diff --git a/upb/util/BUILD b/upb/util/BUILD
new file mode 100644
index 0000000000..3a4b42ce18
--- /dev/null
+++ b/upb/util/BUILD
@@ -0,0 +1,8 @@
+
+cc_library(
+    name = "required_fields",
+    srcs = ["required_fields.c"],
+    hdrs = ["required_fields.h"],
+    deps = ["//:reflection"],
+    visibility = ["//visibility:public"],
+)
diff --git a/upb/util/required_fields.c b/upb/util/required_fields.c
new file mode 100644
index 0000000000..4b857dea44
--- /dev/null
+++ b/upb/util/required_fields.c
@@ -0,0 +1,181 @@
+/*
+ * 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.
+ */
+
+#include "upb/util/required_fields.h"
+
+#include <setjmp.h>
+#include <stdarg.h>
+#include <stdio.h>
+
+#include "upb/reflection.h"
+
+// Must be last.
+#include "upb/port_def.inc"
+
+typedef struct {
+  upb_arena *arena;
+  char **fields;
+  size_t count;
+  size_t size;
+  const upb_symtab *ext_pool;
+  jmp_buf err;
+} Find_Ctx;
+
+UPB_PRINTF(2, 3)
+static char *upb_arena_printf(upb_arena *arena, const char *fmt, ...) {
+  va_list args;
+  va_start(args, fmt);
+  size_t n = vsnprintf(NULL, 0, fmt, args);
+  va_end(args);
+
+  char *p = upb_arena_malloc(arena, n + 1);
+  if (!p) return NULL;
+
+  va_start(args, fmt);
+  vsnprintf(p, n + 1, fmt, args);
+  va_end(args);
+  return p;
+}
+
+UPB_PRINTF(2, 3)
+static char *upb_asprintf(char *old, const char *fmt, ...) {
+  va_list args;
+  va_start(args, fmt);
+  size_t n = vsnprintf(NULL, 0, fmt, args);
+  va_end(args);
+
+  size_t old_size = old ? strlen(old) : 0;
+  char *p = realloc(old, old_size + n + 1);
+  if (!p) return NULL;
+
+  va_start(args, fmt);
+  vsnprintf(p + old_size, n + 1, fmt, args);
+  va_end(args);
+  return p;
+}
+
+// Allocate on the heap instead of the arena, since the accumulated
+// allocations can grow quite large (O(all fields), not just set).
+bool append_str(Find_Ctx *ctx, const char *prefix, const upb_fielddef *f) {
+  if (ctx->count == ctx->size) {
+    size_t new_size = UPB_MAX(4, ctx->size * 2);
+    char **new_fields = upb_arena_realloc(ctx->arena, ctx->fields,
+                                          ctx->size * sizeof(*ctx->fields),
+                                          new_size * sizeof(*ctx->fields));
+    if (!new_fields) return false;
+    ctx->fields = new_fields;
+    ctx->size = new_size;
+  }
+  char *s = upb_arena_printf(ctx->arena, "%s.%s", prefix, upb_fielddef_name(f));
+  if (!s) return false;
+  ctx->fields[ctx->count++] = s;
+  return true;
+}
+
+// Really we should use a map key for maps, but proto2 doesn't so we don't.
+char *alloc_prefix(const char *prefix, const upb_fielddef *f, int index) {
+  char *ret = NULL;
+  if (upb_fielddef_isextension(f)) {
+    ret = upb_asprintf(ret, "(%s)", upb_fielddef_fullname(f));
+  } else {
+    ret = upb_asprintf(ret, "%s", upb_fielddef_name(f));
+  }
+
+  if (index >= 0) {
+    ret = upb_asprintf(ret, "[%d]", index);
+  }
+
+  ret = upb_asprintf(ret, ".");
+  return ret;
+}
+
+bool find_in_msg(Find_Ctx *ctx, const char *prefix, const upb_msg *msg,
+                 const upb_msgdef *m) {
+  // OPT: add markers in the schema for where we can avoid iterating:
+  // 1. messages with no required fields.
+  // 2. messages that cannot possibly reach any required fields.
+  for (int i = 0, n = upb_msgdef_fieldcount(m); i < n; i++) {
+    const upb_fielddef *f = upb_msgdef_field(m, i);
+    if (upb_fielddef_label(f) != UPB_LABEL_REQUIRED) continue;
+    if (!upb_msg_has(msg, f)) {
+      if (!append_str(ctx, prefix, f)) return false;;
+    }
+  }
+
+  size_t iter = UPB_MSG_BEGIN;
+  const upb_fielddef *f;
+  upb_msgval val;
+  while (upb_msg_next(msg, m, ctx->ext_pool, &f, &val, &iter)) {
+    // Skip non-submessage fields.
+    if (!upb_fielddef_issubmsg(f)) continue;
+    const upb_msgdef *sub_m = upb_fielddef_msgsubdef(f);
+    if (upb_fielddef_ismap(f)) {
+      const upb_fielddef *val_f = upb_msgdef_field(sub_m, 1);
+      const upb_msgdef *val_m = upb_fielddef_msgsubdef(val_f);
+      if (!val_m) continue;
+      const upb_map *map = val.map_val;
+      size_t iter = UPB_MAP_BEGIN;
+      int i = 0;
+      while (upb_mapiter_next(map, &iter)) {
+        char *sub_prefix = alloc_prefix(prefix, f, i++);
+        if (!sub_prefix) return false;
+        upb_msgval map_val = upb_mapiter_value(map, iter);
+        bool ok = find_in_msg(ctx, sub_prefix, map_val.msg_val, val_m);
+        free(sub_prefix);
+        if (!ok) return false;
+      }
+    } else if (upb_fielddef_isseq(f)) {
+      const upb_array *arr = val.array_val;
+      for (size_t i = 0, n = upb_array_size(arr); i < n; i++) {
+        upb_msgval elem = upb_array_get(arr, i);
+        char *sub_prefix = alloc_prefix(prefix, f, i);
+        if (!sub_prefix) return false;
+        bool ok = find_in_msg(ctx, sub_prefix, elem.msg_val, sub_m);
+        free(sub_prefix);
+        if (!ok) return false;
+      }
+    } else {
+      char *sub_prefix = alloc_prefix(prefix, f, -1);
+      if (!sub_prefix) return false;
+      bool ok = find_in_msg(ctx, sub_prefix, val.msg_val, sub_m);
+      free(sub_prefix);
+      if (!ok) return false;
+    }
+  }
+
+  return true;
+}
+
+bool upb_msg_findunsetrequired(const upb_msg *msg, const upb_msgdef *m,
+                               const upb_symtab *ext_pool, char ***fields,
+                               size_t *count, upb_arena *arena) {
+  Find_Ctx ctx = {arena, NULL, 0, 0, ext_pool};
+  if (!find_in_msg(&ctx, "", msg, m)) return false;
+  *count = ctx.count;
+  *fields = ctx.fields;
+  return true;
+}
diff --git a/upb/util/required_fields.h b/upb/util/required_fields.h
new file mode 100644
index 0000000000..42e3d22d34
--- /dev/null
+++ b/upb/util/required_fields.h
@@ -0,0 +1,50 @@
+/*
+ * 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_REQUIRED_FIELDS_H_
+#define UPB_UTIL_REQUIRED_FIELDS_H_
+
+#include "upb/def.h"
+
+/* Must be last. */
+#include "upb/port_def.inc"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+bool upb_msg_findunsetrequired(const upb_msg *msg, const upb_msgdef *m,
+                               const upb_symtab *ext_pool, char ***fields,
+                               size_t *count, upb_arena *arena);
+
+#ifdef __cplusplus
+}  /* extern "C" */
+#endif
+
+#include "upb/port_undef.inc"
+
+#endif  /* UPB_UTIL_REQUIRED_FIELDS_H_ */