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

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Message-ID: <CAH7mPvgqLf5x5QvdP1u1hpJCD+p2vy3aj=nt0RsHQH+aKTdovA@mail.gmail.com>
Date:   Sat, 23 Dec 2017 22:28:37 -0500
From:   Nick Desaulniers <nick.desaulniers@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Minchan Kim <minchan@...nel.org>, Nitin Gupta <ngupta@...are.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-mm@...ck.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] zsmalloc: use U suffix for negative literals being shifted

On Sat, Dec 23, 2017 at 10:24 PM, Matthew Wilcox <willy@...radead.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