what does (size before relaxing) in a GCC .map file mean and how does it work?

Are there duplicate strings, or strings where one is identical to the 
ending of another?

It was just a guess, but I do know that compilers such as GCC are able 
to share identical strings. If some string is idential to the trailing 
part of another, it can be omitted entirely as the corresponding part of 
the longer one can be used instead. It won't work with shared prefixes 
though, since they miss the \0 in the proper place.

Since both sizes are shown in the map file, it likely is caused by some 
optimization done by the linker though.

(size before relaxing) GCC wrote:

> There are no duplicate strings, but indeed there are strings where one
> is identical to the ending of another (like: "first" and "the first").

It's calles string merging.  The trigger is the right sections flags set 
by the compiler, which then are avaluated by the linker in order to 
merge the strings.  Look at the generated assembler code.  GCC puts the 
strings into sections with flags like "aMS".  "M" allows merging and "S" 
marks it as a section containing strings.

pooter wrote:
> the size after relaxing should be always smaller than before

No, not necessarily.  It can be the same size.  And depending on what 
relaxations are performed, code sie can even increase.

An example for increasing code size is when a jump / call is out of 
reach and a jump pad just after the function is added that forwards to 
the the  jump target, and the original jump jumps to the pad.