> 2025年5月6日 11:34，Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> 写道：
>
> Hi,
>
> 在 2025/05/06 10:34, Coly Li 写道:
>> For the above code, I don’t see how only unlikely(previous) help on branch prediction and prefetch,
>> IMHO the following form may help to achieve expected unlikely() result,
>> if (unlikely(previous && !stripe_ahead_of_reshape(mddev, conf, sh))) {
>> Thanks.
>
> What you mean you don't see *only unlikely*, for example:
>
> int test0(int a, int b)
> {
> if (a && b)
> return 1;
> return 0;
> }
>
> int test1(int a, int b)
> {
> if (unlikely(a) && b)
> return 1;
>
> return 0;
> }
>
> You can see unlikely will generate setne/movzbl/test assemble code:
>
> test0:
> 0x0000000000401130 <+10>: cmpl $0x0,-0x4(%rbp)
> 0x0000000000401134 <+14>: je 0x401143 <test0+29>
> 0x0000000000401136 <+16>: cmpl $0x0,-0x8(%rbp)
> 0x000000000040113a <+20>: je 0x401143 <test0+29>
>
> test1:
> 0x0000000000401154 <+10>: cmpl $0x0,-0x4(%rbp)
> 0x0000000000401158 <+14>: setne %al
> 0x000000000040115b <+17>: movzbl %al,%eax
> 0x000000000040115e <+20>: test %rax,%rax
> 0x0000000000401161 <+23>: je 0x401170 <test1+38>
> 0x0000000000401163 <+25>: cmpl $0x0,-0x8(%rbp)
>
> BTW, what you suggested is the same as:
>
> if (unlikely(previous) && unlikely(!stripe_ahead_of_reshape(mddev, conf, sh)))
I seldom use unlikely() this way, it is confused as well.
Anyway since the result seems to be similar as including whole checking statement by unlikely(), I don’t insist on this.
Thanks for the patient explanation.
Coly Li
