Re: -Wsizeof-array-div in mm/hugetlb.c

["Dávid Bolvanský" <david.bolvansky@gmail.com>]

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You can use extra parens.

size(arr) / (size(int))

ut 17. 9. 2019 o 23:06 Mike Kravetz <mike.kravetz@oracle.com> napísal(a):

> On 9/17/19 12:34 AM, Nathan Chancellor wrote:
> > Hi all,
> >
> > Clang recently added a new diagnostic in r371605, -Wsizeof-array-div,
> > that tries to warn when sizeof(X) / sizeof(Y) does not compute the
> > number of elements in an array X (i.e., sizeof(Y) is wrong). See that
> > commit for more details:
> >
> >
> https://github.com/llvm/llvm-project/commit/3240ad4ced0d3223149b72a4fc2a4d9b67589427
> >
> > There is a warning in mm/hugetlb.c in hugetlb_fault_mutex_hash:
> >
> > mm/hugetlb.c:4055:40: warning: expression does not compute the number of
> > elements in this array; element type is 'unsigned long', not 'u32' (aka
> > 'unsigned int') [-Wsizeof-array-div]
> >         hash = jhash2((u32 *)&key, sizeof(key)/sizeof(u32), 0);
> >                                           ~~~ ^
> > mm/hugetlb.c:4049:16: note: array 'key' declared here
> >         unsigned long key[2];
> >                       ^
> > 1 warning generated.
> >
> > Should this warning be silenced? What is the reasoning behind having key
> > be an array of unsigned longs but representing it as an array of u32s?
>
> Well, the second argument to jhash2 is "the number of u32's in the key".
> This is the reason for the sizeof(key)/sizeof(u32) calculation.  It
> certainly
> is not trying to calculate the number of elements in the array as
> suggested by
> the warning.
>
> > Would it be better to avoid the cast and have it just be an array of
> > u32s directly?
>
> I did not write this code, but it is much easier to do the assignments
> (below)
> to build the key if the array is unsigned long as opposed to u32.
>
> struct address_space *mapping;
> pgoff_t idx;
> unsigned long key[2];
>
>         key[0] = (unsigned long) mapping;
>         key[1] = idx;
>
> > u32s directly? I am not familiar with this code so I may be naive for
> > asking such questions but we'd like to get these warnings cleaned up so
> > that this warning can be useful down the road.
>
> I suppose it would be possible to change 'key' to be something else besides
> an array (such as struct or union) to eliminate the warning.  But, I would
> prefer to have some type of directive to indicate the code is ok as is.  It
> is not trying to calculate the number of elements in the array as suspected
> by the clang diagnostic.
>
> --
> Mike Kravetz
>

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