XCHG is slow ?

Vladimir Kozlov vladimir.kozlov at oracle.com
Fri Jul 6 09:20:08 PDT 2012


On 7/6/12 9:03 AM, Vitaly Davidovich wrote:
> Ah I see - thanks for the explanation. Do you still align instructions for fetching though or don't care about unaligned 
> reads?

We align head of inner loops to 16 bytes on x86 (OptoLoopAlignment flag).

> 
> What specific type of patching is done that you mention? I'm not familiar with what (un)verified entry points actually 
> means.

https://wikis.oracle.com/display/HotSpotInternals/Overview+of+CompiledIC+and+CompiledStaticCall

Vladimir

> 
> Thanks
> 
> Sent from my phone
> 
> On Jul 6, 2012 11:44 AM, "Vladimir Kozlov" <vladimir.kozlov at oracle.com <mailto:vladimir.kozlov at oracle.com>> wrote:
> 
>     Padding is used for different purpose. In the case from this thread it is used to align call instruction so that the
>     address part of instruction is aligned to 4 bytes which we need for atomic patching (same cacheline and 4-bytes store
>     instruction movl which is atomic).
> 
>     Padding is also used for adding space between unverified and verified entry points in a compiled method code. Padding
>     need to be safe so that if the beginning of padding is overwritten by patching it still be nop. If you overwrite 0F in
>     "0F 1F 00H" the rest "1F 00H" will not be NOP anymore.
> 
>     Vladimir
> 
>     On 7/6/12 8:08 AM, Vitaly Davidovich wrote:
>      > Vladimir mentioned earlier that the xchg ax ax was a 2 byte pad, for what I assume is to align the call
>     instruction on a
>      > 16 byte boundary to facilitate instruction fetch/decode. I'm sure he can give a more definitive answer.
>      >
>      > Sent from my phone
>      >
>      > On Jul 6, 2012 9:29 AM, "changren" <changren at taobao.com <mailto:changren at taobao.com> <mailto:changren at taobao.com
>     <mailto:changren at taobao.com>>> wrote:
>      >
>      > As far as I know, lock is automatically applied only if a memory operand is present; reg to reg should not.
>      > Yes, lock doesn't apply to reg reg case.
>      >
>      > As mentioned earlier, xchg eax eax translates to same thing as nop - reg can be used to change instruction size for
>      > alignment purposes.
>      > I am curious to know why using nop ptr need patching safe padding and what is that.
>      >
>      > Thanks
>      > Joseph
>      >
>      > ÓÚ 2012-7-6 20:42, Vitaly Davidovich дµÀ:
>      >>
>      >> Hi Joseph,
>      >>
>      >> As far as I know, lock is automatically applied only if a memory operand is present; reg to reg should not. As
>      >> mentioned earlier, xchg eax eax translates to same thing as nop - reg can be used to change instruction size for
>      >> alignment purposes.
>      >>
>      >> Cheers,
>      >> Vitaly
>      >>
>      >> Sent from my phone
>      >>
>      >> On Jul 6, 2012 3:20 AM, "changren" <changren at taobao.com <mailto:changren at taobao.com> <mailto:changren at taobao.com
>     <mailto:changren at taobao.com>>> wrote:
>      >>
>      >> 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
>      >>
>      >>
>      >> ÓÚ 2012-7-4 9:30, Vladimir Kozlov дµÀ:
>      >>
>      >> 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
>      >>
>      >> R¨¦mi Forax wrote:
>      >>
>      >> On 07/04/2012 01:44 AM, Vladimir Kozlov wrote:
>      >>
>      >> R¨¦mi,
>      >>
>      >> 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 :)
>      >>
>      >> R¨¦mi
>      >>
>      >> # {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
>      >>
>      >>
>      >>
>      >> R¨¦mi 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.
>      >>
>      >> R¨¦mi
>      >>
>      >> [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}
>      >> ...
>      >>
>      >>
>      >>
>      >>
>      >>
>      >>
>      >> .
>      >>
>      >>
>      >>
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