gcc precompiled头怪异的行为与-c选项 [英] gcc precompiled headers weird behaviour with -c option

问题描述

短小故事：

我无法使预编译的标头使用gcc -c选项正常工作。

I can't make precompiled headers work properly with gcc -c option.

长故事：

我在Linux上使用gcc-4.4.1，在一个非常大的项目中尝试预编译头之前，我决定在简单程序。他们有点工作，但我不满意结果，我确定我的设置有问题。

Folks, I'm using gcc-4.4.1 on Linux and before trying precompiled headers in a really large project I decided to test them on simple program. They "kinda work" but I'm not happy with results and I'm sure there is something wrong about my setup.

首先，我写了一个简单的程序（main.cpp）来测试它们是否工作：

First of all, I wrote a simple program(main.cpp) to test if they work at all:

#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/type_traits.hpp>

int main()
{
  return 0;
}

$ b <如下：

Then I created the precompiled headers file pre.h(in the same directory) as follows:

#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/type_traits.hpp>

...并编译：

$ g++ -I. pre.h

（pre.h.gch已创建）

(pre.h.gch was created)

之后，我测量了有无预编译头文件的编译时间：

After that I measured compile time with and without precompiled headers:

与pch

$ time g++ -I. -include pre.h main.cpp

real    0m0.128s
user    0m0.088s
sys  0m0.048s

不含pch

$ time g++ -I. main.cpp 

real    0m0.838s
user    0m0.784s
sys  0m0.056s

到目前为止这么好！ 速度快了7倍，令人印象深刻！现在让我们尝试一些更现实的东西。所有我的源是用-c选项构建的，由于某种原因，我不能让pch玩得很好。

So far so good! Almost 7 times faster, that's impressive! Now let's try something more realistic. All my sources are built with -c option and for some reason I can't make pch play nicely with it. You can reproduce this with the following steps below...

我创建了测试模块foo.cpp，如下所示：

I created the test module foo.cpp as follows:

#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/type_traits.hpp>

int whatever()
{
  return 0;
}

这里是我尝试构建模块foo.cpp的时间，无pch：

Here are the timings of my attempts to build the module foo.cpp with and without pch:

with pch

$ time g++ -I. -include pre.h -c foo.cpp 

real    0m0.357s
user    0m0.348s
sys 0m0.012s

不含pch

$ time g++ -I. -c foo.cpp 

real    0m0.330s
user    0m0.292s
sys 0m0.044s

这很奇怪，看起来没有加速了！（我跑了几次计时）。结果是预编译头在这种情况下根本没有使用，我检查了-H选项（g ++ -I。输出-include pre.h -c foo.cpp -H没有列出pre.h.

That's quite strange, looks like there is no speed up at all!(I ran timings for several times). It turned out precompiled headers were not used at all in this case, I checked it with -H option(output of "g++ -I. -include pre.h -c foo.cpp -H" didn't list pre.h.gch at all).

推荐答案

> Ok，我想我找到了解决方案： -fpch-preprocess 应与 -c 选项一起使用。

Ok, I think I've found the solution: -fpch-preprocess should be used alongside with -c option. It works like a charm!

/ p>

with pch

$ time g++ -I. -include pre.h -c foo.cpp -fpch-preprocess

real    0m0.028s
user    0m0.016s
sys 0m0.016s

without pch

$ time g++ -I. -c foo.cpp 

real    0m0.330s
user    0m0.292s
sys 0m0.044s

更新：我在gcc帮助邮件列表上提出了同样的问题，Ian Lance Taylor用我的distcc / ccache解释了这个奇怪的行为。这些工具首先预处理源，这就是为什么这个选项是必需的。

Update: I asked the same question on the gcc help mailing list and Ian Lance Taylor explained this strange behavior by my usage of distcc/ccache. These tools first preprocess the source that's why this options is required.

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