On Wed, Mar 06, 2019 at 06:14:09PM -0800, Bart Van Assche wrote: > On 3/6/19 5:24 PM, Jason Gunthorpe wrote: > > 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) > > Hi Jason, > > I don't think that gcc accepts something like is_signed_type(typeof(a)) so > I'm not sure that the is_signed_type() macro is useful in this context. > > > > + 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) > > > > ? > > I have tested this patch. With this patch applied no warnings are reported > while building the mlx5 driver and the tests in lib/test_overflow.c pass.
Bart, My simple patch passes too :). > > Thanks, > > Bart.
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