On Wed, 29 Oct 2025, David Laight wrote:
> Since the overflow check always triggers when the divisor is zero
> move the check for divide by zero inside the overflow check.
> This means there is only one test in the normal path.
>
> Signed-off-by: David Laight <david.laight.linux@gmail.com>
Reviewed-by: Nicolas Pitre <npitre@baylibre.com>
> ---
>
> V3 contained a different patch 2 that did different chenges to the
> error paths.
>
> lib/math/div64.c | 19 +++++++++----------
> 1 file changed, 9 insertions(+), 10 deletions(-)
>
> diff --git a/lib/math/div64.c b/lib/math/div64.c
> index 0ebff850fd4d..1092f41e878e 100644
> --- a/lib/math/div64.c
> +++ b/lib/math/div64.c
> @@ -212,12 +212,16 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
>
> #endif
>
> - /* make sure d is not zero, trigger runtime exception otherwise */
> - if (unlikely(d == 0)) {
> - unsigned long zero = 0;
> + if (unlikely(n_hi >= d)) {
> + /* trigger runtime exception if divisor is zero */
> + if (d == 0) {
> + unsigned long zero = 0;
>
> - OPTIMIZER_HIDE_VAR(zero);
> - return ~0UL/zero;
> + OPTIMIZER_HIDE_VAR(zero);
> + return ~0UL/zero;
> + }
> + /* overflow: result is unrepresentable in a u64 */
> + return ~0ULL;
> }
>
> int shift = __builtin_ctzll(d);
> @@ -234,11 +238,6 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 d)
> */
> }
>
> - if (n_hi >= d) {
> - /* overflow: result is unrepresentable in a u64 */
> - return -1;
> - }
> -
> /* Do the full 128 by 64 bits division */
>
> shift = __builtin_clzll(d);
> --
> 2.39.5
>
>