<Chapter Id="compiler">
<DocInfo>
<AuthorGroup>
<Author>
<FirstName>Brian</FirstName>
<Surname>Gallew</Surname>
</Author>
</AuthorGroup>
<Date>Transcribed 1998-02-12</Date>
</DocInfo>

<Title>GCC Default Optimizations</Title>

<Para>
<Note>
<Para>
Contributed by <ULink url="mailto:geek+@cmu.edu">Brian Gallew</ULink>
</Para>
</Note>

<Para>
Configuring gcc to use certain flags by default is a simple matter of
editing the 
<FileName>/usr/local/lib/gcc-lib/<Replaceable>platform</Replaceable>/<Replaceable>version</Replaceable>/specs</FileName>
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
"*<Replaceable>section_name</Replaceable>:" (e.g. "*asm:").
The second line is a list of flags,
and the third line is blank.

<Para>
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:
<ProgramListing>
- ----------SECTION----------
*cc1:


- ----------SECTION----------
</ProgramListing>
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:
<ProgramListing>
- ----------SECTION----------
*cc1:
- -m486 -fomit-frame-pointer

- ----------SECTION----------
</ProgramListing>
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:
<ProgramListing>
- ----------SECTION----------
*cc1:
%{!m386:-m486} -fomit-frame-pointer

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

<Para>
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.

</Chapter>
