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()
Date: Fri, 13 Sep 2019 16:41:24 +0300 [thread overview]
Message-ID: <b9471f7165bf57e348729a09e07d7055@ispras.ru> (raw)
In-Reply-To: <85d9e45a-9631-a139-2d65-86a6753a35e6@ispras.ru>
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
prev parent reply other threads:[~2019-09-13 13:41 UTC|newest]
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
2019-09-13 13:41 ` efremov [this message]
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