了解汇编程序的原因之一是,有时可以使用汇编程序来编写比用高级语言(特别是C语言)编写的代码性能更好的代码。然而,我也听人说过很多次,尽管这并非完全错误,但实际上可以使用汇编程序来生成性能更好的代码的情况极其罕见,并且需要汇编方面的专业知识和经验。

这个问题甚至没有涉及到这样一个事实,即汇编程序指令将是特定于机器的、不可移植的,或者汇编程序的任何其他方面。当然,除了这一点之外,了解汇编还有很多很好的理由,但这是一个需要示例和数据的具体问题,而不是关于汇编程序与高级语言的扩展论述。

谁能提供一些具体的例子,说明使用现代编译器汇编代码比编写良好的C代码更快,并且您能否用分析证据支持这一说法?我相信这些案例确实存在,但我真的很想知道这些案例到底有多深奥,因为这似乎是一个有争议的问题。


当前回答

在Amiga上,CPU和图形/音频芯片会为了访问特定区域的RAM(具体来说是前2MB的RAM)而争斗。因此,当你只有2MB RAM(或更少)时,显示复杂的图形加上播放声音会杀死CPU的性能。

在汇编程序中,你可以巧妙地交错你的代码,使CPU只在图形/音频芯片内部繁忙时(即当总线空闲时)才尝试访问RAM。因此,通过重新排序指令,巧妙地使用CPU缓存,总线定时,你可以实现一些使用任何高级语言都不可能实现的效果,因为你必须为每个命令定时,甚至在这里或那里插入nop,以使不同的芯片不受彼此的雷达影响。

这也是为什么CPU的NOP (No Operation -什么都不做)指令实际上可以让你的整个应用程序运行得更快的另一个原因。

当然,这种技术取决于特定的硬件设置。这就是为什么许多Amiga游戏无法适应更快的cpu的主要原因:指令的计时错误。

其他回答

Actually you can build large scale programs in a large model mode segaments may be restricted to 64kb code but you can write many segaments, people give the argument against ASM as it is an old language and we don't need to preserve memory anymore, If that were the case why would we be packing our PC's with memory, the only Flaw I can find with ASM is that it is more or less Processor based so most programs written for the intel architecture Most likely would not run on An AMD Architecture. As for C being faster than ASM there is no language faster than ASM and ASM can do many thing's C and other HLL's can not do at processor level. ASM is a difficult language to learn but once you learn it no HLL can translate it better than you. If you could only see some of the things HLL's Do to you code, and understand what it is doing, you would wonder why More people don't use ASM and why assembers are no longer being updated ( For general public use anyway). So no C is not faster than ASM. Even experiences C++ programmers still use and write code Chunks in ASM added to there C++ code for speed. Other Languages Also that some people think are obsolete or possibly no good is a myth at times for instance Photoshop is written in Pascal/ASM 1st release of souce has been submitted to the technical history museum, and paintshop pro is written still written in Python,TCL and ASM ... a common denominator of these to "Fast and Great image processors is ASM, although photoshop may have Upgraded to delphi now it is still pascal. and any speed problems are comming from pascal but this is because we like the way programs look and not what they do now days. I would like to make a Photoshop Clone in pure ASM which I have been working on and its comming along rather well. not code,interpret,arange,rewwrite,etc.... Just code and go process complete.

在处理器速度以MHz为单位,屏幕尺寸低于100万像素的时代,一个众所周知的更快显示的技巧是展开循环:为屏幕的每个扫描行写操作。它避免了维护循环索引的开销!再加上检测屏幕刷新,它非常有效。 这是C编译器不会做的事情……(虽然通常可以在速度优化和规模优化之间进行选择,但我认为前者使用了一些类似的技巧。)

我知道有些人喜欢用汇编语言编写Windows应用程序。他们声称他们更快(很难证明)和更小(确实如此!)。 显然,虽然这样做很有趣,但可能会浪费时间(当然,学习目的除外!),特别是对于GUI操作…… 现在,也许某些操作(比如在文件中搜索字符串)可以通过精心编写的汇编代码进行优化。

只有在使用编译器不支持的特殊用途指令集时。

为了最大限度地利用具有多个管道和预测分支的现代CPU的计算能力,您需要以这样一种方式来构造汇编程序:a)人类几乎不可能编写b)甚至更不可能维护。

此外,更好的算法、数据结构和内存管理将为您提供至少一个数量级的性能,而不是在汇编中进行的微观优化。

使用SIMD指令的矩阵操作可能比编译器生成的代码更快。

Walter Bright的《optimization Immutable and Purity》可能值得一看,它不是一个概要测试,但向您展示了手写和编译器生成ASM之间的区别。Walter Bright写优化编译器,所以值得一看他的其他博客文章。