javascript - JavaScript 中的分支

Question

考虑这段代码:

if (a) 
    doSomething(a)
else if (b)
    doSomething(b)
else
    doSomething(c)

我可以使用 javascript 逻辑运算符将其等效地重写为:

(a && doSomething(a) || (!a && b && doSomething(b)) || (!a && !b && doSomething(c)))

我知道它不太可读，但这会以某种方式优化以前的版本吗？

由于运算符&&和||返回实际的表达式值，所以比较会减少吗？

Answer 1

我很好奇，想通过 v8 反汇编程序 (d8 --print_code) 运行这两个函数(简化版)。

第一个版本:

--- Raw source ---
(a, b) {
    if (a) {
        return foo(a);
    } else if (b) {
        return bar(b);
    }
}


--- Code ---
source_position = 47
kind = FUNCTION
name = x
Instructions (size = 284)
 44 REX.W movq rax,[rbp+0x18]
 48 call 0x235a8cb18560     ;; debug: statement 60
                            ;; debug: position 64
                            ;; code: TO_BOOLEAN_IC, UNINITIALIZED (id = 8)
 53 REX.W testq rax,rax
 56 jz 184  (0x235a8cb66518)
 62 REX.W movq rcx,0x1d21fad2d781    ;; object: 0x1d21fad2d781 <String[3]: foo>
 72 REX.W movq rdx,[rsi+0x27]
 76 call 0x235a8cb368c0     ;; debug: statement 77
                            ;; debug: position 84
                            ;; code: contextual, LOAD_IC, UNINITIALIZED
 81 push rax
 82 REX.W movq r10,0x144d04104121    ;; object: 0x144d04104121 <undefined>
 92 push r10
 94 push [rbp+0x18]
 97 REX.W leaq rdx,[r12+r12*2]
101 REX.W movq rdi,[rsp+0x10]
106 call 0x235a8cb15d80     ;; code: CALL_IC, DEFAULT
111 REX.W movq rsi,[rbp-0x8]
115 REX.W addq rsp,0x8
119 REX.W movq rbx,0x7ed0fc04b11    ;; object: 0x7ed0fc04b11 Cell for 6144
129 addl [rbx+0xb],0xd1
133 jns 166  (0x235a8cb66506)
135 push rax
136 call InterruptCheck  (0x235a8cb3ac60)    ;; code: BUILTIN
141 pop rax
142 REX.W movq rbx,0x7ed0fc04b11    ;; object: 0x7ed0fc04b11 Cell for 6144
152 REX.W movq r10,0x180000000000
162 REX.W movq [rbx+0x7],r10
166 REX.W movq rsp,rbp      ;; debug: statement 141
                            ;; js return
169 pop rbp
170 ret 0x18
173 int3
174 int3
175 int3
176 int3
177 int3
178 int3
179 jmp 266  (0x235a8cb6656a)
184 REX.W movq rax,[rbp+0x10]
188 call 0x235a8cb18560     ;; debug: statement 103
                            ;; debug: position 107
                            ;; code: TO_BOOLEAN_IC, UNINITIALIZED (id = 24)
193 REX.W testq rax,rax
196 jz 266  (0x235a8cb6656a)
202 REX.W movq rcx,0x1d21fad2d7a1    ;; object: 0x1d21fad2d7a1 <String[3]: bar>
212 REX.W movq rdx,[rsi+0x27]
216 call 0x235a8cb368c0     ;; debug: statement 120
                            ;; debug: position 127
                            ;; code: contextual, LOAD_IC, UNINITIALIZED
221 push rax
222 REX.W movq r10,0x144d04104121    ;; object: 0x144d04104121 <undefined>
232 push r10
234 push [rbp+0x10]
237 xorl rdx,rdx
239 REX.W leaq rdx,[rdx+r12*4]
243 REX.W movq rdi,[rsp+0x10]
248 call 0x235a8cb15d80     ;; code: CALL_IC, DEFAULT
253 REX.W movq rsi,[rbp-0x8]
257 REX.W addq rsp,0x8
261 jmp 119  (0x235a8cb664d7)
266 REX.W movq rax,[r13-0x58]
270 jmp 119  (0x235a8cb664d7)
275 nop

“优化”版本:

--- Raw source ---
(a, b) {
  return a && foo(a) || (!a && b && bar(b))
}

--- Code ---
source_position = 154
kind = FUNCTION
name = y
Instructions (size = 372)

 44 REX.W movq rax,[rbp+0x18]
 48 push rax
 49 call 0x235a8cb18560     ;; debug: statement 165
                            ;; debug: position 172
                            ;; code: TO_BOOLEAN_IC, UNINITIALIZED (id = 13)
 54 REX.W testq rax,rax
 57 jnz 69  (0x235a8cb66705)
 63 pop rax
 64 jmp 130  (0x235a8cb66742)
 69 REX.W addq rsp,0x8
 73 d0000 REX.W movq rcx,0x1d21fad2d781    ;; object: 0x1d21fad2d781 <String[3]: foo>
 83 REX.W movq rdx,[rsi+0x27]
 87 call 0x235a8cb368c0     ;; debug: position 177
                            ;; code: contextual, LOAD_IC, UNINITIALIZED
 92 push rax
 93 40000 REX.W movq r10,0x144d04104121    ;; object: 0x144d04104121 <undefined>
103 push r10
105 push [rbp+0x18]
108 REX.W leaq rdx,[r12+r12*2]
112 REX.W movq rdi,[rsp+0x10]
117 call 0x235a8cb15d80     ;; code: CALL_IC, DEFAULT
122 REX.W movq rsi,[rbp-0x8]
126 REX.W addq rsp,0x8
130 push rax
131 call 0x235a8cb18560     ;; code: TO_BOOLEAN_IC, UNINITIALIZED (id = 9)
136 REX.W testq rax,rax
139 jz 151  (0x235a8cb66757)
145 pop rax
146 jmp 296  (0x235a8cb667e8)
151 REX.W addq rsp,0x8
155 REX.W movq rax,[rbp+0x18]
159 call 0x235a8cb18560     ;; debug: position 189
                            ;; code: TO_BOOLEAN_IC, UNINITIALIZED (id = 35)
164 REX.W testq rax,rax
167 jnz 179  (0x235a8cb66773)
173 REX.W movq rax,[r13-0x40]
177 jmp 183  (0x235a8cb66777)
179 REX.W movq rax,[r13-0x38]
183 push rax
184 call 0x235a8cb18560     ;; code: TO_BOOLEAN_IC, UNINITIALIZED (id = 31)
189 REX.W testq rax,rax
192 jnz 204  (0x235a8cb6678c)
198 pop rax
199 jmp 212  (0x235a8cb66794)
204 REX.W addq rsp,0x8
208 REX.W movq rax,[rbp+0x10]
212 push rax
213 call 0x235a8cb18560     ;; debug: position 194
                            ;; code: TO_BOOLEAN_IC, UNINITIALIZED (id = 27)
218 REX.W testq rax,rax
221 jnz 233  (0x235a8cb667a9)
227 pop rax
228 jmp 296  (0x235a8cb667e8)
233 REX.W addq rsp,0x8
237 REX.W movq rcx,0x1d21fad2d7a1    ;; object: 0x1d21fad2d7a1 <String[3]: bar>
247 REX.W movq rdx,[rsi+0x27]
251 call 0x235a8cb368c0     ;; debug: position 199
                            ;; code: contextual, LOAD_IC, UNINITIALIZED
256 push rax
257 REX.W movq r10,0x144d04104121    ;; object: 0x144d04104121 <undefined>
267 push r10
269 push [rbp+0x10]
272 xorl rdx,rdx
274 REX.W leaq rdx,[rdx+r12*4]
278 REX.W movq rdi,[rsp+0x10]
283 call 0x235a8cb15d80     ;; code: CALL_IC, DEFAULT
288 REX.W movq rsi,[rbp-0x8]
292 REX.W addq rsp,0x8
296 REX.W movq rbx,0x7ed0fc04b31    ;; object: 0x7ed0fc04b31 Cell for 6144
306 addl [rbx+0xb],0xd1
310 jns 343  (0x235a8cb66817)
312 push rax
313 call InterruptCheck  (0x235a8cb3ac60)    ;; code: BUILTIN
318 pop rax
319 REX.W movq rbx,0x7ed0fc04b31    ;; object: 0x7ed0fc04b31 Cell for 6144
329 REX.W movq r10,0x180000000000
339 REX.W movq [rbx+0x7],r10
343 REX.W movq rsp,rbp      ;; debug: statement 207
                            ;; js return

即使您不阅读 x86 汇编程序，很明显“优化”版本实际上具有更多比较和更多分支(查找jmp、jz 等)。

所以你的问题的答案

will this somehow be optimizing the previous version?

是否，至少在v8中是这样(我猜其他引擎在这方面没有太大不同)。

这是 foo(a || b || c) 的 dis:

--- Raw source ---
(a, b, c) {
    return foo(a || b || c);
}


--- Code ---
source_position = 220
kind = FUNCTION
name = z
Instructions (size = 228)
 44 REX.W movq rcx,0x36f71e52d781    ;; object: 0x36f71e52d781 <String[3]: foo>
 54 REX.W movq rdx,[rsi+0x27]
 58 call 0x117d9bb368c0     ;; debug: statement 236
                            ;; debug: position 243
                            ;; code: contextual, LOAD_IC, UNINITIALIZED
 63 push rax
 64 REX.W movq r10,0xb18ce704121    ;; object: 0xb18ce704121 <undefined>
 74 push r10
 76 REX.W movq rax,[rbp+0x20]
 80 push rax
 81 call 0x117d9bb18560     ;; debug: position 247
                            ;; code: TO_BOOLEAN_IC, UNINITIALIZED (id = 19)
 86 REX.W testq rax,rax
 89 jz 101  (0x117d9bb66945)
 95 pop rax
 96 jmp 109  (0x117d9bb6694d)
101 REX.W addq rsp,0x8
105 REX.W movq rax,[rbp+0x18]
109 push rax
110 call 0x117d9bb18560     ;; debug: position 252
                            ;; code: TO_BOOLEAN_IC, UNINITIALIZED (id = 15)
115 REX.W testq rax,rax
118 jnz 131  (0x117d9bb66963)
124 REX.W addq rsp,0x8
128 push [rbp+0x10]
131 REX.W leaq rdx,[r12+r12*2]
135 REX.W movq rdi,[rsp+0x10]
140 call 0x117d9bb15d80     ;; code: CALL_IC, DEFAULT
145 REX.W movq rsi,[rbp-0x8]
149 REX.W addq rsp,0x8
153 REX.W movq rbx,0xdf65a404b41    ;; object: 0xdf65a404b41 Cell for 6144
163 addl [rbx+0xb],0xd1
167 jns 200  (0x117d9bb669a8)
169 push rax
170 call InterruptCheck  (0x117d9bb3ac60)    ;; code: BUILTIN
175 pop rax
176 REX.W movq rbx,0xdf65a404b41    ;; object: 0xdf65a404b41 Cell for 6144
186 REX.W movq r10,0x180000000000
196 REX.W movq [rbx+0x7],r10
200 REX.W movq rsp,rbp      ;; debug: statement 261
                            ;; js return
203 pop rbp
204 ret 0x20
207 int3
208 int3
209 int3
210 int3
211 int3
212 int3
213 REX.W movq rax,[r13-0x58]
217 jmp 153  (0x117d9bb66979)
222 nop