Re: [avr-gcc-list] Pointer register allocation optimizer

[Georg-Johann Lay]

Wouter van Gulik schrieb:
> Georg-Johann Lay schreef op 2012-07-27 16:06:
> > Wouter van Gulik schrieb:
> > > Hi list,
> > > This code:
> > > char* f(char* p)
> > > {
> > >   p++;
> > >   return p;
> > > }
> > > Results in:
> > >
> > >     mov r18,r24
> > >     mov r19,r25
> > >     subi r18,lo8(-(1))
> > >     sbci r19,hi8(-(1))
> > >     mov r24,r18
> > >     mov r25,r19
> > >     ret
> > > When compiling with avr-gcc -O[23s] -mmcu=avr5 -S main.c
>
> Oops, copy paste error; for avr5 movw is used to move the pointer 
> registers. Still it does a useless move.
>
> > Looks very much like PR52278, which is still open.
>
> I think this is the same, for sanity I also checked with int and long 
> (against my Ubuntu gcc-avr 4.5.3) and it yields the same result; first 
> move the register then the add, then move it back.
>
> What I wonder: why is r18 picked? Clearly r26, or r30 are way better 
> choices. Maybe this is fixed in 4.7.1 already, don't have a 4.7+ at the 
> moment.
>
> > According to Vladimir, the register allocator (RA) should work smooth
> > with SUBREGs, but obviously, it does not.
>
> Clearly.
>
> > You can try -fno-split-wide-types, but that might have other 
> > disadvantages.
>
> That gives the expected result.
>
> HTH,
>
> Wouter

FYI, there is also the following post in gcc@ ML that
addresses exactly that artifact:

http://gcc.gnu.org/ml/gcc/2012-02/msg00313.html

Johann