On Tue, Apr 10, 2018 at 08:34:37AM +0200, Christophe Leroy wrote:
> This reverts commit 6ad966d7303b70165228dba1ee8da1a05c10eefe.
>
> That commit was pointless, because csum_add() sums two 32 bits
> values, so the sum is 0x1fffffffe at the maximum.
> And then when adding upper part (1) and lower part (0xfffffffe),
> the result is 0xffffffff which doesn't carry.
> Any lower value will not carry either.
>
> And behind the fact that this commit is useless, it also kills the
> whole purpose of having an arch specific inline csum_add()
> because the resulting code gets even worse than what is obtained
> with the generic implementation of csum_add()
:-)
> And the reverted implementation for PPC64 gives:
>
> 0000000000000240 <.csum_add>:
> 240: 7c 84 1a 14 add r4,r4,r3
> 244: 78 80 00 22 rldicl r0,r4,32,32
> 248: 7c 80 22 14 add r4,r0,r4
> 24c: 78 83 00 20 clrldi r3,r4,32
> 250: 4e 80 00 20 blr
If you really, really, *really* want to optimise this you could
make it:
rldimi r3,r3,0,32
rldimi r4,r4,0,32
add r3,r3,r4
srdi r3,r3,32
blr
which is the same size, but has a shorter critical path length. Very
analogous to how you fold 64->32.
Segher
