linux-kernel - Re: [PATCH v2] lib/memweight.c: open codes bitmap

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Date:   Fri, 13 Sep 2019 16:41:24 +0300
From:   efremov <efremov@...ras.ru>
To:     Denis Efremov <efremov@...ras.ru>
Cc:     Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org,
        Akinobu Mita <akinobu.mita@...il.com>, Jan Kara <jack@...e.cz>,
        linux-kernel@...r.kernel.org, Matthew Wilcox <matthew@....cx>,
        dm-devel@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-media@...r.kernel.org, Erdem Tumurov <erdemus@...il.com>,
        Vladimir Shelekhov <vshel@....nsk.su>
Subject: Re: [PATCH v2] lib/memweight.c: open codes bitmap_weight()

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