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 <[email protected]> > Cc: Leon Romanovsky <[email protected]> > Cc: Rasmus Villemoes <[email protected]> > Fixes: 0c66847793d1 ("overflow.h: Add arithmetic shift helper") # v4.19 > Signed-off-by: Bart Van Assche <[email protected]> > 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
