Re: [PATCH 1/8] io-wq: code clean for io_wq_add_work_after()

[Pavel Begunkov <asml.silence@gmail.com>]

On 9/27/21 7:17 AM, Hao Xu wrote:
> Remove a local variable.

It's there to help alias analysis, which usually can't do anything
with pointer heavy logic. Compare ASMs below, before and after
respectively:
	testq	%rax, %rax	# next

replaced with
	cmpq	$0, (%rdi)	#, node_2(D)->next

One extra memory dereference and a bigger binary


=====================================================

wq_list_add_after:
# fs/io_uring.c:271: 	struct io_wq_work_node *next = pos->next;
	movq	(%rsi), %rax	# pos_3(D)->next, next
# fs/io_uring.c:273: 	pos->next = node;
	movq	%rdi, (%rsi)	# node, pos_3(D)->next
# fs/io_uring.c:275: 	if (!next)
	testq	%rax, %rax	# next
# fs/io_uring.c:274: 	node->next = next;
	movq	%rax, (%rdi)	# next, node_5(D)->next
# fs/io_uring.c:275: 	if (!next)
	je	.L5927	#,
	ret	
.L5927:
# fs/io_uring.c:276: 		list->last = node;
	movq	%rdi, 8(%rdx)	# node, list_8(D)->last
	ret	

=====================================================

wq_list_add_after:
# fs/io-wq.h:48: 	node->next = pos->next;
	movq	(%rsi), %rax	# pos_3(D)->next, _5
# fs/io-wq.h:48: 	node->next = pos->next;
	movq	%rax, (%rdi)	# _5, node_2(D)->next
# fs/io-wq.h:49: 	pos->next = node;
	movq	%rdi, (%rsi)	# node, pos_3(D)->next
# fs/io-wq.h:50: 	if (!node->next)
	cmpq	$0, (%rdi)	#, node_2(D)->next
	je	.L5924	#,
	ret	
.L5924:
# fs/io-wq.h:51: 		list->last = node;
	movq	%rdi, 8(%rdx)	# node, list_4(D)->last
	ret	


> 
> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
>  fs/io-wq.h | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/io-wq.h b/fs/io-wq.h
> index bf5c4c533760..8369a51b65c0 100644
> --- a/fs/io-wq.h
> +++ b/fs/io-wq.h
> @@ -33,11 +33,9 @@ static inline void wq_list_add_after(struct io_wq_work_node *node,
>  				     struct io_wq_work_node *pos,
>  				     struct io_wq_work_list *list)
>  {
> -	struct io_wq_work_node *next = pos->next;
> -
> +	node->next = pos->next;
>  	pos->next = node;
> -	node->next = next;
> -	if (!next)
> +	if (!node->next)
>  		list->last = node;
>  }
>  
> 

-- 
Pavel Begunkov