在听StackOverflow播客的时候,经常有人说“真正的程序员”是用C语言编写的,而C语言的速度要快得多,因为它“接近机器”。把前面的断言留到另一篇文章,C有什么特别之处,使它比其他语言更快?或者换句话说:什么能阻止其他语言编译成二进制代码,使其运行速度与C语言一样快?


当前回答

在过去,只有两种类型的语言:编译型和解释型。

编译语言利用“编译器”读取语言语法并将其转换为相同的汇编语言代码,这可以直接在CPU上进行。解释型语言使用了几种不同的方案,但从本质上讲,语言语法被转换成一种中间形式,然后在“解释器”(用于执行代码的环境)中运行。

因此,在某种意义上,在代码和机器之间存在另一个“层”——解释器。而且,在计算机中,越多就意味着使用更多的资源。翻译速度较慢,因为他们必须执行更多的操作。

More recently, we've seen more hybrid languages like Java, that employ both a compiler and an interpreter to make them work. It's complicated, but a JVM is faster, more sophisticated and way more optimized than the old interpreters, so it stands a much better change of performing (over time) closer to just straight compiled code. Of course, the newer compilers also have more fancy optimizing tricks so they tend to generate way better code than they used to as well. But most optimizations, most often (although not always) make some type of trade-off such that they are not always faster in all circumstances. Like everything else, nothing comes for free, so the optimizers must get their boast from somewhere (although often times it using compile-time CPU to save runtime CPU).

Getting back to C, it is a simple language, that can be compiled into fairly optimized assembly and then run directly on the target machine. In C, if you increment an integer, it's more than likely that it is only one assembler step in the CPU, in Java however, it could end up being a lot more than that (and could include a bit of garbage collection as well :-) C offers you an abstraction that is way closer to the machine (assembler is the closest), but you end up having to do way more work to get it going and it is not as protected, easy to use or error friendly. Most other languages give you a higher abstraction and take care of more of the underlying details for you, but in exchange for their advanced functionality they require more resources to run. As you generalize some solutions, you have to handle a broader range of computing, which often requires more resources.

保罗。

其他回答

令人惊讶的是,“C/ c++必须比Java快,因为Java是解释性的”的神话仍然活跃。有几年前的文章,也有最近的文章,用概念或测量来解释为什么这种情况并不总是如此。

当前的虚拟机实现(顺便说一下,不仅仅是JVM)可以利用程序执行期间收集的信息来动态地优化代码,使用各种技术:

将频繁的方法呈现给机器代码, 内联小方法, 锁紧调整

以及各种其他调整,这些调整是基于了解代码实际在做什么,以及它运行的环境的实际特征。

一些c++算法比C快,其他语言中的算法或设计模式的一些实现可能比C快。

当人们说C语言很快,然后转向谈论其他语言时,他们通常是在用C语言的性能作为基准。

实际上,在某些应用程序(数字)中,甚至C也可以被击败,我指的不是汇编语言,而是老的、经常被嘲笑的Fortran。原因是,Fortran保证没有指针别名。

只需在您的IDE中逐步检查机器代码,您就会看到为什么它更快(如果它更快的话)。它省去了很多手把手的环节。很有可能你的Cxx也会被告知不要使用,在这种情况下,它应该是相同的。

编译器优化被高估了,就像几乎所有关于语言速度的看法一样。

优化生成的代码只会对热点代码产生影响,也就是说,没有函数调用(显式或隐式)的紧凑算法。在其他地方,收效甚微。

这不是语言的问题,而是工具和库的问题。C语言可用的库和编译器比新语言要老得多。你可能认为这会让它们变慢,但事实恰恰相反。

这些库是在处理能力和内存非常重要的时候编写的。它们必须写得非常高效,才能发挥作用。C编译器的开发人员也花了很长时间为不同的处理器进行各种巧妙的优化。C语言的成熟和广泛采用使得它比同时期的其他语言具有显著的优势。它还使C语言在速度上比那些不像C语言那样强调原始性能的新工具更有优势。