Re: Why is memory footprint smaller when compiled static?

[Mark Johnson <mark_h_johnson@raytheon.com>]

Preben Randhol wrote:
> [snip - static bigger on disk]
> When I run the two programs and examine the memory usage of the two
> programs I find :
> 
>    3296 kb for the static version
>    4412 kb for the dynamic version
> 
> Why is there such a big difference? Is it because the compiler can do
> better job at optimising the code when you build it statically.
> 
Hmm. I doubt the compiler (gcc) has anything to do with this. The linker
(ld) is the likely culprit and you should see two effects...
 - each static program has a "smaller footprint" because functions and
procedures in the library that are not used can be omitted from the
generated main program. However,...
 - your overall memory usage of the system can be higher when you run
several of these statically linked programs because they do not share
the run time library.
The second effect can be a pretty substantial by the way. I have seen
some systems implemented with a "server" and a separate main program for
each "display". With statically linked main programs, the memory usage
of the system is unacceptable when you get 5-10 displays up. With shared
libraries, the memory usage is OK. YMMV.

> I also did the same without the -O3 switch and then the memory usage
> was:
> 
>    3412 kb for the static version
>    4512 kb for the dynamic version
> 
It is somewhat interesting that the dynamic version went up by 100 kb
and the static version went up by 116 kb, but you might have just gotten
"lucky" with an extra segment or two going up by a 4k page in the static
version. Check the link map (something like - gnatmake ... -largs
-Wl,-Map,glosa.map) if you want to get more insight in how the linker is
putting the application together.
  --Mark