Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8;
 format=flowed
Content-Transfer-Encoding: 8bit
Date:   Fri, 13 Sep 2019 16:41:24 +0300
From:   efremov <efremov@ispras.ru>
To:     Denis Efremov <efremov@ispras.ru>
Cc:     Matthew Wilcox <willy@infradead.org>, akpm@linux-foundation.org,
        Akinobu Mita <akinobu.mita@gmail.com>, Jan Kara <jack@suse.cz>,
        linux-kernel@vger.kernel.org, Matthew Wilcox <matthew@wil.cx>,
        dm-devel@redhat.com, linux-fsdevel@vger.kernel.org,
        linux-media@vger.kernel.org, Erdem Tumurov <erdemus@gmail.com>,
        Vladimir Shelekhov <vshel@iis.nsk.su>
Subject: Re: [PATCH v2] lib/memweight.c: open codes bitmap_weight()
Organization: ISPRAS
In-Reply-To: <85d9e45a-9631-a139-2d65-86a6753a35e6@ispras.ru>
References: <20190821074200.2203-1-efremov@ispras.ru>
 <20190824100102.1167-1-efremov@ispras.ru>
 <20190825061158.GC28002@bombadil.infradead.org>
 <ba051566-0343-ea75-0484-8852f65a15da@ispras.ru>
 <20190826183956.GF15933@bombadil.infradead.org>
 <85d9e45a-9631-a139-2d65-86a6753a35e6@ispras.ru>
Message-ID: <b9471f7165bf57e348729a09e07d7055@ispras.ru>
User-Agent: Roundcube Webmail/1.1.2
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Sorry, no question, pointer alignment of course.

Denis Efremov писал 2019-09-13 14:48:
> Hi,
> 
> Sorry for reviving this conversation, but it looks to me like
> this function could be reduced to a single bitmap_weight call:
> 
> static inline size_t memweight(const void *ptr, size_t bytes)
> {
>         BUG_ON(bytes >= UINT_MAX / BITS_PER_BYTE);
>         return bitmap_weight(ptr, bytes * BITS_PER_BYTE);
> }
> 
> Comparing to the current implementation
> https://elixir.bootlin.com/linux/latest/source/lib/memweight.c#L11
> this results in a signification simplification.
> 
> __bitmap_weight already count last bits with hweight_long as we
> discussed earlier.
> 
> int __bitmap_weight(const unsigned long *bitmap, unsigned int bits)
> {
> 	...
> 	if (bits % BITS_PER_LONG)
> 		w += hweight_long(bitmap[k] & BITMAP_LAST_WORD_MASK(bits));
> 	...
> }
> 
> and __arch_hweight* functions use popcnt instruction.
> 
> I've briefly tested the equivalence of 2 implementations on x86_64 with
> fuzzing here: 
> https://gist.github.com/evdenis/95a8b9b8041e09368b31c3a9510491a5
> 
> What do you think making this function static inline and moving it
> to include/linux/string.h? I could prepare a patch for it and add some 
> tests for
> memweight and bitmap_weight. Or maybe I miss something again?
> 
> Best regards,
> Denis