Messages in this thread | | | From | Andy Lutomirski <> | Date | Fri, 24 Mar 2017 09:54:46 -0700 | Subject | Re: locking/atomic: Introduce atomic_try_cmpxchg() |
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On Fri, Mar 24, 2017 at 9:41 AM, Peter Zijlstra <peterz@infradead.org> wrote: > On Fri, Mar 24, 2017 at 03:21:40PM +0100, Peter Zijlstra wrote: >> On Fri, Mar 24, 2017 at 01:44:00PM +0100, Dmitry Vyukov wrote: > >> > So I would suggest to change it to a safer and less surprising >> > alternative: >> > >> > diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h >> > index fb961db51a2a..81fb985f51f4 100644 >> > --- a/arch/x86/include/asm/cmpxchg.h >> > +++ b/arch/x86/include/asm/cmpxchg.h >> > @@ -212,7 +212,8 @@ extern void __add_wrong_size(void) >> > default: \ >> > __cmpxchg_wrong_size(); \ >> > } \ >> > - *_old = __old; \ >> > + if (!success) \ >> > + *_old = __old; \ >> > success; \ >> > }) >> >> I've no immediate objection, I'll double check what, if anything, it >> does for code gen. > > 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
Can you re-send the better asm you got earlier?
If I pretend to be a dumb compiler, I wonder if you'd get better results with:
if (!success) { *_old = __old; return false; } else { return true; }
or however you jam that into a statement expression. That way you aren't relying on the compiler to merge the branches.
> > Which is rather unfortunate...
-- Andy Lutomirski AMA Capital Management, LLC
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