Re: Why is my GNU-based m68k program too big?

[Kai Tuschner <tuschner@mathworks.com>]

Christopher Bahns wrote:

> Hello,
>
> I am converting a program (based on the MC68306) from the Microtec
> Research (MRI) compiler to the GNU compiler. Things in general are going
> pretty well, but I'm having a problem: The size of the code generated by
> GNU is too big to fit in my flash memory, whereas the code generated by
> MRI does fit.
>
> I have 96k of flash available. The MRI-built program uses all but the
> last 2572 bytes, so it is rather close to reaching the limit. However,
> the GNU-built program is 11080 bytes over limit, causing my link to
> fail. The total difference in sizes between the two compilers is 13652
> bytes, or an increase of 14.3% when going from MRI to GNU. These numbers
> just apply to the executable code, which is the ".text" section with
> GNU, and with MRI it is the combination of the "code", "const",
> "literals", "strings", "??INITDATA", and "ioports" sections. The others
> sections are ok, but the general trend seems to be that if two
> corresponding sections are not the same size, the GNU version is always
> bigger. In this particular case it is too big for my program to work.
>
> My configuration: Windows 98SE, Cygnus Cygwin 1.0, binutils-2.9.1
> (patched), gcc-2.95.2, newlib-1.8.2, target=m68k-coff.
>
> I built the compiler myself without modifying the default configuration
> for this target. When building my program I use the "-O2" option, which
> makes the program quite small in comparison to other optimization
> settings ("-O3" is larger, and "-Os" is only slightly smaller). I'm not
> sure whether it's the difference in the sizes of the project's
> functions, or those of the MRI and GNU run-time libraries that are
> linked in. But, I suspect that it's the run-time libraries since the
> project's functions are built from essentially the same source. The
> differences in the compilers may account for a small difference here,
> but I assume that if the source code is identical, that the size of code
> generated by GNU (with level 2 optimizations) should be very near the
> size of code generated by MRI.
>
> So, I'll assume that it's the difference in the implementation of the
> run-time libraries. But, how do I proceed?

You might use the size command to figure out if the gcc is larger than the MRI
compiled version of your function (e.g. >>size my_func.o). This might answer the
question if the standard libraries or your object file is the cause of the over
size.

> Has anyone else had this
> problem, or been at all concerned about the sizes of the executables
> generated by the GNU compiler (particularly for m68k targets)? I think
> I'll try to rebuild the compiler with the "-O2" option and see if that
> makes a difference. Has anyone done this, or have any insight into
> whether the resulting libraries will work, or whether the optimizations
> will make a significant difference?
>
> Thanks a lot for any help. The help that I've gotten here so far has
> been very helpful.
> Chris
>
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