Re: [PATCH] zsmalloc: use U suffix for negative literals being shifted

[Nick Desaulniers]

On Sat, Dec 23, 2017 at 10:24 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> On Sat, Dec 23, 2017 at 09:33:40PM -0500, Nick Desaulniers wrote:
>> Fixes warnings about shifting unsigned literals being undefined
>> behavior.
>
> Do you mean signed literals?


A sorry, s/unsigned/negative signed/g.  The warning is:

mm/zsmalloc.c:1059:20: warning: shifting a negative signed value is undefined
      [-Wshift-negative-value]
                        link->next = -1 << OBJ_TAG_BITS;
                                     ~~ ^

>
>>                        */
>> -                     link->next = -1 << OBJ_TAG_BITS;
>> +                     link->next = -1U << OBJ_TAG_BITS;
>>               }
>
> I don't understand what -1U means.  Seems like a contradiction in terms,
> a negative unsigned number.  Is this supposed to be ~0U?

$ ag \\-1U[^L]

The code base is full of that literal.  I think of it as:

(unsigned) -1