是由编译器优化的字符串形成吗？ [英] Is string formation optimized by the compiler?

问题描述

我试图回答另一个关于==运算符的问题，我创建了这个代码：

  NSString * aString = @ 你好; 
 NSString * bString = aString; 
 NSString * cString = @Hello; 
 
 if（aString == bString）
 NSLog（@CHECK 1）; 
 
 if（bString == cString）
 NSLog（@CHECK 2）; 
 
 if（[aString isEqual：bString]）
 NSLog（@CHECK 3）; 
 
 if（[aString isEqual：cString]）
 NSLog（@CHECK 4）; 
 
 NSLog（@％i，aString）; 
 NSLog（@％i，bString）; 
 NSLog（@％i，cString）; 
  

但结果令人惊讶：

 等于[6599：10b]检查1 
等于[6599：10b]检查2 
等于[6599：10b]检查3 
等[6599： 10b] CHECK 4 
 Equal [6599：10b] 8240 
 Equal [6599：10b] 8240 
 Equal [6599：10b] 8240 
  
 
 
解决方案
有明显的字符串正在进行，至少在一个单一的编译单元。我建议您通过 man gcc 进行简短的访问，在此期间您访问所有使用字符串。你会发现几个与文字 NSString 和他们的免费桥接对象直接相关的选项， CFString  s：
 
 
 
 
 
   -fconstant-string-class  =  class-name  设置用于实例化 @...文字的类的名称。除非你使用GNU运行时，它默认为 NSConstantString 。 （如果你不知道自己是不是，你不是。）
 
 
   -fconstant-cfstrings  strong>启用 CFSTR（...）时使用内置函数创建 CFString  > 
 
 
 
 
您可以使用 -fwritable-strings 禁用C字符串文字的唯一化已弃用。我不能想出一个选项的组合，将停止在$ Objective-C文件中 NSString 字面量的统一。 （任何人想要和Pascal字符串文字说话？）
 
 
 
你看到 -fconstant-cfstrings  code> CFString.h 定义 CFSTR（）宏用于创建 CFString  literals：
  #ifdef __CONSTANT_CFSTRINGS__ 
 #define CFSTR（cStr）（（CFStringRef）__builtin___CFStringMakeConstantString cStr））
 #else 
 #define CFSTR（cStr）__CFStringMakeConstantString（cStr）
 #endif 
  
如果你看一下 __ CFStringMakeConstantString（） //www.opensource.apple.com/source/CF/CF-299.33/String.subproj/CFString.crel =nofollow noreferrer>  CFString.c  ，你会看到函数确实使用非常大的 CFMutableDictionary 执行uniquing：
  if（（result =（CFStringRef）CFDictionaryGetValue（constantStringTable，cStr）））{
 __CFSpinUnlock（& _CFSTRLock）; 
} 
 //。 。 。 
 return result; 
  
另请参阅对问题的回复，字符串常量和字符串之间的区别是什么？ 
 
I was trying to answer another question about the == operator and I created this code:
NSString *aString = @"Hello";
NSString *bString = aString;
NSString *cString = @"Hello";

if (aString == bString)
    NSLog(@"CHECK 1");

if (bString == cString)
    NSLog(@"CHECK 2");

if ([aString isEqual:bString])
    NSLog(@"CHECK 3");

if ([aString isEqual:cString])
    NSLog(@"CHECK 4");

NSLog(@"%i", aString);
NSLog(@"%i", bString);
NSLog(@"%i", cString);
But was surprised at the results:
Equal[6599:10b] CHECK 1
Equal[6599:10b] CHECK 2
Equal[6599:10b] CHECK 3
Equal[6599:10b] CHECK 4
Equal[6599:10b] 8240
Equal[6599:10b] 8240
Equal[6599:10b] 8240
Is there some compiler trickery going on here?
解决方案
There is clearly string uniquing going on, at least within a single compilation unit. I recommend you take a brief tour through man gcc during which you visit all uses of "string". You'll find a few options that are directly relevant to literal NSStrings and their toll-free-bridged counterparts, CFStrings:


-fconstant-string-class=class-name sets the name of the class used to instantiate @"..." literals. It defaults to NSConstantString unless you're using the GNU runtime. (If you don't know if you are, you aren't.)
-fconstant-cfstrings enables use of a builtin to create CFStrings when you write CFSTR(...).


You can disable uniquing for C string literals using -fwritable-strings, though this option is deprecated. I couldn't come up with a combination of options that would stop the uniquing of NSString literals in an Objective-C file. (Anyone want to speak to Pascal string literals?)

You see -fconstant-cfstrings coming into play in CFString.h's definition of the CFSTR() macro used to create CFString literals:
    #ifdef __CONSTANT_CFSTRINGS__
    #define CFSTR(cStr)  ((CFStringRef) __builtin___CFStringMakeConstantString ("" cStr ""))
    #else
    #define CFSTR(cStr)  __CFStringMakeConstantString("" cStr "")
    #endif
If you look at the implementation of the non-builtin __CFStringMakeConstantString() in CFString.c, you'll see that the function does indeed perform uniquing using a very large CFMutableDictionary:
    if ((result = (CFStringRef)CFDictionaryGetValue(constantStringTable, cStr))) {
        __CFSpinUnlock(&_CFSTRLock);
    }
    // . . .
    return result;
See also responses to the question, "What's the difference between a string constant and a string literal?"

                        
这篇关于是由编译器优化的字符串形成吗？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！