[PATCH -mm] kstrtox: delete division from inner parsing loop

[PATCH -mm] kstrtox: delete division from inner parsing loop

[Alexey Dobriyan]

Overflow conditions can be checked on the fly resulting in at most
2 very predictable branches and zero division instructions:

	_parse_integer_limit:
			...
		mov rax, rdx
		mul rsi
		seto bl
		add rax, rcx
		mov rdx, rax
		setb al
			...

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---
 lib/kstrtox.c | 14 +++++---------
 1 file changed, 5 insertions(+), 9 deletions(-)

diff --git a/lib/kstrtox.c b/lib/kstrtox.c
index 97be2a39f537..efd55db0533a 100644
--- a/lib/kstrtox.c
+++ b/lib/kstrtox.c
@@ -71,15 +71,11 @@ unsigned int _parse_integer_limit(const char *s, unsigned int base, unsigned lon
 
 		if (val >= base)
 			break;
-		/*
-		 * Check for overflow only if we are within range of
-		 * it in the max base we support (16)
-		 */
-		if (unlikely(res & (~0ull << 60))) {
-			if (res > div_u64(ULLONG_MAX - val, base))
-				rv |= KSTRTOX_OVERFLOW;
-		}
-		res = res * base + val;
+
+		if (check_mul_overflow(res, base, &res))
+			rv |= KSTRTOX_OVERFLOW;
+		if (check_add_overflow(res, val, &res))
+			rv |= KSTRTOX_OVERFLOW;
 		rv++;
 		s++;
 	}
-- 
2.54.0

Re: [PATCH -mm] kstrtox: delete division from inner parsing loop

[Dmitry Antipov]

On 7/3/26 1:25 PM, Alexey Dobriyan wrote:

> Overflow conditions can be checked on the fly resulting in at most
> 2 very predictable branches and zero division instructions:
> 
> 	_parse_integer_limit:
> 			...
> 		mov rax, rdx
> 		mul rsi
> 		seto bl
> 		add rax, rcx
> 		mov rdx, rax
> 		setb al

Trust my word, there is a huge world beyond x86.

Dmitry