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

[Georg-Johann Lay]

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
> Which is bad since it could have been:
>
>         adiw r24, 1
>         ret
>
> I tested against WinAVR's 4.3.3 and avr-gcc-4.7-185693-mingw32
> Both generate the same code.
>
> Should I create a bug report?
>
> Wouter

Looks very much like PR52278, which is still open.

According to Vladimir, the register allocator (RA) should work smooth
with SUBREGs, but obviously, it does not.

Maybe it's also related to PR49491, yet another RA optimization flaw
showing up for avr.

You can try -fno-split-wide-types, but that might have other disadvantages.

Johann