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

Pavel Begunkov
2021-09-28T11:09:04Z

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