Re: [avr-gcc-list] GCC-AVR Register optimisations

[Wouter van Gulik]

Wouter van Gulik schreef:

> Note that in some cases it could be very interesting to use r27, or Y, 
> register.

Should have written R28 of course.

Since gcc seems down at the moment I did some more testing.

Now consider this example:
void main(void)
{
        char *p = x;
        foo(p); p+=65;
        foo(p); p+=65;
        foo(p); p+=65;
        foo(p); p+=65;
        foo(p); p+=65;
        foo(p); p+=65;
        foo(p); p+=65;
        foo(p); p+=65;
        foo(p); p+=65;
        foo(p); p+=65;
}
This must be done using a subi/sbci pare.

But the compiler now seems to realize that p is a constant offset to x. 
So we now get:

main:
/* prologue: frame size=0 */
        push r16
        push r17
/* prologue end (size=2) */
        lds r16,x
        lds r17,(x)+1
        movw r24,r16
        call foo
        movw r24,r16
        subi r24,lo8(-(65))
        sbci r25,hi8(-(65))
        call foo
        movw r24,r16
        subi r24,lo8(-(130))
        sbci r25,hi8(-(130))

Here x is stored in r16 and the cumulative offset is added to R24

But if the compiler can realize this... Then why not do this for adds 
within the adiw range?!?
So for p++/p+=1 we would get something like:

        movw r24, r16
        adiw r24, 1
        call foo
        movw r24, r16
        adiw r24, 2
etc..

This is just as small as the earlier suggested use of R28!

Wouter