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

[Hao Xu <haoxu@linux.alibaba.com>]

在 2021/9/28 下午7:08, Pavel Begunkov 写道:
> 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
hmm, this is definitely not good, not sure why this is not optimised to
cmpq $0, %rax (haven't touched assembly for a long time..)
> 	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;
>>   }
>>   
>>
>