linux-kernel - Re: [PATCH v21 2/6] block: Fix bio_flagged() so that gcc can better optimise it

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Message-ID: <20230523080720.3eovz5wbwmpckhsm@quack3>
Date:   Tue, 23 May 2023 10:07:20 +0200
From:   Jan Kara <jack@...e.cz>
To:     David Howells <dhowells@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Christoph Hellwig <hch@....de>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [PATCH v21 2/6] block: Fix bio_flagged() so that gcc can better
 optimise it

On Mon 22-05-23 21:57:40, David Howells wrote:
> Fix bio_flagged() so that multiple instances of it, such as:
> 
> 	if (bio_flagged(bio, BIO_PAGE_REFFED) ||
> 	    bio_flagged(bio, BIO_PAGE_PINNED))
> 
> can be combined by the gcc optimiser into a single test in assembly
> (arguably, this is a compiler optimisation issue[1]).
> 
> The missed optimisation stems from bio_flagged() comparing the result of
> the bitwise-AND to zero.  This results in an out-of-line bio_release_page()
> being compiled to something like:
> 
>    <+0>:     mov    0x14(%rdi),%eax
>    <+3>:     test   $0x1,%al
>    <+5>:     jne    0xffffffff816dac53 <bio_release_pages+11>
>    <+7>:     test   $0x2,%al
>    <+9>:     je     0xffffffff816dac5c <bio_release_pages+20>
>    <+11>:    movzbl %sil,%esi
>    <+15>:    jmp    0xffffffff816daba1 <__bio_release_pages>
>    <+20>:    jmp    0xffffffff81d0b800 <__x86_return_thunk>
> 
> However, the test is superfluous as the return type is bool.  Removing it
> results in:
> 
>    <+0>:     testb  $0x3,0x14(%rdi)
>    <+4>:     je     0xffffffff816e4af4 <bio_release_pages+15>
>    <+6>:     movzbl %sil,%esi
>    <+10>:    jmp    0xffffffff816dab7c <__bio_release_pages>
>    <+15>:    jmp    0xffffffff81d0b7c0 <__x86_return_thunk>
> 
> instead.
> 
> Also, the MOVZBL instruction looks unnecessary[2] - I think it's just
> 're-booling' the mark_dirty parameter.
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> Reviewed-by: John Hubbard <jhubbard@...dia.com>
> cc: Jens Axboe <axboe@...nel.dk>
> cc: linux-block@...r.kernel.org
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370 [1]
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108371 [2]
> Link: https://lore.kernel.org/r/167391056756.2311931.356007731815807265.stgit@warthog.procyon.org.uk/ # v6

Sure. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  include/linux/bio.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index b3e7529ff55e..7f53be035cf0 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -229,7 +229,7 @@ static inline void bio_cnt_set(struct bio *bio, unsigned int count)
>  
>  static inline bool bio_flagged(struct bio *bio, unsigned int bit)
>  {
> -	return (bio->bi_flags & (1U << bit)) != 0;
> +	return bio->bi_flags & (1U << bit);
>  }
>  
>  static inline void bio_set_flag(struct bio *bio, unsigned int bit)
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR