diff --git a/cache.h b/cache.h
index ac5ab4ef9d3..f29dbbadf77 100644
--- a/cache.h
+++ b/cache.h
@@ -677,81 +677,6 @@ void initialize_repository_version(int hash_algo, int reinit);
 void sanitize_stdfds(void);
 int daemonize(void);
 
-#define alloc_nr(x) (((x)+16)*3/2)
-
-/**
- * Dynamically growing an array using realloc() is error prone and boring.
- *
- * Define your array with:
- *
- * - a pointer (`item`) that points at the array, initialized to `NULL`
- *   (although please name the variable based on its contents, not on its
- *   type);
- *
- * - an integer variable (`alloc`) that keeps track of how big the current
- *   allocation is, initialized to `0`;
- *
- * - another integer variable (`nr`) to keep track of how many elements the
- *   array currently has, initialized to `0`.
- *
- * Then before adding `n`th element to the item, call `ALLOC_GROW(item, n,
- * alloc)`.  This ensures that the array can hold at least `n` elements by
- * calling `realloc(3)` and adjusting `alloc` variable.
- *
- * ------------
- * sometype *item;
- * size_t nr;
- * size_t alloc
- *
- * for (i = 0; i < nr; i++)
- * 	if (we like item[i] already)
- * 		return;
- *
- * // we did not like any existing one, so add one
- * ALLOC_GROW(item, nr + 1, alloc);
- * item[nr++] = value you like;
- * ------------
- *
- * You are responsible for updating the `nr` variable.
- *
- * If you need to specify the number of elements to allocate explicitly
- * then use the macro `REALLOC_ARRAY(item, alloc)` instead of `ALLOC_GROW`.
- *
- * Consider using ALLOC_GROW_BY instead of ALLOC_GROW as it has some
- * added niceties.
- *
- * DO NOT USE any expression with side-effect for 'x', 'nr', or 'alloc'.
- */
-#define ALLOC_GROW(x, nr, alloc) \
-	do { \
-		if ((nr) > alloc) { \
-			if (alloc_nr(alloc) < (nr)) \
-				alloc = (nr); \
-			else \
-				alloc = alloc_nr(alloc); \
-			REALLOC_ARRAY(x, alloc); \
-		} \
-	} while (0)
-
-/*
- * Similar to ALLOC_GROW but handles updating of the nr value and
- * zeroing the bytes of the newly-grown array elements.
- *
- * DO NOT USE any expression with side-effect for any of the
- * arguments.
- */
-#define ALLOC_GROW_BY(x, nr, increase, alloc) \
-	do { \
-		if (increase) { \
-			size_t new_nr = nr + (increase); \
-			if (new_nr < nr) \
-				BUG("negative growth in ALLOC_GROW_BY"); \
-			ALLOC_GROW(x, new_nr, alloc); \
-			memset((x) + nr, 0, sizeof(*(x)) * (increase)); \
-			nr = new_nr; \
-		} \
-	} while (0)
-
 /* Initialize and use the cache information */
 struct lock_file;
 void preload_index(struct index_state *index,
diff --git a/git-compat-util.h b/git-compat-util.h
index 58d7708296b..eb4b27f4846 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1034,31 +1034,11 @@ FILE *fopen_or_warn(const char *path, const char *mode);
  */
 int xstrncmpz(const char *s, const char *t, size_t len);
 
-/*
- * FREE_AND_NULL(ptr) is like free(ptr) followed by ptr = NULL. Note
- * that ptr is used twice, so don't pass e.g. ptr++.
+/**
+ * Common allocation utils, including ones xdiff uses, and thus are
+ * split into this file for sharing with external projects.
  */
-#define FREE_AND_NULL(p) do { free(p); (p) = NULL; } while (0)
-
-#define ALLOC_ARRAY(x, alloc) (x) = xmalloc(st_mult(sizeof(*(x)), (alloc)))
-#define CALLOC_ARRAY(x, alloc) (x) = xcalloc((alloc), sizeof(*(x)))
-#define REALLOC_ARRAY(x, alloc) (x) = xrealloc((x), st_mult(sizeof(*(x)), (alloc)))
-
-#define COPY_ARRAY(dst, src, n) copy_array((dst), (src), (n), sizeof(*(dst)) + \
-	BUILD_ASSERT_OR_ZERO(sizeof(*(dst)) == sizeof(*(src))))
-static inline void copy_array(void *dst, const void *src, size_t n, size_t size)
-{
-	if (n)
-		memcpy(dst, src, st_mult(size, n));
-}
-
-#define MOVE_ARRAY(dst, src, n) move_array((dst), (src), (n), sizeof(*(dst)) + \
-	BUILD_ASSERT_OR_ZERO(sizeof(*(dst)) == sizeof(*(src))))
-static inline void move_array(void *dst, const void *src, size_t n, size_t size)
-{
-	if (n)
-		memmove(dst, src, st_mult(size, n));
-}
+#include "git-shared-util.h"
 
 /*
  * These functions help you allocate structs with flex arrays, and copy
diff --git a/git-shared-util.h b/git-shared-util.h
new file mode 100644
index 00000000000..7b4479a0f72
--- /dev/null
+++ b/git-shared-util.h
@@ -0,0 +1,104 @@
+#ifndef GIT_SHARED_UTIL_H
+#define GIT_SHARED_UTIL_H
+
+/*
+ * FREE_AND_NULL(ptr) is like free(ptr) followed by ptr = NULL. Note
+ * that ptr is used twice, so don't pass e.g. ptr++.
+ */
+#define FREE_AND_NULL(p) do { free(p); (p) = NULL; } while (0)
+
+#define ALLOC_ARRAY(x, alloc) (x) = xmalloc(st_mult(sizeof(*(x)), (alloc)))
+#define CALLOC_ARRAY(x, alloc) (x) = xcalloc((alloc), sizeof(*(x)))
+#define REALLOC_ARRAY(x, alloc) (x) = xrealloc((x), st_mult(sizeof(*(x)), (alloc)))
+
+#define COPY_ARRAY(dst, src, n) copy_array((dst), (src), (n), sizeof(*(dst)) + \
+	BUILD_ASSERT_OR_ZERO(sizeof(*(dst)) == sizeof(*(src))))
+static inline void copy_array(void *dst, const void *src, size_t n, size_t size)
+{
+	if (n)
+		memcpy(dst, src, st_mult(size, n));
+}
+
+#define MOVE_ARRAY(dst, src, n) move_array((dst), (src), (n), sizeof(*(dst)) + \
+	BUILD_ASSERT_OR_ZERO(sizeof(*(dst)) == sizeof(*(src))))
+static inline void move_array(void *dst, const void *src, size_t n, size_t size)
+{
+	if (n)
+		memmove(dst, src, st_mult(size, n));
+}
+
+#define alloc_nr(x) (((x)+16)*3/2)
+
+/**
+ * Dynamically growing an array using realloc() is error prone and boring.
+ *
+ * Define your array with:
+ *
+ * - a pointer (`item`) that points at the array, initialized to `NULL`
+ *   (although please name the variable based on its contents, not on its
+ *   type);
+ *
+ * - an integer variable (`alloc`) that keeps track of how big the current
+ *   allocation is, initialized to `0`;
+ *
+ * - another integer variable (`nr`) to keep track of how many elements the
+ *   array currently has, initialized to `0`.
+ *
+ * Then before adding `n`th element to the item, call `ALLOC_GROW(item, n,
+ * alloc)`.  This ensures that the array can hold at least `n` elements by
+ * calling `realloc(3)` and adjusting `alloc` variable.
+ *
+ * ------------
+ * sometype *item;
+ * size_t nr;
+ * size_t alloc
+ *
+ * for (i = 0; i < nr; i++)
+ * 	if (we like item[i] already)
+ * 		return;
+ *
+ * // we did not like any existing one, so add one
+ * ALLOC_GROW(item, nr + 1, alloc);
+ * item[nr++] = value you like;
+ * ------------
+ *
+ * You are responsible for updating the `nr` variable.
+ *
+ * If you need to specify the number of elements to allocate explicitly
+ * then use the macro `REALLOC_ARRAY(item, alloc)` instead of `ALLOC_GROW`.
+ *
+ * Consider using ALLOC_GROW_BY instead of ALLOC_GROW as it has some
+ * added niceties.
+ *
+ * DO NOT USE any expression with side-effect for 'x', 'nr', or 'alloc'.
+ */
+#define ALLOC_GROW(x, nr, alloc) \
+	do { \
+		if ((nr) > alloc) { \
+			if (alloc_nr(alloc) < (nr)) \
+				alloc = (nr); \
+			else \
+				alloc = alloc_nr(alloc); \
+			REALLOC_ARRAY(x, alloc); \
+		} \
+	} while (0)
+
+/*
+ * Similar to ALLOC_GROW but handles updating of the nr value and
+ * zeroing the bytes of the newly-grown array elements.
+ *
+ * DO NOT USE any expression with side-effect for any of the
+ * arguments.
+ */
+#define ALLOC_GROW_BY(x, nr, increase, alloc) \
+	do { \
+		if (increase) { \
+			size_t new_nr = nr + (increase); \
+			if (new_nr < nr) \
+				BUG("negative growth in ALLOC_GROW_BY"); \
+			ALLOC_GROW(x, new_nr, alloc); \
+			memset((x) + nr, 0, sizeof(*(x)) * (increase)); \
+			nr = new_nr; \
+		} \
+	} while (0)
+#endif
