Re: [Qemu-devel] [PATCH] util: Fix MIN_NON_ZERO

[Eric Blake <eblake@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 1519 bytes --]

On 07/12/2016 12:48 AM, Fam Zheng wrote:
> MIN_NON_ZERO(0, 1) is evaluated to 0. Rewrite the macro to fix it.

Huh?

Old expansion, in various stages:

(((0) != 0 && (0) < (1)) ? (0) : (1))
((0 && 1) ? 0 : 1)
(0 ? 0 : 1)
1

Maybe you meant MIN_NON_ZERO(1, 0), which evaluates to:

(((1) != 0 && (1) < (0)) ? (1) : (0))
((1 && 0) ? 1 : 0)
(0 ? 1 : 0)
0

in which case, you are correct that there is a bug.

> 
> Reported-by: Miroslav Rezanina <mrezanin@redhat.com>
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  include/qemu/osdep.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index e63da28..e4c6ae6 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -151,7 +151,8 @@ extern int daemon(int, int);
>  /* Minimum function that returns zero only iff both values are zero.
>   * Intended for use with unsigned values only. */
>  #ifndef MIN_NON_ZERO
> -#define MIN_NON_ZERO(a, b) (((a) != 0 && (a) < (b)) ? (a) : (b))
> +#define MIN_NON_ZERO(a, b) ((a) == 0 ? (b) : \
> +                                ((b) == 0 ? (a) : (MIN(a, b))))

Another way might be:

(!(a) == !(b) ? MIN(a, b) : (a) + (b))

but you have to put more mental thought into that.  I'm just fine with
your version.

With the commit message fixed,
Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 604 bytes --]