linux-kernel - Re: [PATCH] iov_iter: optimise iter type checking

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Date:   Sun, 6 Dec 2020 16:01:14 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iov_iter: optimise iter type checking

On 21/11/2020 14:37, Pavel Begunkov wrote:
> The problem here is that iov_iter_is_*() helpers check types for
> equality, but all iterate_* helpers do bitwise ands. This confuses
> compilers, so even if some cases were handled separately with
> iov_iter_is_*(), corresponding ifs in iterate*() right after are not
> eliminated.
> 
> E.g. iov_iter_npages() first handles discards, but iterate_all_kinds()
> still checks for discard iter type and generates unreachable code down
> the line.

Ping. This one should be pretty simple

> 
>            text    data     bss     dec     hex filename
> before:   24409     805       0   25214    627e lib/iov_iter.o
> after:    23977     805       0   24782    60ce lib/iov_iter.o
> 
> Reviewed-by: Jens Axboe <axboe@...nel.dk>
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  include/linux/uio.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index 72d88566694e..c5970b2d3307 100644
> --- a/include/linux/uio.h
> +++ b/include/linux/uio.h
> @@ -57,27 +57,27 @@ static inline enum iter_type iov_iter_type(const struct iov_iter *i)
>  
>  static inline bool iter_is_iovec(const struct iov_iter *i)
>  {
> -	return iov_iter_type(i) == ITER_IOVEC;
> +	return iov_iter_type(i) & ITER_IOVEC;
>  }
>  
>  static inline bool iov_iter_is_kvec(const struct iov_iter *i)
>  {
> -	return iov_iter_type(i) == ITER_KVEC;
> +	return iov_iter_type(i) & ITER_KVEC;
>  }
>  
>  static inline bool iov_iter_is_bvec(const struct iov_iter *i)
>  {
> -	return iov_iter_type(i) == ITER_BVEC;
> +	return iov_iter_type(i) & ITER_BVEC;
>  }
>  
>  static inline bool iov_iter_is_pipe(const struct iov_iter *i)
>  {
> -	return iov_iter_type(i) == ITER_PIPE;
> +	return iov_iter_type(i) & ITER_PIPE;
>  }
>  
>  static inline bool iov_iter_is_discard(const struct iov_iter *i)
>  {
> -	return iov_iter_type(i) == ITER_DISCARD;
> +	return iov_iter_type(i) & ITER_DISCARD;
>  }
>  
>  static inline unsigned char iov_iter_rw(const struct iov_iter *i)
> 

-- 
Pavel Begunkov