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SubjectRe: [PATCH] zsmalloc: use U suffix for negative literals being shifted
On Sat, Dec 23, 2017 at 10:24 PM, Matthew Wilcox <willy@infradead.org> 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

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