Using precompiled headers in gcc

Question

I am investigating using precompiled headers to reduce our compile times.

I have read the documentaiton on the subject here: https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html, where I read the following:

Only one precompiled header can be used in a particular compilation.

On the project whose build time I would like to improve, there are often very Long lists of includes. The above leads me to Think that to get the most performance improvements, I would have to make a collection of common includes, put them into a single Header file, compile and include that Header file.

On the other hand, I prefer to list my dependancies in particular file explicitly, so I would be inclined to include first the precompiled Header, followed by the Manual list of actual Header files.

I have two questions related to this:

1. Is my analysis and approach correct? Have I interpreted the statement correctly?

2. Doing this, I will use this file (say stdafx.h) in many places, thereby including files I don't need. I would like to explicitly list my dependencies however, for code documentation purposes. Where I to do something like the following:

   #ifdef USE_PRECOMPILED_HEADERS 
   #include "stdafx.h"
   #else
   #include "dep1.h"
   #include "dep2.h"
   #endif
   

   I could periodically run a build without pre-compiled headers to check if all my dependencis are listed. This is a bit clunky however. Does anyone have a better solution?

If anyone has Information to help us obtain better results in our Investigation, I am happy to hear them.

Answer 1

1. Yes, your observation is absolutely fine! You "would have to make a collection of common includes, put them into a single Header file, compile and include that Header file". This common header file is generally named as stdafx.h (although you can name it anything you want!)

2. I am afraid I don't really understand this part of the question.

EDIT :

Do you also want the standard headers (like iostream, map, vector, etc.) to be included as dependencies in the code documentation?

Generally this must be a NO. Hence, you must include only those header files in stdafx.h which are not under your control (i.e., [1] standard language includes [2] includes from dependent modules (mostly exposed interface headers)). Rest all includes (whose source is in the current project/module) must be explicitly included in each header file wherever required, and not put in the pre-compiled stdafx.h.

Answer 2

The above leads me to Think that to get the most Performance improvements, I would have to make a collection of common includes, put them into a single Header file, compile and include that Header file.

Yes, this observation is correct: You put most (all?) includes in one single header file, which is then precompiled.

Which, in turn, means that...

• any compilation without the aid of that header being precompiled will take ages;
• you are relying on naming conventions or other means (documentation?) to make the information link between things referenced in your individual translation unit and their declaration.

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I don't much like precompiled headers for those reasons...