diff --git a/xdiff/xmacros.h b/xdiff/xmacros.h
index 0977d1615a..8487bb396f 100644
--- a/xdiff/xmacros.h
+++ b/xdiff/xmacros.h
@@ -58,4 +58,14 @@ do { \
 /* Allocate an array of nr zeroed out elements, returns NULL on failure */
 #define XDL_CALLOC_ARRAY(p, nr)	((p) = xdl_calloc(nr, sizeof(*(p))))
 
+/*
+ * Ensure array p can accommodate at least nr elements, growing the
+ * array and updating alloc (which is the number of allocated
+ * elements) as necessary. Frees p and returns -1 on failure, returns
+ * 0 on success
+ */
+#define XDL_ALLOC_GROW(p, nr, alloc)	\
+	(-!((nr) <= (alloc) ||		\
+	    ((p) = xdl_alloc_grow_helper((p), (nr), &(alloc), sizeof(*(p))))))
+
 #endif /* #if !defined(XMACROS_H) */
diff --git a/xdiff/xprepare.c b/xdiff/xprepare.c
index b016570c48..c84549f6c5 100644
--- a/xdiff/xprepare.c
+++ b/xdiff/xprepare.c
@@ -111,7 +111,6 @@ static int xdl_classify_record(unsigned int pass, xdlclassifier_t *cf, xrecord_t
 	long hi;
 	char const *line;
 	xdlclass_t *rcrec;
-	xdlclass_t **rcrecs;
 
 	line = rec->ptr;
 	hi = (long) XDL_HASHLONG(rec->ha, cf->hbits);
@@ -127,14 +126,8 @@ static int xdl_classify_record(unsigned int pass, xdlclassifier_t *cf, xrecord_t
 			return -1;
 		}
 		rcrec->idx = cf->count++;
-		if (cf->count > cf->alloc) {
-			cf->alloc *= 2;
-			if (!(rcrecs = (xdlclass_t **) xdl_realloc(cf->rcrecs, cf->alloc * sizeof(xdlclass_t *)))) {
-
+		if (XDL_ALLOC_GROW(cf->rcrecs, cf->count, cf->alloc))
 				return -1;
-			}
-			cf->rcrecs = rcrecs;
-		}
 		cf->rcrecs[rcrec->idx] = rcrec;
 		rcrec->line = line;
 		rcrec->size = rec->size;
@@ -163,7 +156,7 @@ static int xdl_prepare_ctx(unsigned int pass, mmfile_t *mf, long narec, xpparam_
 	unsigned long hav;
 	char const *blk, *cur, *top, *prev;
 	xrecord_t *crec;
-	xrecord_t **recs, **rrecs;
+	xrecord_t **recs;
 	xrecord_t **rhash;
 	unsigned long *ha;
 	char *rchg;
@@ -190,12 +183,8 @@ static int xdl_prepare_ctx(unsigned int pass, mmfile_t *mf, long narec, xpparam_
 		for (top = blk + bsize; cur < top; ) {
 			prev = cur;
 			hav = xdl_hash_record(&cur, top, xpp->flags);
-			if (nrec >= narec) {
-				narec *= 2;
-				if (!(rrecs = (xrecord_t **) xdl_realloc(recs, narec * sizeof(xrecord_t *))))
-					goto abort;
-				recs = rrecs;
-			}
+			if (XDL_ALLOC_GROW(recs, nrec + 1, narec))
+				goto abort;
 			if (!(crec = xdl_cha_alloc(&xdf->rcha)))
 				goto abort;
 			crec->ptr = prev;
diff --git a/xdiff/xutils.c b/xdiff/xutils.c
index 115b2b1640..9e36f24875 100644
--- a/xdiff/xutils.c
+++ b/xdiff/xutils.c
@@ -432,3 +432,20 @@ int xdl_fall_back_diff(xdfenv_t *diff_env, xpparam_t const *xpp,
 
 	return 0;
 }
+
+void* xdl_alloc_grow_helper(void *p, long nr, long *alloc, size_t size)
+{
+	void *tmp = NULL;
+	size_t n = ((LONG_MAX - 16) / 2 >= *alloc) ? 2 * *alloc + 16 : LONG_MAX;
+	if (nr > n)
+		n = nr;
+	if (SIZE_MAX / size >= n)
+		tmp = xdl_realloc(p, n * size);
+	if (tmp) {
+		*alloc = n;
+	} else {
+		xdl_free(p);
+		*alloc = 0;
+	}
+	return tmp;
+}
diff --git a/xdiff/xutils.h b/xdiff/xutils.h
index fba7bae03c..fd0bba94e8 100644
--- a/xdiff/xutils.h
+++ b/xdiff/xutils.h
@@ -42,6 +42,7 @@ int xdl_emit_hunk_hdr(long s1, long c1, long s2, long c2,
 int xdl_fall_back_diff(xdfenv_t *diff_env, xpparam_t const *xpp,
 		       int line1, int count1, int line2, int count2);
 
-
+/* Do not call this function, use XDL_ALLOC_GROW instead */
+void* xdl_alloc_grow_helper(void* p, long nr, long* alloc, size_t size);
 
 #endif /* #if !defined(XUTILS_H) */