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

From: René Scharfe <[email-masked]>
Date: Thu, 14 May 2026 22:17:52 +0200

On 5/14/26 9:12 PM, Junio C Hamano wrote:
> René Scharfe <[email-masked]> writes:
> 
>> Provide a variant of st_add() that wraps __builtin_add_overflow() to
>> help Clang optimize it.  Use it on all platforms for simplicity.
>> ...
>> +/* 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
> 
> Makes me wonder if we tweaked unsigned_add_overflows() to take an
> extra *dst parameter to match __builtin_add_overflow(), which of
> course requires us to all of 18 callsites, it might make the whole
> thing a bit simpler.  New uses of unsigned_add_overflows(), if we
> ever add them, would automatically benefit, right?

Hmm.  It sounds like a lot of churn, but it would make sure that
we use the checked result and not check a + b and then go on and
use x + y because the code de-synced at some point.

How to do it, though?  It needs to be generic and evaluate its
arguments only once.  Perhaps like this?

diff --git a/git-compat-util.h b/git-compat-util.h
index ca89cfb0b3..27fbb622d7 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -103,6 +103,21 @@ struct strbuf;
 #define unsigned_add_overflows(a, b) \
     ((b) > maximum_unsigned_value_of_type(a) - (a))
 
+static bool uint_add_overflow(uintmax_t a, uintmax_t b,
+			      uintmax_t *out, size_t out_size)
+{
+	if (b > UINTMAX_MAX - a)
+		return true;
+	a += b;
+	if (a > (UINTMAX_MAX >> (bitsizeof(uintmax_t) - CHAR_BIT * out_size)))
+		return true;
+	*out = a;
+	return false;
+}
+
+#define UINT_ADD_OVERFLOW(a, b, out) \
+	uint_add_overflow((a), (b), (out), sizeof(a))
+
 /*
  * Returns true if the multiplication of "a" and "b" will
  * overflow. The types of "a" and "b" must match and must be unsigned.
@@ -616,10 +631,11 @@ int git_open_cloexec(const char *name, int flags);
 
 static inline size_t st_add(size_t a, size_t b)
 {
-	if (unsigned_add_overflows(a, b))
+	size_t ret;
+	if (UINT_ADD_OVERFLOW(a, b, &ret))
 		die("size_t overflow: %"PRIuMAX" + %"PRIuMAX,
 		    (uintmax_t)a, (uintmax_t)b);
-	return a + b;
+	return ret;
 }
 #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))