diff --git a/tree-diff.c b/tree-diff.c
index edb4de9b72..e164e532b2 100644
--- a/tree-diff.c
+++ b/tree-diff.c
@@ -14,6 +14,16 @@
  */
 #define S_IFXMIN_NEQ	S_DIFFTREE_IFXMIN_NEQ
 
+#define FAST_ARRAY_ALLOC(x, nr) do { \
+	if ((nr) <= 2) \
+		(x) = xalloca((nr) * sizeof(*(x))); \
+	else \
+		ALLOC_ARRAY((x), nr); \
+} while(0)
+#define FAST_ARRAY_FREE(x, nr) do { \
+	if ((nr) > 2) \
+		free((x)); \
+} while(0)
 
 static struct combine_diff_path *ll_diff_tree_paths(
 	struct combine_diff_path *p, const unsigned char *sha1,
@@ -265,7 +275,7 @@ static struct combine_diff_path *emit_path(struct combine_diff_path *p,
 	if (recurse) {
 		const unsigned char **parents_sha1;
 
-		parents_sha1 = xalloca(nparent * sizeof(parents_sha1[0]));
+		FAST_ARRAY_ALLOC(parents_sha1, nparent);
 		for (i = 0; i < nparent; ++i) {
 			/* same rule as in emitthis */
 			int tpi_valid = tp && !(tp[i].entry.mode & S_IFXMIN_NEQ);
@@ -277,7 +287,7 @@ static struct combine_diff_path *emit_path(struct combine_diff_path *p,
 		strbuf_add(base, path, pathlen);
 		strbuf_addch(base, '/');
 		p = ll_diff_tree_paths(p, sha1, parents_sha1, nparent, base, opt);
-		xalloca_free(parents_sha1);
+		FAST_ARRAY_FREE(parents_sha1, nparent);
 	}
 
 	strbuf_setlen(base, old_baselen);
@@ -402,8 +412,8 @@ static struct combine_diff_path *ll_diff_tree_paths(
 	void *ttree, **tptree;
 	int i;
 
-	tp     = xalloca(nparent * sizeof(tp[0]));
-	tptree = xalloca(nparent * sizeof(tptree[0]));
+	FAST_ARRAY_ALLOC(tp, nparent);
+	FAST_ARRAY_ALLOC(tptree, nparent);
 
 	/*
 	 * load parents first, as they are probably already cached.
@@ -531,8 +541,8 @@ static struct combine_diff_path *ll_diff_tree_paths(
 	free(ttree);
 	for (i = nparent-1; i >= 0; i--)
 		free(tptree[i]);
-	xalloca_free(tptree);
-	xalloca_free(tp);
+	FAST_ARRAY_FREE(tptree, nparent);
+	FAST_ARRAY_FREE(tp, nparent);
 
 	return p;
 }