diff --git a/diff.c b/diff.c
index 469e18aed20ed0..750d1b1a6c36b5 100644
--- a/diff.c
+++ b/diff.c
@@ -3374,6 +3374,17 @@ void diff_set_mnemonic_prefix(struct diff_options *options, const char *a, const
 		options->b_prefix = b;
 }
 
+void diff_set_noprefix(struct diff_options *options)
+{
+	options->a_prefix = options->b_prefix = "";
+}
+
+void diff_set_default_prefix(struct diff_options *options)
+{
+	options->a_prefix = "a/";
+	options->b_prefix = "b/";
+}
+
 struct userdiff_driver *get_textconv(struct repository *r,
 				     struct diff_filespec *one)
 {
@@ -4674,10 +4685,9 @@ void repo_diff_setup(struct repository *r, struct diff_options *options)
 		options->flags.ignore_untracked_in_submodules = 1;
 
 	if (diff_no_prefix) {
-		options->a_prefix = options->b_prefix = "";
+		diff_set_noprefix(options);
 	} else if (!diff_mnemonic_prefix) {
-		options->a_prefix = "a/";
-		options->b_prefix = "b/";
+		diff_set_default_prefix(options);
 	}
 
 	options->color_moved = diff_color_moved_default;
@@ -5261,8 +5271,7 @@ static int diff_opt_no_prefix(const struct option *opt,
 
 	BUG_ON_OPT_NEG(unset);
 	BUG_ON_OPT_ARG(optarg);
-	options->a_prefix = "";
-	options->b_prefix = "";
+	diff_set_noprefix(options);
 	return 0;
 }
 
diff --git a/diff.h b/diff.h
index 8d770b1d579c8e..2af10bc5851115 100644
--- a/diff.h
+++ b/diff.h
@@ -497,6 +497,8 @@ void diff_tree_combined(const struct object_id *oid, const struct oid_array *par
 void diff_tree_combined_merge(const struct commit *commit, struct rev_info *rev);
 
 void diff_set_mnemonic_prefix(struct diff_options *options, const char *a, const char *b);
+void diff_set_noprefix(struct diff_options *options);
+void diff_set_default_prefix(struct diff_options *options);
 
 int diff_can_quit_early(struct diff_options *);