performance - 通用序列和生成器的极端性能问题

Question

我正在尝试创建我自己的通用序列类型。我可以创建的显示性能问题的最简单示例是:

// Warning, this code will cause Xcode to hang if you run this in a playground
// Instead I recommend running it in the REPL  or running it with the command
// line tool

struct TakeGenerator<T: GeneratorType>: GeneratorType {
    typealias Element = T.Element

    var generator: T

    mutating func next() -> Element? {
        return self.generator.next()
    }
}

struct Take<T: SequenceType>: SequenceType {
    typealias Generator = TakeGenerator<T.Generator>

    let seq: T
    let test: ((element: T.Generator.Element) -> Bool)?

    func generate() -> Generator {
        return Generator(generator: seq.generate())
    }
}

// NOTE: The times below reflect how long it takes to run this code from 
// the command line with only that line uncommented (other than the type
// declarations)
//
// The command I use is: `time xcrun swift <filename>`

var array = [1,2,3,4]
Take(seq: array, test: {$0 < 3}) // 0.104 seconds to execute
Take(seq: [1,2,3,4], test: nil) // 0.100 seconds to execute
Take(seq: ([1,2,3,4] as [Int]), test: {$0 < 3}) // 0.140 seconds to execute
Take(seq: [1,2,3,4], test: {$0 < 3}) // 17.939 seconds to execute

注意:显然这是一个无用的类型，因为它只是生成与生成的类型相同的东西，但我的真实类型稍微复杂一些，更有用。

如您在我的代码注释中所见，最新版本的构造函数存在极端的性能问题。花费的不是十分之一秒，而是将近 18 秒。

我什至没有做任何实际的生成。我想这与确定类型有关，因为只有数组是文字并且提供了测试方法时才会出现问题。如果我将测试属性声明更改为硬编码为 Int，也没有性能问题:

let test: ((element: Int) -> Bool)?

任何人都可以阐明可能导致此性能问题的原因以及我如何解决它吗？

Answer 1

这绝对是一个类型推断问题。使用swiftc test.swift编译占用大部分时间；运行 ./test 是即时的。此外，示例 显示大部分时间花在

Call graph:
    2677 Thread_113719   DispatchQueue_1: com.apple.main-thread  (serial)
      2677 start  (in libdyld.dylib) + 1  [0x7fff946715c9]
        2677 main  (in swift) + 1725  [0x105919a2d]
          2677 frontend_main(llvm::ArrayRef<char const*>, char const*, void*)  (in swift) + 1960  [0x10591b908]
            2677 swift::CompilerInstance::performSema()  (in swift) + 2064  [0x105b7de80]
              2677 swift::performTypeChecking(swift::SourceFile&, swift::TopLevelContext&, unsigned int)  (in swift) + 1464  [0x106830d08]
                2677 swift::TypeChecker::typeCheckTopLevelCodeDecl(swift::TopLevelCodeDecl*)  (in swift) + 136  [0x10687d518]
                  2677 swift::ASTVisitor<(anonymous namespace)::StmtChecker, void, swift::Stmt*, void, void, void, void>::visit(swift::Stmt*)  (in swift) + 291  [0x10687d683]
                    2677 swift::TypeChecker::typeCheckExpression(swift::Expr*&, swift::DeclContext*, swift::Type, swift::Type, bool, swift::FreeTypeVariableBinding, swift::ExprTypeCheckListener*)  (in swift) + 971  [0x10683a2ab]
                      2677 swift::constraints::ConstraintSystem::solve(llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::FreeTypeVariableBinding)  (in swift) + 363  [0x10681e8fb]
                        2677 swift::constraints::ConstraintSystem::solve(llvm::SmallVectorImpl<swift::constraints::Solution>&, swift::FreeTypeVariableBinding)  (in swift) + 945  [0x10681eb41]
                          ...

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