xte*_*ern 19 java performance jit x86-64
如您所知，Math.abs(Integer.MIN_VALUE) == Integer.MIN_VALUE为了防止出现负值，该safeAbs方法已在我的项目中实现：
public static int safeAbs(int i) {
i = Math.abs(i);
return i < 0 ? 0 : i;
}
我将性能与以下各项进行了比较：
public static int safeAbs(int i) {
return i == Integer.MIN_VALUE ? 0 : Math.abs(i);
}
并且第一个比第二个慢几乎6倍（第二个性能几乎与“纯” Math.abs（int）相同）。从我的角度来看，字节码没有显着差异，但是我猜想差异存在于JIT“汇编”代码中：
“慢”版本：
0x00007f0149119720: mov %eax,0xfffffffffffec000(%rsp)
0x00007f0149119727: push %rbp
0x00007f0149119728: sub $0x20,%rsp
0x00007f014911972c: test %esi,%esi
0x00007f014911972e: jl 0x7f0149119734
0x00007f0149119730: mov %esi,%eax
0x00007f0149119732: jmp 0x7f014911973c
0x00007f0149119734: neg %esi
0x00007f0149119736: test %esi,%esi
0x00007f0149119738: jl 0x7f0149119748
0x00007f014911973a: mov %esi,%eax
0x00007f014911973c: add $0x20,%rsp
0x00007f0149119740: pop %rbp
0x00007f0149119741: test %eax,0x1772e8b9(%rip) ; {poll_return}
0x00007f0149119747: retq
0x00007f0149119748: mov %esi,(%rsp)
0x00007f014911974b: mov $0xffffff65,%esi
0x00007f0149119750: nop
0x00007f0149119753: callq 0x7f01490051a0 ; OopMap{off=56}
;*ifge
; - math.FastAbs::safeAbsSlow@6 (line 16)
; {runtime_call}
0x00007f0149119758: callq 0x7f015f521d20 ; {runtime_call}
“普通”版本：
# {method} {0x00007f31acf28cd8} 'safeAbsFast' '(I)I' in 'math/FastAbs'
# parm0: rsi = int
# [sp+0x30] (sp of caller)
0x00007f31b08c7360: mov %eax,0xfffffffffffec000(%rsp)
0x00007f31b08c7367: push %rbp
0x00007f31b08c7368: sub $0x20,%rsp
0x00007f31b08c736c: cmp $0x80000000,%esi
0x00007f31b08c7372: je 0x7f31b08c738e
0x00007f31b08c7374: mov %esi,%r10d
0x00007f31b08c7377: neg %r10d
0x00007f31b08c737a: test %esi,%esi
0x00007f31b08c737c: mov %esi,%eax
0x00007f31b08c737e: cmovl %r10d,%eax
0x00007f31b08c7382: add $0x20,%rsp
0x00007f31b08c7386: pop %rbp
0x00007f31b08c7387: test %eax,0x162c2c73(%rip) ; {poll_return}
0x00007f31b08c738d: retq
0x00007f31b08c738e: mov %esi,(%rsp)
0x00007f31b08c7391: mov $0xffffff65,%esi
0x00007f31b08c7396: nop
0x00007f31b08c7397: callq 0x7f31b07b11a0 ; OopMap{off=60}
;*if_icmpne
; - math.FastAbs::safeAbsFast@3 (line 17)
; {runtime_call}
0x00007f31b08c739c: callq 0x7f31c5863d20 ; {runtime_call}
基准代码：
0x00007f0149119720: mov %eax,0xfffffffffffec000(%rsp)
0x00007f0149119727: push %rbp
0x00007f0149119728: sub $0x20,%rsp
0x00007f014911972c: test %esi,%esi
0x00007f014911972e: jl 0x7f0149119734
0x00007f0149119730: mov %esi,%eax
0x00007f0149119732: jmp 0x7f014911973c
0x00007f0149119734: neg %esi
0x00007f0149119736: test %esi,%esi
0x00007f0149119738: jl 0x7f0149119748
0x00007f014911973a: mov %esi,%eax
0x00007f014911973c: add $0x20,%rsp
0x00007f0149119740: pop %rbp
0x00007f0149119741: test %eax,0x1772e8b9(%rip) ; {poll_return}
0x00007f0149119747: retq
0x00007f0149119748: mov %esi,(%rsp)
0x00007f014911974b: mov $0xffffff65,%esi
0x00007f0149119750: nop
0x00007f0149119753: callq 0x7f01490051a0 ; OopMap{off=56}
;*ifge
; - math.FastAbs::safeAbsSlow@6 (line 16)
; {runtime_call}
0x00007f0149119758: callq 0x7f015f521d20 ; {runtime_call}
结果（Linux x86-64、7820HQ在oracle jdk 8和11上检查，结果非常相似）。
Benchmark Mode Cnt Score Error Units
SafeAbsMicroBench.safeAbsFast avgt 10 6435155.516 ± 47130.767 ns/op
SafeAbsMicroBench.safeAbsSlow avgt 10 35646411.744 ± 776173.621 ns/op
有人可以解释为什么第一个代码比第二个要慢得多吗？
safeAbsSlow和safeAbsFast方法所生成的本机代码有所不同。
safeAbsSlow （C2，第4级）：
0x0000023d12ec4b14: add eax,ecx
0x0000023d12ec4b16: inc ebx
0x0000023d12ec4b18: cmp ebx,989680h
0x0000023d12ec4b1e: jnl 23d12ec4b4eh ; jump if `ebx` was not less than `10_000_000`
0x0000023d12ec4b20: mov ecx,dword ptr [r9+rbx*4+10h]
0x0000023d12ec4b25: test ecx,ecx
0x0000023d12ec4b27: jnl 23d12ec4b14h ; jump if `ecx` was not less-than `0`
0x0000023d12ec4b29: neg ecx
0x0000023d12ec4b2b: test ecx,ecx
0x0000023d12ec4b2d: jnl 23d12ec4b14h ; jump if `ecx` was not less-than `0`
safeAbsFast （C2，第4级）：
0x000001d89e8a4b20: mov ecx,dword ptr [r9+rdi*4+10h]
0x000001d89e8a4b25: cmp ecx,80000000h
0x000001d89e8a4b2b: je 1d89e8a4b66h ; jump if `ecx` was equal to `2147483648`
0x000001d89e8a4b2d: mov r11d,ecx
0x000001d89e8a4b30: neg r11d
0x000001d89e8a4b33: test ecx,ecx
0x000001d89e8a4b35: cmovl ecx,r11d
0x000001d89e8a4b39: add eax,ecx
0x000001d89e8a4b3b: inc edi
0x000001d89e8a4b3d: cmp edi,989680h
0x000001d89e8a4b43: jl 1d89e8a4b20h ; jump if `edi` was less than `10_000_000`
从上面我们可以看到，safeAbsSlow条件跳转比更大safeAbsFast。
尤其是因为Math.abs内联到中的实现safeAbsFast没有条件跳转：
0x000001d89e8a4b2d: mov r11d,ecx
0x000001d89e8a4b30: neg r11d
0x000001d89e8a4b33: test ecx,ecx
0x000001d89e8a4b35: cmovl ecx,r11d
结果，与数据集同时具有正值和负值且分散在整个数组中的slow版本相比，该 normal版本中的分支丢失更多。以下是使用perfLinux探查器收集的相应统计信息：
Benchmark Mode Cnt Score Error Units
safeAbsFast avgt 10 9611659.726 ± 1429082.431 ns/op
safeAbsFast:branch-misses avgt 2869.853 #/op
safeAbsFast:branches avgt 12492918.020 #/op
safeAbsFast:cycles avgt 28212203.936 #/op
safeAbsFast:instructions avgt 92352048.153 #/op
safeAbsSlow avgt 10 44524180.366 ± 6324887.086 ns/op
safeAbsSlow:branch-misses avgt 5006493.144 #/op
safeAbsSlow:branches avgt 17496069.911 #/op
safeAbsSlow:cycles avgt 126413171.674 #/op
safeAbsSlow:instructions avgt 67549877.558 #/op
相反，这是排序后的数据集的结果：
Benchmark Mode Cnt Score Error Units
safeAbsFast avgt 10 9026800.584 ± 528992.157 ns/op
safeAbsFast:branch-misses avgt 2785.463 #/op
safeAbsFast:branches avgt 12474751.905 #/op
safeAbsFast:cycles avgt 27379727.603 #/op
safeAbsFast:instructions avgt 92418075.715 #/op
safeAbsSlow avgt 10 6981828.374 ± 2375480.834 ns/op
safeAbsSlow:branch-misses avgt 2801.022 #/op
safeAbsSlow:branches avgt 17496585.992 #/op
safeAbsSlow:cycles avgt 19478382.113 #/op
safeAbsSlow:instructions avgt 67589946.278 #/op
slow当对数据集进行排序时，以前的版本甚至变得更快（在这种情况下，将代价高昂的分支丢失最小化）。
环境：
openjdk version "12-internal" 2019-03-19
OpenJDK Runtime Environment (slowdebug build 12-internal+0-adhoc.jdk12)
OpenJDK 64-Bit Server VM (slowdebug build 12-internal+0-adhoc.jdk12, mixed mode)
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