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Removing unused sections in dynamically linked executables

AWLaFramboise@aol.com AWLaFramboise@aol.com
Fri Jan 10 08:28:00 GMT 2003 
Hi,
 
I am looking for a way to remove unused sections from
dynamically-linked executables.
 
In my particular instance, I am working
with C++ code.  It is common in my code for many functions to be defined,
for purposes of fullfilling interface requirements, that are never used.
(Example: I may write a container class that defines many methods for
insertion, deletion, etc, but the client code only uses a small subset
of these.)  Ordinarily, with GNU development tools, these functions are
retained in the final executable, even though nothing actually uses them.
It has occured to me that it is very possible there may be, in some
particular executable, more unused functions than functions actually
being used!
 
Currently, I have found two ways to accomplish this,
neither of which are acceptable:
 
 1) Put each function in a separate
translation unit before compilation.  This is unacceptable because it is
overly intrusive into design, and will obfuscate and inhibit good style
in my project.  (Example: It would be silly, in my opinion, to have a
container class with 30 accessors be spread over 30 files.  It would also
have severe performance penalties at compiletime when having to recompile
depended-on headers 29 times more than necessary.)
 
 2) When compile
translation units, put each 'object' (function, variable, whatever)
into a separate section (in GCC, -ffunction-sections, -fdata-sections),
then link statically and collect unused sections (--gc-sections).
This is unacceptable because, in most cases, I do not actually want
to link statically, and the penalties of code duplication from static
linking would outweigh any benefit I get from removing unused objects.

I must admit it is not clear to me why the restriction is placed that
--gc-sections may only be used when statically linking.  I understand that
if -export-dynamic is specified, code at runtime may attempt to resolve
previously unused symbols, so --gc-sections may cause a problem here.
But what about when this option is not specified, in the usual case,
when there is no possible runtime dependency (as far as I know) upon
unused symbols?  Why may we not garbage collect sections that could not
possibly (as far as I can tell) ever be used?
 
Honestly, in searching
for an answer to this question, I was quite suprised that there was very
little interest in doing this kind of an optimization, or that noone seems
to have a problem with substancial unused code being in an executable.
Yet, as someone who often finds himself limited in storage and bandwidth
resources, I greatly value storage efficiency..
 
Platforms which are
of particular interest to me are i386-linux-elf and i386-win32-pe, but
information with regards to other platforms is interesting as well.
 
I would appreciate any help with regards to this issue.  Any pointers,
code, suggestions, comments, referals, references to other sources,
whatever, would be greatly appreciated.

 
 Thanks,
 
Aaron W. LaFramboise

awlaframboise@aol.com
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