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From: Denis Efremov <efremov@ispras.ru>
To: Matthew Wilcox <willy@infradead.org>
Cc: 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()
Date: Fri, 13 Sep 2019 14:48:15 +0300
Message-ID: <85d9e45a-9631-a139-2d65-86a6753a35e6@ispras.ru> (raw)
In-Reply-To: <20190826183956.GF15933@bombadil.infradead.org>

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

  reply index

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-21  7:42 [PATCH] lib/memweight.c: optimize by inlining bitmap_weight() Denis Efremov
2019-08-22  1:25 ` Andrew Morton
2019-08-22  7:30   ` Denis Efremov
2019-08-24 10:01 ` [PATCH v2] lib/memweight.c: open codes bitmap_weight() Denis Efremov
2019-08-25  6:11   ` Matthew Wilcox
2019-08-25 11:39     ` Denis Efremov
2019-08-26 18:39       ` Matthew Wilcox
2019-09-13 11:48         ` Denis Efremov [this message]
2019-09-13 13:41           ` efremov

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