该编译器照顾无用codeS一样，如果（0）？ [英] Does the compiler take care the useless codes like if(0)?

问题描述

最近，我在编译的ffmpeg codeS windows下使用英特尔编译器VS2010。
对于以下codeS：

Recently, I am compiling the ffmpeg codes under windows use the VS2010 with intel compiler. For the following codes:

void ff_dcadsp_init(DCADSPContext *s)
{
    s->lfe_fir = dca_lfe_fir_c;
    if (ARCH_ARM) ff_dcadsp_init_arm(s);
}

宏 ARCH_ARM 定义为 0 。

当我编译它在Linux下，没有在 ff_dcadsp_init_arm（）目标文件中没有的功能，同时它在Windows下？所以，我要确保，如果编译器会做一些无用的codeS和如何设置英特尔编译器的。

When I compile it under linux, there is no function in ff_dcadsp_init_arm() in the object file, while it does under windows? So I want to make sure if the compiler will do something about the useless codes and how to set it for INTEL compiler.

推荐答案

通常情况下，大多数编译器能够采取那种在死者的code的护理，有效去除特定的指令/块。不过，据我所知是没有的保证的并可能编译器和/或提供的优化级别设置不同的。

Usually, most compilers are capable of taking care of that kind of the dead code, effectively removing that particular instruction / block. However, AFAIK it is not guaranteed and may differ between compilers and / or supplied optimization level settings.

然而，如果你希望它在preprocessing国家本身进行处理（应该是一个更好的方法，恕我直言，如 ARCH_ARM 是一个宏），你可以使用的＃如果 preprocessor声明，以消除编译器优化级别的依赖性。

However, if you want this to be handled in the preprocessing state itself (should be a better approach, IMHO, as ARCH_ARM is a MACRO), you can make use of #if preprocessor statements, to remove the dependency on compiler optimization level.

例如，在一般

void ff_dcadsp_init(DCADSPContext *s)
{
    //something common code

    #if ARCH_ARM                 //this {#if...#endif} block 
        ff_dcadsp_init_arm(s);   //only get compiled if ARCH_ARM evaluates to non-zero
    #endif

   //something more common code
}

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