Re: locking/atomic: Introduce atomic_try_cmpxchg()

[Peter Zijlstra]

On Fri, Mar 24, 2017 at 06:51:15PM +0100, Dmitry Vyukov wrote:
> On Fri, Mar 24, 2017 at 6:23 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Fri, Mar 24, 2017 at 09:54:46AM -0700, Andy Lutomirski wrote:
> >> > So the first snipped I tested regressed like so:
> >> >
> >> >
> >> > 0000000000000000 <T_refcount_inc>:                              0000000000000000 <T_refcount_inc>:
> >> >    0:   8b 07                   mov    (%rdi),%eax                 0:   8b 17                   mov    (%rdi),%edx
> >> >    2:   83 f8 ff                cmp    $0xffffffff,%eax            2:   83 fa ff                cmp    $0xffffffff,%edx
> >> >    5:   74 13                   je     1a <T_refcount_inc+0x1a>    5:   74 1a                   je     21 <T_refcount_inc+0x21>
> >> >    7:   85 c0                   test   %eax,%eax                   7:   85 d2                   test   %edx,%edx
> >> >    9:   74 0d                   je     18 <T_refcount_inc+0x18>    9:   74 13                   je     1e <T_refcount_inc+0x1e>
> >> >    b:   8d 50 01                lea    0x1(%rax),%edx              b:   8d 4a 01                lea    0x1(%rdx),%ecx
> >> >    e:   f0 0f b1 17             lock cmpxchg %edx,(%rdi)           e:   89 d0                   mov    %edx,%eax
> >> >   12:   75 ee                   jne    2 <T_refcount_inc+0x2>     10:   f0 0f b1 0f             lock cmpxchg %ecx,(%rdi)
> >> >   14:   ff c2                   inc    %edx                       14:   74 04                   je     1a <T_refcount_inc+0x1a>
> >> >   16:   75 02                   jne    1a <T_refcount_inc+0x1a>   16:   89 c2                   mov    %eax,%edx
> >> >   18:   0f 0b                   ud2                               18:   eb e8                   jmp    2 <T_refcount_inc+0x2>
> >> >   1a:   c3                      retq                              1a:   ff c1                   inc    %ecx
> >> >                                                                   1c:   75 03                   jne    21 <T_refcount_inc+0x21>
> >> >                                                                   1e:   0f 0b                   ud2
> >> >                                                                   20:   c3                      retq
> >> >                                                                   21:   c3                      retq
> >>

> This seems to help ;)
> 
> #define try_cmpxchg(ptr, pold, new) __atomic_compare_exchange_n(ptr, pold, new, 0, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST)

That gets me:

0000000000000000 <T_refcount_inc>:
   0:   8b 07                   mov    (%rdi),%eax
   2:   89 44 24 fc             mov    %eax,-0x4(%rsp)
   6:   8b 44 24 fc             mov    -0x4(%rsp),%eax
   a:   83 f8 ff                cmp    $0xffffffff,%eax
   d:   74 1c                   je     2b <T_refcount_inc+0x2b>
   f:   85 c0                   test   %eax,%eax
  11:   75 07                   jne    1a <T_refcount_inc+0x1a>
  13:   8b 44 24 fc             mov    -0x4(%rsp),%eax
  17:   0f 0b                   ud2    
  19:   c3                      retq   
  1a:   8d 50 01                lea    0x1(%rax),%edx
  1d:   8b 44 24 fc             mov    -0x4(%rsp),%eax
  21:   f0 0f b1 17             lock cmpxchg %edx,(%rdi)
  25:   75 db                   jne    2 <T_refcount_inc+0x2>
  27:   ff c2                   inc    %edx
  29:   74 e8                   je     13 <T_refcount_inc+0x13>
  2b:   c3                      retq  


Which is even worse... (I did double check it actually compiled)