diff --git a/git-compat-util.h b/git-compat-util.h
index 5024814bd4..8809776407 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))
diff --git a/strbuf.c b/strbuf.c
index 3e04addc22..8610965d53 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -106,12 +106,10 @@ void strbuf_attach(struct strbuf *sb, void *buf, size_t len, size_t alloc)
 void strbuf_grow(struct strbuf *sb, size_t extra)
 {
 	int new_buf = !sb->alloc;
-	if (unsigned_add_overflows(extra, 1) ||
-	    unsigned_add_overflows(sb->len, extra + 1))
-		die("you want to use way too much memory");
+	size_t new_len = st_add3(sb->len, extra, 1);
 	if (new_buf)
 		sb->buf = NULL;
-	ALLOC_GROW(sb->buf, sb->len + extra + 1, sb->alloc);
+	ALLOC_GROW(sb->buf, new_len, sb->alloc);
 	if (new_buf)
 		sb->buf[0] = '\0';
 }