diff --git a/cache.h b/cache.h
index 5e7381eb1e..6761554e6c 100644
--- a/cache.h
+++ b/cache.h
@@ -225,6 +225,21 @@ extern void verify_non_filename(const char *prefix, const char *name);
 
 #define alloc_nr(x) (((x)+16)*3/2)
 
+/*
+ * Realloc the buffer pointed at by variable 'x' so that it can hold
+ * at least 'nr' entries; the number of entries currently allocated
+ * is 'alloc', using the standard growing factor alloc_nr() macro.
+ *
+ * DO NOT USE any expression with side-effect for 'x' or 'alloc'.
+ */
+#define ALLOC_GROW(x, nr, alloc) \
+	do { \
+		if ((nr) >= alloc) { \
+			alloc = alloc_nr(alloc); \
+			x = xrealloc((x), alloc * sizeof(*(x))); \
+		} \
+	} while(0)
+
 /* Initialize and use the cache information */
 extern int read_index(struct index_state *);
 extern int read_index_from(struct index_state *, const char *path);
diff --git a/dir.c b/dir.c
index f543f50f42..5ba6030e9a 100644
--- a/dir.c
+++ b/dir.c
@@ -271,27 +271,26 @@ int excluded(struct dir_struct *dir, const char *pathname)
 	return 0;
 }
 
-struct dir_entry *dir_add_name(struct dir_struct *dir, const char *pathname, int len)
-{
+static struct dir_entry *dir_entry_new(const char *pathname, int len) {
 	struct dir_entry *ent;
 
-	if (cache_name_pos(pathname, len) >= 0)
-		return NULL;
-
-	if (dir->nr == dir->alloc) {
-		int alloc = alloc_nr(dir->alloc);
-		dir->alloc = alloc;
-		dir->entries = xrealloc(dir->entries, alloc*sizeof(ent));
-	}
 	ent = xmalloc(sizeof(*ent) + len + 1);
 	ent->ignored = ent->ignored_dir = 0;
 	ent->len = len;
 	memcpy(ent->name, pathname, len);
 	ent->name[len] = 0;
-	dir->entries[dir->nr++] = ent;
 	return ent;
 }
 
+struct dir_entry *dir_add_name(struct dir_struct *dir, const char *pathname, int len)
+{
+	if (cache_name_pos(pathname, len) >= 0)
+		return NULL;
+
+	ALLOC_GROW(dir->entries, dir->nr, dir->alloc);
+	return dir->entries[dir->nr++] = dir_entry_new(pathname, len);
+}
+
 enum exist_status {
 	index_nonexistent = 0,
 	index_directory,