diff --git a/git-compat-util.h b/git-compat-util.h
index ae1bdc90a4..5b1d15fe4f 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -614,12 +614,30 @@ static inline bool strip_suffix(const char *str, const char *suffix,
 int git_open_cloexec(const char *name, int flags);
 #define git_open(name) git_open_cloexec(name, O_RDONLY)
 
-static inline size_t st_add(size_t a, size_t b)
+
+/*
+ * Help Clang; GCC generates the same instructions for both variants on
+ * x64 and aarch64.
+ */
+#ifdef __clang__
+#define st_add_overflow __builtin_add_overflow
+#else
+static inline bool st_add_overflow(size_t a, size_t b, size_t *out)
 {
 	if (unsigned_add_overflows(a, b))
+		return true;
+	*out = a + b;
+	return false;
+}
+#endif
+
+static inline size_t st_add(size_t a, size_t b)
+{
+	size_t result;
+	if (st_add_overflow(a, b, &result))
 		die("size_t overflow: %"PRIuMAX" + %"PRIuMAX,
 		    (uintmax_t)a, (uintmax_t)b);
-	return a + b;
+	return result;
 }
 #define st_add3(a,b,c)   st_add(st_add((a),(b)),(c))
 #define st_add4(a,b,c,d) st_add(st_add3((a),(b),(c)),(d))