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Chapter 56. gcc Default Optimizations

Note: Contributed by Brian Gallew

Configuring gcc to use certain flags by default is a simple matter of editing the /usr/local/lib/gcc-lib/platform/version/specs file. The format of this file pretty simple. The file is broken into sections, each of which is three lines long. The first line is "*section_name:" (e.g. "*asm:"). The second line is a list of flags, and the third line is blank.

The easiest change to make is to append the desired default flags to the list in the appropriate section. As an example, let's suppose that I have linux running on a '486 with gcc 2.7.2 installed in the default location. In the file /usr/local/lib/gcc-lib/i486-linux/2.7.2/specs, 13 lines down I find the following section:

- ----------SECTION----------
*cc1:


- ----------SECTION----------
As you can see, there aren't any default flags. If I always wanted compiles of C code to use "-m486 -fomit-frame-pointer", I would change it to look like:
- ----------SECTION----------
*cc1:
- -m486 -fomit-frame-pointer

- ----------SECTION----------
If I wanted to be able to generate 386 code for another, older linux box lying around, I'd have to make it look like this:
- ----------SECTION----------
*cc1:
%{!m386:-m486} -fomit-frame-pointer

- ----------SECTION----------
This will always omit frame pointers, any will build 486-optimized code unless -m386 is specified on the command line.

You can actually do quite a lot of customization with the specs file. Always remember, however, that these changes are global, and affect all users of the system.

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