Re: [avr-gcc-list] Question about code size

[Dave Hylands]

Hi Joerg,

> > The numbers are the size in bytes of the .text section for the
> > function above. It appears that the compiler is promoting the
> > unsigned char's to ints.
>
> It has to do this internally, but it correctly removed the high byte
> handling in all three compiler versions then, as it does not
> contribute to the result.  Your disassembled code is a bit misleading,
> as it does not fill in the relocatable offsets of the branches
> correctly.

Yeah - In this particular case, I was just compiling a snippet of C
code. Normally I would dissassemble the fully located elf file.

> The difference between 4.1.x and 4.2.0 is that in your
> version, instead of immediately testing the loop count against 0, it
> jumps to the end of the loop first.  This is probably the consequence
> of using -Os, as it is correct that way: it takes more time but one
> less instruction.
>
> The major difference is that it no longer recognizes it could reorder
> the loop counter to count down rather than up.  If you manually
> reorder the loop like this:
>
> unsigned char getch(void);
> void getNch(unsigned char count)
> {
>     for(;count > 0; count--)
>         getch();
> }
>
> ....you get the same result as in 3.x.
>
> Would that be considered an optimization already that is worth a GCC
> bug report?

To me, I don't really care which way it counts (internally). I was
shocked to see a working bootloader grow by 60% when all I did was go
to a newer compiler. A big portion of the increase was due to the way
it was inlining stuff. The failure to optimize out the int promotions
i small potatoes in comparison to the inlining decision.

I'd be happy to file a bug a report. I just wanted to make sure I
wasn't overlooking something obvious.

> > Aside: I've just moved my development platform from Windows to
> > linux.  Where would I go to get the latest patches for
> > gcc/binutils/et al.?
>
> See Bingo600's script, it's referenced in a "sticky" article on top of
> the AVR-GCC forum of http://www.avrfreaks.net/ .

Thanks, I'll check it out.

--
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/