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SubjectRe: [PATCH] Avoid that check_shl_overflow() triggers a compiler warning when building with W=1
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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@mellanox.com>
> Cc: Leon Romanovsky <leonro@mellanox.com>
> Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Fixes: 0c66847793d1 ("overflow.h: Add arithmetic shift helper") # v4.19
> Signed-off-by: Bart Van Assche <bvanassche@acm.org>
> 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

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