Messages in this thread | | | Subject | Re: [PATCH] Avoid that check_shl_overflow() triggers a compiler warning when building with W=1 | From | Bart Van Assche <> | Date | Wed, 6 Mar 2019 18:14:09 -0800 |
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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 <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)
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.
Thanks,
Bart.
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