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

Hao Xu
2021-09-29T07:36:03Z

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;
>> }
>>
>>
>