On Fri, Mar 24, 2017 at 7:08 PM, Peter Zijlstra <pet...@infradead.org> wrote: > On Fri, Mar 24, 2017 at 06:51:15PM +0100, Dmitry Vyukov wrote: >> On Fri, Mar 24, 2017 at 6:23 PM, Peter Zijlstra <pet...@infradead.org> 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)
For me gcc 7.0.1 generates: 0000000000000330 <T_refcount_inc1>: 330: 55 push %rbp 331: 8b 07 mov (%rdi),%eax 333: 48 89 e5 mov %rsp,%rbp 336: 83 f8 ff cmp $0xffffffff,%eax 339: 74 12 je 34d <T_refcount_inc1+0x1d> 33b: 85 c0 test %eax,%eax 33d: 74 10 je 34f <T_refcount_inc1+0x1f> 33f: 8d 50 01 lea 0x1(%rax),%edx 342: f0 0f b1 17 lock cmpxchg %edx,(%rdi) 346: 75 ee jne 336 <T_refcount_inc1+0x6> 348: 83 fa ff cmp $0xffffffff,%edx 34b: 74 04 je 351 <T_refcount_inc1+0x21> 34d: 5d pop %rbp 34e: c3 retq 34f: 31 c0 xor %eax,%eax 351: 0f 0b ud2 353: 5d pop %rbp 354: c3 retq with: if (!success) \ *_old = __old; \ 0000000000000320 <T_refcount_inc1>: 320: 8b 0f mov (%rdi),%ecx 322: 55 push %rbp 323: 48 89 e5 mov %rsp,%rbp 326: 83 f9 ff cmp $0xffffffff,%ecx 329: 74 2d je 358 <T_refcount_inc1+0x38> 32b: 85 c9 test %ecx,%ecx 32d: 74 25 je 354 <T_refcount_inc1+0x34> 32f: 8d 71 01 lea 0x1(%rcx),%esi 332: 89 c8 mov %ecx,%eax 334: f0 0f b1 37 lock cmpxchg %esi,(%rdi) 338: 89 c2 mov %eax,%edx 33a: 74 20 je 35c <T_refcount_inc1+0x3c> 33c: 83 fa ff cmp $0xffffffff,%edx 33f: 74 17 je 358 <T_refcount_inc1+0x38> 341: 85 d2 test %edx,%edx 343: 74 0f je 354 <T_refcount_inc1+0x34> 345: 8d 72 01 lea 0x1(%rdx),%esi 348: 89 d0 mov %edx,%eax 34a: f0 0f b1 37 lock cmpxchg %esi,(%rdi) 34e: 74 0a je 35a <T_refcount_inc1+0x3a> 350: 89 c2 mov %eax,%edx 352: eb e8 jmp 33c <T_refcount_inc1+0x1c> 354: 31 c9 xor %ecx,%ecx 356: 0f 0b ud2 358: 5d pop %rbp 359: c3 retq 35a: 89 d1 mov %edx,%ecx 35c: 83 fe ff cmp $0xffffffff,%esi 35f: 74 f5 je 356 <T_refcount_inc1+0x36> 361: 5d pop %rbp 362: c3 retq with __atomic_compare_exchange_n: exactly the same as the original code. But I don't have an answer for runtime patching of LOCK. Looks like something to fix in gcc.
