Re: [PATCH] xen: Use __auto_type

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

On 2025-05-05 14:46, Andrew Cooper wrote:
> In macros it is common to declare local variables using typeof(param) 
> in order
> to ensure that side effects are only evaluated once.  A consequence of 
> this is
> double textural expansion of the parameter, which can get out of hand 
> very
> quickly with nested macros.
> 
> A GCC extension, __auto_type, is now avaialble in the new toolchain 
> baseline
> and avoids the double textural expansion.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> CC: Anthony PERARD <anthony.perard@vates.tech>
> CC: Michal Orzel <michal.orzel@amd.com>
> CC: Jan Beulich <jbeulich@suse.com>
> CC: Julien Grall <julien@xen.org>
> CC: Roger Pau Monné <roger.pau@citrix.com>
> CC: Stefano Stabellini <sstabellini@kernel.org>
> CC: Roberto Bagnara <roberto.bagnara@bugseng.com>
> CC: Nicola Vetrini <nicola.vetrini@bugseng.com>
> CC: consulting@bugseng.com <consulting@bugseng.com>
> 
> The resulting build is identical.
> 
> RFC.  This requires a MISRA change, as it currently manifests as a R1.1
> violation.  Nevertheless, I think we want to start using in places 
> where we
> currently use typeof(expression of <initilaiser>).
> 
> Eclair run on this patch (expecting a failure):
>   
> https://gitlab.com/xen-project/hardware/xen-staging/-/pipelines/1800631949
> 

Hi,

to make the analysis pass you need a couple of hunks in 
eclair_analysis/ECLAIR/toolchain.ecl:

-name_selector+={auto_type, "^__auto_type$"}

and add auto_type to the STD.tokenext config below around line 25, then 
later

-name_selector+={ext_auto_type, "^ext_auto_type$"}

and add "ext_auto_type" to the -config lines below

around line 125, along with a reference to the gcc docs above the 
configurations and in C-language-toolchain.rst

This is an extension, so it's usable without further MISRA impact.

> Min toolchain check:
>   https://godbolt.org/z/f9WjooPYj
> 
> GCC Manual:
>   
> https://www.gnu.org/software/c-intro-and-ref/manual/html_node/Auto-Type.html
> ---
>  xen/include/xen/macros.h | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/xen/include/xen/macros.h b/xen/include/xen/macros.h
> index cd528fbdb127..b5e5ff4b1c2f 100644
> --- a/xen/include/xen/macros.h
> +++ b/xen/include/xen/macros.h
> @@ -71,18 +71,18 @@
>  /* Hide a value from the optimiser. */
>  #define HIDE(x)                                 \
>      ({                                          \
> -        typeof(x) _x = (x);                     \
> +        __auto_type _x = (x);                   \
>          asm volatile ( "" : "+r" (_x) );        \
>          _x;                                     \
>      })
> 
>  #define ABS(x) ({                              \
> -    typeof(x) x_ = (x);                        \
> +    __auto_type x_ = (x);                      \
>      (x_ < 0) ? -x_ : x_;                       \
>  })
> 
>  #define SWAP(a, b) \
> -   do { typeof(a) t_ = (a); (a) = (b); (b) = t_; } while ( 0 )
> +   do { __auto_type t_ = (a); (a) = (b); (b) = t_; } while ( 0 )
> 
>  #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]) + 
> __must_be_array(x))
> 
> @@ -110,15 +110,15 @@
>   */
>  #define min(x, y)                               \
>      ({                                          \
> -        const typeof(x) _x = (x);               \
> -        const typeof(y) _y = (y);               \
> +        const __auto_type _x = (x);             \
> +        const __auto_type _y = (y);             \
>          (void)(&_x == &_y); /* typecheck */     \
>          _x < _y ? _x : _y;                      \
>      })
>  #define max(x, y)                               \
>      ({                                          \
> -        const typeof(x) _x = (x);               \
> -        const typeof(y) _y = (y);               \
> +        const __auto_type _x = (x);             \
> +        const __auto_type _y = (y);             \
>          (void)(&_x == &_y); /* typecheck */     \
>          _x > _y ? _x : _y;                      \
>      })
> 
> base-commit: 78ce2be733b1e45e2e190c1765fe31da318d435f

-- 
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253