T to be complete.
 template <typename T, typename = void>
-class SmallVectorTemplateCommon : public SmallVectorBase {
+class SmallVectorTemplateCommon
+    : public SmallVectorBase<SmallVectorSizeType<T>> {
+  using Base = SmallVectorBase<SmallVectorSizeType<T>>;
+
   /// Find the address of the first element.  For this pointer math to be valid
   /// with small-size of 0 for T with lots of alignment, it's important that
   /// SmallVectorStorage is properly-aligned even for small-size of 0.
@@ -92,21 +113,20 @@
   // Space after 'FirstEl' is clobbered, do not add any instance vars after it.
 
 protected:
-  SmallVectorTemplateCommon(size_t Size)
-      : SmallVectorBase(getFirstEl(), Size) {}
+  SmallVectorTemplateCommon(size_t Size) : Base(getFirstEl(), Size) {}
 
   void grow_pod(size_t MinCapacity, size_t TSize) {
-    SmallVectorBase::grow_pod(getFirstEl(), MinCapacity, TSize);
+    Base::grow_pod(getFirstEl(), MinCapacity, TSize);
   }
 
   /// Return true if this is a smallvector which has not had dynamic
   /// memory allocated for it.
-  bool isSmall() const { return BeginX == getFirstEl(); }
+  bool isSmall() const { return this->BeginX == getFirstEl(); }
 
   /// Put this vector in a state of being small.
   void resetToSmall() {
-    BeginX = getFirstEl();
-    Size = Capacity = 0; // FIXME: Setting Capacity to 0 is suspect.
+    this->BeginX = getFirstEl();
+    this->Size = this->Capacity = 0; // FIXME: Setting Capacity to 0 is suspect.
   }
 
 public:
@@ -124,6 +144,10 @@
   using pointer = T *;
   using const_pointer = const T *;
 
+  using Base::capacity;
+  using Base::empty;
+  using Base::size;
+
   // forward iterator creation methods.
   iterator begin() { return (iterator)this->BeginX; }
   const_iterator begin() const { return (const_iterator)this->BeginX; }
@@ -137,7 +161,9 @@
   const_reverse_iterator rend() const { return const_reverse_iterator(begin());}
 
   size_type size_in_bytes() const { return size() * sizeof(T); }
-  size_type max_size() const { return size_type(-1) / sizeof(T); }
+  size_type max_size() const {
+    return std::min(this->SizeTypeMax(), size_type(-1) / sizeof(T));
+  }
 
   size_t capacity_in_bytes() const { return capacity() * sizeof(T); }
 
@@ -232,18 +258,21 @@
 // Define this out-of-line to dissuade the C++ compiler from inlining it.
 template <typename T, bool TriviallyCopyable>
 void SmallVectorTemplateBase<T, TriviallyCopyable>::grow(size_t MinSize) {
-  if (MinSize > UINT32_MAX)
+  // Ensure we can fit the new capacity.
+  // This is only going to be applicable when the capacity is 32 bit.
+  if (MinSize > this->SizeTypeMax())
     report_bad_alloc_error("SmallVector capacity overflow during allocation");
 
   // Ensure we can meet the guarantee of space for at least one more element.
   // The above check alone will not catch the case where grow is called with a
   // default MinCapacity of 0, but the current capacity cannot be increased.
-  if (this->capacity() == size_t(UINT32_MAX))
+  // This is only going to be applicable when the capacity is 32 bit.
+  if (this->capacity() == this->SizeTypeMax())
     report_bad_alloc_error("SmallVector capacity unable to grow");
 
   // Always grow, even from zero.
   size_t NewCapacity = size_t(NextPowerOf2(this->capacity() + 2));
-  NewCapacity = std::min(std::max(NewCapacity, MinSize), size_t(UINT32_MAX));
+  NewCapacity = std::min(std::max(NewCapacity, MinSize), this->SizeTypeMax());
   T *NewElts = static_cast<T*>(llvm::safe_malloc(NewCapacity*sizeof(T)));
 
   // Move the elements over.
diff --git a/llvm/lib/Support/SmallVector.cpp b/llvm/lib/Support/SmallVector.cpp
--- a/llvm/lib/Support/SmallVector.cpp
+++ b/llvm/lib/Support/SmallVector.cpp
@@ -37,24 +37,30 @@
                   sizeof(unsigned) * 2 + sizeof(void *) * 2,
               "wasted space in SmallVector size 1");
 
-/// grow_pod - This is an implementation of the grow() method which only works
-/// on POD-like datatypes and is out of line to reduce code duplication.
-/// This function will report a fatal error if it cannot increase capacity.
-void SmallVectorBase::grow_pod(void *FirstEl, size_t MinCapacity,
-                               size_t TSize) {
-  // Ensure we can fit the new capacity in 32 bits.
-  if (MinCapacity > UINT32_MAX)
+static_assert(sizeof(SmallVector<char, 0>) ==
+                  sizeof(void *) * 2 + sizeof(void *),
+              "1 byte elements have word-sized type for size and capacity");
+
+// Note: Moving this function into the header may cause performance regression.
+template <class Size_T>
+void SmallVectorBase<Size_T>::grow_pod(void *FirstEl, size_t MinCapacity,
+                                       size_t TSize) {
+  // Ensure we can fit the new capacity.
+  // This is only going to be applicable when the capacity is 32 bit.
+  if (MinCapacity > SizeTypeMax())
     report_bad_alloc_error("SmallVector capacity overflow during allocation");
 
   // Ensure we can meet the guarantee of space for at least one more element.
   // The above check alone will not catch the case where grow is called with a
   // default MinCapacity of 0, but the current capacity cannot be increased.
-  if (capacity() == size_t(UINT32_MAX))
+  // This is only going to be applicable when the capacity is 32 bit.
+  if (capacity() == SizeTypeMax())
     report_bad_alloc_error("SmallVector capacity unable to grow");
 
+  // In theory 2*capacity can overflow if the capacity is 64 bit, but the
+  // original capacity would never be large enough for this to be a problem.
   size_t NewCapacity = 2 * capacity() + 1; // Always grow.
-  NewCapacity =
-      std::min(std::max(NewCapacity, MinCapacity), size_t(UINT32_MAX));
+  NewCapacity = std::min(std::max(NewCapacity, MinCapacity), SizeTypeMax());
 
   void *NewElts;
   if (BeginX == FirstEl) {
@@ -70,3 +76,6 @@
   this->BeginX = NewElts;
   this->Capacity = NewCapacity;
 }
+
+template class llvm::SmallVectorBase<uint32_t>;
+template class llvm::SmallVectorBase<uint64_t>;