linux-kernel - Re: [PATCH v9 4/8] block: Fix bio_flagged() so that gcc can better optimise it

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Date:   Tue, 24 Jan 2023 11:28:25 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>
CC:     Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>,
        "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>,
        <linux-fsdevel@...r.kernel.org>, <linux-block@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, "Christoph Hellwig" <hch@....de>
Subject: Re: [PATCH v9 4/8] block: Fix bio_flagged() so that gcc can better
 optimise it

On 1/24/23 09:01, 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>
> 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
> ---
>   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 c1da63f6c808..10366b8bdb13 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -227,7 +227,7 @@ 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)
> 

I don't know how you noticed that this was even a problem! Neatly
fixed.

Reviewed-by: John Hubbard <jhubbard@...dia.com>

thanks,
-- 
John Hubbard
NVIDIA