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