Re: [XEN PATCH][for-4.19 1/9] xen/include: add macro LOWEST_POW2

[Julien Grall <julien@xxxxxxx>]

Hi,

On 06/10/2023 09:26, Nicola Vetrini wrote:
> The purpose of this macro is to encapsulate the well-known expression
> 'x & -x', that in 2's complement architectures on unsigned integers will
> give 2^ffs(x), where ffs(x) is the position of the lowest set bit in x.
>
> A deviation for ECLAIR is also introduced.
Can you explain why this is a deviation in ECLAIR rather than one with 
/* SAF-* */ (or whichever name we decide to rename to)? Is this because 
the code is correct from MISRA perspective but the tool is confused?
> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
> ---
>   automation/eclair_analysis/ECLAIR/deviations.ecl | 6 ++++++
>   xen/include/xen/macros.h                         | 6 ++++--
>   2 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/automation/eclair_analysis/ECLAIR/deviations.ecl 
> b/automation/eclair_analysis/ECLAIR/deviations.ecl
> index d8170106b449..016164643105 100644
> --- a/automation/eclair_analysis/ECLAIR/deviations.ecl
> +++ b/automation/eclair_analysis/ECLAIR/deviations.ecl
> @@ -274,6 +274,12 @@ still non-negative."
>   -config=MC3R1.R10.1,etypes+={safe, 
> "stmt(operator(logical)||node(conditional_operator||binary_conditional_operator))", 
> "dst_type(ebool||boolean)"}
>   -doc_end
>   
> +-doc_begin="The macro LOWEST_POW2 encapsulates a well-known pattern to obtain the value
> +2^ffs(x) for unsigned integers on two's complement architectures
> +(all the architectures supported by Xen satisfy this requirement)."
> +-config=MC3R1.R10.1,reports+={safe, 
> "any_area(any_loc(any_exp(macro(^LOWEST_POW2$))))"}
> +-doc_end
> +
>   ### Set 3 ###
>   
>   #
> diff --git a/xen/include/xen/macros.h b/xen/include/xen/macros.h
> index d0caae7db298..bb9a1c9a53d0 100644
> --- a/xen/include/xen/macros.h
> +++ b/xen/include/xen/macros.h
> @@ -8,8 +8,10 @@
>   #define DIV_ROUND(n, d) (((n) + (d) / 2) / (d))
>   #define DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
>   
> -#define MASK_EXTR(v, m) (((v) & (m)) / ((m) & -(m)))
> -#define MASK_INSR(v, m) (((v) * ((m) & -(m))) & (m))
> +#define LOWEST_POW2(x) ((x) & -(x))
> +
> +#define MASK_EXTR(v, m) (((v) & (m)) / LOWEST_POW2(m))
> +#define MASK_INSR(v, m) (((v) * LOWEST_POW2(m)) & (m))
>   
>   #define count_args_(dot, a1, a2, a3, a4, a5, a6, a7, a8, x, ...) x
>   #define count_args(args...) \
--
Julien Grall