Binary Size: function-sections, data-sections, etc.

[Martin Nowak]

On Tue, 20 Dec 2011 19:14:03 +0100, dsimcha <dsimcha at yahoo.com> wrote:

> I started poking around and examining the details of how the GNU linker  
> works, to solve some annoying issues with LDC.  In the process I the  
> following things that may be useful low-hanging fruit for reducing  
> binary size:
>
> 1.  If you have an ar library of object files, by default no dead code  
> elimination is apparently done within an object file, or at least not  
> nearly as much as one would expect.  Each object file in the ar library  
> either gets pulled in or doesn't.
>
> 2.  When something is compiled with -lib, DMD writes libraries with one  
> object file **per function**, to get around this.  GDC and LDC don't.   
> However, if you compile the object files and then manually make an  
> archive with the ar command (which is common in a lot of build  
> processes, such as gtkD's), this doesn't apply.
>
> 3.  The defaults can be overridden if you compile your code with  
> -ffunction-sections and -fdata-sections (DMD doesn't support this, GDC  
> and LDC do) and link with --gc-sections.  -ffunction-sections and  
> -fdata-sections cause each function or piece of static data to be  
> written as its own section in the object file, instead of having one  
> giant section that's either pulled in or not.  --gc-sections garbage  
> collects unused sections, resulting in much smaller binaries especially  
> when the sections are fine-grained.
>
Only newer versions of binutils actually support --gc-sections.
There also was a bug that it clears the EH sections.

> On one project I'm working on, I compiled all the libs I use with GDC  
> using -ffunction-sections -fdata-sections.  The stripped binary is 5.6  
> MB when I link the app without --gc-sections, or 3.5 MB with  
> --gc-sections.  Quite a difference.  The difference would be even larger  
> if Phobos were compiled w/ -ffunction-sections and -fdata-sections.   
> (See  
> https://bitbucket.org/goshawk/gdc/issue/293/ffunction-sections-fdata-sections-for  
> ).
>
> DMD can't compile libraries with -ffunction-sections or -fdata-sections  
> and due to other details of my build process that are too complicated to  
> explain here, the results from DMD aren't directly comparable to those  
> from GDC.  However, --gc-sections reduces the DMD binaries from 11 MB to  
> 9 MB.
>
> Bottom line:  If we want to reduce D's binary size there are two pieces  
> of low-hanging fruit:
>
> 1.  Make -L--gc-sections the default in dmd.conf on Linux and probably  
> other Posix OS's.
>
> 2.  Add -ffunction-sections and -fdata-sections or equivalents to DMD  
> and compile Phobos with these enabled.  I have no idea how hard this  
> would be, but I imagine it would be easy for someone who's already  
> familiar with object file formats.