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.
