diff --git a/cache.h b/cache.h
index b1fd3d58ab..4d014541ab 100644
--- a/cache.h
+++ b/cache.h
@@ -1023,6 +1023,16 @@ extern const struct object_id null_oid;
 
 static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 {
+	/*
+	 * This is a temporary optimization hack. By asserting the size here,
+	 * we let the compiler know that it's always going to be 20, which lets
+	 * it turn this fixed-size memcmp into a few inline instructions.
+	 *
+	 * This will need to be extended or ripped out when we learn about
+	 * hashes of different sizes.
+	 */
+	if (the_hash_algo->rawsz != 20)
+		BUG("hash size not yet supported by hashcmp");
 	return memcmp(sha1, sha2, the_hash_algo->rawsz);
 }