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
不要轻信别人的话,看看在代码的任何性能关键部分,C语言和你选择的语言的反汇编。我认为你可以在Visual Studio运行时的反汇编窗口中看到已分解的. net。如果Java使用windbg有点棘手,应该是可能的,尽管如果你用。net来做,许多问题将是相同的。
如果没有必要的话,我不喜欢用C来编写,但我认为,这些回答中吹捧除C之外其他语言的速度的许多主张可以放在一边,只需用C和您选择的高级语言分解相同的例程,特别是如果涉及大量数据(这在性能关键型应用程序中很常见)。Fortran在其专业领域可能是个例外,不知道。它比C高吗?
第一次比较JITed代码和本地代码时,我解决了。net代码是否能与C代码运行得相当的所有问题。额外的抽象层次和所有的安全检查都带来了巨大的成本。同样的成本可能也适用于Java,但不要相信我的话,在性能至关重要的地方尝试一下。(有没有人足够了解JITed Java来在内存中找到一个编译过的过程?这当然是可能的)
其他回答
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。
撇开诸如热点优化、预编译元算法和各种形式的并行等高级优化技术不提,语言的基本速度与支持通常在内部循环中指定的操作所需的隐含的幕后复杂性密切相关。
也许最明显的方法是对间接内存引用进行有效性检查——比如检查指针是否为空,检查索引是否符合数组边界。大多数高级语言隐式地执行这些检查,但C不这样做。然而,这并不一定是这些其他语言的基本限制——一个足够聪明的编译器可能能够通过某种形式的循环不变代码运动,从算法的内部循环中删除这些检查。
C语言(在类似程度上与c++密切相关)更基本的优势是严重依赖基于堆栈的内存分配,这本质上是快速的分配、回收和访问。在C(和c++)中,主调用堆栈可用于分配原语、数组和聚合(结构/类)。
虽然C语言确实提供了动态分配任意大小和生命周期的内存的能力(使用所谓的“堆”),但默认情况下是避免这样做的(而是使用堆栈)。
诱人的是,有时可以在其他编程语言的运行时环境中复制C内存分配策略。asm.js已经证明了这一点,它允许用C或c++编写的代码被翻译成JavaScript的子集,并以接近本机的速度安全地运行在web浏览器环境中。
As somewhat of an aside, another area where C and C++ outshine most other languages for speed is the ability to seamlessly integrate with native machine instruction sets. A notable example of this is the (compiler and platform dependent) availability of SIMD intrinsics which support the construction of custom algorithms that take advantage of the now nearly ubiquitous parallel processing hardware -- while still utilizing the data allocation abstractions provided by the language (lower-level register allocation is managed by the compiler).
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。
1)正如其他人所说,C为你做的更少。没有初始化变量,没有数组边界检查,没有内存管理等。其他语言中的这些特性会消耗C语言不需要的内存和CPU周期。
2) Answers saying that C is less abstracted and therefore faster are only half correct I think. Technically speaking, if you had a "sufficiently advanced compiler" for language X, then language X could approach or equal the speed of C. The difference with C is that since it maps so obviously (if you've taken an architecture course) and directly to assembly language that even a naive compiler can do a decent job. For something like Python, you need a very advanced compiler to predict the probable types of objects and generate machine code on the fly -- C's semantics are simple enough that a simple compiler can do well.
C的设计者们已经做出了取舍。也就是说,他们决定把速度放在安全之上。C不会
检查数组下标边界 检查未初始化的变量值 检查内存泄漏 检查空指针解引用
当你索引到一个数组时,在Java中,它接受虚拟机中的一些方法调用,绑定检查和其他健全检查。这是有效的,绝对没问题,因为它在应有的地方增加了安全性。但是在C语言中,即使是非常微不足道的东西也不会被放在安全的地方。例如,C不要求memcpy检查要复制的区域是否重叠。它并不是一种用于编写大型商业应用程序的语言。
但是这些设计决策并不是C语言中的bug。它们是被设计出来的,因为它允许编译器和库编写者从计算机中获得每一点性能。下面是C语言的精神——C语言的基本原理文档是这样解释的:
C code can be non-portable. Although it strove to give programmers the opportunity to write truly portable programs, the Committee did not want to force programmers into writing portably, to preclude the use of C as a ``high-level assembler'': the ability to write machine-specific code is one of the strengths of C. Keep the spirit of C. The Committee kept as a major goal to preserve the traditional spirit of C. There are many facets of the spirit of C, but the essence is a community sentiment of the underlying principles upon which the C language is based. Some of the facets of the spirit of C can be summarized in phrases like Trust the programmer. Don't prevent the programmer from doing what needs to be done. Keep the language small and simple. Provide only one way to do an operation. Make it fast, even if it is not guaranteed to be portable. The last proverb needs a little explanation. The potential for efficient code generation is one of the most important strengths of C. To help ensure that no code explosion occurs for what appears to be a very simple operation, many operations are defined to be how the target machine's hardware does it rather than by a general abstract rule. An example of this willingness to live with what the machine does can be seen in the rules that govern the widening of char objects for use in expressions: whether the values of char objects widen to signed or unsigned quantities typically depends on which byte operation is more efficient on the target machine.