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

[Wouter van Gulik]

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