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From: David Sterba <dsterba@suse.cz>
To: Christoph Hellwig <hch@infradead.org>
Cc: David Sterba <dsterba@suse.com>,
	linux-btrfs@vger.kernel.org,
	"Gustavo A . R . Silva" <gustavoars@kernel.org>
Subject: Re: [PATCH] btrfs: add special case to setget helpers for 64k pages
Date: Fri, 2 Jul 2021 13:06:30 +0200	[thread overview]
Message-ID: <20210702110630.GE2610@twin.jikos.cz> (raw)
In-Reply-To: <YN67+nvpQBfiLXzh@infradead.org>

On Fri, Jul 02, 2021 at 08:10:50AM +0100, Christoph Hellwig wrote:
> > +	if (INLINE_EXTENT_BUFFER_PAGES == 1) {				\
> >  		return get_unaligned_le##bits(token->kaddr + oip);	\
> > +	} else {							\
> 
> No need for an else after the return and thus no need for all the
> reformatting.

That leads to worse code, compiler does not eliminate the block that
would otherwise be in the else block. Measured on x86_64 with
instrumented code to force INLINE_EXTENT_BUFFER_PAGES = 1 this adds
+1100 bytes of code and has impact on stack consumption.

That the code that is in two branches that do not share any code is
maybe not pretty but the compiler did what I expected.  The set/get
helpers get called a lot and are performance sensitive.

This patch pre (original version), post (with dropped else):

1156210   19305   14912 1190427  122a1b pre/btrfs.ko
1157386   19305   14912 1191603  122eb3 post/btrfs.ko

DELTA: +1176

And effect on function stacks:

btrfs_set_token_32                   +8 (48 -> 56)
btrfs_set_token_64                  +16 (48 -> 64)
btrfs_set_32                        +32 (32 -> 64)
btrfs_set_16                        +32 (32 -> 64)
btrfs_set_token_16                   +8 (48 -> 56)
btrfs_set_64                        +40 (32 -> 72)
btrfs_set_token_8                    -8 (48 -> 40)

  reply	other threads:[~2021-07-02 11:09 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-01 16:00 [PATCH] btrfs: add special case to setget helpers for 64k pages David Sterba
2021-07-01 21:57 ` Gustavo A. R. Silva
2021-07-01 23:59   ` Qu Wenruo
2021-07-02  0:09     ` Gustavo A. R. Silva
2021-07-02  0:21       ` Qu Wenruo
2021-07-02  0:39         ` Gustavo A. R. Silva
2021-07-02  0:39           ` Qu Wenruo
2021-07-02  1:09             ` Gustavo A. R. Silva
2021-07-02 10:22           ` David Sterba
2021-07-02  7:10 ` Christoph Hellwig
2021-07-02 11:06   ` David Sterba [this message]
2021-07-05  8:33     ` Christoph Hellwig
2021-07-08 14:34       ` David Sterba
2021-07-14 23:37         ` Gustavo A. R. Silva
2021-07-28 15:32           ` David Sterba
2021-07-28 16:00             ` David Sterba

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