[Bug target/43892] PowerPC suboptimal "add with carry" optimization

[joakim.tjernlund at infinera dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43892

--- Comment #34 from Joakim Tjernlund <joakim.tjernlund at infinera dot com> ---
(In reply to Christophe Leroy from comment #28)
> Looks like we have a way to do it. Works at least with GCC 5.5, 8.2, 9.2,
> 10.1
> 
> unsigned long g(unsigned long a, unsigned long b)
> {
> 	unsigned long long s = (unsigned long long)a + (unsigned long long)b;
> 
> 	return (s >> 32) + s;
> }
> 
> 00000020 <g>:
>   20:	7c 63 20 14 	addc    r3,r3,r4
>   24:	7c 63 01 94 	addze   r3,r3
>   28:	4e 80 00 20 	blr
> 

Does that work when placed in a loop to?
Something close to the example in the first comment:

for(;len; --len)
   sum = add32carry(sum, *++buf);


        addic 3, 3, 0 /* clear carry */
.L31:
        lwzu 0,4(9)
        adde 3, 3, 0 /* add with carry */
        bdnz .L31

        addze 3, 3 /* add in final carry */