Diğer cevabın işaret ettiği gibi , vaka değerleri bitişik olduğundan (seyrek olarak), çeşitli testleriniz için oluşturulan bayt kodu bir anahtar tablosu kullanır (bayt kodu talimatı)tableswitch
) kullanır.
Ancak, JIT işine başladığında ve bayt kodunu derlemeye derlediğinde, tableswitch
talimat her zaman bir işaretçi dizisi ile sonuçlanmaz: bazen anahtar tablosu neye benzediğine dönüşür lookupswitch
( if
/ else if
yapısına benzer ).
JIT (hotspot JDK 1.7) tarafından oluşturulan montajın ayrıştırılması, 17 veya daha az vaka olduğunda, 18'den fazla (daha verimli) olduğunda bir dizi işaretçi olup olmadığını art arda kullandığını gösterir.
Bu sihirli sayının 18 kullanılmasının nedeni MinJumpTableSize
JVM bayrağının varsayılan değerine ( kodda 352 satır civarında) iniyor gibi görünüyor .
Hotspot derleyici listesindeki sorunu gündeme getirdim ve geçmiş testlerin mirası gibi görünüyor . Daha fazla karşılaştırma yapıldıktan sonra bu varsayılan değerin JDK 8'de kaldırıldığını unutmayın .
Son olarak, yöntem çok uzun hale geldiğinde (testlerimde> 25 vaka), artık varsayılan JVM ayarları ile çizgiselleştirilmez - bu, o noktada performans düşüşünün en olası sebebidir.
5 vakada, ayrıştırılmış kod şöyle görünür (cmp / je / jg / jmp talimatlarını, if / goto için montajı dikkat edin):
[Verified Entry Point]
# {method} 'multiplyByPowerOfTen' '(DI)D' in 'javaapplication4/Test1'
# parm0: xmm0:xmm0 = double
# parm1: rdx = int
# [sp+0x20] (sp of caller)
0x00000000024f0160: mov DWORD PTR [rsp-0x6000],eax
; {no_reloc}
0x00000000024f0167: push rbp
0x00000000024f0168: sub rsp,0x10 ;*synchronization entry
; - javaapplication4.Test1::multiplyByPowerOfTen@-1 (line 56)
0x00000000024f016c: cmp edx,0x3
0x00000000024f016f: je 0x00000000024f01c3
0x00000000024f0171: cmp edx,0x3
0x00000000024f0174: jg 0x00000000024f01a5
0x00000000024f0176: cmp edx,0x1
0x00000000024f0179: je 0x00000000024f019b
0x00000000024f017b: cmp edx,0x1
0x00000000024f017e: jg 0x00000000024f0191
0x00000000024f0180: test edx,edx
0x00000000024f0182: je 0x00000000024f01cb
0x00000000024f0184: mov ebp,edx
0x00000000024f0186: mov edx,0x17
0x00000000024f018b: call 0x00000000024c90a0 ; OopMap{off=48}
;*new ; - javaapplication4.Test1::multiplyByPowerOfTen@72 (line 83)
; {runtime_call}
0x00000000024f0190: int3 ;*new ; - javaapplication4.Test1::multiplyByPowerOfTen@72 (line 83)
0x00000000024f0191: mulsd xmm0,QWORD PTR [rip+0xffffffffffffffa7] # 0x00000000024f0140
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@52 (line 62)
; {section_word}
0x00000000024f0199: jmp 0x00000000024f01cb
0x00000000024f019b: mulsd xmm0,QWORD PTR [rip+0xffffffffffffff8d] # 0x00000000024f0130
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@46 (line 60)
; {section_word}
0x00000000024f01a3: jmp 0x00000000024f01cb
0x00000000024f01a5: cmp edx,0x5
0x00000000024f01a8: je 0x00000000024f01b9
0x00000000024f01aa: cmp edx,0x5
0x00000000024f01ad: jg 0x00000000024f0184 ;*tableswitch
; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
0x00000000024f01af: mulsd xmm0,QWORD PTR [rip+0xffffffffffffff81] # 0x00000000024f0138
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@64 (line 66)
; {section_word}
0x00000000024f01b7: jmp 0x00000000024f01cb
0x00000000024f01b9: mulsd xmm0,QWORD PTR [rip+0xffffffffffffff67] # 0x00000000024f0128
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@70 (line 68)
; {section_word}
0x00000000024f01c1: jmp 0x00000000024f01cb
0x00000000024f01c3: mulsd xmm0,QWORD PTR [rip+0xffffffffffffff55] # 0x00000000024f0120
;*tableswitch
; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
; {section_word}
0x00000000024f01cb: add rsp,0x10
0x00000000024f01cf: pop rbp
0x00000000024f01d0: test DWORD PTR [rip+0xfffffffffdf3fe2a],eax # 0x0000000000430000
; {poll_return}
0x00000000024f01d6: ret
18 vakada, montaj şu şekilde görünür (kullanılan işaretçi dizisine dikkat edin ve tüm karşılaştırmalara olan ihtiyacı bastırır: jmp QWORD PTR [r8+r10*1]
doğrudan doğru çarpmaya atlar) - performans iyileştirmesinin olası nedeni budur:
[Verified Entry Point]
# {method} 'multiplyByPowerOfTen' '(DI)D' in 'javaapplication4/Test1'
# parm0: xmm0:xmm0 = double
# parm1: rdx = int
# [sp+0x20] (sp of caller)
0x000000000287fe20: mov DWORD PTR [rsp-0x6000],eax
; {no_reloc}
0x000000000287fe27: push rbp
0x000000000287fe28: sub rsp,0x10 ;*synchronization entry
; - javaapplication4.Test1::multiplyByPowerOfTen@-1 (line 56)
0x000000000287fe2c: cmp edx,0x13
0x000000000287fe2f: jae 0x000000000287fe46
0x000000000287fe31: movsxd r10,edx
0x000000000287fe34: shl r10,0x3
0x000000000287fe38: movabs r8,0x287fd70 ; {section_word}
0x000000000287fe42: jmp QWORD PTR [r8+r10*1] ;*tableswitch
; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
0x000000000287fe46: mov ebp,edx
0x000000000287fe48: mov edx,0x31
0x000000000287fe4d: xchg ax,ax
0x000000000287fe4f: call 0x00000000028590a0 ; OopMap{off=52}
;*new ; - javaapplication4.Test1::multiplyByPowerOfTen@202 (line 96)
; {runtime_call}
0x000000000287fe54: int3 ;*new ; - javaapplication4.Test1::multiplyByPowerOfTen@202 (line 96)
0x000000000287fe55: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe8b] # 0x000000000287fce8
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@194 (line 92)
; {section_word}
0x000000000287fe5d: jmp 0x000000000287ff16
0x000000000287fe62: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe86] # 0x000000000287fcf0
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@188 (line 90)
; {section_word}
0x000000000287fe6a: jmp 0x000000000287ff16
0x000000000287fe6f: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe81] # 0x000000000287fcf8
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@182 (line 88)
; {section_word}
0x000000000287fe77: jmp 0x000000000287ff16
0x000000000287fe7c: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe7c] # 0x000000000287fd00
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@176 (line 86)
; {section_word}
0x000000000287fe84: jmp 0x000000000287ff16
0x000000000287fe89: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe77] # 0x000000000287fd08
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@170 (line 84)
; {section_word}
0x000000000287fe91: jmp 0x000000000287ff16
0x000000000287fe96: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe72] # 0x000000000287fd10
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@164 (line 82)
; {section_word}
0x000000000287fe9e: jmp 0x000000000287ff16
0x000000000287fea0: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe70] # 0x000000000287fd18
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@158 (line 80)
; {section_word}
0x000000000287fea8: jmp 0x000000000287ff16
0x000000000287feaa: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe6e] # 0x000000000287fd20
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@152 (line 78)
; {section_word}
0x000000000287feb2: jmp 0x000000000287ff16
0x000000000287feb4: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe24] # 0x000000000287fce0
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@146 (line 76)
; {section_word}
0x000000000287febc: jmp 0x000000000287ff16
0x000000000287febe: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe6a] # 0x000000000287fd30
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@140 (line 74)
; {section_word}
0x000000000287fec6: jmp 0x000000000287ff16
0x000000000287fec8: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe68] # 0x000000000287fd38
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@134 (line 72)
; {section_word}
0x000000000287fed0: jmp 0x000000000287ff16
0x000000000287fed2: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe66] # 0x000000000287fd40
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@128 (line 70)
; {section_word}
0x000000000287feda: jmp 0x000000000287ff16
0x000000000287fedc: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe64] # 0x000000000287fd48
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@122 (line 68)
; {section_word}
0x000000000287fee4: jmp 0x000000000287ff16
0x000000000287fee6: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe62] # 0x000000000287fd50
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@116 (line 66)
; {section_word}
0x000000000287feee: jmp 0x000000000287ff16
0x000000000287fef0: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe60] # 0x000000000287fd58
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@110 (line 64)
; {section_word}
0x000000000287fef8: jmp 0x000000000287ff16
0x000000000287fefa: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe5e] # 0x000000000287fd60
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@104 (line 62)
; {section_word}
0x000000000287ff02: jmp 0x000000000287ff16
0x000000000287ff04: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe5c] # 0x000000000287fd68
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@98 (line 60)
; {section_word}
0x000000000287ff0c: jmp 0x000000000287ff16
0x000000000287ff0e: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe12] # 0x000000000287fd28
;*tableswitch
; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
; {section_word}
0x000000000287ff16: add rsp,0x10
0x000000000287ff1a: pop rbp
0x000000000287ff1b: test DWORD PTR [rip+0xfffffffffd9b00df],eax # 0x0000000000230000
; {poll_return}
0x000000000287ff21: ret
Ve son olarak 30 vakaya sahip (aşağıda) movapd xmm0,xmm1
, @cHao tarafından tespit edildiği gibi , kodun ortasına doğru görünen ek hariç, 18 vakaya benziyor - ancak performans düşüşünün en olası nedeni, yöntemin de varsayılan JVM ayarlarıyla satır içi uzun:
[Verified Entry Point]
# {method} 'multiplyByPowerOfTen' '(DI)D' in 'javaapplication4/Test1'
# parm0: xmm0:xmm0 = double
# parm1: rdx = int
# [sp+0x20] (sp of caller)
0x0000000002524560: mov DWORD PTR [rsp-0x6000],eax
; {no_reloc}
0x0000000002524567: push rbp
0x0000000002524568: sub rsp,0x10 ;*synchronization entry
; - javaapplication4.Test1::multiplyByPowerOfTen@-1 (line 56)
0x000000000252456c: movapd xmm1,xmm0
0x0000000002524570: cmp edx,0x1f
0x0000000002524573: jae 0x0000000002524592 ;*tableswitch
; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
0x0000000002524575: movsxd r10,edx
0x0000000002524578: shl r10,0x3
0x000000000252457c: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe3c] # 0x00000000025243c0
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@364 (line 118)
; {section_word}
0x0000000002524584: movabs r8,0x2524450 ; {section_word}
0x000000000252458e: jmp QWORD PTR [r8+r10*1] ;*tableswitch
; - javaapplication4.Test1::multiplyByPowerOfTen@1 (line 56)
0x0000000002524592: mov ebp,edx
0x0000000002524594: mov edx,0x31
0x0000000002524599: xchg ax,ax
0x000000000252459b: call 0x00000000024f90a0 ; OopMap{off=64}
;*new ; - javaapplication4.Test1::multiplyByPowerOfTen@370 (line 120)
; {runtime_call}
0x00000000025245a0: int3 ;*new ; - javaapplication4.Test1::multiplyByPowerOfTen@370 (line 120)
0x00000000025245a1: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe27] # 0x00000000025243d0
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@358 (line 116)
; {section_word}
0x00000000025245a9: jmp 0x0000000002524744
0x00000000025245ae: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe22] # 0x00000000025243d8
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@348 (line 114)
; {section_word}
0x00000000025245b6: jmp 0x0000000002524744
0x00000000025245bb: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe1d] # 0x00000000025243e0
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@338 (line 112)
; {section_word}
0x00000000025245c3: jmp 0x0000000002524744
0x00000000025245c8: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe18] # 0x00000000025243e8
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@328 (line 110)
; {section_word}
0x00000000025245d0: jmp 0x0000000002524744
0x00000000025245d5: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe13] # 0x00000000025243f0
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@318 (line 108)
; {section_word}
0x00000000025245dd: jmp 0x0000000002524744
0x00000000025245e2: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe0e] # 0x00000000025243f8
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@308 (line 106)
; {section_word}
0x00000000025245ea: jmp 0x0000000002524744
0x00000000025245ef: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe09] # 0x0000000002524400
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@298 (line 104)
; {section_word}
0x00000000025245f7: jmp 0x0000000002524744
0x00000000025245fc: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe04] # 0x0000000002524408
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@288 (line 102)
; {section_word}
0x0000000002524604: jmp 0x0000000002524744
0x0000000002524609: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffdff] # 0x0000000002524410
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@278 (line 100)
; {section_word}
0x0000000002524611: jmp 0x0000000002524744
0x0000000002524616: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffdfa] # 0x0000000002524418
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@268 (line 98)
; {section_word}
0x000000000252461e: jmp 0x0000000002524744
0x0000000002524623: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffd9d] # 0x00000000025243c8
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@258 (line 96)
; {section_word}
0x000000000252462b: jmp 0x0000000002524744
0x0000000002524630: movapd xmm0,xmm1
0x0000000002524634: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffe0c] # 0x0000000002524448
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@242 (line 92)
; {section_word}
0x000000000252463c: jmp 0x0000000002524744
0x0000000002524641: movapd xmm0,xmm1
0x0000000002524645: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffddb] # 0x0000000002524428
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@236 (line 90)
; {section_word}
0x000000000252464d: jmp 0x0000000002524744
0x0000000002524652: movapd xmm0,xmm1
0x0000000002524656: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffdd2] # 0x0000000002524430
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@230 (line 88)
; {section_word}
0x000000000252465e: jmp 0x0000000002524744
0x0000000002524663: movapd xmm0,xmm1
0x0000000002524667: mulsd xmm0,QWORD PTR [rip+0xfffffffffffffdc9] # 0x0000000002524438
;*dmul
; - javaapplication4.Test1::multiplyByPowerOfTen@224 (line 86)
; {section_word}
[etc.]
0x0000000002524744: add rsp,0x10
0x0000000002524748: pop rbp
0x0000000002524749: test DWORD PTR [rip+0xfffffffffde1b8b1],eax # 0x0000000000340000
; {poll_return}
0x000000000252474f: ret
switch
açık bir şekilde en uygun çözüm olduğu için tüm ifadelerde tam olarak 22 vaka olacak . : D (Bunu benim yoluma gösterme, lütfen.)