pow函數有四樣寫法，你知道嗎

求次冪是個老生常談的話題了，在任何一門語言裏都是最基礎的部分。在這裏主要以JS爲例。

首先，最容易想到的是暴力法，也就是連續乘上n個x；但是這個的性能顯然很差，我們一般不會考慮這麼寫。而且，爲什麼不用內置函數呢？調內置函數沒什麼難度，隨便舉個例子吧：

var myPow = function(x, n) {
    return Math.pow(x, n);
};

確實，這個沒什麼難度，並且內置函數一般得到了充分的優化，性能也是很有保障的。但是，內置函數一定就快嗎？還能進一步優化嗎？還有沒有別的方法？這些問題是值得思考的。

這是利用數學性質來進行計算的，基於一個很簡單的思路，$x^n=2^{nlog_2x}$：

var myPow = function (x, n) {
  if (n == 0) return 1;
  let tmp = Math.exp(n * Math.log(Math.abs(x)));
  if (x < 0 && n % 2 == 1) tmp *= -1;
  return tmp;
};

而這個用的是快速冪乘法，需要注意的是，要用Math.trunc而不是Math.floor，因爲Math.floor是向下取整，在面對負數的時候也會向下取整，導致無限遞歸；而Math.trunc則是截斷，相當於向0取整：

function pow(x, n) {
  if (n == 0) {
    return 1;
  }
  const tmp = pow(x, Math.trunc(n / 2));
  return n % 2 == 0 ? tmp * tmp : tmp * tmp * x;
}
var myPow = function (x, n) {
  return n > 0 ? pow(x, n) : 1 / pow(x, n);
};

我們已經看到了其他的寫法，按理來說，到這裏已經可以結束了，但是我發現，在執行次數很少的情況下，手寫的代碼反而可能比原生的快，這就很奇怪了。所以我打算看看V8的源碼，能不能看出點東西來：

CodeStubAssembler::Node* MathBuiltinsAssembler::MathPow(Node* context,
                                                        Node* base,
                                                        Node* exponent) {
  Node* base_value = TruncateTaggedToFloat64(context, base);
  Node* exponent_value = TruncateTaggedToFloat64(context, exponent);
  Node* value = Float64Pow(base_value, exponent_value);
  return ChangeFloat64ToTagged(value);
}
Node* Float64Pow(Node* a, Node* b) {
    return AddNode(machine()->Float64Pow(), a, b);
}

Node* Float64Exp(Node* a) { return AddNode(machine()->Float64Exp(), a); }
Node* Float64Log(Node* a) { return AddNode(machine()->Float64Log(), a); }

得，話都說到這份上了，用的是硬件自帶的運算，咱也沒轍。所以只能進一步深入，看看能不能從彙編層面看出點東西來，用d8編譯一下，看看輸出的結果。

內容很長，放到最後面了。簡單說一下結論：在執行次數很少的情況下（我這裏測試大概是10000次以下），原生代碼可能不會成爲所謂的“熱點”，也就是不會被V8編譯成彙編指令；而手寫的代碼會被編譯成彙編指令。所以，在這個情況下，是有可能存在手寫比原生快的情況的；而隨着執行次數的增加，原生代碼也會變成彙編指令，並且條數更少，自然也就更快了。

附錄

原生

Instructions (size = 460)
00000061000C2E60     0  488d1df9ffffff REX.W leaq rbx,[rip+0xfffffff9]
00000061000C2E67     7  483bd9         REX.W cmpq rbx,rcx
00000061000C2E6A     a  7418           jz 00000061000C2E84  <+0x24>
00000061000C2E6C     c  48ba6800000000000000 REX.W movq rdx,0000000000000068
00000061000C2E76    16  49bae0c0d5cdfb7f0000 REX.W movq r10,00007FFBCDD5C0E0  (Abort)    ;; off heap target
00000061000C2E80    20  41ffd2         call r10
00000061000C2E83    23  cc             int3l
00000061000C2E84    24  8b59d0         movl rbx,[rcx-0x30]
00000061000C2E87    27  4903dd         REX.W addq rbx,r13
00000061000C2E8A    2a  f6430701       testb [rbx+0x7],0x1
00000061000C2E8E    2e  740d           jz 00000061000C2E9D  <+0x3d>
00000061000C2E90    30  49baa0f9cacdfb7f0000 REX.W movq r10,00007FFBCDCAF9A0  (CompileLazyDeoptimizedCode)    ;; off heap target
00000061000C2E9A    3a  41ffe2         jmp r10
00000061000C2E9D    3d  55             push rbp
00000061000C2E9E    3e  4889e5         REX.W movq rbp,rsp
00000061000C2EA1    41  56             push rsi
00000061000C2EA2    42  57             push rdi
00000061000C2EA3    43  48ba4200000000000000 REX.W movq rdx,0000000000000042
00000061000C2EAD    4d  4c8b15c4ffffff REX.W movq r10,[rip+0xffffffc4]
00000061000C2EB4    54  41ffd2         call r10
00000061000C2EB7    57  cc             int3l
00000061000C2EB8    58  4883ec10       REX.W subq rsp,0x10
00000061000C2EBC    5c  488975b0       REX.W movq [rbp-0x50],rsi
00000061000C2EC0    60  488b55d0       REX.W movq rdx,[rbp-0x30]
00000061000C2EC4    64  f6c201         testb rdx,0x1
00000061000C2EC7    67  0f8516010000   jnz 00000061000C2FE3  <+0x183>
00000061000C2ECD    6d  81fa002d3101   cmpl rdx,0x1312d00
00000061000C2ED3    73  0f8c0b000000   jl 00000061000C2EE4  <+0x84>
00000061000C2ED9    79  488b45d8       REX.W movq rax,[rbp-0x28]
00000061000C2EDD    7d  488be5         REX.W movq rsp,rbp
00000061000C2EE0    80  5d             pop rbp
00000061000C2EE1    81  c20800         ret 0x8
00000061000C2EE4    84  48b9b93a240861000000 REX.W movq rcx,0000006108243AB9    ;; object: 0x006108243ab9 <Object map = 0000006108280A09>
00000061000C2EEE    8e  8b4903         movl rcx,[rcx+0x3]
00000061000C2EF1    91  4903cd         REX.W addq rcx,r13
00000061000C2EF4    94  8b495b         movl rcx,[rcx+0x5b]
00000061000C2EF7    97  49ba0000000001000000 REX.W movq r10,0000000100000000
00000061000C2F01    a1  4c3bd1         REX.W cmpq r10,rcx
00000061000C2F04    a4  7715           ja 00000061000C2F1B  <+0xbb>
00000061000C2F06    a6  48ba0200000000000000 REX.W movq rdx,0000000000000002
00000061000C2F10    b0  4c8b1561ffffff REX.W movq r10,[rip+0xffffff61]
00000061000C2F17    b7  41ffd2         call r10
00000061000C2F1A    ba  cc             int3l
00000061000C2F1B    bb  bff13f2408     movl rdi,0000000008243FF1    ;; (compressed) object: 0x006108243ff1 <JSFunction pow (sfi = 00000061081C7729)>
00000061000C2F20    c0  3bf9           cmpl rdi,rcx
00000061000C2F22    c2  0f85c7000000   jnz 00000061000C2FEF  <+0x18f>
00000061000C2F28    c8  488bca         REX.W movq rcx,rdx
00000061000C2F2B    cb  d1f9           sarl rcx, 1
00000061000C2F2D    cd  83c101         addl rcx,0x1
00000061000C2F30    d0  0f80c5000000   jo 00000061000C2FFB  <+0x19b>
00000061000C2F36    d6  493b6560       REX.W cmpq rsp,[r13+0x60] (external value (StackGuard::address_of_jslimit()))
00000061000C2F3A    da  0f8713000000   ja 00000061000C2F53  <+0xf3>
00000061000C2F40    e0  e940000000     jmp 00000061000C2F85  <+0x125>
00000061000C2F45    e5  660f1f840000000000 nop
00000061000C2F4E    ee  6690           nop
00000061000C2F50    f0  488bca         REX.W movq rcx,rdx
00000061000C2F53    f3  81f980969800   cmpl rcx,0x989680
00000061000C2F59    f9  0f8d17000000   jge 00000061000C2F76  <+0x116>
00000061000C2F5F    ff  488bd1         REX.W movq rdx,rcx
00000061000C2F62   102  83c201         addl rdx,0x1
00000061000C2F65   105  0f809c000000   jo 00000061000C3007  <+0x1a7>
00000061000C2F6B   10b  493b6560       REX.W cmpq rsp,[r13+0x60] (external value (StackGuard::address_of_jslimit()))
00000061000C2F6F   10f  77df           ja 00000061000C2F50  <+0xf0>
00000061000C2F71   111  e942000000     jmp 00000061000C2FB8  <+0x158>
00000061000C2F76   116  48b80008000000000000 REX.W movq rax,0000000000000800
00000061000C2F80   120  e958ffffff     jmp 00000061000C2EDD  <+0x7d>
00000061000C2F85   125  48894da8       REX.W movq [rbp-0x58],rcx
00000061000C2F89   129  33c0           xorl rax,rax
00000061000C2F8B   12b  48bea10d240861000000 REX.W movq rsi,0000006108240DA1    ;; object: 0x006108240da1 <NativeContext[247]>
00000061000C2F95   135  48bbd0ec24cffb7f0000 REX.W movq rbx,00007FFBCF24ECD0    ;; external reference (Runtime::StackGuard)
00000061000C2F9F   13f  488bd0         REX.W movq rdx,rax
00000061000C2FA2   142  488bfe         REX.W movq rdi,rsi
00000061000C2FA5   145  49ba00a8edcdfb7f0000 REX.W movq r10,00007FFBCDEDA800  (CEntry_Return1_DontSaveFPRegs_ArgvOnStack_NoBuiltinExit)    ;; off heap target
00000061000C2FAF   14f  41ffd2         call r10
00000061000C2FB2   152  488b4da8       REX.W movq rcx,[rbp-0x58]
00000061000C2FB6   156  eb9b           jmp 00000061000C2F53  <+0xf3>
00000061000C2FB8   158  488955a8       REX.W movq [rbp-0x58],rdx
00000061000C2FBC   15c  488b1dd4ffffff REX.W movq rbx,[rip+0xffffffd4]
00000061000C2FC3   163  33c0           xorl rax,rax
00000061000C2FC5   165  48bea10d240861000000 REX.W movq rsi,0000006108240DA1    ;; object: 0x006108240da1 <NativeContext[247]>
00000061000C2FCF   16f  4c8b15d1ffffff REX.W movq r10,[rip+0xffffffd1]
00000061000C2FD6   176  41ffd2         call r10
00000061000C2FD9   179  488b55a8       REX.W movq rdx,[rbp-0x58]
00000061000C2FDD   17d  e96effffff     jmp 00000061000C2F50  <+0xf0>
00000061000C2FE2   182  90             nop
00000061000C2FE3   183  49c7c500000000 REX.W movq r13,0x0
00000061000C2FEA   18a  e851f00300     call 0000006100102040    ;; eager deoptimization bailout
00000061000C2FEF   18f  49c7c501000000 REX.W movq r13,0x1
00000061000C2FF6   196  e845f00300     call 0000006100102040    ;; eager deoptimization bailout
00000061000C2FFB   19b  49c7c502000000 REX.W movq r13,0x2
00000061000C3002   1a2  e839f00300     call 0000006100102040    ;; eager deoptimization bailout
00000061000C3007   1a7  49c7c503000000 REX.W movq r13,0x3
00000061000C300E   1ae  e82df00300     call 0000006100102040    ;; eager deoptimization bailout
00000061000C3013   1b3  49c7c504000000 REX.W movq r13,0x4
00000061000C301A   1ba  e821f00700     call 0000006100142040    ;; lazy deoptimization bailout
00000061000C301F   1bf  49c7c505000000 REX.W movq r13,0x5
00000061000C3026   1c6  e815f00700     call 0000006100142040    ;; lazy deoptimization bailout
00000061000C302B   1cb  90             nop

手寫

Instructions (size = 580)
0000028E000C2F20     0  488d1df9ffffff REX.W leaq rbx,[rip+0xfffffff9]
0000028E000C2F27     7  483bd9         REX.W cmpq rbx,rcx
0000028E000C2F2A     a  7418           jz 0000028E000C2F44  <+0x24>
0000028E000C2F2C     c  48ba6800000000000000 REX.W movq rdx,0000000000000068
0000028E000C2F36    16  49bae0c0d5cdfb7f0000 REX.W movq r10,00007FFBCDD5C0E0  (Abort)    ;; off heap target
0000028E000C2F40    20  41ffd2         call r10
0000028E000C2F43    23  cc             int3l
0000028E000C2F44    24  8b59d0         movl rbx,[rcx-0x30]
0000028E000C2F47    27  4903dd         REX.W addq rbx,r13
0000028E000C2F4A    2a  f6430701       testb [rbx+0x7],0x1
0000028E000C2F4E    2e  740d           jz 0000028E000C2F5D  <+0x3d>
0000028E000C2F50    30  49baa0f9cacdfb7f0000 REX.W movq r10,00007FFBCDCAF9A0  (CompileLazyDeoptimizedCode)    ;; off heap target
0000028E000C2F5A    3a  41ffe2         jmp r10
0000028E000C2F5D    3d  55             push rbp
0000028E000C2F5E    3e  4889e5         REX.W movq rbp,rsp
0000028E000C2F61    41  56             push rsi
0000028E000C2F62    42  57             push rdi
0000028E000C2F63    43  48ba4200000000000000 REX.W movq rdx,0000000000000042
0000028E000C2F6D    4d  4c8b15c4ffffff REX.W movq r10,[rip+0xffffffc4]
0000028E000C2F74    54  41ffd2         call r10
0000028E000C2F77    57  cc             int3l
0000028E000C2F78    58  4883ec10       REX.W subq rsp,0x10
0000028E000C2F7C    5c  488975b0       REX.W movq [rbp-0x50],rsi
0000028E000C2F80    60  488b55d0       REX.W movq rdx,[rbp-0x30]
0000028E000C2F84    64  f6c201         testb rdx,0x1
0000028E000C2F87    67  0f8576010000   jnz 0000028E000C3103  <+0x1e3>
0000028E000C2F8D    6d  81fa002d3101   cmpl rdx,0x1312d00
0000028E000C2F93    73  0f8c0b000000   jl 0000028E000C2FA4  <+0x84>
0000028E000C2F99    79  488b45d8       REX.W movq rax,[rbp-0x28]
0000028E000C2F9D    7d  488be5         REX.W movq rsp,rbp
0000028E000C2FA0    80  5d             pop rbp
0000028E000C2FA1    81  c20800         ret 0x8
0000028E000C2FA4    84  48b9b93a24088e020000 REX.W movq rcx,0000028E08243AB9    ;; object: 0x028e08243ab9 <Object map = 0000028E08280A09>
0000028E000C2FAE    8e  8b7903         movl rdi,[rcx+0x3]
0000028E000C2FB1    91  4903fd         REX.W addq rdi,r13
0000028E000C2FB4    94  448b472f       movl r8,[rdi+0x2f]
0000028E000C2FB8    98  49ba0000000001000000 REX.W movq r10,0000000100000000
0000028E000C2FC2    a2  4d3bd0         REX.W cmpq r10,r8
0000028E000C2FC5    a5  7715           ja 0000028E000C2FDC  <+0xbc>
0000028E000C2FC7    a7  48ba0200000000000000 REX.W movq rdx,0000000000000002
0000028E000C2FD1    b1  4c8b1560ffffff REX.W movq r10,[rip+0xffffff60]
0000028E000C2FD8    b8  41ffd2         call r10
0000028E000C2FDB    bb  cc             int3l
0000028E000C2FDC    bc  8b7f43         movl rdi,[rdi+0x43]
0000028E000C2FDF    bf  4c8b15d4ffffff REX.W movq r10,[rip+0xffffffd4]
0000028E000C2FE6    c6  4c3bd7         REX.W cmpq r10,rdi
0000028E000C2FE9    c9  7712           ja 0000028E000C2FFD  <+0xdd>
0000028E000C2FEB    cb  488b15d7ffffff REX.W movq rdx,[rip+0xffffffd7]
0000028E000C2FF2    d2  4c8b153fffffff REX.W movq r10,[rip+0xffffff3f]
0000028E000C2FF9    d9  41ffd2         call r10
0000028E000C2FFC    dc  cc             int3l
0000028E000C2FFD    dd  8b490b         movl rcx,[rcx+0xb]
0000028E000C3000    e0  4c8b15b3ffffff REX.W movq r10,[rip+0xffffffb3]
0000028E000C3007    e7  4c3bd1         REX.W cmpq r10,rcx
0000028E000C300A    ea  7712           ja 0000028E000C301E  <+0xfe>
0000028E000C300C    ec  488b15b6ffffff REX.W movq rdx,[rip+0xffffffb6]
0000028E000C3013    f3  4c8b151effffff REX.W movq r10,[rip+0xffffff1e]
0000028E000C301A    fa  41ffd2         call r10
0000028E000C301D    fd  cc             int3l
0000028E000C301E    fe  41b909412408   movl r9,0000000008244109    ;; (compressed) object: 0x028e08244109 <JSFunction abs (sfi = 0000028E081C7919)>
0000028E000C3024   104  443bc9         cmpl r9,rcx
0000028E000C3027   107  0f85e2000000   jnz 0000028E000C310F  <+0x1ef>
0000028E000C302D   10d  b9493f2408     movl rcx,0000000008243F49    ;; (compressed) object: 0x028e08243f49 <JSFunction log (sfi = 0000028E081C7639)>
0000028E000C3032   112  3bcf           cmpl rcx,rdi
0000028E000C3034   114  0f85e1000000   jnz 0000028E000C311B  <+0x1fb>
0000028E000C303A   11a  b9bd3e2408     movl rcx,0000000008243EBD    ;; (compressed) object: 0x028e08243ebd <JSFunction exp (sfi = 0000028E081C7571)>
0000028E000C303F   11f  413bc8         cmpl rcx,r8
0000028E000C3042   122  0f85df000000   jnz 0000028E000C3127  <+0x207>
0000028E000C3048   128  488bca         REX.W movq rcx,rdx
0000028E000C304B   12b  d1f9           sarl rcx, 1
0000028E000C304D   12d  83c101         addl rcx,0x1
0000028E000C3050   130  0f80dd000000   jo 0000028E000C3133  <+0x213>
0000028E000C3056   136  493b6560       REX.W cmpq rsp,[r13+0x60] (external value (StackGuard::address_of_jslimit()))
0000028E000C305A   13a  0f8713000000   ja 0000028E000C3073  <+0x153>
0000028E000C3060   140  e940000000     jmp 0000028E000C30A5  <+0x185>
0000028E000C3065   145  660f1f840000000000 nop
0000028E000C306E   14e  6690           nop
0000028E000C3070   150  488bca         REX.W movq rcx,rdx
0000028E000C3073   153  81f980969800   cmpl rcx,0x989680
0000028E000C3079   159  0f8d17000000   jge 0000028E000C3096  <+0x176>
0000028E000C307F   15f  488bd1         REX.W movq rdx,rcx
0000028E000C3082   162  83c201         addl rdx,0x1
0000028E000C3085   165  0f80b4000000   jo 0000028E000C313F  <+0x21f>
0000028E000C308B   16b  493b6560       REX.W cmpq rsp,[r13+0x60] (external value (StackGuard::address_of_jslimit()))
0000028E000C308F   16f  77df           ja 0000028E000C3070  <+0x150>
0000028E000C3091   171  e942000000     jmp 0000028E000C30D8  <+0x1b8>
0000028E000C3096   176  48b80008000000000000 REX.W movq rax,0000000000000800
0000028E000C30A0   180  e9f8feffff     jmp 0000028E000C2F9D  <+0x7d>
0000028E000C30A5   185  48894da8       REX.W movq [rbp-0x58],rcx
0000028E000C30A9   189  33c0           xorl rax,rax
0000028E000C30AB   18b  48bea10d24088e020000 REX.W movq rsi,0000028E08240DA1    ;; object: 0x028e08240da1 <NativeContext[247]>
0000028E000C30B5   195  48bbd0ec24cffb7f0000 REX.W movq rbx,00007FFBCF24ECD0    ;; external reference (Runtime::StackGuard)
0000028E000C30BF   19f  488bd0         REX.W movq rdx,rax
0000028E000C30C2   1a2  488bfe         REX.W movq rdi,rsi
0000028E000C30C5   1a5  49ba00a8edcdfb7f0000 REX.W movq r10,00007FFBCDEDA800  (CEntry_Return1_DontSaveFPRegs_ArgvOnStack_NoBuiltinExit)    ;; off heap target
0000028E000C30CF   1af  41ffd2         call r10
0000028E000C30D2   1b2  488b4da8       REX.W movq rcx,[rbp-0x58]
0000028E000C30D6   1b6  eb9b           jmp 0000028E000C3073  <+0x153>
0000028E000C30D8   1b8  488955a8       REX.W movq [rbp-0x58],rdx
0000028E000C30DC   1bc  488b1dd4ffffff REX.W movq rbx,[rip+0xffffffd4]
0000028E000C30E3   1c3  33c0           xorl rax,rax
0000028E000C30E5   1c5  48bea10d24088e020000 REX.W movq rsi,0000028E08240DA1    ;; object: 0x028e08240da1 <NativeContext[247]>
0000028E000C30EF   1cf  4c8b15d1ffffff REX.W movq r10,[rip+0xffffffd1]
0000028E000C30F6   1d6  41ffd2         call r10
0000028E000C30F9   1d9  488b55a8       REX.W movq rdx,[rbp-0x58]
0000028E000C30FD   1dd  e96effffff     jmp 0000028E000C3070  <+0x150>
0000028E000C3102   1e2  90             nop
0000028E000C3103   1e3  49c7c500000000 REX.W movq r13,0x0
0000028E000C310A   1ea  e831ef0300     call 0000028E00102040    ;; eager deoptimization bailout
0000028E000C310F   1ef  49c7c501000000 REX.W movq r13,0x1
0000028E000C3116   1f6  e825ef0300     call 0000028E00102040    ;; eager deoptimization bailout
0000028E000C311B   1fb  49c7c502000000 REX.W movq r13,0x2
0000028E000C3122   202  e819ef0300     call 0000028E00102040    ;; eager deoptimization bailout
0000028E000C3127   207  49c7c503000000 REX.W movq r13,0x3
0000028E000C312E   20e  e80def0300     call 0000028E00102040    ;; eager deoptimization bailout
0000028E000C3133   213  49c7c504000000 REX.W movq r13,0x4
0000028E000C313A   21a  e801ef0300     call 0000028E00102040    ;; eager deoptimization bailout
0000028E000C313F   21f  49c7c505000000 REX.W movq r13,0x5
0000028E000C3146   226  e8f5ee0300     call 0000028E00102040    ;; eager deoptimization bailout
0000028E000C314B   22b  49c7c506000000 REX.W movq r13,0x6
0000028E000C3152   232  e8e9ee0700     call 0000028E00142040    ;; lazy deoptimization bailout
0000028E000C3157   237  49c7c507000000 REX.W movq r13,0x7
0000028E000C315E   23e  e8ddee0700     call 0000028E00142040    ;; lazy deoptimization bailout
0000028E000C3163   243  90             nop