diff --git a/objectivec/GPBExtensionInternals.m b/objectivec/GPBExtensionInternals.m
new file mode 100644
index 0000000..634c336
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+++ b/objectivec/GPBExtensionInternals.m
@@ -0,0 +1,380 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#import "GPBExtensionInternals.h"
+
+#import <objc/runtime.h>
+
+#import "GPBCodedInputStream_PackagePrivate.h"
+#import "GPBCodedOutputStream.h"
+#import "GPBDescriptor_PackagePrivate.h"
+#import "GPBMessage_PackagePrivate.h"
+#import "GPBUtilities_PackagePrivate.h"
+
+static id NewSingleValueFromInputStream(GPBExtensionDescriptor *extension,
+                                        GPBCodedInputStream *input,
+                                        GPBExtensionRegistry *extensionRegistry,
+                                        GPBMessage *existingValue)
+    __attribute__((ns_returns_retained));
+
+GPB_INLINE size_t DataTypeSize(GPBDataType dataType) {
+  switch (dataType) {
+    case GPBDataTypeBool:
+      return 1;
+    case GPBDataTypeFixed32:
+    case GPBDataTypeSFixed32:
+    case GPBDataTypeFloat:
+      return 4;
+    case GPBDataTypeFixed64:
+    case GPBDataTypeSFixed64:
+    case GPBDataTypeDouble:
+      return 8;
+    default:
+      return 0;
+  }
+}
+
+static size_t ComputePBSerializedSizeNoTagOfObject(GPBDataType dataType, id object) {
+#define FIELD_CASE(TYPE, ACCESSOR)                                     \
+  case GPBDataType##TYPE:                                              \
+    return GPBCompute##TYPE##SizeNoTag([(NSNumber *)object ACCESSOR]);
+#define FIELD_CASE2(TYPE)                                              \
+  case GPBDataType##TYPE:                                              \
+    return GPBCompute##TYPE##SizeNoTag(object);
+  switch (dataType) {
+    FIELD_CASE(Bool, boolValue)
+    FIELD_CASE(Float, floatValue)
+    FIELD_CASE(Double, doubleValue)
+    FIELD_CASE(Int32, intValue)
+    FIELD_CASE(SFixed32, intValue)
+    FIELD_CASE(SInt32, intValue)
+    FIELD_CASE(Enum, intValue)
+    FIELD_CASE(Int64, longLongValue)
+    FIELD_CASE(SInt64, longLongValue)
+    FIELD_CASE(SFixed64, longLongValue)
+    FIELD_CASE(UInt32, unsignedIntValue)
+    FIELD_CASE(Fixed32, unsignedIntValue)
+    FIELD_CASE(UInt64, unsignedLongLongValue)
+    FIELD_CASE(Fixed64, unsignedLongLongValue)
+    FIELD_CASE2(Bytes)
+    FIELD_CASE2(String)
+    FIELD_CASE2(Message)
+    FIELD_CASE2(Group)
+  }
+#undef FIELD_CASE
+#undef FIELD_CASE2
+}
+
+static size_t ComputeSerializedSizeIncludingTagOfObject(
+    GPBExtensionDescription *description, id object) {
+#define FIELD_CASE(TYPE, ACCESSOR)                                   \
+  case GPBDataType##TYPE:                                            \
+    return GPBCompute##TYPE##Size(description->fieldNumber,          \
+                                  [(NSNumber *)object ACCESSOR]);
+#define FIELD_CASE2(TYPE)                                            \
+  case GPBDataType##TYPE:                                            \
+    return GPBCompute##TYPE##Size(description->fieldNumber, object);
+  switch (description->dataType) {
+    FIELD_CASE(Bool, boolValue)
+    FIELD_CASE(Float, floatValue)
+    FIELD_CASE(Double, doubleValue)
+    FIELD_CASE(Int32, intValue)
+    FIELD_CASE(SFixed32, intValue)
+    FIELD_CASE(SInt32, intValue)
+    FIELD_CASE(Enum, intValue)
+    FIELD_CASE(Int64, longLongValue)
+    FIELD_CASE(SInt64, longLongValue)
+    FIELD_CASE(SFixed64, longLongValue)
+    FIELD_CASE(UInt32, unsignedIntValue)
+    FIELD_CASE(Fixed32, unsignedIntValue)
+    FIELD_CASE(UInt64, unsignedLongLongValue)
+    FIELD_CASE(Fixed64, unsignedLongLongValue)
+    FIELD_CASE2(Bytes)
+    FIELD_CASE2(String)
+    FIELD_CASE2(Group)
+    case GPBDataTypeMessage:
+      if (GPBExtensionIsWireFormat(description)) {
+        return GPBComputeMessageSetExtensionSize(description->fieldNumber,
+                                                 object);
+      } else {
+        return GPBComputeMessageSize(description->fieldNumber, object);
+      }
+  }
+#undef FIELD_CASE
+#undef FIELD_CASE2
+}
+
+static size_t ComputeSerializedSizeIncludingTagOfArray(
+    GPBExtensionDescription *description, NSArray *values) {
+  if (GPBExtensionIsPacked(description)) {
+    size_t size = 0;
+    size_t typeSize = DataTypeSize(description->dataType);
+    if (typeSize != 0) {
+      size = values.count * typeSize;
+    } else {
+      for (id value in values) {
+        size +=
+            ComputePBSerializedSizeNoTagOfObject(description->dataType, value);
+      }
+    }
+    return size + GPBComputeTagSize(description->fieldNumber) +
+           GPBComputeRawVarint32SizeForInteger(size);
+  } else {
+    size_t size = 0;
+    for (id value in values) {
+      size += ComputeSerializedSizeIncludingTagOfObject(description, value);
+    }
+    return size;
+  }
+}
+
+static void WriteObjectIncludingTagToCodedOutputStream(
+    id object, GPBExtensionDescription *description,
+    GPBCodedOutputStream *output) {
+#define FIELD_CASE(TYPE, ACCESSOR)                      \
+  case GPBDataType##TYPE:                               \
+    [output write##TYPE:description->fieldNumber        \
+                  value:[(NSNumber *)object ACCESSOR]]; \
+    return;
+#define FIELD_CASE2(TYPE)                                       \
+  case GPBDataType##TYPE:                                       \
+    [output write##TYPE:description->fieldNumber value:object]; \
+    return;
+  switch (description->dataType) {
+    FIELD_CASE(Bool, boolValue)
+    FIELD_CASE(Float, floatValue)
+    FIELD_CASE(Double, doubleValue)
+    FIELD_CASE(Int32, intValue)
+    FIELD_CASE(SFixed32, intValue)
+    FIELD_CASE(SInt32, intValue)
+    FIELD_CASE(Enum, intValue)
+    FIELD_CASE(Int64, longLongValue)
+    FIELD_CASE(SInt64, longLongValue)
+    FIELD_CASE(SFixed64, longLongValue)
+    FIELD_CASE(UInt32, unsignedIntValue)
+    FIELD_CASE(Fixed32, unsignedIntValue)
+    FIELD_CASE(UInt64, unsignedLongLongValue)
+    FIELD_CASE(Fixed64, unsignedLongLongValue)
+    FIELD_CASE2(Bytes)
+    FIELD_CASE2(String)
+    FIELD_CASE2(Group)
+    case GPBDataTypeMessage:
+      if (GPBExtensionIsWireFormat(description)) {
+        [output writeMessageSetExtension:description->fieldNumber value:object];
+      } else {
+        [output writeMessage:description->fieldNumber value:object];
+      }
+      return;
+  }
+#undef FIELD_CASE
+#undef FIELD_CASE2
+}
+
+static void WriteObjectNoTagToCodedOutputStream(
+    id object, GPBExtensionDescription *description,
+    GPBCodedOutputStream *output) {
+#define FIELD_CASE(TYPE, ACCESSOR)                             \
+  case GPBDataType##TYPE:                                      \
+    [output write##TYPE##NoTag:[(NSNumber *)object ACCESSOR]]; \
+    return;
+#define FIELD_CASE2(TYPE)               \
+  case GPBDataType##TYPE:               \
+    [output write##TYPE##NoTag:object]; \
+    return;
+  switch (description->dataType) {
+    FIELD_CASE(Bool, boolValue)
+    FIELD_CASE(Float, floatValue)
+    FIELD_CASE(Double, doubleValue)
+    FIELD_CASE(Int32, intValue)
+    FIELD_CASE(SFixed32, intValue)
+    FIELD_CASE(SInt32, intValue)
+    FIELD_CASE(Enum, intValue)
+    FIELD_CASE(Int64, longLongValue)
+    FIELD_CASE(SInt64, longLongValue)
+    FIELD_CASE(SFixed64, longLongValue)
+    FIELD_CASE(UInt32, unsignedIntValue)
+    FIELD_CASE(Fixed32, unsignedIntValue)
+    FIELD_CASE(UInt64, unsignedLongLongValue)
+    FIELD_CASE(Fixed64, unsignedLongLongValue)
+    FIELD_CASE2(Bytes)
+    FIELD_CASE2(String)
+    FIELD_CASE2(Message)
+    case GPBDataTypeGroup:
+      [output writeGroupNoTag:description->fieldNumber value:object];
+      return;
+  }
+#undef FIELD_CASE
+#undef FIELD_CASE2
+}
+
+static void WriteArrayIncludingTagsToCodedOutputStream(
+    NSArray *values, GPBExtensionDescription *description,
+    GPBCodedOutputStream *output) {
+  if (GPBExtensionIsPacked(description)) {
+    [output writeTag:description->fieldNumber
+              format:GPBWireFormatLengthDelimited];
+    size_t dataSize = 0;
+    size_t typeSize = DataTypeSize(description->dataType);
+    if (typeSize != 0) {
+      dataSize = values.count * typeSize;
+    } else {
+      for (id value in values) {
+        dataSize +=
+            ComputePBSerializedSizeNoTagOfObject(description->dataType, value);
+      }
+    }
+    [output writeRawVarintSizeTAs32:dataSize];
+    for (id value in values) {
+      WriteObjectNoTagToCodedOutputStream(value, description, output);
+    }
+  } else {
+    for (id value in values) {
+      WriteObjectIncludingTagToCodedOutputStream(value, description, output);
+    }
+  }
+}
+
+void GPBExtensionMergeFromInputStream(GPBExtensionDescriptor *extension,
+                                      BOOL isPackedOnStream,
+                                      GPBCodedInputStream *input,
+                                      GPBExtensionRegistry *extensionRegistry,
+                                      GPBMessage *message) {
+  GPBExtensionDescription *description = extension->description_;
+  GPBCodedInputStreamState *state = &input->state_;
+  if (isPackedOnStream) {
+    NSCAssert(GPBExtensionIsRepeated(description),
+              @"How was it packed if it isn't repeated?");
+    int32_t length = GPBCodedInputStreamReadInt32(state);
+    size_t limit = GPBCodedInputStreamPushLimit(state, length);
+    while (GPBCodedInputStreamBytesUntilLimit(state) > 0) {
+      id value = NewSingleValueFromInputStream(extension,
+                                               input,
+                                               extensionRegistry,
+                                               nil);
+      [message addExtension:extension value:value];
+      [value release];
+    }
+    GPBCodedInputStreamPopLimit(state, limit);
+  } else {
+    id existingValue = nil;
+    BOOL isRepeated = GPBExtensionIsRepeated(description);
+    if (!isRepeated && GPBDataTypeIsMessage(description->dataType)) {
+      existingValue = [message getExistingExtension:extension];
+    }
+    id value = NewSingleValueFromInputStream(extension,
+                                             input,
+                                             extensionRegistry,
+                                             existingValue);
+    if (isRepeated) {
+      [message addExtension:extension value:value];
+    } else {
+      [message setExtension:extension value:value];
+    }
+    [value release];
+  }
+}
+
+void GPBWriteExtensionValueToOutputStream(GPBExtensionDescriptor *extension,
+                                          id value,
+                                          GPBCodedOutputStream *output) {
+  GPBExtensionDescription *description = extension->description_;
+  if (GPBExtensionIsRepeated(description)) {
+    WriteArrayIncludingTagsToCodedOutputStream(value, description, output);
+  } else {
+    WriteObjectIncludingTagToCodedOutputStream(value, description, output);
+  }
+}
+
+size_t GPBComputeExtensionSerializedSizeIncludingTag(
+    GPBExtensionDescriptor *extension, id value) {
+  GPBExtensionDescription *description = extension->description_;
+  if (GPBExtensionIsRepeated(description)) {
+    return ComputeSerializedSizeIncludingTagOfArray(description, value);
+  } else {
+    return ComputeSerializedSizeIncludingTagOfObject(description, value);
+  }
+}
+
+// Note that this returns a retained value intentionally.
+static id NewSingleValueFromInputStream(GPBExtensionDescriptor *extension,
+                                        GPBCodedInputStream *input,
+                                        GPBExtensionRegistry *extensionRegistry,
+                                        GPBMessage *existingValue) {
+  GPBExtensionDescription *description = extension->description_;
+  GPBCodedInputStreamState *state = &input->state_;
+  switch (description->dataType) {
+    case GPBDataTypeBool:     return [[NSNumber alloc] initWithBool:GPBCodedInputStreamReadBool(state)];
+    case GPBDataTypeFixed32:  return [[NSNumber alloc] initWithUnsignedInt:GPBCodedInputStreamReadFixed32(state)];
+    case GPBDataTypeSFixed32: return [[NSNumber alloc] initWithInt:GPBCodedInputStreamReadSFixed32(state)];
+    case GPBDataTypeFloat:    return [[NSNumber alloc] initWithFloat:GPBCodedInputStreamReadFloat(state)];
+    case GPBDataTypeFixed64:  return [[NSNumber alloc] initWithUnsignedLongLong:GPBCodedInputStreamReadFixed64(state)];
+    case GPBDataTypeSFixed64: return [[NSNumber alloc] initWithLongLong:GPBCodedInputStreamReadSFixed64(state)];
+    case GPBDataTypeDouble:   return [[NSNumber alloc] initWithDouble:GPBCodedInputStreamReadDouble(state)];
+    case GPBDataTypeInt32:    return [[NSNumber alloc] initWithInt:GPBCodedInputStreamReadInt32(state)];
+    case GPBDataTypeInt64:    return [[NSNumber alloc] initWithLongLong:GPBCodedInputStreamReadInt64(state)];
+    case GPBDataTypeSInt32:   return [[NSNumber alloc] initWithInt:GPBCodedInputStreamReadSInt32(state)];
+    case GPBDataTypeSInt64:   return [[NSNumber alloc] initWithLongLong:GPBCodedInputStreamReadSInt64(state)];
+    case GPBDataTypeUInt32:   return [[NSNumber alloc] initWithUnsignedInt:GPBCodedInputStreamReadUInt32(state)];
+    case GPBDataTypeUInt64:   return [[NSNumber alloc] initWithUnsignedLongLong:GPBCodedInputStreamReadUInt64(state)];
+    case GPBDataTypeBytes:    return GPBCodedInputStreamReadRetainedBytes(state);
+    case GPBDataTypeString:   return GPBCodedInputStreamReadRetainedString(state);
+    case GPBDataTypeEnum:     return [[NSNumber alloc] initWithInt:GPBCodedInputStreamReadEnum(state)];
+    case GPBDataTypeGroup:
+    case GPBDataTypeMessage: {
+      GPBMessage *message;
+      if (existingValue) {
+        message = [existingValue retain];
+      } else {
+        GPBDescriptor *decriptor = [extension.msgClass descriptor];
+        message = [[decriptor.messageClass alloc] init];
+      }
+
+      if (description->dataType == GPBDataTypeGroup) {
+        [input readGroup:description->fieldNumber
+                 message:message
+            extensionRegistry:extensionRegistry];
+      } else {
+        // description->dataType == GPBDataTypeMessage
+        if (GPBExtensionIsWireFormat(description)) {
+          // For MessageSet fields the message length will have already been
+          // read.
+          [message mergeFromCodedInputStream:input
+                           extensionRegistry:extensionRegistry];
+        } else {
+          [input readMessage:message extensionRegistry:extensionRegistry];
+        }
+      }
+
+      return message;
+    }
+  }
+
+  return nil;
+}