ios - 在 Swift 中，为什么 "let _ = this"比 "this != nil"快？

Question

所以我的问题是为什么 let _ = this 比 this != nil 快？

例子:

这是:

let this : Bool? = true //

let start = DispatchTime.now()
for _ in 0...100000000  {
    guard this != nil else { continue }
}
let end = DispatchTime.now()

let nanoTime = end.uptimeNanoseconds - start.uptimeNanoseconds
let timeInterval = Double(nanoTime)
print("Time \(timeInterval)") 

         // Time 5426559135.0
         // Time 5428084767.0
         // Time 5327325459.0

慢于:

let this : Bool? = true //

let start = DispatchTime.now()
for _ in 0...100000000  {
    guard let _ = this else { continue }
}
let end = DispatchTime.now()

let nanoTime = end.uptimeNanoseconds - start.uptimeNanoseconds
let timeInterval = Double(nanoTime)
print("Time \(timeInterval)")

          // Time 257045414.0
          // Time 261933863.0
          // Time 263465919.0

Answer 1

正在关注 Jonathan的 response我检查了实际的反汇编说明。 这是结果: 对于代码:

let this : Bool? = nil
this != nil

我们得到:

    0x100001290 <+0>:  pushq  %rbp
    0x100001291 <+1>:  movq   %rsp, %rbp
    0x100001294 <+4>:  subq   $0x30, %rsp
    0x100001298 <+8>:  leaq   0x2c7259(%rip), %rdx      ; type metadata for Swift.Bool
    0x10000129f <+15>: leaq   0x2b66ca(%rip), %rcx      ; protocol witness table for Swift.Bool : Swift.Equatable in Swift
    0x1000012a6 <+22>: leaq   -0x18(%rbp), %rax
    0x1000012aa <+26>: leaq   -0x8(%rbp), %r8
    0x1000012ae <+30>: movb   $0x2, 0x2f940b(%rip)
    0x1000012b5 <+37>: movb   0x2f9404(%rip), %r9b      ; test2.this : Swift.Optional<Swift.Bool>
    0x1000012bc <+44>: movb   %r9b, -0x8(%rbp)
    0x1000012c0 <+48>: movb   $0x2, -0x10(%rbp)
    0x1000012c4 <+52>: movb   -0x10(%rbp), %r9b
    0x1000012c8 <+56>: movb   %r9b, -0x18(%rbp)
    0x1000012cc <+60>: movl   %edi, -0x1c(%rbp)
    0x1000012cf <+63>: movq   %r8, %rdi
    0x1000012d2 <+66>: movq   %rsi, -0x28(%rbp)
    0x1000012d6 <+70>: movq   %rax, %rsi
    0x1000012d9 <+73>: callq  0x10004df10               ; Swift.!= infix <A where A: Swift.Equatable> (Swift.Optional<A>, Swift.Optional<A>) -> Swift.Bool
    0x1000012de <+78>: xorl   %r10d, %r10d
    0x1000012e1 <+81>: movb   %al, -0x29(%rbp)
    0x1000012e4 <+84>: movl   %r10d, %eax
    0x1000012e7 <+87>: addq   $0x30, %rsp
    0x1000012eb <+91>: popq   %rbp
    0x1000012ec <+92>: retq

和为了:

let this : Bool? = nil
let _ = this

有:

    0x1000012d0 <+0>:  pushq  %rbp
    0x1000012d1 <+1>:  movq   %rsp, %rbp
    0x1000012d4 <+4>:  xorl   %eax, %eax
    0x1000012d6 <+6>:  movb   $0x2, 0x2f93e3(%rip)
    0x1000012dd <+13>: movl   %edi, -0x4(%rbp)
    0x1000012e0 <+16>: movq   %rsi, -0x10(%rbp)
    0x1000012e4 <+20>: popq   %rbp
    0x1000012e5 <+21>: retq   

另外，谢谢Code Different用于指向优化级别。

将值从 [-Onone] 更改为 [-O -whole-module-optimisation] 将导致生成的 asm 发生以下变化:

let this : Bool? = nil
let _ = this

有

    0x100001490 <+0>:  pushq  %rbp
    0x100001491 <+1>:  movq   %rsp, %rbp
    0x100001494 <+4>:  movb   $0x2, 0x3d9595(%rip)      ; gCRAnnotations + 63
    0x10000149b <+11>: xorl   %eax, %eax
    0x10000149d <+13>: popq   %rbp
    0x10000149e <+14>: retq   

和

let this : Bool? = nil
this != nil

到

    0x100001490 <+0>:  pushq  %rbp
    0x100001491 <+1>:  movq   %rsp, %rbp
    0x100001494 <+4>:  movb   $0x2, 0x3d9595(%rip)      ; gCRAnnotations + 63
    0x10000149b <+11>: xorl   %eax, %eax
    0x10000149d <+13>: popq   %rbp
    0x10000149e <+14>: retq   

所以结果指令实际上是相同的，执行它们的时间应该非常接近。