Subject: [PATCH v2 0/2] use __builtin_add_overflow() in st_add() with Clang

From: René Scharfe <[email-masked]>
Date: Mon, 18 May 2026 22:25:00 +0200

Changes since v2:
- Pass variable instead of st_add3() expression to ALLOC_GROW.
- Add the helper st_add_overflow() that mimics __builtin_add_overflow()
  for size_t to avoid duplicating most of the definition of st_add().

  strbuf: use st_add3() in strbuf_grow()
  use __builtin_add_overflow() in st_add() with Clang

 git-compat-util.h | 22 ++++++++++++++++++++--
 strbuf.c          |  6 ++----
 2 files changed, 22 insertions(+), 6 deletions(-)

Interdiff against v1:
diff --git a/git-compat-util.h b/git-compat-util.h
index aa088d04bb..5b1d15fe4f 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -614,25 +614,31 @@ 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)
 
-/* Help Clang; GCC generates the same code for both variants. */
-#if defined(__clang__)
-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)
 {
-	size_t sum;
-	if (__builtin_add_overflow(a, b, &sum))
-		die("size_t overflow: %"PRIuMAX" + %"PRIuMAX,
-		    (uintmax_t)a, (uintmax_t)b);
-	return sum;
+	if (unsigned_add_overflows(a, b))
+		return true;
+	*out = a + b;
+	return false;
 }
-#else
+#endif
+
 static inline size_t st_add(size_t a, size_t b)
 {
-	if (unsigned_add_overflows(a, 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;
 }
-#endif
 #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 bb04d3910e..8610965d53 100644
--- a/strbuf.c
+++ b/strbuf.c
@@ -106,9 +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;
+	size_t new_len = st_add3(sb->len, extra, 1);
 	if (new_buf)
 		sb->buf = NULL;
-	ALLOC_GROW(sb->buf, st_add3(sb->len, extra, 1), sb->alloc);
+	ALLOC_GROW(sb->buf, new_len, sb->alloc);
 	if (new_buf)
 		sb->buf[0] = '\0';
 }
-- 
2.54.0