diff --git a/upb/util/BUILD b/upb/util/BUILD
index 94eb9d86c0..48657a3167 100644
--- a/upb/util/BUILD
+++ b/upb/util/BUILD
@@ -5,6 +5,8 @@ load(
     "upb_proto_reflection_library",
 )
 
+# Def to Proto
+
 cc_library(
     name = "def_to_proto",
     srcs = ["def_to_proto.c"],
@@ -47,6 +49,8 @@ cc_test(
     ],
 )
 
+# Required fields
+
 cc_library(
     name = "required_fields",
     srcs = ["required_fields.c"],
@@ -82,3 +86,12 @@ cc_test(
         ":required_fields_test_upb_proto_reflection",
     ],
 )
+
+# Compare
+
+cc_library(
+    name = "compare",
+    srcs = ["compare.c"],
+    hdrs = ["compare.h"],
+    deps = ["//:reflection"],
+)
diff --git a/upb/util/compare.c b/upb/util/compare.c
new file mode 100644
index 0000000000..32588a42bc
--- /dev/null
+++ b/upb/util/compare.c
@@ -0,0 +1,197 @@
+
+#include "upb/util/compare.h"
+
+#include <stdbool.h>
+#include <setjmp.h>
+
+#include "upb/port_def.inc"
+
+struct upb_UnknownFields;
+typedef struct upb_UnknownFields upb_UnknownFields;
+
+typedef struct {
+  uint32_t tag;
+  union {
+    uint64_t varint;
+    uint64_t uint64;
+    uint32_t uint32;
+    upb_strview delimited;
+    upb_UnknownFields* group;
+  } data;
+} upb_UnknownField;
+
+struct upb_UnknownFields {
+  size_t size;
+  size_t capacity;
+  upb_UnknownField* fields;
+};
+
+typedef struct {
+  const char *end;
+  upb_arena *arena;
+  int depth;
+  jmp_buf err;
+} upb_UnknownField_Context;
+
+static void upb_UnknownFields_Grow(upb_UnknownField_Context *ctx,
+                                   upb_UnknownField **base,
+                                   upb_UnknownField **ptr,
+                                   upb_UnknownField **end) {
+  size_t old = (*ptr - *base);
+  size_t new = UPB_MAX(4, old * 2);
+
+  *base = upb_arena_realloc(ctx->arena, *base, old * sizeof(*base),
+                            new * sizeof(*base));
+  if (!*base) UPB_LONGJMP(ctx->err, kUpb_UnknownCompareResult_OutOfMemory);
+
+  *ptr = *base + old;
+  *end = *base + new;
+}
+
+static const char *upb_UnknownFields_ParseVarint(const char *ptr,
+                                                 const char *limit,
+                                                 uint64_t *val) {
+  uint8_t byte;
+  int bitpos = 0;
+  *val = 0;
+
+  do {
+    // Unknown field data must be valid.
+    UPB_ASSERT(bitpos < 70 && ptr < limit);
+    byte = *ptr;
+    *val |= (uint64_t)(byte & 0x7F) << bitpos;
+    ptr++;
+    bitpos += 7;
+  } while (byte & 0x80);
+
+  return ptr;
+}
+
+static upb_UnknownFields *upb_UnknownFields_DoBuild(
+    upb_UnknownField_Context *ctx, const char **buf) {
+  upb_UnknownField *arr_base = NULL;
+  upb_UnknownField *arr_ptr = NULL;
+  upb_UnknownField *arr_end = NULL;
+  const char *ptr = *buf;
+  while (ptr < ctx->end) {
+    if (arr_ptr == arr_end) {
+      upb_UnknownFields_Grow(ctx, &arr_base, &arr_ptr, &arr_end);
+    }
+    upb_UnknownField *field = arr_ptr;
+    arr_ptr++;
+    uint64_t val;
+    ptr = upb_UnknownFields_ParseVarint(ptr, ctx->end, &val);
+    UPB_ASSERT(val <= UINT32_MAX);
+    field->tag = val;
+    switch (field->tag & 7) {
+      case UPB_WIRE_TYPE_VARINT:
+        ptr = upb_UnknownFields_ParseVarint(ptr, ctx->end, &field->data.varint);
+        break;
+      case UPB_WIRE_TYPE_64BIT:
+        UPB_ASSERT(ctx->end - ptr >= 8);
+        memcpy(&field->data.uint64, ptr, 8);
+        ptr += 8;
+        break;
+      case UPB_WIRE_TYPE_32BIT:
+        UPB_ASSERT(ctx->end - ptr >= 4);
+        memcpy(&field->data.uint32, ptr, 4);
+        ptr += 8;
+        break;
+      case UPB_WIRE_TYPE_DELIMITED: {
+        uint64_t size;
+        ptr = upb_UnknownFields_ParseVarint(ptr, ctx->end, &size);
+        UPB_ASSERT(ctx->end - ptr >= size);
+        field->data.delimited.data = ptr;
+        field->data.delimited.size = size;
+        break;
+      }
+      case UPB_WIRE_TYPE_START_GROUP:
+        if (--ctx->depth == 0) {
+          UPB_LONGJMP(ctx->err, kUpb_UnknownCompareResult_MaxDepthExceeded);
+        }
+        field->data.group = upb_UnknownFields_DoBuild(ctx, &ptr);
+        ctx->depth++;
+        break;
+      case UPB_WIRE_TYPE_END_GROUP:
+        goto done;
+      default:
+        UPB_UNREACHABLE();
+    }
+  }
+
+done:
+  *buf = ptr;
+  upb_UnknownFields *ret = upb_arena_malloc(ctx->arena, sizeof(*ret));
+  if (!ret) UPB_LONGJMP(ctx->err, kUpb_UnknownCompareResult_OutOfMemory);
+  ret->fields = arr_base;
+  ret->size = arr_ptr - arr_base;
+  ret->capacity = arr_end - arr_base;
+  return ret;
+}
+
+static upb_UnknownFields *upb_UnknownFields_Build(upb_UnknownField_Context *ctx,
+                                                  const char *buf,
+                                                  size_t size) {
+  ctx->end = buf + size;
+  upb_UnknownFields *fields = upb_UnknownFields_DoBuild(ctx, &buf);
+  UPB_ASSERT(buf == ctx->end);
+  return fields;
+}
+
+static bool upb_UnknownFields_IsEqual(const upb_UnknownFields *uf1,
+                                      const upb_UnknownFields *uf2) {
+  if (uf1->size != uf2->size) return false;
+  for (size_t i = 0, n = uf1->size; i < n; i++) {
+    upb_UnknownField *f1 = &uf1->fields[i];
+    upb_UnknownField *f2 = &uf2->fields[i];
+    if (f1->tag != f2->tag) return false;
+    switch (f1->tag & 7) {
+      case UPB_WIRE_TYPE_VARINT:
+        if (f1->data.varint != f2->data.varint) return false;
+        break;
+      case UPB_WIRE_TYPE_64BIT:
+        if (f1->data.uint64 != f2->data.uint64) return false;
+        break;
+      case UPB_WIRE_TYPE_32BIT:
+        if (f1->data.uint32 != f2->data.uint32) return false;
+        break;
+      case UPB_WIRE_TYPE_DELIMITED:
+        if (!upb_strview_eql(f1->data.delimited, f2->data.delimited)) {
+          return false;
+        }
+        break;
+      case UPB_WIRE_TYPE_START_GROUP:
+        if (!upb_UnknownFields_IsEqual(f1->data.group, f2->data.group)) {
+          return false;
+        }
+        break;
+      default:
+        UPB_UNREACHABLE();
+    }
+  }
+  return true;
+}
+
+upb_UnknownCompareResult upb_Message_UnknownFieldsAreEqual(const upb_msg *msg1,
+                                                           const upb_msg *msg2,
+                                                           int max_depth) {
+  size_t size1, size2;
+  const char *buf1 = upb_msg_getunknown(msg1, &size1);
+  const char *buf2 = upb_msg_getunknown(msg2, &size2);
+  if (size1 == 0 && size2 == 0) return kUpb_UnknownCompareResult_Equal;
+  if (size1 == 0 || size2 == 0) return kUpb_UnknownCompareResult_NotEqual;
+  if (memcmp(buf1, buf2, size1) == 0) return kUpb_UnknownCompareResult_Equal;
+
+  upb_UnknownField_Context ctx = {
+    .arena = upb_arena_new(),
+    .depth = max_depth,
+  };
+  if (!ctx.arena) return kUpb_UnknownCompareResult_OutOfMemory;
+
+  upb_UnknownFields *uf1 = upb_UnknownFields_Build(&ctx, buf1, size1);
+  upb_UnknownFields *uf2 = upb_UnknownFields_Build(&ctx, buf2, size2);
+  bool ret = upb_UnknownFields_IsEqual(uf1, uf2);
+  upb_arena_free(ctx.arena);
+  return ret ? kUpb_UnknownCompareResult_Equal
+             : kUpb_UnknownCompareResult_NotEqual;
+}
diff --git a/upb/util/compare.h b/upb/util/compare.h
new file mode 100644
index 0000000000..944e71683a
--- /dev/null
+++ b/upb/util/compare.h
@@ -0,0 +1,61 @@
+/*
+ * Copyright (c) 2009-2021, Google LLC
+ * All rights reserved.
+ *
+ * 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 LLC 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 Google LLC 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.
+ */
+
+#ifndef UPB_UTIL_COMPARE_H_
+#define UPB_UTIL_COMPARE_H_
+
+#include "upb/def.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Returns true if unknown fields from the two messages are equal when sorted
+// and varints are made canonical.
+//
+// These semantics are unfortunate, as the comparison is lossy without schema
+// data:
+//  1. We don't know whether delimited fields are sub-messages. Unknown
+//     sub-messages will therefore not have their fields sorted and varints
+//     canonicalized.
+//  2. We don't know about oneof/non-repeated fields, which should semantically
+//     discard every value except the last.
+typedef enum {
+    kUpb_UnknownCompareResult_Equal = 0,
+    kUpb_UnknownCompareResult_NotEqual = 1,
+    kUpb_UnknownCompareResult_OutOfMemory = 2,
+    kUpb_UnknownCompareResult_MaxDepthExceeded = 3,
+} upb_UnknownCompareResult;
+upb_UnknownCompareResult upb_Message_UnknownFieldsAreEqual(const upb_msg *msg1,
+                                                           const upb_msg *msg2,
+                                                           int max_depth);
+
+#ifdef __cplusplus
+}  /* extern "C" */
+#endif
+
+#endif  /* UPB_UTIL_COMPARE_H_ */