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@xxxxxxxxxxxxxxxxx>
---
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;
}
