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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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@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)

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.



[Index of Archives]     [Linux USB Devel]     [Video for Linux]     [Linux Audio Users]     [Photo]     [Yosemite News]     [Yosemite Photos]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux