------- Additional Comments From pinskia at gcc dot gnu dot org  2005-01-09 
01:30 -------
One issue I find is that avr does not define_insn_and_split (or just 
define_split) which will greatly 
improve the code generation because the hi part of the QI would be zero and you 
don't need to set it 
four times.
        ldi r25,hi8(2)
        ldi r25,hi8(1)
        ldi r26,hlo8(1)
        ldi r27,hhi8(1)

The main difference between -Os and -O0 (at least before 4.0.0) is that we use 
more registers at -Os 
which causes the prologue and eplogue to do more so my recommendation will help 
a lot.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11180

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