Swift性能優化分析

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"自從2014年Apple發佈Swift語言以來，歷時六年多，Swift已經發布到5.3版本，在5.0版本已經ABI stability，5.2版本也已經module stability，不管是語言還是基礎庫都日趨穩定，目前國內外大廠也都積極擁抱Swift陣營。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"絕大多數公司選擇Swift語言開發iOS應用，主要原因是因爲Swift相比Objc有更快的運行效率，更加安全的類型檢測，更多現代語言的特性提升開發效率；這一系列的優點使Swift語言的熱度越來越高。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"大多數人知道Swift語言相比於Objc語言運行效率更高，但是卻不知道爲什麼效率更高，在這裏我們Swift編譯層探討一下Swift語言高效的原因。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"更加高效的數據類型","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在開始討論Swift數據類型之前，我們先討論一下Swift的函數派發機制；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"靜態派發、動態派發、消息派發(static dispatch、dynamic dispatch、message dispatch)","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"動態派發(dynamic dispatch): 動態派發是指編譯期無法確定應該調用哪個方法，需要在運行時才能確定方法的調用。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"靜態派發(static dispatch)：是在編譯期就能確定的調用方法的派發方式。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"除了上面兩種方式之外，在Swift裏面還會使用Objc的消息派發(message dispatch))機制；Objc採用了運行時採用","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"obj_msgsend","attrs":{}}],"attrs":{}},{"type":"text","text":"進行消息派發，所以Objc的一些動態特性在Swift裏面也可以被限制的使用。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"靜態派發相比於動態派發更快，而且靜態派發還會進行內聯等一些優化，減少函數的尋址及內存地址的偏移計算等一系列操作，使函數的執行速度更快，性能更高。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"數據類型(struct/class)","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們都知道，內存分配可以分爲堆區(Heap)和棧區(Stack)。由於棧區內存是連續的，內存的分配和銷燬是通過入棧和出棧操作進行的，速度要高於堆區。堆區存儲高級數據類型，在數據初始化時，查找沒有使用的內存，銷燬時再從內存中清除，所以堆區的數據存儲不一定是連續的。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"類(class)和結構體(struct)在內存分配上是不同的，基本數據類型和結構體默認分配在棧區，而像類這種高級數據類型存儲在堆區，且堆區數據存儲不是線程安全的，在頻繁的數據讀寫操作時，要進行加鎖操作。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"我們在swift文檔裏面能看到對結構的描述，結構體是值類型(Value Type)，當值類型的數據賦值給一個變量或常量，或者傳遞給一個函數時，是值拷貝；","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"例如：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"struct Resolution {\n var width = 0\n var height = 0\n}\n\nlet hd = Resolution(width: 1920, height: 1080)\nvar cinema = hd\ncinema.width = 2048\n\nprint(\"cinema is now \\(cinema.width) pixels wide\")\n// Prints \"cinema is now 2048 pixels wide\"\n\nprint(\"hd is still \\(hd.width) pixels wide\")\n// Prints \"hd is still 1920 pixels wide\"\n","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通過這個例子我們能清楚的看到，當","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"hd","attrs":{}}],"attrs":{}},{"type":"text","text":"賦值給","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"cinema","attrs":{}}],"attrs":{}},{"type":"text","text":"時，是將","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"hd","attrs":{}}],"attrs":{}},{"type":"text","text":"中存儲的值拷貝給","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"cinema","attrs":{}}],"attrs":{}},{"type":"text","text":",所以當給","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"cinema","attrs":{}}],"attrs":{}},{"type":"text","text":"的","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"width","attrs":{}}],"attrs":{}},{"type":"text","text":"屬性賦值的時候，並不會改變","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"hd","attrs":{}}],"attrs":{}},{"type":"text","text":"中的屬性值，如下圖所示：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/3e/3e1791647b891f0003e136893c087aea.png","alt":null,"title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"image","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"結構體除了屬性的存儲更安全、效率更高之外，其函數的派發也更高效。由於結構體不能被繼承，也就是結構體的類型被","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"final","attrs":{}}],"attrs":{}},{"type":"text","text":"修飾，根據我們對於動態派發及靜態派發的描述，那麼其內部函數應該是屬於靜態派發，在編譯期就確定了函數的執行方式，其函數的調用通過內聯(inline)的方式進行優化，其內存連續，減少了函數的尋址及內存地址的偏移計算，其運行相比於動態派發更加高效。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"協議類型(protocol type)","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"多態是面向對象的一大特性，在結構體中不能通過繼承或者引用語言的多態，swift就引入了協議(protocol)，通過協議來實現了結構體的多態特性，這也是swift面向協議編程的核心所在。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"對於類(class)來說，每個類都會創建一個虛擬函數表指針，這個指針則指向一個v-table表，也就是虛函數表，表內存儲着該類的函數指針數組，擁有繼承關係的子類會在虛函數表內通過繼承順序(C++可以實現多繼承)去展示虛函數表指針。類裏面方法的派發則是根據v-table表裏面函數指針來進行派發。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"而結構體(struct)沒有繼承，也就是說結構體並沒有v-table表用於函數的派發。爲了實現這一特性，在結構體的協議(protocol)的實現裏添加了Protocol Witness Table用於管理協議類型的方法派發。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"編譯過程","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"上面介紹了一些swift在數據結構上的一些優化，除了數據結構優化之外，swift在編譯過程也進行了大量的優化，其中最核心的優化，是在編譯過程中引入","attrs":{}},{"type":"codeinline","content":[{"type":"text","text":"SIL","attrs":{}}],"attrs":{}},{"type":"text","text":"。","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"codeinline","content":[{"type":"text","text":"SIL","attrs":{}}],"attrs":{}},{"type":"text","text":"，Swift Intermediate Language，是爲了優化swift編譯過程而設計的中間語言，主要包含了以下功能：","attrs":{}}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"一系列的高級別優化保障，用於對運行時和診斷行爲提供可預測的基線；","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"對swift語言數據流分析強制要求，對不滿足強制要求的問題產生診斷。例如變量和結構體必須明確初始化，代碼可達性即方法return的檢測，switch的覆蓋率；","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"確保高級別優化。包含retain/release優化，動態方法的去虛擬化，閉包內聯，內存初始化提升和泛型方法實例 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IR之間有巨大的抽象鴻溝(Wide abstraction gap between source and LLVM IR );","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"IR不適合源碼級別的分析(IR isn't suitable for source-level analysis );","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"CFG(Control Flow Graph)缺少精準度(CFG lacks fidelity );","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","text":"CFG偏離主道(CFG is off the hot path );","attrs":{}}]}],"attrs":{}},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","text":"在CFG和IR降級中會出現重複分析(Duplicated effort in CFG and IR 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