ios – 在Swift中,为什么“让_ = this”比“this!= nil”更快?
发布时间:2020-12-14 19:46:37 所属栏目:百科 来源:网络整理
导读:所以我的问题是为什么让_ =这比这快!= nil? 例: 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 -
所以我的问题是为什么让_ =这比这快!= nil?
例: 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 解决方法
在
Jonathan的
response之后,我检查了实际的反汇编指令.
结果如下: 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 并为: 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指向优化级别. 将值从[-Onone]更改为[-O -whole-module-optimization]将导致生成的asm按以下方式更改: 该 let this : Bool? = nil let _ = this 具有 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 至 0x100001490 <+0>: pushq %rbp 0x100001491 <+1>: movq %rsp,%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. (编辑:李大同) 【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容! |