如您所知，Math.abs(Integer.MIN_VALUE) == Integer.MIN_VALUE為了防止出現(xiàn)負(fù)值，該safeAbs方法在我的項(xiàng)目中實(shí)現(xiàn)了： public static int safeAbs(int i) { i = Math.abs(i); return i < 0 ? 0 : i; }我將性能與以下性能進(jìn)行了比較： public static int safeAbs(int i) { return i == Integer.MIN_VALUE ? 0 : Math.abs(i); }第一個(gè)幾乎比第二個(gè)慢 6 倍（第二個(gè)性能幾乎與“純”Math.abs(int) 相同）。從我的角度來(lái)看，字節(jié)碼沒(méi)有顯著差異，但我猜差異存在于 JIT“匯編”代碼中：“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}
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Smart貓小萌
TA貢獻(xiàn)1911條經(jīng)驗(yàn) 獲得超7個(gè)贊
safeAbsSlow和方法生成的本機(jī)代碼存在差異safeAbsFast。
safeAbsSlow（C2，4 級(jí)）：
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（C2，4 級(jí)）：
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`
從上面我們可以看出，safeAbsSlow比 具有更多的條件跳轉(zhuǎn)safeAbsFast。
這尤其是因?yàn)镸ath.abs內(nèi)聯(lián)到的實(shí)現(xiàn)safeAbsFast沒(méi)有條件跳轉(zhuǎn)：
0x000001d89e8a4b2d: mov r11d,ecx
0x000001d89e8a4b30: neg r11d
0x000001d89e8a4b33: test ecx,ecx
0x000001d89e8a4b35: cmovl ecx,r11d
因此，與數(shù)據(jù)集同時(shí)具有分散在數(shù)組中的正值和負(fù)值時(shí)的版本slow相比，該 版本中的分支未命中次數(shù)要多得多。normal以下是使用 Linux 分析器收集的相應(yīng)統(tǒng)計(jì)信息perf：
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，這是排序數(shù)據(jù)集的結(jié)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
當(dāng)數(shù)據(jù)集排序時(shí)，以前的slow版本變得更快（在這種情況下，代價(jià)高昂的分支未命中被最小化）。
環(huán)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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