Re: [PATCH] mm: migrate: Use bool instead of int for the return value of PageMovable

[Chen Gang]

On 7/12/16 15:48, Michal Hocko wrote:
> On Tue 12-07-16 03:47:42, Chen Gang wrote:
> [...]
>> In our case, the 2 output size are same, but under x86_64, the insns are
>> different. After uses bool, it uses push/pop instead of branch, for me,
>> it should be a little better for catching.
> 
> The code generated for bool version looks much worse. Look at the fast
> path. Gcc tries to reuse the retq from the fast path in the bool case
> and so it has to push rbp and rbx on the stack.
> 
> That being said, gcc doesn't seem to generate a better code for bool so
> I do not think this is really worth it.
>

The code below also merge 3 statements into 1 return statement, although
for me, it is a little more readable, it will generate a little bad code.
That is the reason why the output looks a little bad.

In our case, for gcc 6.0, using bool instead of int for bool function
will get the same output under x86_64.

In our case, for gcc 4.8, using bool instead of int for bool function
will get a little better output under x86_64.

Thanks.
 
>> The orig:
>>
>>   0000000000001290 <PageMovable>:
>>       1290:       48 8b 47 08             mov    0x8(%rdi),%rax
>>       1294:       83 e0 03                and    $0x3,%eax
>>       1297:       48 83 f8 02             cmp    $0x2,%rax
>>       129b:       74 03                   je     12a0 <__SetPageMovable+0x12a0>
>>       129d:       31 c0                   xor    %eax,%eax
>>       129f:       c3                      retq
>>       12a0:       55                      push   %rbp
>>       12a1:       48 89 e5                mov    %rsp,%rbp
>>       12a4:       e8 00 00 00 00          callq  12a9 <__SetPageMovable+0x12a9>
>>       12a9:       48 85 c0                test   %rax,%rax
>>       12ac:       74 17                   je     12c5 <__SetPageMovable+0x12c5>
>>       12ae:       48 8b 50 68             mov    0x68(%rax),%rdx
>>       12b2:       48 85 d2                test   %rdx,%rdx
>>       12b5:       74 0e                   je     12c5 <__SetPageMovable+0x12c5>
>>       12b7:       48 83 7a 68 00          cmpq   $0x0,0x68(%rdx)
>>       12bc:       b8 01 00 00 00          mov    $0x1,%eax
>>       12c1:       74 02                   je     12c5 <__SetPageMovable+0x12c5>
>>       12c3:       5d                      pop    %rbp
>>       12c4:       c3                      retq
>>       12c5:       31 c0                   xor    %eax,%eax
>>       12c7:       5d                      pop    %rbp
>>       12c8:       c3                      retq
>>       12c9:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
>>
>> The new:
>>
>>   0000000000001290 <PageMovable>:
>>       1290:       48 8b 47 08             mov    0x8(%rdi),%rax
>>       1294:       55                      push   %rbp
>>       1295:       48 89 e5                mov    %rsp,%rbp
>>       1298:       53                      push   %rbx
>>       1299:       31 db                   xor    %ebx,%ebx
>>       129b:       83 e0 03                and    $0x3,%eax
>>       129e:       48 83 f8 02             cmp    $0x2,%rax
>>       12a2:       74 05                   je     12a9 <__SetPageMovable+0x12a9>
>>       12a4:       89 d8                   mov    %ebx,%eax
>>       12a6:       5b                      pop    %rbx
>>       12a7:       5d                      pop    %rbp
>>       12a8:       c3                      retq
>>       12a9:       e8 00 00 00 00          callq  12ae <__SetPageMovable+0x12ae>
>>       12ae:       48 85 c0                test   %rax,%rax
>>       12b1:       74 f1                   je     12a4 <__SetPageMovable+0x12a4>
>>       12b3:       48 8b 40 68             mov    0x68(%rax),%rax
>>       12b7:       48 85 c0                test   %rax,%rax
>>       12ba:       74 e8                   je     12a4 <__SetPageMovable+0x12a4>
>>       12bc:       48 83 78 68 00          cmpq   $0x0,0x68(%rax)
>>       12c1:       0f 95 c3                setne  %bl
>>       12c4:       89 d8                   mov    %ebx,%eax
>>       12c6:       5b                      pop    %rbx
>>       12c7:       5d                      pop    %rbp
>>       12c8:       c3                      retq
>>       12c9:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
>>
>> Thanks.
>> -- 
>> Chen Gang (éå)
>>
>> Managing Natural Environments is the Duty of Human Beings.
> 

-- 
Chen Gang (éå)

Managing Natural Environments is the Duty of Human Beings.