RE: New precompiled headers support in gnustep-make

[Nicola Pero]

I updated Renaissance to use this -- on my machine it now builds twice as fast 
with GCC 3.4.6, so precompiled headers seem to work fine if your compiler 
supports it! :-)

Thanks



-----Original Message-----
From: Nicola Pero <address@hidden>
Sent: Tue, January 16, 2007 3:20 am
To: address@hidden
Subject: New precompiled headers support in gnustep-make

I implemented (experimental) precompiled headers support in gnustep-make.

Precompiled headers can speed up the compilation of your project significantly 
if you're using a gcc that supports it.

Here is how you use them in gnustep-make:

 0. install gnustep-make from subversion/trunk.  Make sure you reconfigure it.  
Make sure that ./configure tells you that precompiled headers are enabled (you 
need GCC >= 3.4 to get them). ;-)

 1. create the header to precompile.  In practice, create a new header that 
include all the headers that you repeatedly include in all the files of your 
project, especially library headers that never change (eg, 
Foundation/Foundation.h, AppKit/AppKit.h, etc).

 2. now modify your source so that in all files to compile, you include this 
main header at the beginning, *before anything else happens*.

 3. your project should still compile fine; you have just refactored your 
headers a bit, putting includes for all the commonly included headers into a 
single header to include.  Make sure it still compiles.  You can time how long 
it takes if you want. ;-)

 4. now add the line

xxx_OBJC_PRECOMPILED_HEADERS = MyPrecompiledHeader.h

to your GNUmakefile (in the same place as your xxx_OBJC_FILES etc declarations 
appear).  Obviously replace 'xxx' with the name of the tool/library/app/... 
that would use the precompiled header, and 'MyPrecompiledHeader.h' with the 
name of your precompiled header.

 5. now try compiling.  GNUstep-make will precompile your header first, and put 
it into a special ./obj/PrecompiledHeaders/ObjC/ directory.  Then it will 
compile the other files as usual, but using -I./obj/PrecompiledHeaders/ObjC/ so 
that gcc will find the precompiled file and use it.

You should see a considerable speed-up in compilation times if you include the 
precompiled header often enough in your project (and if it's big enough).

The features of the current implementation include:

 * everything works as usual on platforms where precompiled headers are not 
available.

 * support for multiple precompiled headers; eg, you can have part of your 
project files include MyPrecompiledHeader1.h and part of your project files 
include MyPrecompiledHeader2.h.

 * support for multiple languages; eg, you can precompile MyPrecompiledHeader.h 
for both ObjC and C, and the right one will automatically be used at runtime.

 * only the precompile header(s) that you specify, for the languages that you 
specify, will be precompiled.

 * you can add/remove compilation flags to each precompiled file if you need 
them to be precompiled with specific flags (the usual xxx_FILE_FLAGS and 
xxx_FILE_FILTER_OUT_FLAGS are available).

Suggestions/comments/feedback welcome!  Please try it out with your project and 
let me know. :-)

Important: This is all experimental, so variable names/implementation/makefile 
API are subject to change without notice for a while.

Thanks



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