linux-kernel - Re: [PATCH v4 1/5] lib/bitmap: add bitmap_{set,get}

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Fri, 4 Aug 2023 23:05:19 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Alexander Potapenko <glider@...gle.com>, catalin.marinas@....com,
        will@...nel.org, pcc@...gle.com, andreyknvl@...il.com,
        linux@...musvillemoes.dk, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
        syednwaris@...il.com, william.gray@...aro.org,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v4 1/5] lib/bitmap: add bitmap_{set,get}_value()

On Fri, Aug 04, 2023 at 12:55:01PM -0700, Yury Norov wrote:
> On Fri, Aug 04, 2023 at 06:07:00PM +0200, Alexander Potapenko wrote:

> > > > ~BITMAP_FIRST_WORD_MASK(start));
> > >
> > > As I said, ~BITMAP_FIRST_WORD_MASK() is the same as BITMAP_LAST_WORD_MASK()
> > > and vise-versa.
> > 
> > Surprisingly, ~BITMAP_FIRST_WORD_MASK() generates better code than
> > BITMAP_LAST_WORD_MASK().
>  
> Wow... If that's consistent across different compilers/arches, we'd
> just drop the latter. Thanks for pointing that. I'll check.

It might be...

...

> >         map[index] &= (fit ? (~(GENMASK(nbits - 1, 0) << offset)) :
> > ~BITMAP_FIRST_WORD_MASK(start));

...that both parts of ternary are using negation. So compiler may use negation
only once. (Haven't looked at the assembly.)

-- 
With Best Regards,
Andy Shevchenko