[PATCH 2/2] use __builtin_add_overflow() in st_add() with Clang

["René Scharfe" <l.s.r@web.de>]

Clang and GCC optimize away comparisons of overflow checks by checking
the carry flag on x64.  GCC does the same on ARM64, but Clang currently
(version 22.1) doesn't.

Provide a variant of st_add() that wraps __builtin_add_overflow() to
help Clang optimize it.  Use it on all platforms for simplicity.

On an Apple M1 I get a nice speedup for a command that builds lots of
strings using a strbuf, which exercises the st_add3() in strbuf_grow()
for every line of output:

Benchmark 1: ./git_main cat-file --batch-all-objects --batch-check='%(objectname)'
  Time (mean ± σ):     119.8 ms ±   0.2 ms    [User: 113.0 ms, System: 5.8 ms]
  Range (min … max):   119.6 ms … 120.4 ms    24 runs

Benchmark 2: ./git cat-file --batch-all-objects --batch-check='%(objectname)'
  Time (mean ± σ):     114.6 ms ±   0.1 ms    [User: 107.6 ms, System: 6.0 ms]
  Range (min … max):   114.4 ms … 114.9 ms    25 runs

Summary
  ./git cat-file --batch-all-objects --batch-check='%(objectname)' ran
    1.05 ± 0.00 times faster than ./git_main cat-file --batch-all-objects --batch-check='%(objectname)'

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
---
 git-compat-util.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/git-compat-util.h b/git-compat-util.h
index ae1bdc90a4..aa088d04bb 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -614,6 +614,17 @@ 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)
+{
+	size_t sum;
+	if (__builtin_add_overflow(a, b, &sum))
+		die("size_t overflow: %"PRIuMAX" + %"PRIuMAX,
+		    (uintmax_t)a, (uintmax_t)b);
+	return sum;
+}
+#else
 static inline size_t st_add(size_t a, size_t b)
 {
 	if (unsigned_add_overflows(a, b))
@@ -621,6 +632,7 @@ static inline size_t st_add(size_t a, size_t b)
 		    (uintmax_t)a, (uintmax_t)b);
 	return a + b;
 }
+#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))
 
-- 
2.54.0