Index: llvm/include/llvm/Support/LEB128.h
===================================================================
--- llvm/include/llvm/Support/LEB128.h
+++ llvm/include/llvm/Support/LEB128.h
@@ -168,6 +168,89 @@
   ///@}
 };
 
+/// Decodes LEB128-encoded bytes into a value.
+/// Matches the interface of std output iterators.
+template <class ValueT> class LEB128OutputIterator {
+  /// The value into which the decoded bytes are written.
+  ValueT &Value;
+
+  /// Whether this iterator is outputting a signed or unsigned value.
+  bool IsSigned;
+
+  /// Contains the error message of the last error. Null if there was no error.
+  const char *Error;
+
+  /// The bit index inside Value where we'll write the next decoded bits.
+  unsigned Shift;
+
+  /// Returns true if we've seen a byte without a continuation bit.
+  bool IsComplete;
+
+public:
+  /// Boilerplate typedefs for C++ iterators.
+  ///@{
+  using iterator_category = std::output_iterator_tag;
+  using value_type = void;
+  using difference_type = void;
+  using pointer = void;
+  using reference = void;
+  ///@}
+
+  /// Constructs an output iterator that decodes LEB128-encoded bytes and writes
+  /// them into the specified \p Value.
+  LEB128OutputIterator(ValueT &Value, bool IsSigned)
+      : Value(Value), IsSigned(IsSigned), Error(nullptr), Shift(0),
+        IsComplete(false) {
+    // Initially zero before any decoded bits are written into it.
+    this->Value = 0;
+  }
+
+  /// Decodes one byte from a stream of LEB128-encoded bytes, and stores the
+  /// decoded bits in the Value that was used to construct the output iterator.
+  LEB128OutputIterator &operator=(uint8_t Byte) {
+    assert(!IsComplete && "Already saw final LEB128 byte. Can't decode more.");
+    assert(!Error && "Should abandon LEB128 decoding when an error happens.");
+
+    uint64_t Slice = Byte & 0x7f;
+    if (!IsSigned) {
+      // TODO: Implement an equivalent check for signed values?
+      //       (The existing code did not check this error for signed values, so
+      //       this refactor is leaving it unimplemented.)
+      if (Shift >= 64 || Slice << Shift >> Shift != Slice) {
+        Error = "uleb128 too big for uint64";
+        return *this;
+      }
+    }
+    Value |= Slice << Shift;
+    Shift += 7;
+
+    if (!(Byte & 0x80)) {
+      // No continuation bit, therefore this was the final byte.
+      IsComplete = true;
+      // Sign extend negative numbers if needed.
+      if (IsSigned && Shift < 64 && (Byte & 0x40))
+        Value |= -1ULL << Shift;
+    }
+    return *this;
+  }
+
+  /// No-op. Provided to satisfy the requirements of LegacyOutputIterator.
+  ///@{
+  LEB128OutputIterator &operator*() { return *this; }
+  LEB128OutputIterator &operator++() { return *this; }
+  LEB128OutputIterator &operator++(int) { return *this; }
+  ///@}
+
+  /// Returns string describing the last error that happened during decoding.
+  /// If there was no error, returns nullptr.
+  const char *error() const { return Error; }
+
+  /// Returns true if the stream of LEB128-encoded bytes has reached a byte
+  /// without a continuation bit. False otherwise.
+  /// In other words, returns true if a LEB128 value was completely decoded.
+  bool complete() const { return IsComplete; }
+};
+
 /// Utility function to encode a SLEB128 or ULEB128 value to a buffer. Returns
 /// the length in bytes of the encoded value.
 template <class ValueT>
@@ -216,67 +299,49 @@
   return encodeLEB128(Value, /* IsSigned */ false, p, PadTo);
 }
 
-/// Utility function to decode a ULEB128 value.
-inline uint64_t decodeULEB128(const uint8_t *p, unsigned *n = nullptr,
-                              const uint8_t *end = nullptr,
-                              const char **error = nullptr) {
+/// Utility function to decode a ULEB128/SLEB128 value, where the signedness is
+/// defined by \p IsSigned.
+template <class ValueT>
+ValueT decodeLEB128(const uint8_t *p, bool IsSigned, unsigned *n = nullptr,
+                    const uint8_t *end = nullptr,
+                    const char **error = nullptr) {
+  ValueT Value;
+  LEB128OutputIterator<ValueT> Decoder(Value, IsSigned);
   const uint8_t *orig_p = p;
-  uint64_t Value = 0;
-  unsigned Shift = 0;
-  if (error)
-    *error = nullptr;
-  do {
+  const char *found_error = nullptr;
+  while (!Decoder.complete()) {
     if (p == end) {
-      if (error)
-        *error = "malformed uleb128, extends past end";
-      if (n)
-        *n = (unsigned)(p - orig_p);
-      return 0;
+      if (IsSigned)
+        found_error = "malformed sleb128, extends past end";
+      else
+        found_error = "malformed uleb128, extends past end";
+      break;
     }
-    uint64_t Slice = *p & 0x7f;
-    if (Shift >= 64 || Slice << Shift >> Shift != Slice) {
-      if (error)
-        *error = "uleb128 too big for uint64";
-      if (n)
-        *n = (unsigned)(p - orig_p);
-      return 0;
-    }
-    Value += uint64_t(*p & 0x7f) << Shift;
-    Shift += 7;
-  } while (*p++ >= 128);
+    *Decoder = *p++;
+    if ((found_error = Decoder.error()))
+      break;
+  }
   if (n)
     *n = (unsigned)(p - orig_p);
+  if (error)
+    *error = found_error;
+  if (found_error)
+    return 0;
   return Value;
 }
 
+/// Utility function to decode a ULEB128 value.
+inline uint64_t decodeULEB128(const uint8_t *p, unsigned *n = nullptr,
+                              const uint8_t *end = nullptr,
+                              const char **error = nullptr) {
+  return decodeLEB128<uint64_t>(p, /* IsSigned */ false, n, end, error);
+}
+
 /// Utility function to decode a SLEB128 value.
 inline int64_t decodeSLEB128(const uint8_t *p, unsigned *n = nullptr,
                              const uint8_t *end = nullptr,
                              const char **error = nullptr) {
-  const uint8_t *orig_p = p;
-  int64_t Value = 0;
-  unsigned Shift = 0;
-  uint8_t Byte;
-  if (error)
-    *error = nullptr;
-  do {
-    if (p == end) {
-      if (error)
-        *error = "malformed sleb128, extends past end";
-      if (n)
-        *n = (unsigned)(p - orig_p);
-      return 0;
-    }
-    Byte = *p++;
-    Value |= (uint64_t(Byte & 0x7f) << Shift);
-    Shift += 7;
-  } while (Byte >= 128);
-  // Sign extend negative numbers if needed.
-  if (Shift < 64 && (Byte & 0x40))
-    Value |= (-1ULL) << Shift;
-  if (n)
-    *n = (unsigned)(p - orig_p);
-  return Value;
+  return decodeLEB128<int64_t>(p, /* IsSigned */ true, n, end, error);
 }
 
 /// Utility function to get the size of the ULEB128-encoded value.
Index: llvm/unittests/Support/LEB128Test.cpp
===================================================================
--- llvm/unittests/Support/LEB128Test.cpp
+++ llvm/unittests/Support/LEB128Test.cpp
@@ -370,4 +370,19 @@
   EXPECT_EQ(10u, getULEB128Size(UINT64_MAX));
 }
 
-}  // anonymous namespace
+template <class ValueT>
+ValueT decodeWithCopy(StringRef LEBBytes, bool IsSigned) {
+  ValueT Value;
+  std::copy(LEBBytes.begin(), LEBBytes.end(),
+            LEB128OutputIterator<ValueT>(Value, IsSigned));
+  return Value;
+}
+
+TEST(LEB128Test, OutputIteratorCopy) {
+  // Test using LEB128OutputIterator as the output of a std algorithm.
+  // Mainly checking that the output iterator interface is compatible with std.
+  EXPECT_EQ(decodeWithCopy<int64_t>("\xbf\x7f", /* IsSigned */ true), -65);
+  EXPECT_EQ(decodeWithCopy<uint64_t>("\x80\x01", /* IsSigned */ false), 0x80U);
+}
+
+} // anonymous namespace