diff --git a/ebtree/ebtree.c b/ebtree/ebtree.c
index fb266ec..e82ed59 100644
--- a/ebtree/ebtree.c
+++ b/ebtree/ebtree.c
@@ -30,3 +30,21 @@
 {
 	return __eb_insert_dup(sub, new);
 }
+
+/* compares memory blocks m1 and m2 for up to <len> bytes. Immediately stops at
+ * the first non-matching byte. It returns 0 on full match, non-zero otherwise.
+ * One byte will always be checked so this must not be called with len==0. It
+ * takes 2+5cy/B on x86_64 and is ~29 bytes long.
+ */
+int eb_memcmp(const void *m1, const void *m2, size_t len)
+{
+	const char *p1 = (const char *)m1 + len;
+	const char *p2 = (const char *)m2 + len;
+	ssize_t ofs = -len;
+	char diff;
+
+	do {
+		diff = p1[ofs] - p2[ofs];
+	} while (!diff && ++ofs);
+	return diff;
+}