XCHG is slow ?

[changren]

I noticed this

0x00007fdf91061644: xchg %ax,%ax ;...666690

on modern Intel and AMD cpus, it is recommended to use 0F 1F 00H for 3
nops padding. 66 66 90H is for older AMD cpus.
I saw a related comment regarding the change in assembler_x86.cpp

Don't use "0x0F 0x1F 0x00" - need patching safe padding

what does this mean? if it is related to ax register, we can choose
another register with oldest value, for instance, 0F 1F 01 or choose
other instruction, for instance, lea.
BTW, @Vitaly lock is automatically applied to xchg.

Thanks,
Joseph


于 2012-7-4 9:30, Vladimir Kozlov 写道:
> I agree with you that it is strange. I looked on code and, as you
> said, one with overflow detection executes a lot more instructions and
> stack operations. What is time difference? I ran latest 7u6 (linux64)
> promoted build and got this numbers:
>
> $ bin/jruby -X+C bench/bench_fib_recursive.rb
> 0.170000 0.070000 0.240000 ( 0.100000)
> 0.030000 0.020000 0.050000 ( 0.036000)
> 0.040000 0.020000 0.060000 ( 0.036000)
> 0.040000 0.030000 0.070000 ( 0.036000)
> 0.040000 0.020000 0.060000 ( 0.036000)
>
> Vladimir
>
> Rémi Forax wrote:
>> On 07/04/2012 01:44 AM, Vladimir Kozlov wrote:
>>> Rémi,
>>>
>>> NOP and xchg %ax,%ax are and always were the same instruction and
>>> have the same encoding 0x90. What you see is multibytes nop (2 bytes
>>> in your case) to align following call instruction: 0x66 0x90.
>>>
>>> Vladimir
>>
>> I see and I suppose that a multibytes nop is not slower than a single
>> byte nop.
>>
>> So to explain my problem clearly, I've two codes the first one is
>> fibonacci (ClassicFibo)
>> with an int, the second one is fibonacci with an int plus the
>> overflow detection (FiboSample).
>>
>> My problem is that the code with the overflow detection is faster the
>> code
>> without the overflow detection :)
>>
>> Rémi
>>
>> # {method} 'fibo' '(I)I' in 'ClassicFibo'
>> # parm0: rsi = int
>> # [sp+0x30] (sp of caller)
>> 0x00007fdf91061620: mov %eax,-0x14000(%rsp) ;...89842400 c0feff
>> 0x00007fdf91061627: push %rbp ;...55
>> 0x00007fdf91061628: sub $0x20,%rsp ;...4883ec20
>> ;*synchronization entry
>> ; - ClassicFibo::fibo at -1 (line 4)
>> 0x00007fdf9106162c: mov %esi,%ebp ;...8bee
>> 0x00007fdf9106162e: cmp $0x2,%esi ;...83fe02
>> 0x00007fdf91061631: jl 0x00007fdf91061651 ;...7c1e
>> ;*if_icmpge
>> ; - ClassicFibo::fibo at 2 (line 4)
>> 0x00007fdf91061633: dec %esi ;...ffce
>> ;*isub
>> ; - ClassicFibo::fibo at 9 (line 7)
>> 0x00007fdf91061635: xchg %ax,%ax ;...6690
>> 0x00007fdf91061637: callq 0x00007fdf91038060 ;...e8246afd ff
>> ; OopMap{off=28}
>> ;*invokestatic fibo
>> ; - ClassicFibo::fibo at 10 (line 7)
>> ; {static_call}
>> 0x00007fdf9106163c: mov %eax,(%rsp) ;...890424
>> 0x00007fdf9106163f: mov %ebp,%esi ;...8bf5
>> 0x00007fdf91061641: add $0xfffffffffffffffe,%esi ;...83c6fe
>> ;*isub
>> ; - ClassicFibo::fibo at 15 (line 7)
>> 0x00007fdf91061644: xchg %ax,%ax ;...666690
>> 0x00007fdf91061647: callq 0x00007fdf91038060 ;...e8146afd ff
>> ; OopMap{off=44}
>> ;*invokestatic fibo
>> ; - ClassicFibo::fibo at 16 (line 7)
>> ; {static_call}
>> 0x00007fdf9106164c: add (%rsp),%eax ;...030424
>> ;*iadd
>> ; - ClassicFibo::fibo at 19 (line 7)
>> 0x00007fdf9106164f: jmp 0x00007fdf91061656 ;...eb05
>> 0x00007fdf91061651: mov $0x1,%eax ;...b8010000 00
>> 0x00007fdf91061656: add $0x20,%rsp ;...4883c420
>> 0x00007fdf9106165a: pop %rbp ;...5d
>> 0x00007fdf9106165b: test %eax,0x91c899f(%rip) # 0x00007fdf9a22a000
>> ;...85059f89 1c09
>> ; {poll_return}
>> 0x00007fdf91061661: retq ;...c3
>> ;*invokestatic fibo
>> ; - ClassicFibo::fibo at 10 (line 7)
>> 0x00007fdf91061662: mov %rax,%rsi ;...488bf0
>> 0x00007fdf91061665: jmp 0x00007fdf9106166a ;...eb03
>> 0x00007fdf91061667: mov %rax,%rsi ;...488bf0
>> ;*invokestatic fibo
>> ; - ClassicFibo::fibo at 16 (line 7)
>> 0x00007fdf9106166a: add $0x20,%rsp ;...4883c420
>> 0x00007fdf9106166e: pop %rbp ;...5d
>> 0x00007fdf9106166f: jmpq 0x00007fdf91061220 ;...e9acfbff ff
>> ; {runtime_call}
>> 0x00007fdf91061674: hlt ;...f4
>> 0x00007fdf91061675: hlt ;...f4
>> 0x00007fdf91061676: hlt ;...f4
>> 0x00007fdf91061677: hlt ;...f4
>> 0x00007fdf91061678: hlt ;...f4
>> 0x00007fdf91061679: hlt ;...f4
>> 0x00007fdf9106167a: hlt ;...f4
>> 0x00007fdf9106167b: hlt ;...f4
>> 0x00007fdf9106167c: hlt ;...f4
>> 0x00007fdf9106167d: hlt ;...f4
>> 0x00007fdf9106167e: hlt ;...f4
>> 0x00007fdf9106167f: hlt ;...f4
>> [Stub Code]
>> 0x00007fdf91061680: mov $0x0,%rbx ;...48bb0000 000000
>> ;...000000
>> ; {no_reloc}
>> 0x00007fdf9106168a: jmpq 0x00007fdf9106168a ;...e9fbffff ff
>> ; {runtime_call}
>> 0x00007fdf9106168f: mov $0x0,%rbx ;...48bb0000 000000
>> ;...000000
>> ; {static_stub}
>> 0x00007fdf91061699: jmpq 0x00007fdf91061699 ;...e9fbffff ff
>> ; {runtime_call}
>> [Exception Handler]
>> 0x00007fdf9106169e: jmpq 0x00007fdf9105e4a0 ;...e9fdcdff ff
>> ; {runtime_call}
>> [Deopt Handler Code]
>> 0x00007fdf910616a3: callq 0x00007fdf910616a8 ;...e8000000 00
>> 0x00007fdf910616a8: subq $0x5,(%rsp) ;...48832c24 05
>> 0x00007fdf910616ad: jmpq 0x00007fdf91038c00 ;...e94e75fd ff
>> ; {runtime_call}
>> 0x00007fdf910616b2: hlt ;...f4
>> 0x00007fdf910616b3: hlt ;...f4
>> 0x00007fdf910616b4: hlt ;...f4
>> 0x00007fdf910616b5: hlt ;...f4
>> 0x00007fdf910616b6: hlt ;...f4
>> 0x00007fdf910616b7: hlt ;...f4
>>
>>
>> # {method} 'fibo' '(I)I' in 'FiboSample'
>> # parm0: rsi = int
>> # [sp+0x30] (sp of caller)
>> 0x00007faadd05fd40: mov %eax,-0x14000(%rsp) ;...89842400 c0feff
>> 0x00007faadd05fd47: push %rbp ;...55
>> 0x00007faadd05fd48: sub $0x20,%rsp ;...4883ec20
>> ;*synchronization entry
>> ; - FiboSample::fibo at -1
>> 0x00007faadd05fd4c: mov %esi,(%rsp) ;...893424
>> 0x00007faadd05fd4f: cmp $0x2,%esi ;...83fe02
>> 0x00007faadd05fd52: jl 0x00007faadd05fda1 ;...7c4d
>> ;*if_icmpge
>> ; - FiboSample::fibo at 2
>> 0x00007faadd05fd54: dec %esi ;...ffce
>> ;*isub
>> ; - FiboSample::fibo at 9
>> 0x00007faadd05fd56: nop ;...90
>> 0x00007faadd05fd57: callq 0x00007faadd038060 ;...e80483fd ff
>> ; OopMap{off=28}
>> ;*invokestatic fibo
>> ; - FiboSample::fibo at 10
>> ; {static_call}
>> 0x00007faadd05fd5c: mov %eax,0x4(%rsp) ;...89442404
>> ;*goto
>> ; - FiboSample::fibo at 61
>> 0x00007faadd05fd60: mov (%rsp),%esi ;...8b3424
>> 0x00007faadd05fd63: add $0xfffffffffffffffe,%esi ;...83c6fe
>> ;*isub
>> ; - FiboSample::fibo at 18
>> 0x00007faadd05fd66: nop ;...90
>> 0x00007faadd05fd67: callq 0x00007faadd038060 ;...e8f482fd ff
>> ; OopMap{off=44}
>> ;*invokestatic fibo
>> ; - FiboSample::fibo at 19
>> ; {static_call}
>> 0x00007faadd05fd6c: mov %eax,%r9d ;...448bc8
>> 0x00007faadd05fd6f: mov 0x4(%rsp),%eax ;...8b442404
>> 0x00007faadd05fd73: add %r9d,%eax ;...4103c1
>> ;*goto
>> ; - FiboSample::fibo at 75
>> 0x00007faadd05fd76: mov 0x4(%rsp),%r11d ;...448b5c24 04
>> 0x00007faadd05fd7b: xor %eax,%r11d ;...4433d8
>> 0x00007faadd05fd7e: mov %r9d,%r8d ;...458bc1
>> 0x00007faadd05fd81: xor %eax,%r8d ;...4433c0
>> 0x00007faadd05fd84: and %r8d,%r11d ;...4523d8
>> 0x00007faadd05fd87: test %r11d,%r11d ;...4585db
>> 0x00007faadd05fd8a: jge 0x00007faadd05fda6 ;...7d1a
>> ;*ifge
>> ; - jdart.runtime.RT::addExact at 11 (line 139)
>> ; - FiboSample::fibo at 37
>> 0x00007faadd05fd8c: mov $0xa5,%esi ;...bea50000 00
>> 0x00007faadd05fd91: mov %r9d,(%rsp) ;...44890c24
>> 0x00007faadd05fd95: xchg %ax,%ax ;...6690
>> 0x00007faadd05fd97: callq 0x00007faadd039020 ;...e88492fd ff
>> ; OopMap{off=92}
>> ;*new ; - jdart.runtime.RT::addExact at 14 (line 140)
>> ; - FiboSample::fibo at 37
>> ; {runtime_call}
>> 0x00007faadd05fd9c: callq 0x00007faae7394800 ;...e85f4a33 0a
>> ;*new ; - jdart.runtime.RT::addExact at 14 (line 140)
>> ; - FiboSample::fibo at 37
>> ; {runtime_call}
>> 0x00007faadd05fda1: mov $0x1,%eax ;...b8010000 00
>> 0x00007faadd05fda6: add $0x20,%rsp ;...4883c420
>> 0x00007faadd05fdaa: pop %rbp ;...5d
>> 0x00007faadd05fdab: test %eax,0xab5524f(%rip) # 0x00007faae7bb5000
>> ;...85054f52 b50a
>> ; {poll_return}
>> 0x00007faadd05fdb1: retq ;...c3
>> 0x00007faadd05fdb2: mov 0x8(%rax),%r11d ;...448b5808
>> 0x00007faadd05fdb6: cmp $0xefe4b124,%r11d ;...4181fb24 b1e4ef
>> ; {oop('jdart/runtime/ControlFlowException')}
>> 0x00007faadd05fdbd: jne 0x00007faadd05fdf5 ;...7536
>> ;*invokestatic fibo
>> ; - FiboSample::fibo at 19
>> 0x00007faadd05fdbf: mov 0x20(%rax),%ebp ;...8b6820
>> 0x00007faadd05fdc2: test %ebp,%ebp ;...85ed
>> 0x00007faadd05fdc4: jne 0x00007faadd05fe11 ;...754b
>> ;*getfield value
>> ; - FiboSample::fibo at 70
>> 0x00007faadd05fdc6: mov 0x4(%rsp),%eax ;...8b442404
>> 0x00007faadd05fdca: xor %r9d,%r9d ;...4533c9
>> 0x00007faadd05fdcd: jmp 0x00007faadd05fd76 ;...eba7
>> 0x00007faadd05fdcf: mov 0x8(%rax),%r11d ;...448b5808
>> 0x00007faadd05fdd3: cmp $0xefe4b124,%r11d ;...4181fb24 b1e4ef
>> ; {oop('jdart/runtime/ControlFlowException')}
>> 0x00007faadd05fdda: jne 0x00007faadd05fdf0 ;...7514
>> ;*invokestatic fibo
>> ; - FiboSample::fibo at 10
>> 0x00007faadd05fddc: mov 0x20(%rax),%ebp ;...8b6820
>> 0x00007faadd05fddf: test %ebp,%ebp ;...85ed
>> 0x00007faadd05fde1: jne 0x00007faadd05fe02 ;...751f
>> ;*getfield value
>> ; - FiboSample::fibo at 57
>> 0x00007faadd05fde3: xor %r11d,%r11d ;...4533db
>> 0x00007faadd05fde6: mov %r11d,0x4(%rsp) ;...44895c24 04
>> 0x00007faadd05fdeb: jmpq 0x00007faadd05fd60 ;...e970ffff ff
>> 0x00007faadd05fdf0: mov %rax,%rsi ;...488bf0
>> 0x00007faadd05fdf3: jmp 0x00007faadd05fdf8 ;...eb03
>> 0x00007faadd05fdf5: mov %rax,%rsi ;...488bf0
>> ;*invokestatic fibo
>> ; - FiboSample::fibo at 19
>> 0x00007faadd05fdf8: add $0x20,%rsp ;...4883c420
>> 0x00007faadd05fdfc: pop %rbp ;...5d
>> 0x00007faadd05fdfd: jmpq 0x00007faadd061660 ;...e95e1800 00
>> ; {runtime_call}
>> 0x00007faadd05fe02: mov $0xffffffec,%esi ;...beecffff ff
>> 0x00007faadd05fe07: callq 0x00007faadd039020 ;...e81492fd ff
>> ; OopMap{rbp=NarrowOop off=204}
>> ;*astore_2
>> ; - FiboSample::fibo at 60
>> ; {runtime_call}
>> 0x00007faadd05fe0c: callq 0x00007faae7394800 ;...e8ef4933 0a
>> ;*getfield value
>> ; - FiboSample::fibo at 57
>> ; {runtime_call}
>> 0x00007faadd05fe11: mov $0xffffffec,%esi ;...beecffff ff
>> 0x00007faadd05fe16: nop ;...90
>> 0x00007faadd05fe17: callq 0x00007faadd039020 ;...e80492fd ff
>> ; OopMap{rbp=NarrowOop off=220}
>> ;*astore
>> ; - FiboSample::fibo at 73
>> ; {runtime_call}
>> 0x00007faadd05fe1c: callq 0x00007faae7394800 ;...e8df4933 0a
>> ;*getfield value
>> ; - FiboSample::fibo at 70
>> ; {runtime_call}
>> 0x00007faadd05fe21: hlt ;...f4
>> 0x00007faadd05fe22: hlt ;...f4
>> 0x00007faadd05fe23: hlt ;...f4
>> 0x00007faadd05fe24: hlt ;...f4
>> 0x00007faadd05fe25: hlt ;...f4
>> 0x00007faadd05fe26: hlt ;...f4
>> 0x00007faadd05fe27: hlt ;...f4
>> 0x00007faadd05fe28: hlt ;...f4
>> 0x00007faadd05fe29: hlt ;...f4
>> 0x00007faadd05fe2a: hlt ;...f4
>> 0x00007faadd05fe2b: hlt ;...f4
>> 0x00007faadd05fe2c: hlt ;...f4
>> 0x00007faadd05fe2d: hlt ;...f4
>> 0x00007faadd05fe2e: hlt ;...f4
>> 0x00007faadd05fe2f: hlt ;...f4
>> 0x00007faadd05fe30: hlt ;...f4
>> 0x00007faadd05fe31: hlt ;...f4
>> 0x00007faadd05fe32: hlt ;...f4
>> 0x00007faadd05fe33: hlt ;...f4
>> 0x00007faadd05fe34: hlt ;...f4
>> 0x00007faadd05fe35: hlt ;...f4
>> 0x00007faadd05fe36: hlt ;...f4
>> 0x00007faadd05fe37: hlt ;...f4
>> 0x00007faadd05fe38: hlt ;...f4
>> 0x00007faadd05fe39: hlt ;...f4
>> 0x00007faadd05fe3a: hlt ;...f4
>> 0x00007faadd05fe3b: hlt ;...f4
>> 0x00007faadd05fe3c: hlt ;...f4
>> 0x00007faadd05fe3d: hlt ;...f4
>> 0x00007faadd05fe3e: hlt ;...f4
>> 0x00007faadd05fe3f: hlt ;...f4
>> [Stub Code]
>> 0x00007faadd05fe40: mov $0x0,%rbx ;...48bb0000 000000
>> ;...000000
>> ; {no_reloc}
>> 0x00007faadd05fe4a: jmpq 0x00007faadd05fe4a ;...e9fbffff ff
>> ; {runtime_call}
>> 0x00007faadd05fe4f: mov $0x0,%rbx ;...48bb0000 000000
>> ;...000000
>> ; {static_stub}
>> 0x00007faadd05fe59: jmpq 0x00007faadd05fe59 ;...e9fbffff ff
>> ; {runtime_call}
>> [Exception Handler]
>> 0x00007faadd05fe5e: jmpq 0x00007faadd05e8e0 ;...e97deaff ff
>> ; {runtime_call}
>> [Deopt Handler Code]
>> 0x00007faadd05fe63: callq 0x00007faadd05fe68 ;...e8000000 00
>> 0x00007faadd05fe68: subq $0x5,(%rsp) ;...48832c24 05
>> 0x00007faadd05fe6d: jmpq 0x00007faadd038c00 ;...e98e8dfd ff
>> ; {runtime_call}
>> 0x00007faadd05fe72: hlt ;...f4
>> 0x00007faadd05fe73: hlt ;...f4
>> 0x00007faadd05fe74: hlt ;...f4
>> 0x00007faadd05fe75: hlt ;...f4
>> 0x00007faadd05fe76: hlt ;...f4
>> 0x00007faadd05fe77: hlt ;...f4
>>
>>
>>>
>>> Rémi Forax wrote:
>>>> Hi guys,
>>>> I've found something really weird, c2 sometimes generates
>>>> the assembler instruction xchg %ax, %ax (see the assembly code
>>>> of fibo just before the first recursive call)
>>>> which I believe is equivalent to a nop but slower.
>>>>
>>>> In fact, xchg is really slow in my laptop (Nehalem), slower than
>>>> at least 5/6 instructions like move/xor/and.
>>>> I think it's because xchg is atomic see [1]
>>>>
>>>> I think c2 should never generate xchg or at least replace all xchg
>>>> %r, %r by nop.
>>>>
>>>> Rémi
>>>>
>>>> [1]
>>>> http://www.intel.ru/content/dam/doc/white-paper/intel-microarchitecture-white-paper.pdf
>>>>
>>>>
>>>>
>>>> public class ClassicFibo {
>>>> private static int fibo(int n) {
>>>> if (n < 2) {
>>>> return 1;
>>>> }
>>>> return fibo(n - 1) + fibo(n - 2);
>>>> }
>>>>
>>>> public static void main(String[] args) {
>>>> System.out.println(fibo(40));
>>>> }
>>>> }
>>>>
>>>> # {method} 'fibo' '(I)I' in 'ClassicFibo'
>>>> # parm0: rsi = int
>>>> # [sp+0x30] (sp of caller)
>>>> 0x00007fb409061620: mov %eax,-0x14000(%rsp)
>>>> 0x00007fb409061627: push %rbp
>>>> 0x00007fb409061628: sub $0x20,%rsp ;*synchronization entry
>>>> ; - ClassicFibo::fibo at -1 (line 4)
>>>> 0x00007fb40906162c: mov %esi,%ebp
>>>> 0x00007fb40906162e: cmp $0x2,%esi
>>>> 0x00007fb409061631: jl 0x00007fb409061651 ;*if_icmpge
>>>> ; - ClassicFibo::fibo at 2 (line 4)
>>>> 0x00007fb409061633: dec %esi ;*isub
>>>> ; - ClassicFibo::fibo at 9 (line 7)
>>>> 0x00007fb409061635: xchg %ax,%ax
>>>> 0x00007fb409061637: callq 0x00007fb409038060 ; OopMap{off=28}
>>>> ;*invokestatic fibo
>>>> ; - ClassicFibo::fibo at 10 (line 7)
>>>> ; {static_call}
>>>> ...
>>>>
>>>>
>>>>
>>>>
>>
>>
> .
>


________________________________

This email (including any attachments) is confidential and may be legally privileged. If you received this email in error, please delete it immediately and do not copy it or use it for any purpose or disclose its contents to any other person. Thank you.

本电邮(包括任何附件)可能含有机密资料并受法律保护。如您不是正确的收件人，请您立即删除本邮件。请不要将本电邮进行复制并用作任何其他用途、或透露本邮件之内容。谢谢。