Subject: Re: [PATCH] Revert "powerpc/64: Fix checksum folding in csum_add()"

From: Segher Boessenkool <[email-masked]>
Date: Thu, 17 May 2018 09:38:10 -0500

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