Re: [PATCH] Avoid that check_shl_overflow() triggers a compiler warning when building with W=1

[Jason Gunthorpe]

On Wed, Mar 06, 2019 at 05:01:53PM -0800, Bart Van Assche wrote:
> This patch avoids that the following warning is reported when building
> the mlx5 driver with W=1:
> 
> drivers/infiniband/hw/mlx5/qp.c: In function set_user_rq_size:
> ./include/linux/overflow.h:230:6: warning: comparison of unsigned expression >= 0 is always true [-Wtype-limits]
>    _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0;  \
>       ^
> drivers/infiniband/hw/mlx5/qp.c:5820:6: note: in expansion of macro check_shl_overflow
>   if (check_shl_overflow(rwq->wqe_count, rwq->wqe_shift, &rwq->buf_size))
>       ^~~~~~~~~~~~~~~~~~
> 
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Cc: Leon Romanovsky <leonro@xxxxxxxxxxxx>
> Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> Fixes: 0c66847793d1 ("overflow.h: Add arithmetic shift helper") # v4.19
> Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
>  include/linux/overflow.h | 22 ++++++++++++++++++++--
>  1 file changed, 20 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index 40b48e2133cb..8afe0c0ada6f 100644
> +++ b/include/linux/overflow.h
> @@ -202,6 +202,24 @@
>  
>  #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */
>  
> +/*
> + * Evaluate a >= 0 without triggering a compiler warning if the type of a
> + * is an unsigned type.
> + */
> +#define is_positive(a) ({					\
> +	typeof(a) _minus_one = -1LL;				\
> +	typeof((a) + 0U) _sign_mask = _minus_one > 0 ? 0 :	\

This is probably just is_signed_type(a)

> +				1ULL << (8 * sizeof(a) - 1);	\
> +								\
> +	((a) & _sign_mask) == 0;				\


This is the same sort of obfuscation that Leon was building, do you
think the & is better than his ==, >  version?

Will gcc shortcircuit the warning if we write it as

(is_signed_type(a) && a < 0)

?

Jason